Flood Risk Assessment

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Flood Risk Assessment

For

Proposed Strategic Housing Development

at

Daneswell Place,

(Former Printworks/Smurfit Site),

Botanic Road,

Glasnevin,

Dublin 9.

Document No: WS-02-A

Prepared by:

June 2020

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Unit 1, St. Therese’s Place, Flowerhill, Navan, Co Meath. Phone: 046-9077032 Fax: 046 9077932, e-mails: [email protected]

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1. Approach

This section of the report considers 'The Planning System & Flood Risk Management - Guidelines for Planning Authorities' as they relate to the proposed application.

1.1. Planning System & Flood Risk Management Guidelines

General

“The Planning System and Flood Risk Management Guidelines for Planning Authorities”, November 2009 and its technical appendices outline the requirements for a site specific flood risk assessment.

The proposed residential development is classified as ''highly vulnerable development" according to Table 3.1 of the Guidelines. Table 3.2 of the Guidelines indicates that this type of development is appropriate and compatible with flood zone C i.e. outside the 0.1%AEP flood extents.

1.2. Flood Risk Assessment Stages

This site specific flood risk assessment will use existing flood risk information to determine the flood zone category of the site i.e. to check if the Guidelines Sequential Approach has been applied, see Figure 1 below for details.

Figure 1 – Sequential Approach mechanism in the Planning Process

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Flood risk is normally assessed by a flood risk identification stage followed by an initial flood risk assessment. The Greater Dublin Strategic Drainage Study (GDSDS) states that if the risk of flooding is found to be low, there are no restrictions to development. However, if the risk is found to be medium or above, a more detailed flood impact assessment stage must be approved which includes an assessment of surface water management, flood risk and mitigation measures to be applied.

1.3. Existing Site & Location

The subject site is 1.036 ha. The lands subject to this planning application are located within the wider Daneswell Place Development which is 2.02 ha and is bounded to the north by the remaining buildings of the Smurfit printing works, to the east by Iona Park residential housing, to the south by Iona Road residential housing and to the west by Botanic Road.The topography of the site is generally flat with a slight slope from east to west.

There are currently 24 No. constructed dwellings and a further 11 No. dwelling currently under construction within the wider Daneswell Place Development which were permitted under ABP Ref.: PL.29N.246124, DCC Reg. Ref.: 3665/15 (as amended by DCC Reg. Ref. 4267/17, 2133/18 and 4306/18).

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2. Flood Risk Identification Stage

2.1. General

The flood risk identification stage uses existing information to identify and confirm whether there may be flooding or surface water management issues for the lands in question that may warrant further investigation.

2.2. Information Sources Consulted

Information sources consulted for the identification exercise and an assessment are outlined in Table 1 below.

Information Source Comments Assessment

Predictive and historic flood maps, and benefiting lands maps, such as those available on www.floodmaps.ie

OPW

www.floodmaps.ie website consulted

There were no OPW land commission schemes or

benefitting land zones within the subject site’s boundary. Flood event recorded near the site – 600m north of the proposed site Predictive fluvial, coastal, pluvial

and groundwater flood maps

ECFRAMS No flood risk to the site was identified on any flood risk map. The site is located 600mm from the Tolka River and is 3km from the coast

Topographical maps, in particular digital elevation models produced by aerial survey or ground survey techniques

OSI Maps consulted, site topographic survey undertaken and analysed.

No evidence of flood risk due to topography

Information on existing public sewerage condition and performance

Capacity of existing services in Botanic Road evaluated

Discharge rate of 2.0l/s/ha for proposed site agreed with Dublin DCC drainage division during pre-planning stage

Alluvial deposit maps of the Geological Survey of Ireland. These maps, while not providing full coverage, can indicate areas that have flooded in the past

GSI maps consulted The site consists of soils and subsoils of glacial till and boulder clay

Site Specific Geotechnical Report Site investigation report compiled following on site testing which included:

• Boreholes

• Trial pits

• Percolation tests

• Laboratory testing

From the geotechnical report there is evidence that the site is not at risk from groundwater flooding. It also confirms that the existing subsoil underneath the concrete slab/fill sections has very poor infiltration rates

The Planning System and Flood Risk Management Guidelines for Planning Authorities

Flood Zone and Vulnerability Classification – Risk Management process

Confirmation of Flood Zone Category C

Table 1 – Information sources consulted

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2.3. Source-Pathway-Receptor Model

A Source-Pathway-Receptor model was produced to summarize the possible sources of floodwater, the people and assets (receptors) that could be affected by potential flooding (with specific reference to the proposals) and the pathways by which flood water for a 0.1%AEP (Annual Exceedance Probability) and 1% AEP storms could reach the receptors, see Table 2. It provides the probability and magnitude of the sources, the performance and response of pathways and the consequences to the receptors in the context of the post primary development proposal.

