Transcript
70021598-001 JUNE 2020 INTERNAL
Northamptonshire County Council
NORTHAMPTON NORTH WEST
RELIEF ROAD
Drainage Network Water Quality Assessment
Northamptonshire County Council
NORTHAMPTON NORTH WEST RELIEF ROAD
Drainage Network Water Quality Assessment
TYPE OF DOCUMENT (VERSION) INTERNAL
PROJECT NO. 70021598
OUR REF. NO. 70021598-001
DATE: JUNE 2020
Northamptonshire County Council
NORTHAMPTON NORTH WEST RELIEF ROAD
Drainage Network Water Quality Assessment
WSP
The Mailbox Level 2
100 Wharfside Street
Birmingham
B1 1RT
Phone: 0121 352 4700
WSP.com
NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council
QUALITY CONTROL
Issue/revision First issue Revision 1 Revision 2 Revision 3
Remarks
Date June 2020
Prepared by F Marlow
Signature
Checked by J Goodwin
Signature
Authorised by J Goodwin
Signature
Project number 70021598
Report number V1.0
NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council
CONTENTS
1. INTRODUCTION 1
1.1. PURPOSE OF THE ASSESSMENT 1
1.2. STUDY AREA 1
1.3. METHODOLOGY 2
2. PROPOSED DRAINAGE STRATEGY 4
2.2. PROPOSED WATER TREATMENT 5
3. IMPACT OF ROUTINE RUNOFF ON SURFACE WATER BODIES 7
3.1. METHODOLOGY AND DATA 7
3.2. RESULTS 8
CUMULATIVE ASSESSMENT 9
IMPACT OF INCREASED SPILLAGE RISK ON SURFACE WATER BODIES.
10
3.3. METHDOLOGY AND DATA 10
3.4. RESULTS 10
4. SUMMARY 11
5. REFERENCES 12
TABLES
Table 2-1 – Description of highway drainage catchments for the Proposed Scheme and their
outfall locations. 4
Table 3-1 – HEWRAT assessment results for the Proposed Scheme. 8
Table 3-2 – HEWRAT cumulative assessment results for the Proposed Scheme. 9
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Table 3-3 – Spillage risk assessment results for the Proposed Scheme. 10
FIGURES
Figure 1-1 - Overview of the Site. 2
Figure 2-1 - Overview of the highway drainage elements of the Proposed Scheme. 6
APPENDICES
IMPACT OF ROUTINE RUNOFF ASSESSMENT DATA
RISK OF SPILLAGE ASSESSMENT DATA
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GLOSSARY AND ABBREVIATIONS
Terminology Abbreviation Description
Annual Average Daily Traffic Flow
AADT Total volume of two-way vehicle traffic of a highway or road for a year divided by 365 days passing a specific point in a 24-hour period.
Baseline The existing conditions which form the basis or start point of the environmental assessment.
Catchment The area contributing flow to a point on a drainage system.
The Construction Industry Research and Information Association
CIRIA The construction industry research and information association.
Design Manual for Roads and Bridges
DMRB A series of 15 volumes that provide standards, advice notes and other documents relating to the design, assessment and operation of trunk roads, including motorways in the UK.
Environmental Quality Standards
EQS Maximum permissible annual average concentrations of potentially hazardous chemicals, as defined under the WFD.
Environmental Statement Addendum
ESA The Environmental Statement for the Environment Impact Assessment process was written in June 2019. An addendum has been submitted in June 2020 to incorporate further changes to the Proposed Scheme and the changes to the DMRB released in August 2019.
Heavy Goods Vehicles
HGVs Large vehicles with a weight over 3.5 tonnes.
Highways England Water Risk Assessment Tool
HEWRAT A Microsoft Excel application designed to assess the acute and chronic risks of pollution related to the intermittent nature of road runoff and associated with soluble and sediment bound pollutants.
Mitigation Term used to indicate avoidance, remediation or alleviation of adverse impacts.
North West Relief Road
NWRR Northampton North West Relief Road if the subject of this study. Scheme
Proposed Scheme The scheme design for the Northampton North West Relief Road used as the basis for environmental assessment and reporting.
Residual impacts Residual impact means the environmental impact after the provision of mitigation measures, if any.
