Appendix B Hydrology and Physical Environment Assessment

Appendix B Hydrology and Physical Environment Assessment January 2018 Version: Final Draft for EA/NE Review

2 Appendix B Hydrology and Physical Environment Assessment

Contents

B.1. Introduction ................................................................................................... 4

B.1.1. Drought orders on the Test and Itchen ................................................. 4

B.1.2. Gaters Mill and Lower Itchen drought orders ....................................... 5

B.1.2.1. Portsmouth Water’s existing operations ........................................ 5

B.1.2.2. Southern Water’s existing operations ............................................ 6

B.1.2.3. Southern Water’s proposed drought order operations .................. 6

B.1.3. Structure of appendix ............................................................................ 8

B.2. Catchment setting ........................................................................................ 9

B.2.1. Catchment overview .............................................................................. 9

B.2.2. Baseline data availability ..................................................................... 11

B.2.3. Hydrology............................................................................................. 12

B.2.4. Geology ............................................................................................... 14

B.2.5. Hydrogeology ...................................................................................... 16

B.3. Hydrological and hydrogeological impact assessment ............................. 19

B.3.1. Approach ............................................................................................. 19

B.3.2. Reference conditions .......................................................................... 23

B.3.3. Environmental impact pathways ......................................................... 23

B.3.4. Impact on flow ..................................................................................... 24

B.3.4.1. Historical context .......................................................................... 25

B.3.4.2. Relationship of flow impact and drought severity ........................ 25

B.3.4.3. Analysis of example drought events ............................................ 28

B.3.4.4. Common Standards Monitoring Guidance .................................. 31

B.3.5. Impact on river hydraulics ................................................................... 31

B.3.5.1. Implications of drought orders on habitat variables ..................... 31

B.3.6. Impact on groundwater heads ............................................................ 35

B.3.6.1. Impact on Chalk aquifer ............................................................... 35

B.3.6.2. Impact on the hydrological functioning of wetlands ..................... 35

B.3.7. Hydrological and hydrogeological impact summary ........................... 38

B.4. Physical environment assessment ............................................................ 41

B.4.1. Geomorphology ................................................................................... 41

B.4.1.1. Baseline ........................................................................................ 41

B.4.1.2. Assessment .................................................................................. 42

B.4.2. Water quality ........................................................................................ 45

B.4.2.1. Chalk aquifer ................................................................................. 48


B.4.2.2. Reach A-B – River Itchen upstream of Allbrook and Highbridge gauging station .............................................................................................. 48

B.4.2.3. Reach B-C – River Itchen downstream of Allbrook and Highbridge gauging station to Riverside Park gauging station.................... 52

B.4.2.4. Reach C-D – Itchen Estuary (part of Southampton Water WFD waterbody) .................................................................................................... 57

B.4.2.1. Water quality summary ................................................................. 60

B.4.3. Environmental pressures .................................................................... 61

B.4.3.1. Abstraction pressures ................................................................... 61

B.4.3.2. Water quality pressures ................................................................ 62

B.5. Cumulative impacts .................................................................................... 62


B.1. Introduction As part of its Draft Drought Plan 2018, Southern Water is required to undertake environmental

assessments of each of the drought permit and drought order options contained in the plan. This

Appendix forms the hydrological / hydrogeological impact and physical environment assessment for

the proposed drought order to temporarily modify the abstraction licence conditions for Southern

Water’s Lower Itchen sources (Otterbourne groundwater (including Twyford Moors) and surface

water abstraction and Twyford groundwater abstraction), as well as temporary modifications to the

Portsmouth Water Gaters Mill surface water abstraction licence on the Lower Itchen.

The document firstly explains the proposed drought order, then the conceptual understanding of the

sources and their hydrological connections to the River Itchen before estimating the hydrological and

hydrogeological impacts of the drought order. The assessment of the physical environment

considers the impacts on river flows, water levels, water quality and geomorphology. Consideration

is also given to other water users.

B.1.1. Drought orders on the Test and Itchen Southern Water’s resources in its Western Area are dominated by the abstractions on the Rivers

Test and Itchen at Testwood and Otterbourne, respectively. A schematic of these two river systems

is presented in Error! Not a valid bookmark self-reference.. Other key water sources, including

the Environment Agency’s Candover Augmentation Scheme boreholes, are also shown for

reference.

The Testwood and Lower Itchen sources are the subject of a forthcoming public inquiry into proposed

abstraction licence changes that would constrain abstraction at times of low river flows such that

drought orders would be required to help maintain water supplies to customers. Southern Water has

therefore developed four drought orders as part of its Draft Drought Plan 2018 to help maintain water

supplies to Western Area on the assumption that these licence changes are enforced. Due to the

connectivity of the water resources in the Hampshire area, it is proposed that these drought orders

are operated in the following order to limit environmental impact:

Testwood drought order;

Candover drought order (to permit use of the Environment Agency’s Candover Augmentation

Scheme boreholes and discharge water to the River Itchen)

Gaters Mill drought order (to vary Portsmouth Water’s abstraction licence); then

Lower Itchen drought order.

This EAR is concerned with the impacts of the Gaters Mill and Lower Itchen drought orders. The

drought order to vary the abstraction licence conditions for the Portsmouth Water Gaters Mill source

will be used in preference to the drought order to vary the abstraction licence conditions for Southern

Water’s Lower Itchen sources with the view to limiting potential impacts on the River Itchen. In

practice, it is likely that these two drought orders would be applied for simultaneously (potentially as

one single drought order application), and as such they have been assessed here in a cumulative

fashion as a combined drought order.


Figure 1 Schematic of River Test and River Itchen

B.1.2. Gaters Mill and Lower Itchen drought orders B.1.2.1. Portsmouth Water’s existing operations

Portsmouth Water abstracts water from the River Itchen at Gaters Mill less than 1 km upstream from

the Riverside gauging station, which itself is located approximately 600 m upstream of the tidal limit

at Woodmill. Abstraction is restricted by a Hands-off Flow (HOF) condition at the Riverside Park flow

gauging station. The Portsmouth Water abstraction licence summary details are provided in Table

1.

Table 1 Portsmouth Water’s Gaters Mill abstraction licence details

Source Licence number Daily (Ml/d)

Annual (Ml/d)

Conditions

Gaters Mill 11/42/22.10/134 45.5 15916.0 Riverside Park gauging station HOF of 194 Ml/d

Data source: EA data request in January 2017

MU5

MU4

MU1

MU3

MU2

Otterbourne

Lasham

Totford

Candover scheme

Easton

Twyford

To tide

Gaters MillMU6

Eastleigh

Southampton

Allbrook & Highbridge

Riverside ParkTestwood

Winchester

Romsey

To tide

Management Unit (MU)

Surface water abstraction

Groundwater abstraction

Groundwater scheme for river augmentation

Legend

Surface water gauging station

HOF location


B.1.2.2. Southern Water’s existing operations

Southern Water operates a number of water sources on the Lower Itchen which are collectively

referred to in this assessment as the ‘Southern Water Lower Itchen sources’. These consist of:

Otterbourne, which comprises of:

- Otterbourne surface water (SW) abstraction

- Otterbourne groundwater (GW) abstraction, which includes abstraction from Twyford

Moors

Twyford GW abstraction.

Revised abstraction licence details for the Lower Itchen abstraction licences were set out in the Site

Action Plan of The Habitats Directive Review of Consents for the River Itchen Special Area of

Conservation (SAC). Time was allowed for Southern Water to bring in replacement sources of water

to maintain water supply resilience, but on 17th November 2016, the Environment Agency gave notice

that it would vary the terms of the Lower Itchen licences under Section 52 (s52) of the Water

Resources Act 1991 (as amended by Section 22 of the Water Act 2003) and Regulation 31 of the

Water Resources (Abstraction and Impounding) Regulations 2006 before these alternative supplies

could be developed. The current licence details (as of September 2017) and the proposed

Environment Agency Section 52 (s52) revisions to the Lower Itchen sources licence conditions are

summarised in


Table 2.

For the purpose of this assessment, the HOF proposed under the s52 abstraction licence conditions

will simply be referred to as the HOF.

B.1.2.3. Southern Water’s proposed drought order operations

Southern Water’s Draft Drought Plan 2018 covers the period 2018-2023 and assumes that the s52

abstraction licence variation has been implemented in full. Water resources modelling identifies that

the s52 abstraction licence conditions will constrain abstraction under low flow and drought

conditions. Therefore, drought orders may be required as set out in


Table 3 to maintain water supplies to customers in the Western Area.

The drought order for the Portsmouth Water Gaters Mill source will be used in preference to the

drought order for Southern Water’s Lower Itchen sources with a view to limiting potential impact on

the River Itchen. In practice, however, it is likely that these drought orders would be applied for

simultaneously, and as such have been assessed here in a cumulative fashion. The Portsmouth

Water drought order will enable Portsmouth Water to continue to provide a bulk treated water supply

to Southern Water in drought conditions of up to 15 Ml/d.


Table 2 Southern Water’s Lower Itchen abstraction licence details1,2,3,4

Source Licence number

Daily (Ml/d)

Annual (Ml/d)

Conditions

Current licence

Otterbourne GW

11/42/22.7/94 71.6 (68.19*)

21,230 (20,002*)

3.4 Ml/d compensation discharge to the Nightingale Stream Twyford

Moors GW

Twyford GW

11/42/22.6/92 36.37 13,319

Otterbourne SW

11/42/22.6/93 45.46 16,638

s52 proposed abstraction licence changes (and used for this assessment)

Lower Itchen sources aggregate

11/42/22.7/94 11/42/22.6/92 11/42/22.6/93

42,000 Ml per year (115.1 Ml/d)

Maximum monthly abstractions:

• June – 4,110 Ml per month (137.0 Ml/d)

• July – 3,940 Ml per month (127.1 Ml/d)

• August – 3,445 Ml per month (111.1 Ml/d)

• September – 2,280 Ml per month (76.0 Ml/d)

and Allbrook & Highbridge gauging station hands-off flow (HOF) of 198 Ml/d.

* Amount for PWS abstraction

1 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Otterbourne (surface water). 2 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Otterbourne (groundwater) 100641 3 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Twyford Moors 4 HSI, 2001. Water Resource Source Filing: Sussex Coast Resource Zone Twyford 100055


Table 3 Summary details of the proposed Lower Itchen Drought Orders

Portsmouth Water’s Gaters Mill source (Gaters Mill drought order)

HOF control Riverside Park

Watercourse River Itchen

Abstraction source Gaters Mill Surface Water

Normal HOF / licence details

Licence details as per Table 1

Proposed drought permit / order

Reduction in the HOF at Riverside Park from 194 to 150 Ml/d

Permit or order Drought order

Yield (Ml/d) 44*

Option 2 – Southern Water’s Lower Itchen sources (Lower Itchen sources drought order)

HOF control Allbrook & Highbridge

Watercourse River Itchen

Abstraction sources Otterbourne SW Otterbourne GW (including Twyford Moors) Twyford GW

Normal HOF / licence details

Licence details as per Table 2

Proposed Drought Permit / Order

Reduction in the HOF at Allbrook & Highbridge from 198 to 160 Ml/d

Permit or Order Drought order

Yield (Ml/d) 38*

*Assessed as the difference between the s52 HOF and the proposed drought order HOF

B.1.3. Structure of appendix This appendix is set out as follows:

Section B.2 Hydrological and hydrogeological impact assessment;

Section B.3 Physical environment assessment; and

Section B.4 Cumulative impacts.


B.2. Catchment setting This section details the understanding of the River Itchen catchment, enabling an assessment of the

impact on the drought orders on hydrology and the physical environment to be undertaken in later

sections.

B.2.1. Catchment overview The River Itchen in Hampshire supports a range of diverse plant and wildlife species5. As such, the

river and many of its tributaries are designated as Sites of Special Scientific Interest (SSSIs) under

Section 28 of the Wildlife and Countryside Act 1981 (as amended and inserted by section 75 and

Schedule 9 of the Countryside and Rights of Way Act 2000), Section 17 of the Water Resources Act,

1991 and Section 4 of the Water Industry Act, 1991. In addition to this, the River Itchen is also

internationally important for its wildlife and habitat and is designated as a Special Area of

Conservation (SAC) under the European Commission Habitats Directive (River Itchen SAC).

The River Itchen supports game fishing, largely provided by brown trout, and to a lesser extent

salmon and sea trout5. Almost the entire river is managed to maintain and facilitate fishing for brown

trout, with fishing for sea trout and Atlantic salmon (Salmo salar) also taking place along the lower

reaches. In the uppermost reaches of the River Itchen, native populations of brown trout (Salmo

trutta) are believed to persist, and bullhead (Cottius gobbio) and brook lamprey (Lampetra planeri)

are notable elements of the natural fish fauna and are of European importance.

The Itchen valley is also important in terms of its landscape, having extensive water meadows and

associated historic landscape features, structures and mills. The water meadows are still used

extensively for agriculture, although they are not generally ‘floated’ in the same way as they would

have been traditionally. In the headwaters, upstream of Itchen Abbas, multiple watercress beds still

operate commercially and there are also a number of fish farms in operation.

As a result of its historic, commercial and amenity value, the river has been progressively and

extensively managed and modified over time. It now comprises a complex array of multiple channels

with mill races and the historic Itchen Navigation, a canal between Southampton and Winchester.