Source Pathway Receptor Likelihood Impact Risk

Tidal Tidal Flooding

from coast >3km away

- Remote - -

Fluvial Flooding from

nearest watercourse (Tolka River)

- Remote – Site

level 13m above peak flood level recorded in 2002 - - Surface Water Drainage – (Pluvial) Flooding from the surcharging of the developments drainage systems

Low points in the roads within the estate and green areas. Residents & visitors property including buildings & vehicles parked in the estate Possible – overland flows will be routed towards Botanic road via the proposed road layout. This would simulate the existing run-off process Medium Low Surface Water - Pluvial Flooding from external sources – overland flows

Low points in the roads within the estate and green areas. Residents & visitors property including buildings & vehicles parked in the estate Remote– Site elevation is higher than surrounding / neighboring area Surface Water - Pluvial Flooding from external sources – overland flows into basement Basement carparks containing property of residents, cars, bikes etc Possible –Acco Channels installed at top and bottom of entry ramp; proposed levels to direct water away from entry ramp Medium Low Surface Water – External Sources Flooding from external sources (External Roads)

Low points in the roads within the estate and green areas. Residents & visitors property including buildings & vehicles parked in the estate Remote – Site elevation above surrounding roads Groundwater flooding Rising GWL on the site Residents & visitors property, low points in the roads Remote – Refer to Geotechnical report - - ___________________________________________________________________________

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Human or Mechanical Error (Pluvial)

Petrol interceptor and hydrobrake

Low points in the roads within the development and green areas

Possible Low Low

Table 2 – Source-Pathway-Receptor Analysis

2.4. Source-Pathway-Receptor Model Results

It is clear from the above flooding analysis that the proposed site is not at risk from tidal, fluvial, pluvial or groundwater flooding due to its geographic location and topography. Due to proposed levels, if flooding of any road occurs, any water will be kept within the road reservation and/or directed towards the Botanic Road as per the current site layout, eliminating flood risk to buildings.

Thus the type of development proposed is appropriate for this flood zone category. The Guidelines Sequential Approach is therefore met and the 'Avoid' principal achieved and an initial flood risk assessment is not required.

However, there is a moderate risk of pluvial flooding of the site due to the potential surcharging and blockage of the new drainage network. Consequently, an initial flood risk assessment will follow to provide further detail on the causes, effects and possible mitigation measures for the types of flooding identified above.

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3. Initial Flood Risk Assessment Stage

The only flood risks to the proposed residential development, identified from Stage 1 are a low risk of pluvial flooding following development (due to an extreme rainfall event which exceeds the capacity of the surface water drainage network or an extreme rainfall event resulting in overland flows on adjacent public roadways). There is also a low risk to the basement flooding and a low flood risk to the development due to a blockage or mechanical failure of the drainage network.

3.1. Initial Pluvial Flood Risk Assessment

The Source-Pathway-Receptor model identified that there could be potential for pluvial flood risk within the development site related to the future drainage networks serving the proposed development.

The drainage system has potential to cause local flooding unless it is designed in

accordance with the regulations e.g. Greater Dublin Strategic Drainage Study (GDSDS) and to take account of flood exceedance for storms return periods exceeding 1%AEP (Annual Exceedance Probability).

Proper operation and maintenance of the drainage system should also be implemented to reduce the risk of human or mechanical error causing pluvial flood risk from blockages etc.

3.2. Flood Zone Category

Following the assessment of the flood risks to the site and the available information it is considered that the proposed site is located within Flood Zone Category C as defined by the Guidelines and as indicated by the PFRA maps - refer to Appendix A and B.

Therefore, the proposed residential development on the subject site is appropriate for this flood zone category, and a justification test is not required.

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4. Detailed Flood Risk Assessment Stage

4.1. General

As a justification test is not required, a detailed flood risk assessment must be carried out which considers moderate pluvial flood risk in relation to the following;

- Proposed Surface Water Management measures. - Flood Exceedance.

- Impact of proposals on flood risk to adjacent areas. - Effects of climate change.

- Access and Egress during Flood Events - Residual risks.

- Effectiveness of any flood mitigation measures. The following drawings and reports were reviewed:-

- Surface Water Network Calculations - Attenuation storage calculations - Site Services Layout

- Drainage Details

- Stormwater Longsections - Foulwater Longsections

- Site specific Geotechnical Investigation Report - Floodmaps.ie and CFRAMS maps and reports

4.2. Proposed Surface Water Management Measures

The surface water drainage system was designed by using following approach and parameters:-

- Drainage design consists of downpipes, gullies, pipes, manholes, petrol interceptor, underground attenuation system and discharge control at outlet;

- Attenuation with no infiltration to be a combination of 'Stormtech' modular system, concrete storage tanks and oversize pipes;

- Infiltration testing determined that poor soil infiltration rates are present; - Attenuation volume required is 1216m³;

- Climate change factor of 20% has been applied;