Sustainable Drainage System
SUDs A sustainable drainage system (SUDs) is designed to reduce the potential impact of new and existing developments with respect to surface water drainage discharges
WSP NORTHAMPTON NORTH WEST RELIEF ROAD JUNE 2020 Project No.: 70021598 | Our Ref No.: 70021598-001 Northamptonshire County Council
Water Body A body of surface water, or a body of groundwater. The WFD defines discrete surface water bodies, but not necessarily a whole river, while groundwater bodies should be distinct.
Water Framework Directive
WFD Wide-ranging European environmental legislation (2000/60/EC). Addresses inland surface waters, estuarine and coastal waters and groundwater. The fundamental objective of the WFD is to maintain "high status" of waters where it exists, preventing any deterioration in the existing status of waters and achieving at least "good status" in relation to all waters by 2027.
Water quality The chemical and biological status of various parameters within the water column and their interactions, for example dissolved oxygen, indicator metals such as dissolved copper, or suspended solids (the movement of which is determined by hydrological process and forms geomorphological landforms).
NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council
EXECUTIVE SUMMARY
The Drainage Network Water Quality Assessment informs Chapter 15 Road Drainage and the Water
Environment of the Environmental Statement. The assessment has been conducted in accordance
with the Design Manual for Roads and Bridges (DMRB) LA 113; local planning policy; as well as
consultation with the Environment Agency.
The assessment outlines the potential impacts associated with the Proposed Scheme on the
chemical quality of the receiving surface water features and assesses the impact of the proposed
mitigation measures within the surface water management strategy. The assessment uses the
Highways England Water Risk Assessment Tool (HEWRAT) to assess routine runoff and the
method set out in Appendix D of DMRB LA 113 to assess pollution impacts from accidental spillage.
The results of the HEWRAT assessment indicate that the Proposed Scheme passes the
assessment for acute impacts of soluble pollutants and chronic impacts of suspended sediments
with the proposed mitigation measures in place. The results of the assessment of long term pollution
impacts to the receiving water environment indicates that the annual average pollutant
concentrations for zinc and copper are below the Environmental Quality Standard (EQS) thresholds
as set out under the Water Framework Directive (WFD) and as summarised within the DMRB.
The method in Appendix D of DMRB LA 113 assesses the pollution impacts from accidental spillage
on the receiving watercourse. The DMRB recommends that an annual probability of a serious
pollution incident occurring of less than 1% would be acceptable. The Proposed Scheme passes
Method D without taking the proposed mitigation measures into account. The annual probability of a
serious pollution incident occurring is further reduced with the mitigation measures included in the
design.
The results of both the Method A and Method D assessments demonstrate that there will not be a
significant impact on the receiving surface water features as a result of the Proposed Scheme.
Impacts of routine runoff on groundwater bodies (method in Appendix C of DMRB LA 113) is scoped
out of this assessment. The proposed drainage basins are lined to minimise the risk of infiltration
and the superficial geology at the location of the receiving land drains has low permeability and
therefore impacts to groundwater through infiltration are not expected.
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1. INTRODUCTION
1.1. PURPOSE OF THE ASSESSMENT
1.1.1. The Drainage Network Water Quality Assessment informs Chapter 15 Road Drainage and the Water
Environment of the Environmental Statement Addendum (ESA) and supports the planning application
for the Northampton North West Relief Road (Herein referred to as the ‘Proposed Scheme’).
1.1.2. The assessment has been conducted in accordance with the Design Manual for Roads and Bridges
(DMRB) Volume 11, Part 10 (LA 113); local planning policy; as well as consultation with the
Environment Agency (EA).
1.1.3. The Proposed Scheme will increase the impermeable road surface area and alter the current traffic
flow regime through the creation of a new road section consisting of a single two-lane carriageway
and three new roundabouts. These changes have the potential to impact the volume and quality of
surface water runoff to the receiving water features. The purpose of the assessment is to assess the
potential impacts associated with the Proposed Scheme on the chemical quality of the receiving water
features and to assess the impact of the proposed mitigation measures within the surface water
management strategy.