The banks and in-channel vegetation are, in large sections, heavily maintained for ease of access

for fishing activities, although the views and practices on this are progressively changing with

approaches to in-stream and riparian vegetation management gradually becoming more sympathetic

to the riverine habitat.

The Itchen is augmented by a number of spring and pumped water sources along its length.

The study area is shown on Figure 2.

5 Atkins, 2013. Test and Itchen River Restoration Strategy Technical Report


Figure 2 Study area

Based upon: the Ordnance Survey Map by Southern Water by permission of Ordnance Survey on behalf of the controller of Her Majesty's Stationery Office. Crown Copyright 1000019426


B.2.2. Baseline data availability The surface water gauging stations along the River Itchen and its tributaries are listed in Table 4

along with the data availability from the National River Flow Archive6.

Table 4 Surface water flow monitoring along the River Itchen6

Gauge River Gauge location in relation to HOF

Data range Data frequency

Sewards Bridge Cheriton Stream Upstream Allbrook & Highbridge

1956 - 2015 Daily

Drove Lane River Alre Upstream Allbrook & Highbridge

1970 - 2015 Daily

Borough Bridge Candover Stream

Upstream Allbrook & Highbridge

1970 - 2015 Daily

Easton River Itchen Upstream Allbrook & Highbridge

1975 - 2015 Daily


River Itchen HOF location for Southern Water’s Lower Itchen sources

1958 - 2015 Daily

Riverside Park River Itchen

HOF location for Portsmouth Water’s Gaters Mill source. Downstream of Allbrook & Highbridge

1981 - 2015 Daily

Data source: National River Flow Archive, accessed January 2017

The hydrogeological monitoring points within a 5 km radius of Otterbourne are listed in Table 5. All

observation boreholes listed in Table 5 are assumed to be monitoring groundwater levels in the

Chalk unless stated in the monitoring name (as per the table).

The Portsmouth Water Gaters Mill source on the River Itchen is located on low permeability Tertiary

deposits and is unconnected to the underlying Chalk aquifer (see Section B.2.5). As such,

groundwater monitoring near this source is not a relevant parameter for this assessment.

Table 5 Groundwater monitoring sites within 5 km of Otterbourne7

Name Grid reference Approximate distance from Otterbourne (km)

Otterbourne8D SU47032323 0.0

Otterbourne8C SU47032323 0.0

Twyford Moors 1 SU47422320 0.0

Oakwood Copse SU46482371 0.8

Twyford Moors SU47952361 1.0

Highways Road SU46362431 1.3

Four Dell Farm, Shaw SU45572467 2.1

Twyford Reservoir SU49292405 2.3

Martins Fields, Comp SU46432606 3.0

New Barn Farm SU48612592 3.1

6 National River Flow Archive (http://nrfa.ceh.ac.uk/ accessed January 2017) 7 Environment Agency GeoData store (https://data.gov.uk/data/search accessed January 2017)

http://nrfa.ceh.ac.uk/

https://data.gov.uk/data/search


Name Grid reference Approximate distance from Otterbourne (km)

Hazely Down Farm SU50252528 3.8

Hatchers Lane, Henst SU50992408 4.0

5 Bridges Rd Ob7 SU47412732 4.1

Chalk Dale Owslebury SU51152265 4.1

Old Kennels SU45122693 4.2

Upper Sharland, Hur SU43602581 4.3

IBM Ob 14 A31/A3090j SU42832484 4.5

Morestead Hill SU50852575 4.6

Data source: Environment Agency GeoData store accessed January 2017

B.2.3. Hydrology River Itchen

The main channel of the River Itchen is approximately 45 km in length with a surface catchment of

around 470 km2[5]. The headwater tributaries consist of the Cheriton Stream from the south, River

Arle from the east and Candover Stream from the north. These three tributaries converge between

New Alresford and Itchen Stoke. The river then takes its course in multiple channels (including the

Itchen Navigation) through the city of Winchester and flowing broadly in a south-westerly direction

and discharging into Southampton Water. The river becomes tidal downstream of Woodmill.

The Itchen Navigation is an 18th century canal system linking Winchester to the sea at Southampton

and would have transported coal and other goods. It divides the flow of the River Itchen in

Winchester and flows alongside the river through Shawford and Twyford, then disappears in the

Bishopstoke area and reappears as a dry watercourse beside Eastleigh (Southampton) airport

before re-joining the river at Mansbridge. The stretch is internationally important to wildlife as part

of the larger River Itchen SAC.

There are three gauging stations on the River Itchen and three gauging stations on the headwater

tributaries. The flow statistics for these six gauges are shown in Table 6 and highlight the increase

in flow with distance downstream in the catchment. Of the three headwater tributaries, the River

Alre provides most water to the Upper River Itchen due to its very large groundwater catchment,

whilst the flows in the Candover Stream and Cheriton Stream are generally similar. The River Itchen

continues to gain flow from the Chalk between Alresford and Easton, but between Easton and the

Tertiary boundary at Allbrook & Highbridge, this gain in flow is markedly lower. Southern Water’s

Lower Itchen sources are located just upstream of Allbrook & Highbridge. Downstream of Allbrook

& Highbridge, the river flows over low permeability bedrock and connectivity with the Chalk aquifer

is negligible - river flow is influenced by surface water management activities, such as the discharge

from the large Southern Water Chickenhall Wastewater Treatment Works (WwTW) at Eastleigh and

Portsmouth Water’s surface water abstraction at Gaters Mill just upstream of Riverside Park gauging

station. The Monks Brook joins the River Itchen downstream of Riverside Park gauging station, just

above the tidal limit.

The River Itchen gains its water from the Chalk aquifer, which supplies most of the streams and

rivers in the area, as well as most of the water abstracted in the area8. Chalk rivers are characterised

by a baseflow dominant flow regime as shown by the Base Flow Index (BFI): the slow release of

8 Environment Agency, 2012, Catchment Abstraction Management Strategy (CAMS) Conceptualisation


water from the aquifer attenuates rapid recession during periods of low and/or no rainfall events and

recharge to the Chalk can attenuate rapid surface water runoff from high rainfall events.

Typically, Chalk rivers tend to have relatively few tributaries on areas of Chalk outcrop. As a result,

the drainage density is low due to minimal surface runoff. As evident from Table 6, the River Itchen

is subject to a relatively stable flow regime due to a high BFI (which describes the ratio of annual

baseflow in a river to the total annual run-off)5. There are few other permanent surface water features

on the Chalk outcrop because the nature of the soils and depths to the water table are such that all

rainfall either evaporates or infiltrates. There is one unnamed tributary in the vicinity of Otterbourne

that follows the line of the Chalk outcrop and joins the River Itchen to the south of the Otterbourne

abstraction. Based on the geology, it is assumed that this stream is baseflow fed, capturing the

groundwater that overflows as the Chalk dips beneath the Tertiary sediments. Downstream of

Otterbourne, the river network density increases and the BFI reduces, reflecting the presence of

these lower permeability deposits.

There have been two groundwater schemes for river augmentation of the River Itchen, owned,

licensed to and operated by the Environment Agency and its predecessor bodies. The original

Candover Augmentation Scheme was developed in 1976 and was followed by the Further Itchen

River Augmentation Scheme on the River Alre in 1984. The Environment Agency has recently

surrendered its abstraction licence for the Alre scheme and it is understood that the boreholes are

being decommissioned. Further downstream, Southern Water provide up to 3.4 Ml/d compensation

discharge from the Otterbourne GW source to the River Itchen via the Nightingale Stream.

Nightingale Stream is located at Otterbourne, on the east side of the River Itchen.

Table 6 Summary of flow statistics from flow gauges in the River Itchen catchment6

Site Name River Period of record

BFI Mean annual flow (Ml/d)

Q95 (Ml/d)

Comment

Sewards Bridge

Cheriton Stream

1956 - 2015

0.96 58.0 24.0 Cheriton Stream is ephemeral in upper reaches.

Drove Lane River Alre 1970 - 2015

0.98 141.6 89.4 Baseflow dominated regime with narrow flow range

Borough Bridge

Candover Stream

1970 - 2015

0.96 50.8 24.6 Runoff influenced by groundwater abstraction. Impact of the Itchen groundwater scheme for river augmentation is notable during droughts.

Easton River Itchen

1975 - 2015

0.97 375.9 226.6 Largely natural baseflow dominated regime


River Itchen

1958 - 2015

0.96 476.4 259.3 Combination of flows measured at two gauging stations: Allbrook & Highbridge.

Riverside Park

River Itchen

1981 - 2015

0.91 510.4 246.2 Flows artificially influenced by abstractions at Gaters Mill and Otterbourne, and a large sewage treatment works discharge at Chickenhall, Eastleigh.

Data source: National River Flow Archive, accessed January 2017


River Itchen tributaries – Nun’s Walk Stream, Bow Lake Stream, Monks Brook and Allington

Lane Stream

As stated above, the River Itchen has few tributaries over the Chalk outcrop. Aside from the three

headwater tributaries, three tributaries that join the River Itchen further downstream have been

identified as distinct water bodies. Nuns Walk Stream joins the River Itchen immediately

downstream of Easton, Bow Lake Stream joins downstream of Allbrook & Highbridge gauging

station, and Monks Brook joins downstream of Riverside Park gauging station. The fourth main

tributary, Allington Lane Stream, is part of the Itchen waterbody.

There are no gauging stations on Nun’s Walk Stream, Bow Lake Stream or Allington Lane Stream.

Nun’s Walk Stream is anticipated to exhibit similar behaviour to the River Itchen, namely high

baseflow-driven responses from connectivity to the Chalk aquifer. The degree of baseflow

dominance in Bow Lake Stream is less evident - due to the geology (Section B.2.1.4), the upper

reaches are believed to capture groundwater. Monks Brook and Allington Lane Stream traverse

over low permeability deposits. Unlike the River Itchen, Monks Brook reacts quickly to rainfall and

has naturally lower flows. The BFI for Monks Brook is 0.426.

B.2.4. Geology The geology of the study area is shown in Figure 3 and the bedrock and superficial deposits are

listed in


Table 7 and


Table 7 Bedrock geology

Group (Epoch) Formation Lithological description Hydrogeology

Bracklesham Group (Eocene)

Marsh Formation

Sand, silt and clays

Essentially no groundwater

Earnley Sand Formation

Wittering Formation

Thames Group, London Clay (Eocene)

London Clay

Silty clays and sands

Whitecliff Sand Pebble Beds

Durley Sand

Portsmouth Sand

Nursling Sand

Lambeth Group (Palaeocene)

Reading Beds Mottled clays, locally sandy

White Chalk Subgroup (Upper Cretaceous)

Tarrant Chalk Soft white chalk with relatively widely spaced but large flint seams

Highly productive Chalk aquifer

Newhaven Chalk Soft to medium hard smooth white chalks with numerous marl seams and flint bands1

Seaford Chalk Soft white chalk with seams of large nodular and semi tabular flint1

Lewes Nodular Chalk

Interbedded hard to very hard nodular chalks with soft to medium hard grainy chalks and marls1

New Pit Chalk Pure massively bedded chalks with conspicuous marl seams10

Holywell Chalk Medium hard to very hard nodular chalks10

Grey Chalk Subgroup (Upper Cretaceous)

ZigZag Chalk Medium hard greyish, becoming white blocky chalk, with some thin limestones1

Table 8 respectively.

The Chalk is at outcrop for the majority of the River Itchen catchment, which is estimated to be 80%

Chalk5. In the lower catchment, the Chalk is overlain by Tertiary sediments deposited unconformably

on an eroded surface. Gentle folding and erosion brings the older Chalk deposits to the surface, for

example in the core of the Winchester anticline, and can also reduce the Chalk thickness.

As stated earlier, the Chalk is overlain by Tertiary deposits to the south of the catchment, south of

Otterbourne. The deposits in this area comprise the Readings Beds that directly overlie the Chalk

and the London Clay and Bracklesham Beds9. These low permeability formations are essentially

devoid of groundwater and generate rapid runoff into the River Itchen and its tributaries in these

areas5.

9 BGS, 1987 1:50,000 Solid and Drift Geology of Southampton, Sheet number 315


There is a reasonable coverage of superficial deposits, with Alluvial and River Terrace deposits lining

the River Itchen valley. There is a swath of Clay-with-Flints crossing the study area from the north-

west to the south-east. These have been reported to be up to 8 m thick10.

