Plan View: Baile Mhic Ire Interim Measures and... · What the above Baile Mhic Ire Interim Measures Plan View illustrates The interim defence line, its location was selected in order

1

1.0 Baile Mhic Ire Interim Measures

Plan View: Baile Mhic Ire Interim Measures

(Find attached better resolution plan views)

2

Ref. Pts A B C D E F G H I J K

Flood

Level 120.16 119.7 118.85 118.6 118.35 118.12 118.12 117.8 117.77 117.45 117.38 (m) OD

Ground

Level * 119.83 119.1 118.4 117.7 117.4 117.13 117.3 117.3 117 116.68 116.85 (m) OD

Defence

Height 0.33 0.6 0.45 0.9 0.95 0.99 0.82 0.5 0.77^ 0.77^ 0.53^ (m)

Table 1: Flood Levels, Ground Levels and Interim Works Defence Heights

Information

Flood Levels (highlighted above) are the 20-year modelled event levels plus, a 0.3m freeboard.

Note. Ground Levels* (highlighted above) are attained from a specific site survey and Lidar survey, these may vary when on-site.

^ refers to defence levels that may need to be altered to allow for a safe flood event breach (refer below ' Design for Breaching').

Justification for the Interim Works Defence Levels

Objective: To provide interim defences that are practical, structurally safe and do not increase flood risk.

It is considered the interim hard defences may consist of 1000Kg (10KN) sand bags and impermeable liners. Using this information for the interim design,

defending against a 20-year event is considered most suitable, for the following reason: The maximum height (allowing for a factor of safety) for the

interim hard defence measures is considered as 1m (i.e. 5KN in-line hydrostatic flow pressure).

For health and safety reasons, there may be a need for adding an additional non-load bearing high to the above defences heights, to attain 1.2m.

Flood Mechanism for the 20-year event

Based on local information and from the events hydraulically modelled levels, floodwaters first spill from the village bend and out-of-bank up and

downstream of this location, then soon after from the gates at Point B, D and then from the gate at Point A (ree attached plan views).

3

What the above Baile Mhic Ire Interim Measures Plan View illustrates

The interim defence line, its location was selected in order to minimally effect landowners, both residential and agricultural.

The Maroon line is the proposed interim hard defence line, to prevent floodwaters reaching properties from their gardens, flowing through gates and

seeping though walls.

Note. It is possible some units (garages) at the back of some properties may effect this line. This must be confirmed on-site and the line altered, though still

accounting for the 20-year flood levels.

The Green line indicates land-raising, to allow access to/from the existing gateway whilst also mitigating against the 20-year flood levels, this respective

line may vary for best access (again determined on-site), regardless the 20-year flood level must be maintained.

Property areas defended can be seen within the Green hatch polygon, also, the Green diamonds (properties with floor level less that the 20-year event) and

Maroon diamonds (properties with floor levels above the 20-year event though are within a mitigated flood cell).

The Red Diamonds show properties outside the interim defences line with floor levels below the 20-year event (refer to below paragraph, 'Properties

outside of 20-year Interim mitigation measures'). The Pink polygon highlights individual property protection (refer below, for information on property No.

83, 'Properties outside of 20-year Interim mitigation measures').

Navy Diamonds indicate properties outside of schemes defended study area.

Reference points are identified A to K.

Defence line ties into high ground at K.

Piped Outfalls

There are two identified pipe outfalls (identified in red), Out 1 and 2, both would need a non-return valve placed on their outlets. Also, a sump would likely

need to be constructed on the dry side of each piped outlet and pump be available in times of need. A site inspection is needed to confirm these

requirements. Though at this stage, it appears possibly only at Out 2 may require a sump and pump as this outfall appears to be taking surface water from

the road to the river, again a local site inspection is needed to confirm.

A local site inspection is also needed to determine if other pipes outfalls enter the river and back-up onto the mitigated areas, if so, these must have non

return valves fitted and possibly sump and pump system installed also.

4

Interim Hard Defence Design

The above paragraph under 'Justification for Interim Works - Defence Levels' outlines what defence type (1000Kg sand bags and impermeable liners) may

be used in the defences. The Local Authority must ensure whatever defence material and construction process that is used, along the defence line, it must

be suitably designed to take the respective hydrostatic pressure from the river at the above flood levels.