1.1.4. The assessment focusses on the potential risks associated with the operational phase of the Proposed
Scheme and does not consider potential risks during the construction phase. The potential impacts to
the chemical quality of water features during the construction phase are assessed within the Road
Drainage and the Water Environment Chapter of the ESA.
1.2. STUDY AREA
1.2.1. The Proposed Scheme is located to the north of the town of Northampton, an overview of the
Proposed Scheme is provided in Figure 1-1. The Proposed Scheme consists of a single two-way
carriageway design.
1.2.2. The Main Carriageway of the Proposed Scheme runs along a north-south alignment to the west of the
River Nene. To the north, it will connect to Sandy Lane and the A5199 Welford Road, via a new
roundabout. To the south, the Proposed Scheme crosses Brampton Brook and the railway line before
connecting into a new roundabout, east of Grange Farm, which will provide access to the proposed
Dallington Grange residential development. This element of the Proposed Scheme measures
approximately 1570m.
1.2.3. From the new roundabout to the north a second element of the Proposed Scheme, hereafter referred
to as the Causeway, will cross the River Nene parallel to the existing A5119 crossing, and connect to
the A5119 and Brampton Lane via a second roundabout. The existing A5119 crossing will remain but
will be downgraded to a pedestrian and cycle access route only, therefore the Causeway will become
the A5119. This element of the Proposed Scheme measures approximately 320m.
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Figure 1-1 - Overview of the Site
1.2.4. The proposed scheme forms part of a wider strategy to complete the ring road around the northern
sector of Northampton that will ultimately connect the A4500 Weedon Road and M1 Motorway
Junction 16 to the West, with the A34 Kettering Road and Kettering to the north.
1.2.5. The Proposed Scheme will unlock further development of 6,600 houses and over 3,000 jobs in
Northampton and relieve pressure that already exists on the local highway network in northern
Northampton and nearby villages that suffer high volumes of traffic and associated disturbance.
1.3. METHODOLOGY
1.3.1. The assessment of impacts to water quality is defined by the methodology set out in the DMRB.
1.3.2. The Highways England Water Risk Assessment Tool (HEWRAT) has been used to determine the
effect of routine runoff on receiving watercourses. This is the simple assessment method provided in
the DMRB which considers several factors including impermeable road area, and low flows and
dimensions of the receiving watercourse. The HEWRAT has been used to assess all new outfalls as
part of the Proposed Scheme. The HEWRAT assesses the likely quality of scheme-generated surface
water runoff against the Environmental Quality Standards (EQS) given in the Water Framework
Directive. The results of this assessment are summarised in Section 3.
1.3.3. The risk of a potentially polluting spillage event is calculated using equations and factors provided in
Appendix D of DMRB LA 113. This method calculates the probability of a spillage event with an
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associated risk of a serious pollution incident occurring. This firstly calculates the probability of a
spillage occurring with the potential to pollute, and secondly calculates the probability of the pollutant
reaching and impacting a receiving water body.
1.3.4. An assessment of potential risks to groundwater resources as set out in in Appendix C of DMRB LA
113 has not been completed in this assessment because the proposed basins will be lined therefore
infiltration to ground is not expected. In addition, the geology of the area comprises Alluvium and
Whitby Mudstone Formation which is expected to substantially limit potential for infiltration. Therefore,
an assessment of potential risks to groundwater associated with discharge to the land drain which will
receive highway runoff (before discharging to the River Nene) has not been undertaken.
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2. PROPOSED DRAINAGE STRATEGY
2.1.1. Infiltration of runoff to ground is not considered viable therefore the drainage strategy proposes to
drain highway runoff to the River Nene. Attenuation features have been designed to accommodate
the 1 in 200yr flood event with 65% climate change allowance with additional freeboard. This is in
accordance with the guidance in the DMRB CG501i and has been agreed with the Lead Local Flood
Authority (LLFA). All attenuation features will discharge at a restricted rate of 5 l/s (each). This rate
has been agreed with the LLFA.
2.1.2. There are four new outfalls and three new attenuation basins proposed and detailed in Table 2-1. The
location of these elements is shown in Figure 2-1.
2.1.3. The Proposed Scheme does not connect to any existing drainage systems and therefore these are
not scoped into this assessment.