10 Entec, 2003 River Itchen Catchment Groundwater Modelling Study


Table 7 Bedrock geology

Group (Epoch) Formation Lithological description11 Hydrogeology

Bracklesham Group (Eocene)

Marsh Formation

Sand, silt and clays

Essentially no groundwater

Earnley Sand Formation

Wittering Formation

Thames Group, London Clay (Eocene)

London Clay

Silty clays and sands

Whitecliff Sand Pebble Beds

Durley Sand

Portsmouth Sand

Nursling Sand

Lambeth Group (Palaeocene)

Reading Beds Mottled clays, locally sandy

White Chalk Subgroup (Upper Cretaceous)

Tarrant Chalk Soft white chalk with relatively widely spaced but large flint seams

Highly productive Chalk aquifer

Newhaven Chalk Soft to medium hard smooth white chalks with numerous marl seams and flint bands1

Seaford Chalk Soft white chalk with seams of large nodular and semi tabular flint1

Lewes Nodular Chalk

Interbedded hard to very hard nodular chalks with soft to medium hard grainy chalks and marls1

New Pit Chalk Pure massively bedded chalks with conspicuous marl seams10

Holywell Chalk Medium hard to very hard nodular chalks10

Grey Chalk Subgroup (Upper Cretaceous)

ZigZag Chalk Medium hard greyish, becoming white blocky chalk, with some thin limestones1

Table 8 Superficial geology

Deposit (Epoch)

Formation Lithological description11 Occurrence in the study area

Fluvial Deposits (Quaternary)

Alluvium Clay, silt, sand and gravel – soft to firm consolidated

River Itchen valley

Fluvial Deposits (Quaternary)

River Terrace Deposits

Sand and gravel, locally with lenses of silt, clay or peat

River Itchen valley

Residual Deposits Group (Quaternary)

Clay-with-Flints Formation

Clay, silt, sand and gravel – unbedded and heterogeneous

Patches north west to south east across study area

Mass Movement Deposits (Quaternary)

Head Clay, silt, sand and gravel – poorly sorted and stratified

Dry valleys

11 BGS GeoIndex Onshore website (http://mapapps2.bgs.ac.uk/geoindex/home.html)


Deposit (Epoch)

Formation Lithological description11 Occurrence in the study area

Chemical Deposits (Quaternary)

Tufa Inorganic or organic calcium carbonate or silica deposited at or near springs and seepages

River Itchen valley around Otterbourne

Intertidal Deposits (Holocene)

Tidal Flat Deposits

Consolidated soft silty clay, with layers of sand, gravel and peat Estuary

B.2.5. Hydrogeology The hydrogeology of the River Itchen has been comprehensively reviewed as part of the

development of the Test and Itchen groundwater model (T&I GW model)10.

The main aquifer is the Chalk aquifer. Whilst the regional groundwater flow is towards the south12,

the flow direction and groundwater catchments vary seasonally10 and are locally influenced by

abstractions and the River Itchen.

The Chalk aquifer towards the north of Otterbourne is unconfined (except for superficial coverage).

Confining conditions are present towards the south, as the Chalk dips beneath the lower permeability

younger formations of the Reading Beds. These formations act as an aquiclude. As expected, the

transmissivity and storage of the confined aquifer are lower than the unconfined aquifer10.

The Chalk is a ‘dual porosity’ aquifer, consisting of fractures and fissures as well as porous fine

grained matrix, and typically has complex spatial variations in hydraulic conductivity and storage.

Entec10 noted that summer water levels in observation boreholes do not vary much even in a drought

year, and hypothesised that, whilst in some instances this may be due to borehole construction, this

response may reflect reduced permeability with depth.

Chalk permeability is also affected by preferential flow paths which lead to increased dissolution.

Indeed there is geophysical evidence within the catchment of increased fissuring and enhanced

permeability at certain elevations which has been hypothesised to be related to the past or present

groundwater table. There is marked spatial permeability contrasts between the valleys and

interfluves, with higher permeability along the valleys where groundwater flow is concentrated.

Entec10 concluded that the River Itchen is locally karstic and consequently the hydraulic parameters

of the Chalk aquifer are highly variable.

There is believed to be high connectivity between the Chalk aquifer, superficial deposits and the

River Itchen. Baseflow, particularly to the upper reaches, is an important contribution to surface

water flow. This is particularly true in the Alre catchment where groundwater levels are close to the

surface and boreholes can become artesian10. Chalk groundwater levels near the watercourse

typically show relatively flat hydrographs indicating good connectivity with the surface water (or high

storativity from increased fissuring). Moreover, a similar response is observed in the drift deposits

along the river valley. The exception to this is in the vicinity of abstractions and adits (part of the

Otterbourne GW source) where groundwater levels are depressed.

This surface water–groundwater connectivity is highlighted by the conclusions from the Easton

pumping test10 from which it has been estimated that the groundwater source obtains 80% of its

water from the river. Similarly, the diurnal abstraction pattern from the Otterbourne GW source can

be identified in a subdued form in the flow record at Easton gauging station10. It may therefore be

assumed that Otterbourne GW also has a high contribution from surface water. It is worth noting

12 BGS, 1979. Sheet 9 : Hydrogeological Map of Hampshire and the Isle of Wight (1:100,000)


that Entec10 also identified that significant changes in Otterbourne SW, and probably the

groundwater component, resulted in fairly rapid recovery responses in flow at Allbrook & Highbridge.


Figure 3 Conceptualisation of the Lower Itchen

Based upon: the Ordnance Survey Map by Southern Water by permission of Ordnance Survey on behalf of the controller of Her Majesty's Stationery Office. Crown Copyright 1000019426 and British Geological Survey data 2008/00


B.3. Hydrological and hydrogeological impact assessment The Gaters Mill and Lower Itchen drought orders would reduce the HOF conditions at Riverside Park and Allbrook & Highbridge, respectively, so as to enable continued abstraction from Portsmouth Water’s Gaters Mill

source and Southern Water’s Lower Itchen sources during severe drought conditions. The purpose of this section is to assess the potential hydrological impact caused by these drought orders.

Although the Gaters Mill drought order will be utilised in advance of the Lower Itchen drought order, modelling has shown that the two orders will be required in quick succession. Therefore, this assessment considers

the cumulative impact from the two drought orders as a precautionary approach.

B.3.1. Approach For the Test and Itchen drought order environmental assessments, hydrological impacts have been assessed using a combination of Southern Water’s Western Area Aquator water resources model and the T&I GW

model.

Southern Water’s Aquator model was developed for the Water Resource Management Plan (WRMP) 2014 and it has been refined during 2017 for use in Southern Water’s draft WRMP19 and draft Drought Plan 18.

Aquator is an industry standard tool for modelling water demand, abstractions, river flow and supply deficits.

The T&I GW model has been applied to a range of water resources investigations by both the Environment Agency and Southern Water over recent years. Although there are some differences between the modelled

behaviour and observations, for example around Abbotstone, calibration at the Borough Bridge gauges is good and the model is accepted as the best available tool for assessing the complex relationships between

climate, abstractions, groundwater levels and flows.

A schematic summarising the key inputs, outputs and relationships between the two models is show in Figure 4.

Figure 4 Inputs and outputs from the T&I GW model and the Aquator model

Additional details of the modelling tools and approach are set out in a separate method statement, but other key points to be aware of are:

The T&I GW model operates on bi-monthly stress periods. The naturalised river flow inputs to the Aquator model are in daily timesteps which have been interpolated between these bi-monthly outputs.

The impacts of a drought order scenario are compared with a ‘reference condition’ – the situation that would occur during drought but without the drought order in place (described further in Section B.3.2)

The main steps in the hydrological impact assessment are summarised in Figure 5.


Figure 5 Main steps in hydrological impact assessment

In-line with the approach taken for the draft WRMP19, a stochastically-generated climate sequence has been used to help assess potential demand and supply balances and environmental impacts under more severe

and extreme droughts. To generate naturalised flows for the Aquator model (as shown in Figure 4), two climate sequences were simulated in the T&I GW model:

An 80-year historical period from 1918 to 1997; and

A 2000-year stochastic sequence.

The T&I GW model is the best available tool to assess the impacts of groundwater abstraction and augmentation from the Candover boreholes because it includes explicit, three-dimensional representation of groundwater

and surface water processes, whereas the Aquator model is based on simplified assumptions for these interactions. However, because the run times for a 2000-year sequence in the groundwater model are very long,

a selection of droughts from the 80-year and 2000-year sequences were compiled for simulation in the groundwater model. The compiled sequence includes a period of ‘run-in’ to the target droughts followed by a

period of recovery (Table 9). This provides a manageable approach to assessing the differences in impacts on flow and groundwater heads between the reference condition and drought order scenarios.

Table 9 Compiled climate sequence used in the T&I GW model

Simulation year Climatic year Run 163 Year Purpose

1 Average warm up

2 Average warm up

3 Average warm up

4 1919 2719 historical run in

5 1920 2720 historical drought



8 Average recover

9 Average recover

10 Average recover






16 Average recover

17 Average recover

18 Average recover




22 Average recover

23 Average recover

24 Average recover

Step 1: Model reference conditions for historical and stochastic climate sequences

Step 2: Impacts of drought orders on low flows

Step 3: Impacts on habitat variables

Step 4: Impacts on groundwater heads (if relevant)


Simulation year Climatic year Run 163 Year Purpose

25

Stochastic

3704 SWS run in

26 3705 SWS WRMP19 selected 1 in 50




30 Average recover

31 Average recover

32 Average recover

33

Stochastic

4080 SWS run in





38 Average recover

39 Average recover

40 Average recover

41

Stochastic

4132 SWS run in





46 Average recover

47 Average recover

48 Average recover

49

Stochastic

4313 SWS run in





54 Average recover

55 Average recover

56 Average recover

57

Stochastic

3286 SWS run in







92

Average

recover

93 recover

94 recover

This environmental assessment utilises a combination of output from both the Aquator and T&I GW models. It is therefore important to understand how these impacts predicted from both models are correlated and

whether comparison of impacts between models is a valid activity.


Figure 6 presents the predicted surface water impact during the extreme 1:500-year drought (stochastic year 3290) for both of these models at Allbrook & Highbridge. Figure 6 demonstrates that the two models generate

comparable predictions of the surface water impact. Both models are based on naturalised flows in bi-monthly time steps derived from the T&I GW model. The influence of demand, restrictions and abstraction is then

simulated in daily time steps in the Aquator model whereas they are simulated in bi-monthly time steps in the T&I GW model. The other main difference is that the T&I GW model includes an explicit 3-dimensional

representation of groundwater-surface water interactions, whereas in the Aquator model these are based on more simplified assumptions – as informed by the GW model. Bearing these differences in mind, the

comparison shown in


Figure 6 and equivalent interpretation for alternative drought years, provides confidence that the two models indicate similar surface water impacts.


Figure 6 Relationship of predicted surface water impacts between the Aquator and T&I GW model1314

Aquator model runs DP0003_a (without drought orders) and DP0004_a (with drought orders).

Test and Itchen model runs T&I 172(DP0003) (without drought orders) and T&I 173(DP0004) (with drought orders)

B.3.2. Reference conditions During any drought, a number of factors determine the ‘reference conditions’ for river flows. The principal factors are:

Climate

Water demand

Pre-agreed demand restrictions

The deployable output of sources (taking account of licence constraints); and

Southern Water’s water imports and internal transfers.

The reference conditions for the Test and Itchen drought orders are based on water demand and restrictions and the resultant abstractions and transfers in the Hampshire area, assuming that the River Itchen s52

licence changes and the s52 abstraction licence changes for the Testwood abstraction (River Test) are in place. These conditions are simulated for an 80 year historical climate (1918-1997) and a 2000 year stochastic

climate sequence.

With regards to the Candover Augmentation Scheme drought order, the reference condition assumes that no abstraction takes place from the Environment Agency’s Candover Augmentation Scheme boreholes.

B.3.3. Environmental impact pathways The two Lower Itchen drought orders have the potential to affect the environment in subtly different ways. Whilst this impact assessment considers the cumulative impact of the two

drought orders, these differences, set out below, should nevertheless still be noted.

13 Aquator run version ‘a’ outputs are presented in Figure 6 since output from this run was used as inputs to the T&I model (see Figure 4). The Aquator model has since been re-run with minor changes to the assumptions around the triggering of the Gaters Mill drought order. Therefore, output from Aquator versions ‘g and ‘f’ without and with drought orders, respectively) is presented later. However, the implications of this on groundwater impact (for which the T&I model is used) is negligible, therefore the T&I model has not been re-run. 14 Atkins spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx

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Gaters Mill drought order

This drought order has the potential to impact surface freshwater flows in the 1.6 km reach of the River Itchen between Gaters Mill and Woodmill. Downstream of Woodmill, the river is tidal and the small changes in

flow are considered to be negligible in comparison to the influence of the tidal system. Potential environmental impacts in the tidal system are considered further in Appendix D of the EAR.

Over this 1.6 km extent to Woodmill, the river traverses over low permeability Tertiary deposits. It is therefore hydraulically unconnected from the underlying Chalk aquifer, which is over 100 m below surface (the Chalk

is recorded as being 185 m at Bunkers Hill borehole, approximately 12 km west of the Itchen15). Therefore no groundwater impacts are anticipated as a result of this drought order.

Southern Water Lower Itchen sources drought order

The Southern Water Lower Itchen sources drought order has the potential to generate both surface water and groundwater impacts.

The drought order will result in a flow reduction at Allbrook & Highbridge. This flow impact could be translated downstream to the tidal limit at Woodmill. It is assumed in this assessment that Southern Water’s

Chickenhall WwTW at Eastleigh, located between Allbrook & Highbridge gauging station and the Gaters Mill abstraction, will discharge a minimum of 20 Ml/d at low flow conditions and that other minor inflows will be

unchanged.

The spatial extent of any impacts would influence a much greater length of the freshwater River Itchen compared to the Gaters Mill drought order so, for this reason, it is proposed that the Lower Itchen sources drought

order is used second to the Gaters Mill drought order. More broadly, the draft Drought Plan 2018 also indicates that these two drought orders should only be implemented after the Testwood and Candover Augmentation

Scheme drought orders to support the Western Area.

Southern Water’s Lower Itchen sources include groundwater abstractions from Otterbourne GW and Twyford. Therefore, abstraction at these sources beyond that anticipated under reference conditions will result in

additional groundwater drawdown. This impact on the Chalk aquifer has the potential consequence of reducing groundwater-surface water interaction over the extent where the Chalk is unconfined (i.e. north of Allbrook

& Highbridge gauging station), with a resulting impact on surface water flows in this reach.