- Site discharge rate is 4 l/s controlled by a 'Hydrobrake';

- Total site area is 20,200m² with total impermeable area of 13,417m²; Subject site is 10,355m²

- Klargester NSB5 class 1 bypass petrol interceptor provided at inlet to attenuation system

- Minimum public pipe size of 450mm diameter, pipes are designed using the Modified Rational Method with a return period of 30 years and 50mm/hr rainfall intensity;

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5. Assessment of Flood Risk

5.1. Flooding internal to site

Flooding from surcharging of the development's drainage systems:-

The surface water pipe system has been designed using the Modified Rational Method for a 1 in 30 year storm return period with a rainfall intensity of 50mm/hr. It is noted that the overall capacity of the pipes will be greater than the peak flows generated from this intensity,

therefore the pipe system would be considered to exceed the requirements of the GDSDS for a 1 in 30 year return period surcharge check.

The attenuation system has been sized for a 1 in 100 year return period, which exceeds the requirements of the GDSDS and it is designed using the current rainfall depth values available from Met Eireann including 20% increase for the effects of climate change. The NSB5 Klargester petrol interceptor is designed to cater for a peak flow of 501/s which exceeds the predicted peak flow of the incoming drainage system. Furthermore, the petrol interceptor has been located downstream of the attenuation system which would slow and store water during peak flows thereby further reducing any risk of the petrol interceptor capacity being exceeded or any contaminants collected within the interceptor being re-suspended.

As the existing site has very poor infiltration rates, the existing attenuation systems have been tanked (i.e. surrounded in an impermeable bentonite liner) therefore there is no risk that water would de-stabilise or undermine the bearing capacity of the road / carpark spaces. The

proposed attenuation systems for this application will be located against the basement wall, and around Block A but also alongside the existing access road. In the event of the system surcharging the flood water would flow into the road corridor which would act as a channel directing the flood water towards Botanic Avenue.

5.2. Flood Exceedance – Pluvial

Flooding from internal sources- overland flows into basement:-

Site levels have been designed such that overland flow caused by flooding from the site drainage system, or from surface water that fails to enter the site drainage system in extreme events, will not flood driveways or footpaths. Surface water will remain within the roadway which is graded towards Botanic road. This mimics the existing run-off route for the existing site conditions. To prevent any possible overland flow or run off entering the basement at the entry ramp the proposed new road levels will be designed to direct water away from the top of the entry ramp with a drainage channel installed at the bottom of the entry ramp to intercept any rainfall. The basement itself will have a typical basement drainage system consisting of a low level sump and lift pump.

5.3. Human or Mechanical Error – Pluvial

If the petrol interceptor is not adequately cleaned and maintained, there is a risk that the petrol interceptor would become a throttle and cause flooding upstream.

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5.4. Access & Egress during Flood Events

During flood events, access and egress would need to be maintained and overland flow routes and extents would need to be carefully planned. The habitable spaces are located more than 500mm above the top water levels for the Attenuation system for the 1in 100 year event and are at no perceivable risks of flooding.

5.5. Mitigation Measures

Proposed mitigation measures to address residual flood risks are summarized below;

- M1. Surface water retention is provided to accommodate runoff in excess of Qbar for a 1%AEP event.

- M2.The proposed drainage system to be maintained on a regular basis to reduce the risk of blockages and mechanical failure.

- M3.In the event of storms exceeding the design capacity of the drainage system, overland flows will be routed towards the internal road which will act as a channel directing flows towards the Botanic road which simulates the existing site layout. - M4. Road levels designed to direct flow away from top of entry ramp and drainage

channels installed at the start of the ramp to the basement car park.

5.6. Residual Risks

There is a low risk of flooding of the development from surcharging of the development's drainage system. The surface water drainage network is designed to

accommodate surcharging up to a 1 in 100 year event. Localized flooding may occur within the road network during a weather event exceeding the 1 in 100 year flood event. Overland flow paths have been designed to direct high intensity/short duration flood events away from buildings towards the internal road and out to the Botanic road. This simulates the current run-off of the existing site.

6. Conclusion

From reference to the ECFRAMS mapping for the Tolka River, the site is located in Zone 'C', i.e. outside the 0.1%AEP flood extents. A copy of this mapping is included in Appendix 'A'. The proposed development which is deemed 'highly vulnerable' in the 'Planning and Flood Risk Management Guidelines' is deemed appropriate for this zoning classification.

The only risk of flooding to the site is from the drainage system for the development. This is mitigated by designing the drainage system for a 1 in 30 year return period and retaining surface water runoff on site for storm events up to 1% AEP (in excess of the allowable run off).

All of the mitigation measures outlined above should be implemented to ensure flood risk is reduced.

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7. Appendix A - OPW – Preliminary Flood Risk Assessment (PFRA)

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OPW – Local Flood Report

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8. Appendix B

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Flood Risk Assessment