Table 2-1 – Description of highway drainage catchments for the Proposed Scheme and their
outfall locations
Outfall Number Highway Catchment Receiving Watercourse Treatment Features
1 Brampton Lane roundabout (Catchment 1a)
The Causeway (Catchment 1b)
River Nene Catchment 1a discharges directly to the Nene via a new surface water drainage ditch.
Catchment 1b will be attenuated in Basin 1 prior to discharge to the same new surface water drainage ditch.
2 Sandy Lane roundabout and Main Carriageway
River Nene, via an existing land drain
Filter drains and Basin 2
3 Main Carriageway River Nene, via an existing land drain
Filter drains and Basin 3
4 Main Carriageway River Nene Filter drains.
Attenuation will be provided in oversized pipes. Discharge to the Nene will be via a SuDS swale feature also serving the Dallington Grange development. Potential opportunity to combine attenuation with the attenuation basin serving the new Dallington Grange development.
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2.2. PROPOSED WATER TREATMENT
2.2.1. Catchments 1b, 2 and 3 will receive treatment through the use of the attenuation basins. Catchment
4 will receive treatment through the use of a swale. These features will allow for settlement and
entrapment of suspended solids. Filter drains will be implemented along straight sections of the Main
Carriageway and therefore provide treatment of highway runoff for catchments 3 and 4, as well as
some parts of catchment 2 (assumed 50% of the catchment for the purposes of the HEWRAT
assessment). Catchment 1a and 1b will be drained by gullies.
2.2.2. Discharge from catchment 4 will be coordinated with the drainage system serving the Dallington
Grange development. Currently it is proposed that a swale between the borrow pits and the railway
line will receive highway runoff from catchment 4 and divert it towards the River Nene. There may also
be opportunity to discharge runoff from catchment 4 to attenuation basins serving the Dallington
Grange development. The construction of these features and outfall 4 will be coordinated so that
temporary outfalls are not required.
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Figure 2-1 - Overview of the highway drainage elements of the Proposed Scheme
.
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3. IMPACT OF ROUTINE RUNOFF ON SURFACE WATER BODIES
3.1. METHODOLOGY AND DATA
3.1.1. This assessment reviews the effect of pollution from routine runoff on the River Nene. Consideration
has not been given to specific risks to the short section of existing land drain that will receive runoff
from basins 2 and 3 as this is a drainage feature with ephemeral flow and low ecological value.
3.1.2. This assessment uses the Highways England Water Risk Assessment Tool (HEWRAT) which is
described in more detail in Appendix A of DMRB LA 113. This inputs to this tool are:
▪ Details about the receiving watercourse:
• Annual Q95 river flow
• Base Flow Index (BFI)
• River width
• Bed width
• Manning’s n
• Channel side slope
• Channel bed slope
• Presence of downstream structures or sensitive areas
• Ambient background copper concentration
• Water hardness
▪ Details about the proposed scheme / current arrangement:
• Annual Average Daily Traffic flow (AADT)
• Impermeable area
• Permeable area
• Existing mitigation
• Proposed mitigation
3.1.3. Annual Q95 flow is derived from HR Wallingford’s LowFlows2ii software. The software uses a
catchment boundary to determine the low flow data. Catchment boundaries have mostly been derived
from the FEH Web Serviceiii catchment boundaries, with some adjustments based on available LiDAR
dataiv. The FEH Web Service also provides a value for BFI for each downloaded catchment.
3.1.4. Ambient background copper concentration and water hardness is set to the default value for the
HEWRAT assessment for the River Nene as water quality data has not been provided or obtained.
3.1.5. Channel width has been informed by the topographic data used in the hydraulic model for the River
Nene.
3.1.6. AADT was extracted from the traffic model for both the baseline arrangement and the Proposed
Scheme. This data was based on the design year traffic flows expected in 2036.
3.1.7. Impermeable and permeable area, as well as mitigation measures, were taken from the Proposed
Scheme drawings and drainage strategy.
3.1.8. Water treatment elements described in Section 2.2 are included as existing mitigation in this
assessment as they are embedded in the design. Each of these features have treatment efficiencies
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as stated in Table 8.6.4N3 of DMRB CG501 which have been applied in the HEWRAT. For any
treatment features located downstream of another, a 50% reduction in efficiency has been applied as
per guidance in CIRIA C609v.