The nature of the drought order impact will be dependent on the operational split of the groundwater and surface water sources; increasing the component of groundwater abstraction will increase the groundwater

impact. However, the overall impact on surface water flow may decrease as more water is obtained at the expense of aquifer storage.

B.3.4. Impact on flow The potential impact that the Gaters Mill and Lower Itchen sources drought orders would have on flow in the River Itchen has been assessed by comparing the reference condition flows (drought flows with no drought

order in place) to those predicted to arise in a drought with the drought order in place. To do this, the assessment has considered both the historical and stochastic flow timeseries generated from the Southern Water

Aquator model runs DP0003b (without drought orders) and DP0004b (with drought orders).

B.3.4.1. Historical context

Figure 7 provides an initial understanding as to the likely scale and frequency of flow impacts associated with the drought order under historical climate conditions (1918 – 1997). Figure 7 plots the modelled daily mean

historical flows at Allbrook & Highbridge gauging station (the HOF location for the Southern Water Lower Itchen sources) and Riverside Park (the HOF location for the Gaters Mill abstraction) with and without the four

Test and Itchen drought orders in place (Testwood, Candover, Gaters Mill and Lower Itchen). The Testwood and Candover Augmentation Scheme drought orders are assumed to be in

place in advance of the Gaters Mill and Lower Itchen sources drought orders in the model.

Figure 7 demonstrates that both the Lower Itchen and Gaters Mill drought orders would not have been required under the historical climatic conditions examined; neither the Allbrook &

Highbridge HOF nor the Riverside Park HOF are breached. The modelling therefore indicates that these two drought orders are only required for droughts of greater severity. It is important

to note, however, that this assumes the Testwood and Candover Augmentation Schemes drought orders have been implemented first. The Candover Augmentation Scheme drought order would discharge water to the

River Itchen upstream of Easton gauging station, increasing river flow at Otterbourne SW source; this drought order is used on two occasions in the model over the historical period: 1921/22 and 1992.

15 BGS borehole log ID 406528 http://scans.bgs.ac.uk/sobi_scans/boreholes/406528/images/10737902.html, accessed September 2017

http://scans.bgs.ac.uk/sobi_scans/boreholes/406528/images/10737902.html


Figure 7 Historical flow at Allbrook & Highbridge and Riverside Park gauging stations with and without drought orders16

Model run output – DP0003g and DP0004f

B.3.4.2. Relationship of flow impact and drought severity

The Gaters Mill and Lower Itchen sources drought orders are only expected to be required for drought severities greater than those experienced in the historical record. Therefore, the assessment of drought order

impacts requires the use of stochastic time series.

Figure 8 plots the annual minimum flows at the Allbrook & Highbridge and Riverside Park gauging stations, as calculated from the daily mean Aquator flow output, for the two model scenarios (i.e. with and without

drought orders), from the stochastic time sequence. The Y axis has been translated to return periods (as calculated from the Aquator output), plotted in red along the top.

The Allbrook & Highbridge plot shows that the Lower Itchen sources drought order is anticipated to be required during drought severities of approximately 1:150 year or higher. For lower return periods, a series of

sequential demand management measures, bulk supplies, transfers and the implementation of other drought orders cumulatively act to keep river flows above the HOF. These interventions are:

Demand management measures:

Level 1: Drought awareness campaigns;

Level 2: Temporary use bans; and

Level 3: Temporary use bans and non-essential use bans.

Bulk supplies and transfers:

Portsmouth Water bulk supply to a maximum rate of 15 Ml/d (utilised once the ‘Level 1’ demand management intervention has been triggered); then

Utilisation of internal treated water transfers from Southampton West to Southampton East water resource zones to a maximum of 24 Ml/d.

Drought orders:

Testwood drought order; then

Candover Augmentation Scheme drought order.

Beyond approximately a 1:150 year drought event, under reference conditions, the HOF constrains abstraction from Southern Water’s Lower Itchen sources, although this results in large public water supply deficits.

For the most extreme droughts, even with the s52 licence constraints in place and no drought orders implemented, river flows would fall below the HOF. The drought order impact on annual minimum flow is the

difference between the two lines.

The equivalent plot for Riverside Park gauging stations (the HOF location for the Gaters Mill drought order) shows similar features; the flows only fall noticeably below the HOF beyond approximately 1:150 year drought

events.

16 Atkins Spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx

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Riverside Park


Figure 8 Annual minimum flows (daily mean) at Allbrook & Highbridge and Riverside Park gauging stations under the stochastic climate17

Model output from DP0003g and DP0004f. The timeseries for circled drought years are plotted in Figure 9. Analysis has been conducted on annual minimum flow. Thus drought 3290/3291 appears twice in this plot.

17 Atkins Spreadsheet: DP0003g_DP0004f_DP0005b_INQ005_INQ0006_INQ0007_INQ0008f_INQ009g_INQ010g_FFC_v0.1_ITCHEN.xlsx

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Return period (years) Riverside Park HOF 194Ml/d Riverside Park DrO HOF 150 Ml/d

All Drought Orders (DP0004f) No Drought Orders (DP0003g)


B.3.4.3. Analysis of example drought events

The potential impact on flow at the Allbrook & Highbridge and Riverside Park gauging stations during example drought events from the stochastic Aquator modelling sequence has been considered.

Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts


Model runs DP0003g and DP0004f. Drought events of increasing severity left to right, top to bottom. Drought years are marked on Figure 8


presents the modelled flow at Allbrook & Highbridge and Riverside Park gauging stations with and without drought orders in place for droughts of increasing severity. The drought years can be cross-referenced with

Figure 8, which provides context as to drought severity and maximum impact. The flow impact is summarised in Table 10, which also shows the public water supply deficits that are predicted to have occurred if the

drought orders were not in place. The flow data in brackets in Table 10 indicate the equivalent river flow without the drought orders in place.

Three of the stochastic drought years presented have been selected from Southern Water’s draft WRMP19 for 1:200 and 1:500 events. The years used in the draft WRMP19 are:

1 in 200 Events: Stochastic years 3594, 4503, 4564 and 4315

1 in 500 Events: Stochastic years 2995, 3686 and 3290

Of the four 1:200 year drought events, the stochastic year 4315 has been selected as it is the drought year with the lowest flows based on Southern Water’s Aquator modelling. Of the three 1:500 year drought events,

year 3290, which actually extends into 3291, is the most severe.

The other two drought years presented in Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts




(4782 and 2911) represent intermediate return period drought events. Figure 8 indicates a step-increase in flow impact around drought events with return periods of approximately 1:300 year. Years 4782 and 2911

are representative of events either side of this step-increase.

The magnitude and duration of flow impact depends on drought severity, and varies over the course of the drought. This is in part due to the climatic conditions and the supply-demand balance during the drought, but

also due to the operation of the Candover Augmentation Scheme drought order which discharges water to the River Itchen upstream of the Otterbourne SW abstraction.

Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts




highlights that for the approximate 1:200 year drought events considered, the magnitude by which river flows fall below the HOFs is very small (up to a maximum of 7 Ml/d and 12 Ml/d at Allbrook & Highbridge and

Riverside Park, respectively), and that the durations are very short. Flows are maintained at or very close to the two HOFs by operation of the Candover Augmentation Scheme drought order which augments flow in

the River Itchen upstream of the Lower Itchen sources. This augmentation provides a net benefit to flows in the Upper Itchen relative to the situation without this drought order in place.

Under more severe drought events, the Candover Augmentation Scheme drought order becomes constrained as the drought progresses by either the annual groundwater abstraction limit (3,750 Ml/d) or the seasonal

groundwater abstraction restriction (to 20 Ml/d during May-August rather than 27 Ml/d at other times of the year). As a result, the Candover Augmentation Scheme ceases to discharge water to River Itchen and there

is an increased requirement for the Gaters Mill and Lower Itchen sources drought orders. Thus, flow at the Allbrook & Highbridge and Riverside Park falls below the respective HOFs. Under the extreme 1 in 500 year

drought event (stochastic year 3290), a maximum flow impact of 36 Ml/d and 43 Ml/d at Allbrook & Highbridge and Riverside Park, respectively, is predicted. However, this degree of impact is not sustained throughout

the entire drought event.


Table 10 Balance of low flows at Allbrook & Highbridge and Riverside Park gauging stations with public water supply deficits18

River Itchen low flows with (without) drought orders Public Water Supply deficits without drought orders in place

Allbrook & Highbridge Riverside Park Maximum deficit (Ml/d) Duration of deficit (days)

Minimum flow (Ml/d)

Duration below HoF (days) Minimum flow (Ml/d) Duration below HOF (days)

Stochastic

~1:200 (yr 4315) (WRMP year) 198 0 193 2

101 139 (198) (0) (194) (0)

~1:200 (yr 4782) (Aquator) 191 12 182 12

116 224 (198) (0) (194) (0)

~1:300 (yr 2911) (Aquator) 175 129 165 116

123 271 (198) (0) (194) (0)

~1:500 (yr 3290) (WRMP year) 160 306 151 296

147 426 (193) (32) (191) (17)

Statistics from Aquator model runs DP0003g and DP0004f. Years plotted in Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts

18 Atkins spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx




.


Figure 9 Impacts on daily mean flows at Allbrook & Highbridge and Riverside Park gauging station during example droughts19


19 Atkins spreadsheet: DP0003_g and DP0004_f Aquator output comparison_ITCHENGRAPHS.xlsx

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B.3.4.4. Common Standards Monitoring Guidance

The Joint Nature Conservation Committee (JNCC) Common Standards Monitoring Guidance

(CSMG) methodology defines the minimum set of common standards required to consistently

monitor the condition of features of interest in designated conservation sites (SACs and SSSIs) to

help in their conservation and preservation. Flow targets are one component of a CSMG

assessment20. CSMG targets have not been adopted for the River Itchen, but the Environment

Agency and Natural England are working towards agreeing long term flow targets by 202121.

CSMG flow targets are thresholds for a maximum deviation from daily naturalised flow, expressed

for different components of the flow regime (<Qn95, Qn50-Qn95 and Qn10-Q50)22. Naturalised flows

are normally being derived by undertaking a naturalisation process (removal of estimated abstraction

and discharge influences) on Environment Agency monitoring data.

As discussed above, use of the Lower Itchen drought orders is very infrequent and they are not

predicted to be required during historical climatic conditions. Because CSMG compliance is based

on differences from naturalised flows over recent decades rather than extreme (stochastic) droughts,

implementation of the Lower Itchen drought orders will have no impact on CSMG compliance.

B.3.5. Impact on river hydraulics B.3.5.1. Implications of Lower Itchen drought orders on habitat variables

To assess the potential impact of reductions in river flow in the Lower Itchen on key habitat variables,

hydraulic calculations have been carried out for a series of cross-sections between Otterbourne and

Woodmill at the tidal limit of the River Itchen.

Whilst a hydraulic model (ISIS model) is available for this reach of the River Itchen, it was developed

for the purpose of flood risk assessment and thus calibrated to high flows. There are very few spot

flow gauging records from this reach that could be used to assess calibration and therefore

confidence in the hydraulic model calibration under low flows is limited. Consequently, the model

has not been run as a predictive tool but instead has been used as the source of cross-section

profiles and information on longitudinal gradients. These data have then subsequently been used

to support hydraulic calculations to assess the sensitivity to changes in flow of key habitat variables,

specifically velocity and water depth.

As discussed in Section B.2, the lower reaches of the River Itchen comprise multiple channels

including the historic Itchen Navigation, a canal between Southampton and Winchester. Downstream

of Otterbourne, the Allbrook gauging station is located on the Itchen Navigation and the Highbridge

gauging station on the River Itchen. Based on spot flow data from periods of relatively low flow in

1997 and 1998, the average flow split between these two flow gauging stations was ~75% of flow to

the River Itchen and ~25% of flow to the Itchen Navigation. However, the split of flow between the

River Itchen and Itchen Navigation channel, and indeed between the other channels and ditches in

this complex braided system, could vary significantly depending on the operation of river control

structures. The precise split of flows between the River Itchen and other channels is not known in

any detail and this is one reason why the velocity and water depth calculations presented here should

be taken as indicative only.

In addition to the effects of the flow splits, flows in the River Itchen are also influenced by

abstractions, discharges, inflows from tributaries and any baseflow contributions/losses from

groundwater. However, analysis of data from the T&I GW model indicates changes in low flows

20 JNCC (2016) Common Standards Monitoring Guidance for Rivers. September 2016. 21 193 Rivers Itchen & Test: Proposed targets for SAC and SSSI conservation objectives (based on revised Common Standards Guidance) and interim progress goals for uRBMP (RIVPDF193 Additional) (RIV193ADD). http://publications.naturalengland.org.uk/publication/5953871591505920 22 JNCC (2014) Common Standards Monitoring Guidance for Rivers. http://jncc.defra.gov.uk/pdf/CSM_rivers_jan_14.pdf

http://publications.naturalengland.org.uk/publication/5953871591505920

http://jncc.defra.gov.uk/pdf/CSM_rivers_jan_14.pdf


downstream of Otterbourne are dominated by the effects of the discharge from the Chickenhall

WwTW at Eastleigh and the surface water abstraction at Gaters Mill.

