A template flood risk assessment and flood risk report to accompany planning applications.
FLOOD RISK ASSESSMENT
Mr & Mrs Joe Bloggs
Planning Ltd, August 2018
This report has been prepared for Mr & Mrs Joe Bloggs in relation to land at 10 Your Road, Your Village, Surrey, KT100 and no responsibility is accepted to any third party for full or part of this document in connection with this or any other development.
Planning Ltd are providing this flood risk assessment to accompany a planning application by others.
Existing Site and Flood Conditions
Your Road is located off the A25, to the North West of the Your Village Recreation Ground and to the north of Your Village mainline Railway Station. The site is partially inside Flood Risk Zones 2 and 3. The Environment Agency Flood Map places the property in Flood Zone 1 (sites within this zone are susceptible to less than a 1 in 1000 chance of river flooding), however fluvial flood patterns in the area do show the rear garden partially in Flood Zone 2 (1% or greater probability of flooding from a river). Elmbridge Borough Council’s Strategic Flood Risk Assessment (SFRA) Appendix C, shows the property at the outer reaches of Flood Zones 2 / 3.
The building on the site is currently a 3 bedroom, detached single family dwelling. The site is all but flat, however ground level does slope towards the back garden over the depth of the property. Side access is stepped, photo 1 below refers. The property benefits from a substantial grassed rear
garden (see photo 1 below) and a front garden consisting mainly of gravelled drive with mature shrub planting to the boundaries. Side access is largely concrete.
The existing foul and surface water sewerage runs alongside the current property connecting into mains sewers in Your Road.
Elmbridge Borough Council’s Strategic Flood Risk Assessment (SFRA) shows the site in an area of potential risk from groundwater flooding to buildings below ground level. As there are no subterranean features such as basements or swimming pools, it is contended that there is no significant risk to the site.
The Council’s SFRA shows the site in a low risk area for surface water flooding. The Environment Agency’s surface water flood map indicates that the site is at low risk (1 in 1000 years storm event).
The EA’s reservoir flooding map shows the site as being outside of areas designated as being at risk of flooding should a reservoir fail.
We have been unable to find evidence of any historical flood incidents on this site and this has been reinforced by both the EA and Elmbridge Borough Councils SFRA.
Proposed Development and Drainage Strategy
This planning application is for an extension to the rear of the property. The extended footprint will be less than the existing raised patio area, which is currently timber decking on a masonry base. Therefore the proposed development will have minimal impact on the current level of permeable ground surrounding the existing building.
The proposed ground floor finished floor level will adhere to existing levels in the main house. Whilst the surrounding ground levels are essentially flat, there is a difference in level between the front and rear of the property of approximately 400mm, sloping towards the rear garden. The resulting floor levels are set at approximately 300mm above ground level at the front of the house and some 700mm to the rear. The current study and garage to the side of the property are set at marginally above ground level at the front of the property. This application proposes alterations to the side extension to create a continuous floor level equal to the main house. The exception to this will be the proposed garage, which is to be largely used for cycle storage/training and located at the front of the property.
The property is therefore at low risk of flooding and no flood mitigation measures are proposed. Photos 3 and 4 overleaf highlight the difference between surrounding ground and ground floor level.
The overland flood route identified on the EA’s surface water flood map will not be impeded by the proposed extensions or other features.
Infiltration methods are proposed for discharging surface water off the site. Namely, gravel driveways and open jointed porous paving to the rear of the property, with a soakaway in the garden. It is suspected that there is an existing soakaway located in the rear garden. This is to be exposed by the main contractor and inspected for suitability by Building Control Officer.
If required, additional crated soakaway tanks are to hold the volume of the critical 1 in 100 years plus 30% allowance for climate change storm. These units to be sized following percolation tests in accordance with Building Regulations Approved Document H2.
Various alternative sustainable drainage systems have been considered to store excess storm water. Green roofs and ponds were considered impractical and rainwater harvesting tanks on a small residential development are not generally considered good practice or commercially viable.
It is therefore proposed to use gravel or permeable paving to all new areas of hardstanding and a tank soakaway for additional surface water to discharge to ground.
It is proposed to retain the existing discharge method of the foul water into the existing foul sewers in Your Road. Subject to confirmation from Thames Water.
Proposed Flood Warning and Evacuation
The proposed development will not increase the existing flood risk or affect the current evacuation plan, which is to vacate the property via the front door / garage doors to the high ground of Your Road, the opposite direction from the area at risk of flooding to the rear of the property. The applicant is aware of and will utilise the EA flood alert service to forewarn of any possible flood events affecting the property.