3.1.9. All input parameters used in this assessment are presented in Appendix A.
3.1.10. Each of the outfalls in Table 2-1 have been assessed quantitatively using HEWRAT. In addition, a
cumulative assessment has been carried out at the point where discharge from Outfall 4 joins the
River Nene, to identify the cumulative impacts to water quality from the entirety of the Proposed
Scheme on the River Nene.
3.2. RESULTS
3.2.1. The HEWRAT has two stages of assessment: Tier 1 and Tier 2. Tier 1 is a high level assessment
based on river width. If the Tier 1 assessment fails, then Tier 2 is carried out using more channel
dimensions. The assessment is undertaken first without and then with proposed mitigation to assess
the reduction in impact to water quality from copper, zinc and suspended solids. Step 3 of the
assessment is where the mitigation has been included.
3.2.2. The results for each tier indicate whether the test has exceeded the acceptable limits for copper and
zinc and indicate how much sedimentation is required for the test to pass on suspended solids. For
the Proposed Scheme, the Tier 1 assessment was passed and therefore the Tier 2 assessment was
not carried out. Table 3-1 summarises the results for each individual outfall assessed. The full results
suite is presented in Appendix A.
Table 3-1 – HEWRAT assessment results for the Proposed Scheme
Outfall Step Soluble Pollutants Sediment EQS Assessment
Acute impact assessment of copper
Acute impact assessment of zinc
Chronic impact assessment of sediment
Annual average concentration of copper (μg/l) due to road runoff
Annual average concentration of zinc (μg/l) due to road runoff
1 Step 2 Pass Pass Pass 0.00 μg/l - Pass
0.01 μg/l - Pass
Step 3 Pass Pass Pass 0.00 μg/l - Pass
0.01 μg/l - Pass
2 Step 2 Pass Pass Pass 0.01 μg/l - Pass
0.04 μg/l - Pass
Step 3 Pass Pass Pass 0.01 μg/l - Pass
0.03 μg/l - Pass
3 Step 2 Pass Pass Pass 0.00 μg/l - Pass
0.01 μg/l - Pass
Step 3 Pass Pass Pass 0.00 μg/l - Pass
0.01 μg/l - Pass
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Outfall Step Soluble Pollutants Sediment EQS Assessment
Acute impact assessment of copper
Acute impact assessment of zinc
Chronic impact assessment of sediment
Annual average concentration of copper (μg/l) due to road runoff
Annual average concentration of zinc (μg/l) due to road runoff
4 Step 2 Pass Pass Pass 0.00 μg/l - Pass
0.00 μg/l - Pass
Step 3 Pass Pass Pass 0.00 μg/l - Pass
0.00 μg/l - Pass
CUMULATIVE ASSESSMENT
3.2.3. In accordance with DMRB methodology, the HEWRAT assessment should include a cumulative
assessment that considers other outfalls located within 100m and 1km of each that drain to the same
watercourse catchment. With relevance to the Proposed Scheme this applies to outfalls 1, 2, 3 and 4
for the cumulative assessment of outfalls within 1km, and outfalls 2 and 3 for the cumulative
assessment of outfalls within 100m. The results of the cumulative assessment are shown in Table 3-
2 below.
3.2.4. To determine the cumulative effects, the total area of all catchments draining into the River Nene via
each outfall was calculated. The percentage removal of copper and zinc and the percentage
settlement of sediment was determined by weighting the proposed mitigation with the catchment
areas.
Table 3-2 – HEWRAT cumulative assessment results for the Proposed Scheme
Outfalls Step Soluble Pollutants Sediment EQS Assessment
Acute impact assessment of copper
Acute impact assessment of zinc
Chronic impact assessment of sediment
Acute Impact assessment of copper
Acute Impact assessment of zinc
2 & 3 Step 2 Pass Pass Pass 0.02 μg/l - Pass
0.05 μg/l - Pass
Step 3 Pass Pass Pass 0.00 μg/l - Pass
0.04 μg/l - Pass
1, 2, 3 & 4
Step 2 Pass Pass n/a 0.02 μg/l - Pass
0.06 μg/l - Pass
Step 3 Pass Pass n/a 0.02 μg/l - Pass
0.04 μg/l - Pass
3.2.5. The assessment indicates that the Proposed Scheme passes all aspects of the assessment of
potential cumulative effects.