Based on an assumed River Itchen–Itchen Navigation flow split of 75:25 and a low flow discharge

rate of 20 Ml/d at Chickenhall WwTW (as assumed in the Aquator modelling), reach-specific flows

have been estimated for inflow conditions of the Allbrook & Highbridge s52 licence HOF of 198 Ml/d

and equivalent flows for a drought order HOF reduced to 160 Ml/d. Downstream of Gaters Mill, the

assumed inflow conditions are the current abstraction licence HOF of 194 Ml/d and a drought order

HOF reduced to 150 Ml/d (Table 11).

Table 11 Reach-specific flow conditions

Estimated flow (Ml/d)

Otterbourne to Chickenhall WwTW Chickenhall WwTW to Gaters Mill**

Downstream of Gaters Mill

Single channel

Split: 75%- River Itchen*

Split: 25%- Itchen Navigation*

Single channel Single channel

198 149 50 218 194

160 120 40 180 150 *Assumes 75:25 split in total River Itchen flow based on spot flow monitoring in 1997-1998. Allbrook gauging station is on the Itchen Navigation channel, and Highbridge gauging station is on the main River Itchen. **Assumes 20 Ml/d discharge from Chickenhall WwTW.

Thirteen cross-sections between Otterbourne and Woodmill were selected from the hydraulic (ISIS)

model (Figure 10) and, for each of these, based on the cross-section profile and estimated slope

gradient, ‘ratings’ tables based on Mannings calculations were exported from the ISIS model. Depth

and velocity for the reach-specific flows were then calculated from the ratings curves and the results

are summarised in


Table 12.

The main points to take from the results in


Table 12 are:

Velocities at all but three cross-sections are estimated to be above 3 m/s both for the

abstraction licence HOF and the proposed drought order HOF. At the three cross-sections

where velocities are estimated to be below 3 m/s, the change in velocity between the licence

HOF and the reduced drought order HOF is very small (0.01 to 0.02 m/s).

Water depths at all but one cross-section are estimated to be above 0.4 m both for the licence

HOF and the proposed drought order HOF. At the cross-section where the depth is estimated

to be below 0.4 m, the change in depth between the licence HOF and the proposed drought

order HOF is very small (0.04 m).

There are a number of uncertainties that need to be taken into account. Firstly, there are insufficient

spot flow data between Otterbourne and Woodmill with which to assess the accuracy of estimates

of low flow hydraulic characteristics. Secondly, this assessment has been carried out without any

field visits. Therefore, the suitability of cross-sections has been based on map and model data only

and the influence of local controls is unknown. Finally, the Lower Itchen is a highly braided system

with a large number of flow and level control structures. The operation of these control structures

and their influence on flows, velocities and depths in the Lower Itchen is not known. Finally, as noted

in Section 0, even under extreme drought events with 1:200 and 1:300-year return periods, flows

are only predicted to fall below the Allbrook and Highbridge HOF of 198 Ml/d by a relatively small

amount and for only a short duration. Nonetheless, the calculations give a reasonable indication of

the sensitivity of velocities and depths to changes in low flow, and specifically the reduction in flow

due to the drought order changes to the HOFs for the Lower Itchen. The ecological significance of

this is considered in Appendix D of the EAR.


Figure 10 Location of cross sections for hydraulic calculations


Table 12 Depth and velocity calculations for the abstraction licence HOF and proposed drought order

HOF at selected cross-sections

ISIS model cross-section node and reach description

Inflow / HOF (Ml/d)

Flow at Section (Ml/d)

Velocity (m/s)

Velocity change (m/s)

Depth (m)

Depth change (m)

Otterbourne to Chickenhall (River Itchen)

Upstream of Highbridge gauging station

28.008 Otterbourne to Highbridge

198 149 0.41 -0.02 0.64 -0.04

160 120 0.39 0.60


198 149 0.24 -0.02 0.98 -0.09

160 120 0.22 0.89


198 149 0.35 -0.02 0.47 -0.04

160 120 0.33 0.43


198 149 0.37 -0.03 0.40 -0.03

160 120 0.34 0.37

Downstream of Highbridge gauging station

28.058 Highbridge to Chickenhall

198 149 0.20 -0.01 0.85 -0.07

160 120 0.19 0.78


198 198 0.22 -0.01 0.97 -0.07

160 165 0.21 0.90


198 149 0.51 -0.03 0.83 -0.07

160 120 0.48 0.76

Chickenhall to Gaters Mill (River Itchen)

02.261 Chickenhall to Gaters Mill

198 218 0.42 -0.02 0.60 -0.05

160 180 0.40 0.55


198 218 0.55 -0.03 0.77 -0.06

160 180 0.52 0.71


198 218 0.51 -0.02 0.92 -0.06

160 180 0.49 0.86

Downstream of Gaters Mill (River Itchen)

01.020 Gaters Mill to Riverside Park

194 0.47 -0.03 0.69 -0.05

150 0.44 0.64

01.009 Gaters Mill to Riverside Park

194 0.54 -0.03 0.91 -0.06

150 0.51 0.85

01.003 Riverside Park to Woodmill

194 0.55 -0.04 0.69 -0.07

150 0.51 0.62


B.3.6. Impact on groundwater heads To assess the potential impact of the Lower Itchen sources drought order on the Chalk aquifer,

output from the T&I GW model has been analysed. As noted previously, the Gaters Mill drought

order will not impact the Chalk aquifer and therefore this assessment of groundwater heads is purely

related to the Southern Water Lower Itchen sources drought order (see Section B.3.3).

B.3.6.1. Impact on Chalk aquifer

Maintaing groundwater abstraction during drought conditions from the Otterbourne GW and Twyford

GW sources has the potential to impact the Chalk aquifer. However, the degree of impact will

depend on the operational abstraction volume split between Otterbourne SW and groundwater

sources.


Figure 11 presents the potential groundwater drawdown five months into the 3290-3291 stochastic

drought event (the worst drought event on the 2000-year stochastic record). During this first five

months, groundwater abstraction is modelled as being approximately 8 Ml/d greater than it would

have been without the drought orders in place. The drawdown has been calculated by the difference

in groundwater heads under the drought order and the reference conditions for 31/05/3290. It is

noted that Candover Augmentation Scheme drought order is also operational at this point and there

is no change in the modelled Twyford GW abstraction rate from the reference conditions.

Figure 13 indicates a maximum groundwater drawdown impact close to the Otterbourne GW

boreholes of up to 1 m. Decreasing impacts are predicted up to a radius of 6 km where a 0.2 m

drawdown impact is modelled.

Whilst the groundwater model results are sufficient to indicate the approximate extent and magnitude

of impacts during an extreme drought, the exact way in which abstraction from the Lower Itchen

surface water and groundwater sources would be managed is uncertain. Also, the period selected

is not at the end of this most severe drought period. As a result, the predicted impacts on

groundwater heads in the Chalk aquifer should not be regarded as a precise delineation of impact.

B.3.6.2. Impact on the hydrological functioning of wetlands

The main purpose of the T&I GW model is to predict groundwater heads in the Chalk aquifer and

flows in rivers as a result of different abstraction scenarios. The model has not been configured to

represent the local groundwater-surface water interactions that will control wetland water levels. It is

therefore not appropriate to rely on the groundwater model results to assess potential impacts on

the wetland hydrology in any detail.

To provide a high level assessment of the potential hydrological impacts of the drought orders,

modelled gradients between groundwater heads and the ‘wetland surface’ have been considered.

The modelled elevation of the stream cell top has been used as a proxy for the wetland surface as

this will be a key control on groundwater-surface interactions in the model.

The modelling shows that with and without the drought orders operating, baseflow / groundwater

emergence is predicted to continue throughout drought conditions in SSSI Units 84 and 85. The

slight reduction in head due to the drought order could theoretically slightly reduce the upward

gradient for groundwater seepage, but this is considered to be a negligible impact.

At SSSI Unit 83, closer to more of the Otterbourne boreholes, the T&I GW model predicts a more

noticeable response in groundwater head, which fluctuates above and below the modelled stream

cell top. Because the groundwater model is not configured to represent the wetland water table,

these results need to be treated with caution.

As part of the River Itchen Sustainability Study, a more detailed study of abstraction impacts on wet

grasslands was carried out23. This study included the installation of a series of drift piezometers at

three sites in the River Itchen valley, one of which was within the SSSI Unit 83 at Otterbourne. The

study presents a conceptual model for surface water–groundwater interactions in which low

permiability alluvial deposits overly high permability peats / gravels, which in turn overly low

permeability Chalk. The River Itchen, Drift and Chalk are envisaged as being in reasonably good

hydraulic continuiity, with hydraulic heads in the river been transmitted through the high permeability

gravels to the underside of the alluvial deposit layer.

In the Halcrow River Itchen Sustainability Study (2004), a local hydrological model was developed,

focusing on three wet grassland sites (Winnal Moors, Easton and Otterbourne), based on the

conceptual model described above. The local model used inputs from ISIS modelled river levels, 4R

23 Halcrow, 2004. River Itchen Sustainability Study, Wet Grassland Modelling, Technical Ref H(PR)09


modelled rainfall and potential evapotranspiration (from the T&I GW model) and MODFLOW leakage

transfers (also from the T&I GW model).

For the Otterbourne wetland site, results from the local model indicated the east of the site was well-

drained and that abstraction scenarios ranging from naturalised to fully licensed would have ‘no

effect on the soil water regime at the site at all’. On the western half of the site, closer to the River

Itchen, the report noted that despite the close proximity to the Otterbourne boreholes, there were

areas that were surface wet for much of the year. The report concluded that these areas would be

insensitive to climate change or naturalised abstraction regimes but could become drier under

maximum abstraction scenarios.

In summary, the possibility of an impact on water levels in wetlands very close to the abstraction

boreholes cannot be ruled out from available models and previous assessments. However, wetland

water levels at locations close to the River Itchen are likely to be primarily controlled by water levels

in the River Itchen, which have been shown in Section B.3.5 to have a low sensitiivity to changes in

low flows. At locations near to the Otterbourne abstractions but more distant from the River Itchen,

the wetlands are likely to be more free-draining and insensitive to changes in abstraction. Finally, at

the wetland sites further away from the main Otterbourne groundwater abstractions, the groundwater

model suggests that upward head gradients are likely to be maintained. Overall, the hydrological

impacts from the Lower Itchen sources drought order are considered to be very small.


Figure 11 Modelled impacts on Chalk groundwater heads with/without drought orders five months

into a 1:500 year drought


B.3.7. Hydrological and hydrogeological impact summary This environmental assessment has considered the cumulative impacts of two drought orders in the

Lower Itchen: the Gaters Mill drought order, which relates to a reduction in the HOF at Riverside

Park gauging station to enable continued abstraction from Portsmouth Water’s Gaters Mill source

and the Lower Itchen sources drought order, which pertains to a reduction in the HOF at Allbrook &

Highbridge gauging station to enable continued abstraction at the Lower Itchen sources (Otterbourne

SW, Otterbourne GW and Twyford); These two drought orders are part of a collection of four

Southern Water drought orders on the rivers Test and Itchen. The other two, the Testwood drought

order and the Candover Augmentation Scheme drought order, would be applied for in preference to

the Lower Itchen and Gaters Mill drought orders, and for the purpose of this hydrological and

hydrogeological assessment, are assumed to be in place.

Although the Gaters Mill drought order will be utilised in advance of the Lower Itchen sources drought

order, modelling has shown that the two orders will be required in quick succession. Therefore, this

assessment considers the cumulative impact from the two drought orders as a precautionary

approach. It is anticipated that these drought orders would only be required during drought events

of a severity greater than approximately 1:150 year.

The assessment has considered the potential impacts on various parameters in turn. These are

summarised here in a spatial context. These reaches are marked on Figure 12 (presented further

below in Section B4.1).

Impact on the Chalk aquifer (GB40701G505000)

Only the Lower Itchen sources drought order will impact the Chalk aquifer. The Gaters Mill

drought order relates only to surface water abstraction towards the River Itchen tidal limit in

a locality where there is no hydraulic connectivity to the aquifer.

The maximum groundwater impact will depend on the operational abstraction split between

surface water and groundwater abstraction from the Lower Itchen sources.

For drought events with a return period of up to 1:200 years, the magnitude of additional

abstraction as a result of the drought order is negligible relative to the background reference

condition abstraction rates. Impacts on the Chalk aquifer will therefore also be negligible. For

drought events of higher return periods, the duration of additional abstraction will increase.

Taking the worst drought event in the 2000-year stochastic period as an extreme example,

within the first five months of the drought order being implemented, under the modelled

abstraction profile for the drought, additional groundwater drawdown is predicted to be 1 m,

with a 0.2 m impact extending over a 6 km radius.

Impact upstream of Allbrook & Highbridge gauging station (Reach A to B)

Only the Lower Itchen sources drought order will impact this reach.

The impact on river flow is related to the drought characteristics, including severity and

duration, and the operation of the Candover Augmentation scheme drought order.

The maximum impact on flow at Allbrook & Highbridge in a 1:500-year drought event

(stochastic drought year 3290) is predicted to be 36 Ml/d. The minimum flow during this type

of extreme event may reach close to the proposed drought order HOF of 160 Ml/d. However,

this degree of impact is not sustained throughout the drought event.

Surface water flow impacts upstream of the Otterbourne surface water intake are attributable

to baseflow impacts arising from the groundwater component of the Lower Itchen sources.