Also, special consideration must be taken into account by the Local Authority for the design of the interim defences around the village bend, from point I to

K, as secondary lateral forces will need to be included, due to the rivers barrelling velocities into the defence line.

Design for Breaching

It is also recommended that a 'low point' is created in the defence line, to allow for breaching during an exceedance event. This should allow for the safest

possible transfer of floodwaters into the study areas flood cells.

It is suggested this should occur at a point in the defence line that has the most confidence to when out of bank flooding first occurs and at a point for safest

transfer of exceedance flow. This should be confirmed through on-site discussion and assessing of the local topography. Though from previous

information, it would appear this local point may be around the village bend. If so, it is recommended a 0.3m reduction in the above flood level around

point 'I'. This would also look to allow the floodwaters to flow down along the N22 toward the COOP, where during an event of this nature the floodwaters

will likely have also approached the same location from an independent floodwater flow route (i.e. from the river adjacent to the COOP).

Degree of Confidence

- In the modelled flood levels:

It must be noted that there is always uncertainty in hydraulically modelling flood profiles, for this study while it is calibrated on best available data, there is

still uncertainty. This is also why it is very important to have a breaching mode that most safely accommodates for an exceedence event breaching the

defence line.

- Underground Seepage:

Without SI results, there is uncertainty regarding seepage rates and the true benefit of interim hard defences, as it is likely these would not accommodate for

(underground) seepage flow. While the duration of large events through Baile Mhic Ire is traditionally not for long periods, uncertainly currently exists

regarding seepage rates.

5

- Interim Works Standard of Construction

Considering the interim defence line is considered to be constructed with 1000Kg sand bangs, and it is assumed also with impermeable liners. Defending

the flood cells inside the interim defence line against the 20-year event flood risk is hugely dependant on the practicability, standard and quality of the

interim defence line construction. The Design Section takes no responsibility for this area of work.

- Topography

On-site inspection/measurement is needed to determine the above tabled Defence Heights, between the reference points, where 1m should not be exceeded.

Potential benefit

The number properties the 20-year event interim measures potentially benefits are; 14 (fourteen) properties and another 14 (fourteen) with floor levels

within 0.2m. And 1 (one), property 83**, defended from local defences (refer below).

Conclusion

A review of the above proposed interim works needs to be undertook by the distribution list. Largely to ensure constructing such interim measures will not

in any way increase the existing risk caused from all sources of flooding.

Perhaps defending to a lower event return period may be deemed more practical in terms of ensuring flood risk is not increased. Again the interim defence

lines construction practicability, standard and quality is an overriding consideration when considering the possible effects on increasing the existing flood

risk.

6

Properties outside the Interim Defence Measures, and their respective Flood Depth to the 20-year event.

Property No.

83** Residential: 0.124m, recommend local defences to 116.138m OD i.e. grouting and local door flood gates (to average height 0.424m).

93 Business Park: 0.13m, not to mitigation against, as ground levels for a possible interim defence line would require defences greater than 1m.

99 Dairygold – Storage Yard: 0.112m, not to mitigation against, as ground levels for a possible interim defence line would require defences greater

than 1m.

100 County Council Treatment Plant/Yard: 0.675m, outside of studies defended area.

102 Old Storage Yard: 0.484m, outside of studies defended area.

20-year event Property Flood Depths without mitigation from the Interim Measures.

(Positive 'depths' are property flood depths)

Map Reference

Depth of Flooding (20-Year Event)

(m) Property Type

6 0.418

7 0.164 St. Gobnait's Terrace - 11


11 -0.199 St. Gobnait's Terrace - 15









7



36 0.495

39 -0.070

40 -0.074

43 -0.114 The Abbey Hotel

46 -0.184 Tranquilla

50 0.236 Ionad Cultúrtha

57 0.075

58 -0.002 1

59 0.025 2

60 -0.165 3

62 -0.191 5

66 0.397 Esso Garage

67 0.373

68 0.331

83** 0.124** Residential**

93 0.130 Business Park

99 0.112 Dairygold – Storage Yard

100 0.675 County Council Treatment

Plant

102 0.484 Old Storage Yard

Plan View: Baile Mhic Ire Interim Measures and... · What the above Baile Mhic Ire Interim Measures Plan View illustrates The interim defence line, its location was selected in order

Documents