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IMPACT OF INCREASED SPILLAGE RISK ON SURFACE WATER
BODIES.
3.3. METHDOLOGY AND DATA
3.3.1. This assessment reviews the probability of a spillage event occurring and the potential for this event
to pollute receiving water bodies, in this case the River Nene. This is a calculation using a formula
provided in Appendix D of DMRB LA 113. The inputs to this calculation are:
▪ Road grade (motorway, urban trunk road, rural trunk road),
▪ Road type (roundabout, slip road, side road, crossroads, road with no junction),
▪ Road length,
▪ Road location (rural or urban),
▪ AADT for each road type, and
▪ Percentage of heavy goods vehicles (%HGV) use for each road type.
3.3.2. AADT and %HGV data was extracted from the traffic model for both the baseline arrangement and
the Proposed Scheme. This data was based on the design year traffic flows expected in 2036. AADT
for roundabouts was taken as the highest AADT of the arms of the roundabout.
3.3.3. Road grade, type and length was derived from maps and the designations are shown in Appendix B.
3.3.4. Water treatment elements described in Section 2.2 are included as existing mitigation in this
assessment as they are embedded in the design. Each of these features have a spillage risk reducing
factor as stated in Table 8.6.4N3 of CG501 (DMRB) which have been applied in this assessment.
3.3.5. The calculation was carried out for each proposed outfall. All input parameters are presented in
Appendix B.
3.4. RESULTS
3.4.1. A summary of the results of the spillage risk assessment for the Proposed Scheme are presented in
Table 3-3. The Proposed Scheme passes the spillage risk assessment as all outfalls have a
probability of a spillage event occurring of less than 0.5%. Therefore, no additional mitigation is
required in order to minimise spillage risk to an acceptable level. However, mitigation, such as manual
penstocks, is welcomed to reduce the risk of spillage further.
Table 3-3 – Spillage risk assessment results for the Proposed Scheme
Outfall Embedded mitigation Spillage Risk
1a None 0.008%
1b Attenuation basin 0.003%
2 Filter drains, attenuation basin 0.006%
3 Filter drains, attenuation basin 0.000%
4 Filter drains, swale 0.001%
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4. SUMMARY
4.1.1. The introduction of a new highway can impact water quality of nearby water bodies which receive
highway drainage. This assessment determines if the proposed drainage system of the Proposed
Scheme is likely to cause a significant adverse effect to receiving waterbodies. This assessment has
considered impact to surface water quality from changes to routine runoff and from changes to the
risk of a spillage event with potential to pollute waterbodies.
4.1.2. The assessment of routine runoff was conducted using HEWRAT. This assessment determined that
the proposed system is acceptable and does not significantly adversely affect the River Nene.
4.1.3. The assessment of spillage risk was conducted using the calculations provided in Appendix D of
DMRB LA 113 and covered the entire length of the Proposed Scheme and adjoining junctions. This
assessment determined that the proposed system is acceptable for the River Nene.
4.1.4. The proposed mitigation measures that will be included in the design have been taken into
consideration within the assessments. As a result, the proposed mitigation measures are deemed to
be appropriate.
4.1.5. The HEWRAT and spillage risk assessments will also support the standalone WFD assessment and
the Chapter 15 Road Drainage and the Water Environment of the Environmental Statement for the
Proposed Scheme.
4.1.6. The assessment of impacts of routine runoff on groundwater was not carried out as the Proposed
Scheme includes lined basins, preventing infiltration, and the geology of the area is considered to
have a permeability insufficient to pose risk to groundwater.