Baseflow / groundwater emergence is likely to still occur during drought conditions (assessed

at 5 months only) near the edge of the Chalk outcrop, upstream of Allbrook & Highbridge

gauging station. Further up-hydraulic gradient, the reduction in groundwater head may result

in leakage as the vertical hydraulic gradient reverses.

Due to the uncertainties in the groundwater assessment, the upstream extent of impact

(boundary A in Figure 14) is indicative only.

With the drought orders in place, river flow velocity is typically anticipated to be above 3 m/s

and water depth above 0.4 m. Where this is not the case, the change in velocity and water

depth is very small (0.02 m/s and 0.04 m respectively). The ecological significance of this is

considered in Appendix D of the EAR.

Impact between Allbrook & Highbridge and Riverside Park gauging stations (Reach B to C)

The Lower Itchen sources drought order drought order will indirectly impact the surface water

flow throughout this reach. The reduction in flow at Allbrook & Highbridge gauging station

will propagate downstream.

It is assumed that the Chickenhall WwTW at Eastleigh continues to discharge a minimum of

20 Ml/d throughout drought periods, which supports flows in the reach between the discharge

and Gaters Mill. The discharge is also assumed to continue at the same minimum discharge

without the drought orders, and therefore does not change the absolute flow impact induced

by the Lower Itchen sources drought order.

The Gaters Mill drought order will only impact the lower 1 km of this reach of the River Itchen,

downstream of the abstraction intake.

The impact on river flow is related to the drought characteristics, including severity and

duration, and the operation of the Candover Augmentation Scheme drought order.

The maximum impact on flow at the Riverside Park gauging station in a 1:500-year drought

event (stochastic drought year 3290) is predicted to be 43 Ml/d. The minimum flow during

this type of extreme event may reach close to the proposed drought order HOF of 150 Ml/d.

However, this degree of impact is not sustained throughout the drought event.

The flow impact at Riverside Park is due to the cumulative impact of the Gaters Mill and

Lower Itchen sources drought orders.

With the drought orders in place, river flow velocity is typically anticipated to be above 3 m/s.

Where this is not the case, the change in flow velocity is very small (0.01 m/s). Water depth

is predicted to remain above 0.4 m. The ecological significance of this is considered in

Appendix D of the EAR.

Impact downstream of Riverside Park gauging station (Reach C - D)

Both drought orders will impact this reach. However, the impact downstream of Woodmill (the

tidal limit) is considered to be negligible due to the comparative size of the tidal influence. In

the 600 m freshwater reach upstream of Woodmill, impacts will be equivalent to those

indicated above for Reach B to C.

Flow velocity is anticipated to remain above 3 m/s and water depth above 0.4 m under both

drought orders.

Impact on the hydrological functioning of wetlands

Whilst the possibility of an impact on water levels in wetlands very close to the Lower Itchen

sources abstraction boreholes cannot be ruled out, the hydrological impacts are considered

to be small.


- wetland water levels at locations close to the River Itchen are likely to be primarily

controlled by water levels in the River Itchen, which have a low sensitivity to changes in

low flows.

- locations near to the Otterbourne abstractions but more distant from the River Itchen,

are likely to be more free-draining and insensitive to changes in abstraction

- upward head gradients are likely to be maintained at wetland sites further away from the

main Otterbourne groundwater abstraction sites.


B.4. Physical environment assessment

B.4.1. Geomorphology B.4.1.1. Baseline

The River Itchen is a significant sized river flowing through an approximately 400 km2 catchment.

Within the river length, 89 km has designated as a SSSI due to its classic Chalk stream and river,

fen meadow, flood pasture and swamp habitats. In-channel vegetation is particularly important within

this site, being dominated by Ranunculus spp. Habitats adjacent to the river channels are also

important features supporting designations, such as the extensive water meadows, ditches and side

channels as well as areas of wet woodland.

The River Itchen is also designated as a SAC because it supports Habitats Directive Annex I habitats

and Annex II species, primarily Ranunculion fluitantis and Callitricho-Batrachion vegetation, as well

as the populations of southern damselfly (Coenagrion mercuriale) and bullhead (Cottus gobio).

There is significant overlap between the reasons for SSSI and SAC designation on the River Itchen.

Notable tributaries of the River Itchen include the River Arle, Candover Stream and Bow Lake

Stream. Bedrock geology is dominated by Chalk outcrops (80%) but in the downstream, lower

(southern) section of the river, clays and sands of Tertiary deposits overlie the Chalk.

The assessment of the baseline geomorphology for the study area has been informed from a number

of sources, namely: survey work completed as part of the Test and Itchen River Restoration Strategy

and by numerous River Habitat Surveys (RHS). The Test and Itchen River Restoration Strategy

focused specifically on the SSSI reaches within the catchment and included the hydrological reaches

A-B and B-C. The main hydrological reaches in the study area have good RHS coverage but not full

coverage (see Figure 12).

River Itchen upstream of Allbrook & Highbridge gauging station (Reach A - B)

As with most of the River Itchen, this is a multi-threaded channel over its 7.5 km length. Channel

dimensions from the 17 RHS surveys within the overall reach indicate a medium bank height of

channel, with left and right bank heights varying between 0.2 m to 1.2 m. Predominantly, bank

heights equal bankfull height and this is the same for bankfull width when compared to channel water

width, which generally ranged from 7 m to 19 m. The wide range in channel widths observed reflects

the large number of channel threads on the River Itchen which vary in size significantly. The channel

is extensively impounded and overly deepened reflecting both the presence of weirs and sluices

throughout the reach. The channel has a number of riffles, pools and glides which is reflected in

most of the RHS reaches surveyed. The reach has historically suffered from dredging, over-

widening and uniform banks. Siltation is also prevalent and there is a general lack of woody debris.

The shading of the channel is variable overall. It ranges from occasional clumps of trees all the way

to continuous tree coverage along the banks in places. The reach also suffers from inappropriate

control of vegetation and scrub.

The channel is modified in places but not in others: the Habitat Modification Score varies between 2

(predominantly unmodified) and 5 (severely modified). It is modified due to the presence of bridges,

reinforced banks, weirs, sluices, embankments and channel re-sectioning.

River Itchen between Allbrook & Highbridge and Riverside Park gauging stations (Reach B-

C)

As with most of the River Itchen this overall reach is a multi-threaded channel over its 8 km length.

Channel dimensions from the 19 RHS surveys within the overall reach indicate a medium height of


channel, with left and right bank heights varying between 0.4 m to 1.2 m. Predominantly bank heights

equal bankfull height and this is the same for bankfull width when compared to channel water width,

which generally ranged from 9 m to 24 m. The wide range in channel widths observed reflects the

large number of channel threads which vary in size significantly. The overall reach has a good

proportion of the length being fairly natural but with localised issues such as poaching and

modification.

The channel is impounded and over-deepened in places reflecting both the presence of weirs and

sluices through the reach. This is more localised than the upstream reach. The channel has a

number of riffles, pools and glides, which is reflected in most of the RHS reaches surveyed. The

reach has locally suffered from over-widening and modified, uniform banks. The shading of the

channel is variable overall. It ranges from occasional clumps all the way to continuous tree coverage

along the banks in places.

The channel is modified in places but not in others: the Habitat Modification Score varies between 2

(predominantly unmodified) and 5 (severely modified) due to the presence of bridges, reinforced

banks, weirs, sluices, embankments and re-sectioning.

River Itchen downstream of Riverside Park gauging station (Reach C - D)

Downstream of the Riverside Park flow gauging station to Woodmill Lane, the channel has been

modified. Woodmill Lane marks the downstream limit of the freshwater River Itchen as it becomes

tidal downstream of this location. This also marks the point at which the SSSI ends. This section

has probably been modified to channel water down to the original mill at this location. There is

evidence of significant bank protection along the left bank, which is also embanked. Downstream of

Woodmill Lane, the channel widens significantly and is evidently estuarine.

B.4.1.2. Assessment

All the reaches examined above on the River Itchen have been modified to some degree. This has

either been through channel widening, vegetation cutting, dredging, embankments, sluices or weirs.

As a result, the current river system is less resilient to respond to drought conditions than it would

be if it was more of a natural form. In a naturalised form, the cross-section would be more varied

compared to the uniform trapezoidal channel that is commonplace along the River Itchen thus

meaning that, as flow reduces in the river, there will always be deeper sections which remain cooler.

The modifications to the channel is further exacerbated by the fact that numerous channels exist that

are all connected to the main channel thread. In the event of a drought, the fact that there are

multiple channels clearly impacts the overall wetted width as there is only so much more water that

can be distributed across the system. None of the reaches downstream of Otterbourne have been

subject to restoration measures detailed in the Test and Itchen River Restoration Strategy as these

reaches have not been highlighted in updates on the strategy in 2014, 2015 or 2016. Upstream of

Allbrook & Highbridge gauging station a number of reaches have been enhanced as a result of the

undertaking of schemes in the Test and Itchen River Restoration Strategy. These improvements will

help make theses reaches more ecologically resilient to any drought conditions.

Hydrological reach A-B has a greater number of sluices and structures than reach B-C in particular,

and thus the impact on wetted width (and the resultant effects) of the drought order could be

dampened by the fact that there would be backwater effects in many locations leading to higher

volumes of water being maintained in modified reaches compared to the less modified reaches in

reach B-C. Therefore, there is a low risk of change to the wetted width in reach A-B and a medium

risk to B-C. Hydrological reach A-B would also benefit from the upstream discharges of water

provided under the Candover Augmentation Scheme drought order. Modelling has shown that of the

maximum 25 Ml/d drought order augmentation discharge made directly to the River Itchen upstream

of Easton gauging station, 24.5 Ml/d of this flow would reach the Allbrook & Highbridge gauging

stations. Thus, the augmentation scheme drought order would enhance water levels within the


system and therefore impacts on water levels during low flow levels would be mitigated against with

no detrimental impact on the geomorphology. Any effect is amplified in reach B-C as, by the start of

this reach, surface water abstracted under the Lower Itchen sources drought order will have been

removed and thus this reach will have less water within it than the one upstream. Hydraulic

calculations have been produced to show how water depths and flow velocities would be affected

along 13 cross-sections between Otterbourne and Woodmill due to the reduction to the HOF from

198 Ml/d to 160 Ml/d at Allbrook & Highbridge. This analysis demonstrates that there would be water

depth reductions ranging from 6% to 10% and flow velocity reductions ranging from 4% to 8%

reduction across the cross-sections. These calculations show that this drought order would only

lead to small changes in average water depth and flow velocity.

With a reduction in wetted width, there could be an increased risk in fine sedimentation due to lower

flow velocities. This risk is classified, overall, as being of low impact in all the reaches examined as

a consequence of the analysis presented above. The issue of fine sediment entering the River

Itchen is a wider catchment issue due to its modified state. Any reduction in flows caused by the

drought order could lead to increased sedimentation across the catchment. The presence of trees

shading the bank will help to inhibit the potential increases in water temperature as the water levels

become shallower in the summer. Where trees are absent, the impact could be greater due to

increased water temperatures. Potential decreases in wetted widths and water depths will also

impact habitat availability. Emergent in-channel vegetation is a significant feature of the River Itchen,

particularly in the less shaded sections. The risk of habitat decline is of low to medium risk in all the

reaches examined. However, management of the vegetation in all reaches could be improved to

increase resilience.

Reductions in wetted widths and water depths can present potential risks to bank collapse due to

the drying of steep banks. In hydrological reaches A-B and B-C, the risk and significance of bank

collapse is related to the hydrological impact and is considered low.

In hydrological reach C-D, the impact to fluvial geomorphological characteristics from any reductions

in flow are considered to be very low. Firstly, the only non-tidal section is located upstream of

Woodmill Lane and much of this section is ponded and thus not impacted significantly by fluctuating

flows. This is further dampened downstream of Woodmill Lane as the reach becomes tidal and thus

is subject to normal tidal cycles which masks changes to the flow levels.


Figure 12 Location of RHS and water quality monitoring sites

Based upon: the Ordnance Survey Map by Southern Water by permission of Ordnance Survey on behalf of the controller of Her Majesty's Stationery Office. Crown Copyright 1000019426


B.4.2. Water quality This section sets out the baseline water quality of the impacted reaches identified above and

examines changes over time and with respect to river flows. Environmental pressures on river water

quality (such as discharges from wastewater treatment works), which may cause increased

deterioration in water quality with the drought order in place, are discussed separately in Section

B.4.3.

To support the assessment of potentially sensitive environmental features, an understanding has

been developed of the water quality of the river reaches within the hydrological zone of influence of

the drought order. For Water Framework Directive (WFD) classification, the Environment Agency

has set out (according to UKTAG evidence) what pressures, including water quality pressures,

each biological quality element is capable of responding to. For the purposes of this drought order

assessment, the relevant supporting water quality parameters are as follows:

for fish and macroinvertebrates (where identified as sensitive features), the key parameters

are dissolved oxygen saturation and total ammonia concentration; and

for macrophytes and algae (phytobenthos / diatoms) (where identified as sensitive

features), the key parameters are soluble reactive phosphorus.

Potential impacts on water temperature have also been considered.

Environment Agency routine monitoring data were reviewed to provide an overview of water quality

in the hydrological zone of influence. In the River Itchen catchment, within the extent of influence of

the Lower Itchen sources drought orders, there are seven water quality sampling sites, as detailed

in Table 13 and indicated in Figures 14 and 15. Values at the limit of detection were halved in line

with standard Environment Agency practice.