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5. REFERENCES
i Design Manual for Roads and Bridges, Volume 4, Section2, Part 3 (CG 501) ii Wallingford HydroSolutions 2010 iii UK Centre for Ecology & Hydrology 2020, fehweb.ceh.ac.uk iv Defra Survey Data Download 2020, environment.data.gov.uk/DefraDataDownload v Wilson, S., Bray R., Cooper, P., (2004) Sustainable Drainage Systems, Hydraulic, structural and water quality advice CIRIA C609
NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council
Table A-1 – Single outfall HEWRAT input data and assessment for routine runoff
Outfall reference Outfall 1 Outfall 2 Outfall 3 Outfall 4
Section of road Causeway Main Carriageway Main Carriageway Main Carriageway
Easting 473414 473477 473477 473851
Northing 265231 264416 264416 263656
Receiving watercourse
River Nene River Nene River Nene River Nene
AADT DS banding >10,000 and <50,000
>10,000 and <50,000
>10,000 and <50,000
>10,000 and <50,000
Climate region Warm / Dry Warm / Dry Warm / Dry Warm / Dry
Rainfall site Huntingdon Huntingdon Huntingdon Huntingdon
Base flow index 0.53 0.53 0.53 0.53
Hardness (mg CaCO3/l)
Worst case (hard) Worst case (hard) Worst case (hard) Worst case (hard)
Q95 flow (m3/s) 0.14 0.14 0.14 0.14
Impermeable areas drained to the outfall (ha)
0.8027 2.4889 0.7068 0.2686
Permeable area drained to the outfall (ha)
0 0 0 0
River width (m) 14 12 12 12
Downstream structure within 100m of outfall?
No No No No
Discharge in or within 1km upstream of a designated site
No No No No
Proposed Mitigation
Treatment for solubles (%) Copper
50
Treatment for solubles (%) Zinc
22.5 45 70
Outfall reference Outfall 1 Outfall 2 Outfall 3 Outfall 4
Restricted discharge rate (l/s)
5 5 5 5
Settlement of Sediments (%)
19 40 85 100
Tier 1 Results
Step 2
Acute impacts - Copper
Pass Pass Pass Pass
Acute Impacts - Zinc
Pass Pass Pass Pass
Chronic Impact - Sediments
Pass Pass Pass Pass
Annual average concentration - Copper
0 μg/l 0.01 μg/l 0 μg/l 0 μg/l
Annual average concentration - Copper
0.01 μg/l 0.04 μg/l 0.01 μg/l 0 μg/l
Step 3
Acute impacts - Copper
Pass Pass Pass Pass
Acute Impacts - Zinc
Pass Pass Pass Pass
Chronic Impact - Sediments
Pass Pass Pass Pass
Annual average concentration - Copper
0 μg/l 0.01 μg/l 0 μg/l 0 μg/l
Annual average concentration - Copper
0.01 μg/l 0.03 μg/l 0.01 μg/l 0 μg/l
NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council
Table A-2 – Cumulative HEWRAT input data and assessment for routine runoff
Outfall reference Outfall 2 & 3 Outfall 1, 2, 3 & 4
Section of road Main Carriageway Causeway and Main Carriageway
Easting 473477 473851
Northing 264416 263656
Receiving watercourse River Nene River Nene
AADT DS banding >10,000 and <50,000 >10,000 and <50,000
Climate region Warm / Dry Warm / Dry
Rainfall site Huntingdon Huntingdon
Base flow index 0.53 0.53
Hardness (mg CaCO3/l) Worst case (hard) Worst case (hard)
Q95 flow (m3/s) 0.14 0.14
Impermeable areas drained to the outfall (ha)
3.1957 4.2670
Permeable area drained to the outfall (ha)
0 0
River width (m) 12m 12m
Downstream structure within 100m of outfall?
No No
Discharge in or within 1km upstream of a designated site
No No
Proposed Mitigation
Treatment for solubles (%) Copper 3
Treatment for solubles (%) Zinc 27 25
Restricted discharge rate (l/s) 10 20
Settlement of Sediments (%) 50 47
Tier 1 results
Step 2
Acute impacts - Copper Pass Pass
Outfall reference Outfall 2 & 3 Outfall 1, 2, 3 & 4
Acute Impacts - Zinc Pass Pass
Chronic Impact - Sediments Pass n/a
Annual average concentration - Copper
0.02 μg/l 0.02 μg/l
Annual average concentration - Copper
0.05 μg/l 0.06 μg/l
Step 3
Acute impacts - Copper Pass Pass
Acute Impacts - Zinc Pass Pass
Chronic Impact - Sediments Pass n/a
Annual average concentration - Copper
0.02 μg/l 0.02 μg/l
Annual average concentration - Copper
0.04 μg/l 0.04 μg/l
NORTHAMPTON NORTH WEST RELIEF ROAD WSP Project No.: 70021598 | Our Ref No.: 70021598-001 JUNE 2020 Northamptonshire County Council
Table B-1 – Spillage risk assessment for the Proposed Scheme
Road label Catchment Receiving watercourse / drainage system
Length of road draining to outfall
Road Type Junction type Response time
Traffic flow (AADT 2-way)
% HGV Serious spillage factor
Risk of accidental spillage
Probability factor
Risk of pollution incident
1.1 1a River Nene 0.123
Urban trunk road
Roundabout <20 minutes 32677 1.1 5.35 0.009%
0.45 0.004%
1.2 1a River Nene 0.1
Urban trunk road
Roundabout <20 minutes 21819 1.2 5.35 0.005%
0.45 0.002%
1.3 1a River Nene 0.1
Urban trunk road
Roundabout <20 minutes 12031 0.8 5.35 0.002%
0.45 0.001%
Catchment 1a Total Spillage Risk 0.008%
Spillage reduction factor 0
Catchment 1a Total Residual Spillage Risk 0.008%
1.4 1b River Nene 0.023
Urban trunk road
Roundabout <20 minutes 32677 1.1 5.35 0.002%
0.45 0.001%
1.5 1b River Nene 0.077
Urban trunk road
Roundabout <20 minutes 32677 1.1 5.35 0.005%
0.45 0.002%
1.6 1b River Nene 0.058
Urban trunk road
No Junction <20 minutes 32677 1.1 0.31 0.000%
0.45 0.000%
1.7 1b River Nene 0.085
Urban trunk road
Roundabout <20 minutes 32677 1.1 5.35 0.006%
0.45 0.003%
Catchment 1b Total Spillage Risk 0.005%
Spillage reduction factor 0.6
Catchment 1b Total Residual Spillage Risk 0.003%
2.1 2 River Nene 0.197
Urban trunk road
Roundabout <20 minutes 32677 1.1 5.35 0.014%
0.45 0.006%
2.2 2 River Nene 0.015
Urban trunk road
Roundabout <20 minutes 32677 1.1 5.35 0.001%
0.45 0.000%
2.3 2 River Nene 0.1
Urban trunk road
Roundabout <20 minutes 26972 1.1 5.35 0.006%
0.45 0.003%
2.4 2 River Nene 0.1
Urban trunk road
Roundabout <20 minutes 3278 1.3 5.35 0.001%
0.45 0.000%
2.5 2 River Nene 0.1
Urban trunk road
Roundabout <20 minutes 8093 3 5.35 0.005%
0.45 0.002%
Road label Catchment Receiving watercourse / drainage system
Length of road draining to outfall
Road Type Junction type Response time
Traffic flow (AADT 2-way)
% HGV Serious spillage factor
Risk of accidental spillage
Probability factor
Risk of pollution incident
2.6 2 River Nene 0.722
Urban trunk road
No Junction <20 minutes 26972 1.1 0.31 0.002%
0.45 0.001%
2.7 2 River Nene 0.039
Urban trunk road
No Junction <20 minutes 3278 1.3 0.31 0.000%
0.45 0.000%
2.8 2 River Nene 0.043
Urban trunk road
No Junction <20 minutes 8093 3 0.31
0.000% 0.45 0.000%
Catchment 2 Total Spillage Risk 0.013%
Spillage reduction factor 0.48
Catchment 2 Total Residual Spillage Risk 0.006%
3.1 3 River Nene 0.434
Urban trunk road
No Junction <20 minutes 26972 1.1 0.31 0.001%
0.45 0.001%
Spillage reduction factor 0.36
Catchment 3 Total Residual Spillage Risk 0.000%
4.1 4 River Nene 0.083
Urban trunk road
No Junction <20 minutes 26972 1.1 0.31 0.000%
0.45 0.000%
4.2 4 River Nene 0.1
Urban trunk road
Roundabout <20 minutes 26972 1.1 5.35 0.006%
0.45 0.003%
Catchment 4 Total Spillage Risk 0.003%
Spillage reduction factor 0.6
Catchment 4 Total Residual Spillage Risk 0.002%
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