Table 13 Summary of Environment Agency water quality monitoring sites

EA site ID Site name NGR Reach Fish

designation

G0003795 River Itchen St Cross

Bridge

SU4715220591

A-B Salmonid G0003796 Otterbourne Pumping

Station

SU4704423248

G0003793 River Itchen Bishopstoke SU4646119411 B-C

Salmonid

G0003786 River Itchen Gaters Mill SU4537215634

G0003787

River Itchen at Cobden

Bridge SU4376114079

C-D

N/A

G0003781

River Itchen at Kemps

Boatyard SU4383912877 N/A

Table 15 provides a comparison of key water quality data for freshwater sites in Reaches A to C

against WFD Environmental Quality Standards (EQS) set out in Table 14. It should be noted that

this information is provided for interpretive purposes only based on the available dataset from

2005-2016, and does not provide a formal WFD water quality assessment, which is based on

annual datasets. Water quality assessment for the Itchen estuary (Reach C to D) is based on

relevant WFD parameters for transitional water bodies.


Table 14 Relevant WFD Environmental Quality Standards (EQS) for freshwater

Determinand

EQS

High

EQS

Good

EQS

Mod

EQS

Poor Notes

Total ammonia (mg/l)

(EQS is a 90thpercentile) 0.3 0.6 1.1 2.5

Soluble Reactive Phosphorus

(mg/l)

(EQS is an annual average)

0.051 0.092 0.216 1.1

River Itchen St Cross

Bridge

0.052 0.094 0.219 1.107

Otterbourne pumping

station

0.048 0.088 0.209 1.084

Bow Lake Stoke

Common

0.053 0.095 0.222 1.113

River Itchen

Bishopstoke

0.055 0.098 0.227 1.123

River Itchen Gaters

Mill

Dissolved Oxygen (% saturation)

(EQS is a 10th percentile)

80 75 64 50 Salmonid waters

70 60 54 45 Cyprinid waters

pH

(EQS is 5th and 95th percentiles for

High and Good; 10th percentile for

Moderate and Poor) 6 to 9 6 to 9 4.7 4.2

Temperature (°C)

(EQS is a maximum temperature)

20 23 - - Salmonid waters

25 28 - - Cyprinid waters

Table 15 Summary statistics for freshwater sites against EQS

2005-2016 dataset

Site: River Itchen St Cross Bridge

Otterbourne Pumping Station

Bow Lake Stoke Common

River Itchen Bishopstoke

River Itchen Gaters Mill

Total Ammonia

Min 0.015 0.015 0.015 0.015 0.015

Mean 0.032 0.023 0.229 0.020 0.071

Max 0.166 0.074 2.880 0.089 0.384

90% 0.064 0.043 0.551 0.036 0.191

Soluble Reactive Phosphorus

Min 0.010 0.020 0.005 0.010 0.014

Mean 0.064 0.056 0.071 0.051 0.104

Max 0.770 0.141 0.432 0.126 0.557

Dissolved Oxygen

Min 83.5 59.3 31.6 71.1 71.4

Mean 101.8 98.9 83.0 100.1 93.4

Max 134.9 130.3 140.6 130.7 132.2

10% 89.5 89.9 54.7 89.7 83.5

pH

Min 7.32 7.39 5.33 7.40 6.94

Mean 8.00 8.11 7.74 8.08 8.05

Max 8.97 8.72 8.96 8.60 8.59

95% 8.25 8.33 8.25 8.36 8.34

5% 7.71 7.84 7.23 7.73 7.64

Temperature

Min 5.2 4.6 0.5 4.6 4.5

Mean 11.3 11.4 10.9 11.2 11.3

Max 17.9 17.7 21.6 19.0 18.5


Figure 15 Location of estuarine water quality monitoring sites


B.4.2.1. Chalk aquifer

There is little groundwater quality data available for this water body. However, there are no known

pollution incidents in the vicinity and the drought order covering the Southern Water Lower Itchen

groundwater sources will not lead to any saline intrusion. It is possible that there may be a change

in composition in the surface water reaches as a result of reduction in baseflow, but no impact on

groundwater quality is predicted.

B.4.2.2. Reach A-B – River Itchen upstream of Allbrook and Highbridge

gauging station

Water quality analysis for this reach has been undertaken based on the data available at the River Itchen at St. Cross Bridge (G0003795) and at the Otterbourne Pumping Station (G0003796) water quality monitoring sites. pH and Temperature

The average pH values recorded were 8.0 (St. Cross Bridge) and 8.11 (Otterbourne), with the 5 and 95 percentile values in line with WFD High status. Maximum temperatures recorded were 17.9 and 17.7°C, respectively, in line with WFD High status for Salmonid waters. Figure 16 Measured pH at River Itchen at St Cross

Figure 17 Measured pH at River Itchen at Otterbourne Pumping Station


Figure 18 Measured temperature at River Itchen at St Cross

Figure 19 Measured temperature at River Itchen at Otterbourne Pumping Station

Total ammonia concentration

Total ammonia concentration data for the River Itchen at St. Cross Bridge and Otterbourne Pumping Station were reviewed and are presented in Figures 20 and 21 against the relevant WFD standards for a lowland high alkalinity river. Total ammonia concentration measurements were consistently compliant with the WFD standard to support high status (0.3 mg/l) for fish and invertebrates for a lowland high alkalinity river at both sites. Peaks in concentrations were not linked to low flows.


Figure 20 Total ammonia concentration at River Itchen St. Cross Bridge against WFD status bands

Figure 21 Total ammonia concentration at River Itchen Otterbourne Pumping Station against WFD

status bands

Dissolved oxygen saturation

Dissolved oxygen saturation data for the St. Cross Bridge and Otterbourne monitoring sites were

reviewed and presented in Figures 22 and 23 against the relevant WFD standards for a lowland high

alkalinity river with salmonid designation. Dissolved oxygen saturation measurements were

consistently compliant with the WFD standard to support high status (80% saturation; salmonid

designation) for fish and invertebrates for a lowland high alkalinity river at both sites. There was one

instance in the data record at Otterbourne where this standard was not met with the dissolved oxygen

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Tota

l Am

mon

ia m

gN/l

Below Q95 Q95 to Q80 Q80 to Q50 Greater than Q50 No Paired Flow

Bad

Poor

Mod

Good

High

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Tota

l Am

mon

ia m

gN/l


Bad

Poor

Mod

Good

High


saturation value on this occasion indicative of WFD poor status. Dissolved oxygen saturation

displays some moderate seasonality at both sites however this is not linked to low flow conditions.

Figure 22 Dissolved oxygen saturation at River Itchen St. Cross Bridge against WFD status band

Figure 23 Dissolved oxygen saturation at River Itchen Otterbourne Pumping Station against WFD

status band

Soluble reactive phosphorus concentration

Soluble reactive phosphorus concentration data at St. Cross Bridge and Otterbourne were reviewed

and data are presented in Figure 24 and 25 against the relevant WFD site specific standards

provided by the Environment Agency. Soluble reactive phosphorus concentrations were not

completely consistent with the WFD standard to support good status (0.092 and 0.094 mg/l for the

two sites respectively) for fish and invertebrates for a lowland high alkalinity river: a small number of

elevated soluble reactive phosphorus concentrations were observed throughout the two site records,

indicative of WFD moderate status (0.216 and 0.219 mg/l) and one observation indicative of WFD

404550556065707580859095

100105110115120125130135140

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Dis

solv

ed O

xyge

n Sa

tura

tion

(%)


Bad

Poor

Mod

Good

High

404550556065707580859095

100105110115120125130135140

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Dis

solv

ed O

xyge

n Sa

tura

tion

(%)


Bad

Poor

Mod

Good

High


poor status (0.78mg/l at St. Cross). Peak soluble reactive phosphorus concentration values were

rarely linked to low flow conditions at either of the two sites.

Figure 24 Soluble reactive phosphorus concentration at River Itchen St. Cross Bridge against WFD

status bands

Figure 25 Soluble reactive phosphorus concentration at River Itchen Otterbourne Pumping Station

against WFD status bands

B.4.2.3. Reach B-C – River Itchen downstream of Allbrook and Highbridge gauging station

to Riverside Park gauging station

Water quality analysis for this reach was undertaken based on the data available at the River Itchen at Bishopstoke (G0003793) and at Gaters Mill (G0003786) water quality monitoring sites.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Solu

ble

Reac

tive

Pho

spho

rus

mgP

/l


Bad

Poor

Mod

Good

High

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Solu

ble

Reac

tive

Pho

spho

rus

mgP

/l


Bad

Poor

Mod

Good

High


pH and temperature

The average pH values recorded were 8.08 (Bishopstoke) and 8.05 (Gaters Mill), with the 5 and 95percentile values in line with WFD High status. Maximum temperatures recorded were 19.0 and 18.5°C, respectively, in line with WFD High status for Salmonid and Cyprinid waters. Figure 26 Measured pH at River Itchen at Bishopstoke

Figure 27 Measured pH at River Itchen at Gaters Mill

6

6.5

7

7.5

8

8.5

9

pH


Figure 28 Measured temperature at River Itchen at Bishopstoke

Figure 29 Measured temperature at River Itchen at Gaters Mill

Total ammonia concentration

Total ammonia concentration data for the River Itchen at Bishopstoke and Gaters Mill were reviewed and are presented in Figures 30 and 31 against the relevant WFD standards for a lowland high alkalinity river. Total ammonia concentration measurements were consistently compliant with the WFD standard to support high or good status (0.3-0.6 mg/l) at both monitoring sites.


Figure 30 Total ammonia concentration at River Itchen Bishopstoke against WFD status bands

Figure 31 Total ammonia concentration at River Itchen Gaters Mill against WFD status bands

Dissolved oxygen saturation

Dissolved oxygen saturation data for the Bishopstoke and Gaters Mill monitoring sites were reviewed

and presented in Figure and 33 against the relevant WFD standards for a lowland high alkalinity

river (with salmonid designation for Bishopstoke). Dissolved oxygen saturation measurements were

consistently compliant with the WFD standard to support high status (80% saturation, salmonid

designation) for fish and invertebrates for a lowland high alkalinity river at both sites. There was one

instance in both data records where the high status DO standard was not met, with DO saturations

being indicative of WFD moderate status. The dissolved oxygen saturation data displays some

moderate seasonality although this is not linked to low flow conditions.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Tota

l Am

mon

ia m

gN/l


Bad

Poor

Mod

Good

High

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Tota

l Am

mon

ia m

gN/l


Bad

Poor

Mod

Good

High


Figure 32 Dissolved oxygen saturation at River Itchen Bishopstoke against WFD status bands

Figure 33 Dissolved oxygen saturation at River Itchen Gaters Mill against WFD status bands

Soluble reactive phosphorus concentration

Soluble reactive phosphorus concentration values at Bishopstoke and Gaters Mill were reviewed

and data are presented in Figures 34 and 35 against the relevant WFD site specific standards

provided by the Environment Agency. Soluble reactive phosphorus concentration values were not

consistent with the WFD standard to support good status (0.095 and 0.098 mg/l for the two sites

respectively) for fish and invertebrates for a lowland high alkalinity river. Elevated soluble reactive

phosphorus concentrations were observed throughout the two records, indicative of WFD moderate

status (0.222 and 0.227 mg/l) at both sites; prior to 2008 there are also values indicative of WFD

poor status (0.56 mg/l) at Gaters Mill.

404550556065707580859095

100105110115120125130135140

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Dis

solv

ed O

xyge

n Sa

tura

tion

(%)


Bad

Poor

Mod

Good

High


Figure 34 Soluble reactive phosphorus concentration at River Itchen Bishopstone against WFD

status bands

Figure 35 Soluble reactive phosphorus concentration at River Itchen Gaters Mill against WFD status

bands

B.4.2.4. Reach C-D – Itchen Estuary (part of Southampton Water WFD waterbody)

Water quality analysis for this reach has been undertaken based on the data available at River Itchen at Cobden Bridge (G0003787) and River Itchen and Kemps Boatyard (G0003781). The salinity and turbidity conditions for these two sites are presented in Table 16.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Solu

ble

Reac

tive

Pho

spho

rus

mgP

/l


Bad

Poor

Mod

GoodHigh

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

Solu

ble

Reac

tive

Phos

phor

us m

gP/l


Bad

Poor

Mod

GoodHigh


Table 16 Summary of salinity and turbidity statistics

2005-2016 dataset Site: River Itchen at Cobden Bridge

River Itchen at Kemps Boatyard

Salinity (ppt) Min 0.34 2.19

Mean 5.82 22.66

Max 18.70 32.91

Turbidity (suspended solids mg/L) Min 3.00 3.00

Mean 7.29 6.12

Max 23.10 29.10

Dissolved Inorganic Nitrogen

Dissolved Inorganic Nitrogen (DIN) is a measure of the dissolved fractions of ammonia, nitrate and

nitrite present in the water column. DIN standards are specific to each site, being expressed in

micromoles/litre and calculated based on the average annual turbidity and salinity data24. Owing to

the lack of comprehensive data on the three chemical fractions which characterise DIN, it is not

possible to calculate the EQS specific to the sites in the Itchen Estuary (which forms part of the

Southampton Water WFD transitional water body). However, the current WFD DIN status for

Southampton Water is moderate25 and the available DIN data (expressed in mg/l) from the two

estuarine water quality monitoring sites are presented in Figures 36 and 37.

Figure 36 Dissolved inorganic nitrogen (DIN) concentration at River Itchen at Cobden Bridge (Itchen

Estuary)

24 The Water Framework Directive (Standards and Classification) Directions (England and Wales) 2015. Available at http://www.legislation.gov.uk/uksi/2015/1623/pdfs/uksiod_20151623_en_auto.pdf. Accessed 21/11/2017. 25 Environment Agency. Catchment Data Explorer – Southampton Water. Available at http://environment.data.gov.uk/catchment-planning/WaterBody/GB520704202800. Accessed 21/11/2017


Figure 37 Dissolved inorganic nitrogen (DIN) concentration at River Itchen at Kemps Boatyard (Itchen

Estuary)

Dissolved Oxygen Concentrations

The dissolved oxygen concentration EQS for estuarine waters is also site-specific and have been

calculated for the estuarine monitoring sites based on the specific average salinity data, in

accordance to the methodology presented in Section 3 of the Water Framework Directive Directions

201526. The results are presented in Figures 38 and 39 and show that the two sites in the Itchen

Estuary achieve the ‘high’ WFD status for dissolved oxygen concentration in line with the overall

current WFD status classification for Southampton Water (EA, 2015).

Figure 38 Dissolved oxygen concentration at River Itchen at Kemps Boatyard (Itchen Estuary)

26 The Water Framework Directive (Standards and Classification) Directions (England and Wales) 2015. Section 3. Available at http://www.legislation.gov.uk/uksi/2015/1623/pdfs/uksiod_20151623_en_auto.pdf. Accessed 21/11/2017.


Figure 39 Dissolved oxygen concentration at River Itchen at Kemps Boatyard (Itchen Estuary)

B.4.2.1. Water quality summary

Assessment of the risk of water quality deterioration as a result of the Lower Itchen sources drought orders has been undertaken considering the available water quality data and the hydrological and hydrogeological impact within the affected reaches as presented earlier. The findings are summarised in Table 17. Table 17 Summary of water quality deterioration risks for Lower Itchen sources drought orders

Reach Ammonia Dissolved oxygen Soluble reactive

phosphorus

Chalk aquifer Negligible

A-B Negligible Negligible Low

B-C

C-D Negligible for all relevant water quality parameters

There is little groundwater quality monitoring data, but water quality impacts on the Chalk aquifer are predicted to be negligible. Total ammonia concentration values were consistently in line with the WFD standard to support high status for fish and invertebrates on the River Itchen. Therefore, the risk of water quality deterioration with respect to total ammonia is assessed as negligible in all reaches Dissolved oxygen saturation values were consistently in line with WFD high status for all the sites on the River Itchen. Therefore, the risk of water quality deterioration with respect to dissolved oxygen is assessed as negligible across all reaches. Soluble reactive phosphorus (SRP) concentrations fluctuated between high and good status in the majority of the River Itchen sites, with isolated instances in which SRP concentrations were indicative of moderate or poor status and which were linked to low flow conditions. Therefore, the risk of water quality deterioration with respect to SRP is assessed as low in Reaches A to B and B to C.


B.4.3. Environmental pressures B.4.3.1. Abstraction pressures

During a drought, abstractions put pressure on groundwater levels and surface water flow, potentially exacerbating natural low flow conditions. This section considers the other groundwater and surface water abstractions that may impact on the reaches affected by the Lower Itchen sources Drought Orders. The information used in this assessment relating to abstraction was received from the Environment Agency in January 2017.

Groundwater abstractions

Abstractions that are located within the hydrogeological radius of influence defined in Section B.2.3

are listed in Table 18 (excluding the Southern Water Lower Itchen sources abstraction licences).

The Lower Itchen sources drought order has the potential to impact on these groundwater users due

to the reduction in groundwater levels. However, the risk of derogation cannot readily be quantified.

A small change in water level could be significant if the pump is already close to the bottom of the

well. On the other hand, a large change in water level may not derogate others if the well is deep

and there is the potential to lower the pump. The data on well depth and pump intake location is

unknown, and therefore the assessment of risk can only be made based on distance from the

Southern Water proposed drought action abstraction. Given these uncertainties, these abstractions

have been conservatively assessed as being at a medium to high risk of derogation.

Table 18 Other groundwater abstractors within the area of influence

Licence

Number Industry Use description

Maximum

daily

quantity

(Ml/d)

Maximum

annual

quantity

(Ml)

HOF

11/42/25.2/50 PWS Potable Water

Supply - Direct 31.5 7,487 n/a

11/42/21/1

Industrial,

Commercial

And Public

Services

Evaporative

Cooling 0.4 46 n/a

11/42/22.6/95

Industrial,

Commercial

And Public

Services

Spray Irrigation -

Direct 0.1 3.95

Cessation flow

condition when

Allbrook and

Highbridge is

240 Ml/d from May

to November

31/107 Agriculture General Farming &

Domestic 0.1 13.5 n/a

31/108 Agriculture General Farming &

Domestic 0.1 13.5 n/a

Data source: EA data request in January 2017

Note: Northbrook and Lower Upham sources (Portsmouth Water) are listed under the same licence (11/42/25.2/50)


Surface water abstractions

Surface water abstractions that are within the hydrological and hydrogeological radius of influence

defined in Section B.2.3 are listed in Table 19 (excluding the Southern Water and Portsmouth Water

Lower Itchen abstraction licences).

The Lower Itchen sources drought order has the potential to impact on these surface water

abstractors through the reduction in surface water flow with the result that they may be unable to

abstract all of the licensed quantity. Not all of the surface water licences within the area of influence

are impacted by this drought order, in particular those surface water abstractions on the lower River

Test or on tributaries of the lower River Itchen where there will be no groundwater or surface water

impact. Table highlights those licences that may be impacted.

There is some uncertainty over the abstraction volumes for some sources. The data returned by the

Environment Agency records ‘0’ for both daily and annual abstraction limits for some licences.

Furthermore, the units applied to the flow constraint in licence 31/110 are unclear as both l/s and

m3/s are referenced for the same number.

Of the five abstraction licences with HOF conditions, four have a flow constraint that is higher than

that applicable to the Southern Water Lower Itchen sources (198 Ml/d). Therefore, abstraction from

these sources would already have been reduced or ceased by the point the drought order was

implemented, and therefore there is negligible risk to these abstractions.

There are six other surface water abstractions that may be impacted by the drought order. The

abstraction rates for four of these are unknown. Of the remaining two, there is only one reasonably

large abstraction. This is for fish farming, upstream of Otterbourne SW (31/086). There is the

potential this licence could be impacted as a result of groundwater abstraction from the Lower Itchen

sources inducing a reduction in baseflow. However, because the licence is located on the very edge

of the simulated area of impact and that the maximum abstraction limit is less than 10% of the Q95,

the risk to this licence is deemed to be low, especially given the precautionary nature of the

assessment used to generate that boundary.

B.4.3.2. Water quality pressures

Discharges put pressure on water quality during a drought as lower than normal river flows are

experienced. Discharges can however increase river flows to help ameliorate the effect of drought

and drought orders. Discharge data were requested from the Environment Agency in order to

understand the possible environmental impacts of discharges made into the hydrological zone of

influence of the Lower Itchen sources drought orders.

In total, 633 discharge permits were identified within the radius of influence defined in Section B.2.3.

However, not all of these discharges will affect the hydrology or water quality of the impacted reaches

of the Lower Itchen, in particular, discharges to the River Test. Furthermore, many of these

discharges are below 0.5 Ml/d and therefore are not anticipated to individually have any material

environmental effect on the river reaches. Those discharges over 0.5 Ml/d (either dry weather flow

or max daily flow) are identified in Table 20. The largest discharge relates to the WwTW at Eastleigh,

which has a permitted dry weather flow of 32 Ml/d.

B.5. Cumulative impacts The simulated radius of influence for Southern Water’s other drought management options have

been compared against that of the Lower Itchen sources. No other drought management options

are predicted to lead to cumulative impacts with the Lower Itchen sources drought orders.


Portsmouth Water has also confirmed that it does not have any other planned drought management

option that would have any cumulative impacts with the Lower Itchen sources drought orders27.

Therefore no cumulative assessment is required.

B.6. Cumulative impacts No other drought order or permit options or other drought management measures will lead to any

cumulative adverse impacts with the Lower Itchen sources drought orders. The effects of the

Candover Augmentation Scheme drought permit will be beneficial to river flows downstream of the

flow augmentation scheme discharge location on the River Itchen. HRA and WFD screening

assessments have indicated that there would be no cumulative or in-combination effects on

European sites or WFD water bodies downstream of the River Test (i.e. European sites and WFD

water bodies in Southampton Water) with other drought orders, including drought order/permit

options on the Isle of Wight.

27 pers comm. J Burke, Southern Water, February 2017.


Table 19 Other surface water abstractors within the area of influence

Reach Licence number Industry Use description

Maximum

daily

quantity

(Ml/d)

Maximum

annual

quantity

(Ml)

HOF condition Comment

River

Test 11/42/18.16/442 Agriculture

Spray Irrigation -

Direct 2.2 104.6

Reach not impacted

(on the River Test)

River

Test 11/42/18.16/546 PWS

Potable Water

Supply - Direct 136.4 49915.1

Based on flow in the

River Test

Reach not impacted

(on the River Test)

River

Test 11/42/18.16/547 Agriculture

Fish Farm/Cress

Pond Throughflow 45.5 16592.9

Reach not impacted

(on the River Test)

Reach

A - B 31/086 Agriculture

Fish Farm/Cress

Pond Throughflow 10.9 4000.0

Reach impacted by

GW abstraction only

Reach

A - B SO/042/0031/003 Environmental

Transfer Between

Sources (Post

Water Act 2003) 0.0 0.0

Reach impacted by

GW abstraction only

Reach


Transfer Between

Sources (Post

Water Act 2003) 0.0 0.0

Abstraction reduced to

6 Ml/d when flow at

Allbrook and Highbridge

is less than 248 Ml/d

Reach impacted by

GW abstraction only

Reach


Transfer Between

Sources (Post

Water Act 2003) 0.0 0.0

Reach impacted by

GW abstraction only

Reach

A - B SO/042/0031/018

Production Of

Energy

Hydroelectric

Power Generation 119.2 0.0

Abstraction ceased when

flow at Allbrook and

Highbridge is less than

257 Ml/d

Reach impacted by

GW abstraction only

Reach

A - B SO/042/0031/035 Water Supply Heat Pump 0.1 47.4

Reach impacted by

GW abstraction only


Reach Licence number Industry Use description

Maximum

daily

quantity

(Ml/d)

Maximum

annual

quantity

(Ml)

HOF condition Comment

Reach


Transfer Between

Sources (Pre

Water Act 2003) 0.0 0.0

Reach impacted by

GW and SW

abstraction

Reach


Transfer Between

Sources (Post

Water Act 2003) 0.0 0.0

Reach impacted by

GW and SW

abstraction

Reach

B - C 31/110 Environmental

Transfer Between

Sources (Post

Water Act 2003) 1.0 365.0

Abstraction ceased when

flow at Allbrook and

Highbridge is less than

400 m3/s (l/s?)*

Reach impacted by

GW and SW

abstraction

Reach

B - C SO/042/0031/005 Environmental

Transfer Between

Sources (Post

Water Act 2003) 0.0 0.0

Abstraction reduced to

6 Ml/d when flow at

Allbrook and Highbridge

is less than 248 Ml/d

Reach impacted by

GW and SW

abstraction

Reach

B - C 11/42/22.10/120 Agriculture

Spray Irrigation -

Direct 0.1 2.3

Reach not impacted

(on Lower River

Itchen tributary)

* Units uncertain - referenced as m3/s and l/s.


Table 20 Discharge permits within the area of influence

Reach Permit number Use Receiving

watercourse

Dry weather

flow (Ml/d)

Max daily

flow (Ml/d)

Consideration of

water quality

pressure risk

(during baseline

low flow

conditions)

A-B EPR/EB3496WK Domestic property (single) (incl

farm house)

Groundwater via

soakaway Unknown Unknown Negligible

B-C A00154 WwTW/Sewage Treatment Works

(water company) The River Itchen 32 Unknown Low

B-C P06057 WTW/water

collection/treatment/supply

Tributary of River

Itchen Unknown 2.3 Negligible

D-B G00171 Fish + aquaculture/fish farm/cress

farm

Bow Lake stream

via 200mm pipe Unknown 2.2 Negligible

D-B NPSWQD005981 Fish + aquaculture/fish farm/cress

farm

A tributary of Bow

Lake Unknown 2.0 Negligible

D-B EPRNB3235AG Fish + aquaculture/fish farm/cress

farm

Bow Lake and a

tributary of Bow

Lake

Unknown 1.0 Negligible

n/a W00314 WwTW/Sewage Treatment Works

(water company)

River Itchen

Estuary 27.7 Unknown

Negligible (on Itchen

estuary)

n/a NPSWQD002578 Undefined or other

Empress dock,

tributary of River

Itchen

Unknown 3.9 Negligible (on Itchen

estuary)

n/a A00691 WTW/water

collection/treatment/supply The River Test Unknown 2.0

Negligible (Reach

not impacted - on

River Test)

Appendix B Hydrology and Physical Environment Assessment

Documents