A Flood Risk Assessment and SuDS design for 123 residential dwellings over 7.7 hectares.
About the project
Ambiental were asked to undertake a NPPF (National Planning Policy Framework) compliant Flood Risk Assessment and Surface Water Drainage Strategy for the development of 123 residential dwellings on Rose Mead Farm, Horam. The development covered approximately 7.7 hectares.
The planning for the project went on to be approved and the development has been given the go-ahead. Ambiental further supported post planning providing detailed design drawings to discharge the pre-commencement conditions.
Rose Mead Farm Drawing
A little more about the project
The site consisted of undeveloped agricultural land and was classified as a predominantly Greenfield Site. The new development under the National Planning Policy Framework (NPPF) saw the vulnerability classification change from the existing “Less Vulnerable” to “More Vulnerable” under the NPPF guidance.
The redline application boundary of the site being greater than 1ha whilst being in Flood Zone 1 (low risk of flooding) still constituted a need for a Flood Risk Assessment adhering to the national policies. In addition, given the number of units proposed and at the time in accordance with the national policy introduced as of the 6th April 2015, Sustainable Urban Drainage (SuDS) - SuDS for Major Applications. The new policy required SuDS to be considered for planning applications of 10+ residential dwellings. The new guidance would therefore require production of a new assessment which we call a Surface Water Drainage Strategy.
The Manor Farm development proposed 123 units and the new guidance requested that all development of 10+more residential dwellings would be required to provide a Surface Water Drainage Strategy to evidence how surface water would be mitigated post construction of the proposed development.
The FRA was to show that the development could be achieved in a sustainable manner, with an overall reduction of flood risk to the site and surrounding area.
Rose Mead Farm Flood Zone Map (Taken from: Flood Map For Planning)
Prior to the development, the site was used almost exclusively for agriculture. With the change in use moving to primarily residential dwellings, this was likely to cause a change in the way rainfall and surface water reacts within the redline application boundary.
Our Chartered Engineers reviewed how the proposed development increased the impervious material (material that limits/impedes the draining of water), accounted for about 2.31 hectares of space. This is equating to 30% of the total development area. In practical terms, this means that 30% of the area will no longer allow water to flow/drain freely, likely resulting in the water pooling on the surface or travelling to other areas where it wouldn’t before. This is where introduction of Sustainable urban Drainage Systems (SuDS) can become greatly beneficial.
The geological survey showed that the site was mostly underlain by a Wadhurst Clay aquifer which was filled with fractures and discontinuities, making it a fairly ineffective aquifer with very poor infiltration. There were also multiple ponds within the site, suggesting the water table was quite close to the surface.
When Ambiental looked at the local water surface management plan for East Sussex, it indicated that there are opportunities for bespoke infiltration SuDS. Unfortunately, the results of the Geological Survey showed that it’s unlikely to be suitable for infiltration SuDs.
Ambiental, upon receipt of the infiltration tests, met with the Lead Local Authority on site and took part in a design meeting to work with the authority to design a Surface Water Drainage Strategy that would be approved by them, whilst also addressing the client’s aspirations for the development.
Given the scale and importance of the development in the local area, whilst also not being able to identify a means of mitigation for the predicted level of water as per CIRIA SuDS Hierarchy, it meant that a slightly more complex bespoke SuDs design needed to be considered to effectively mitigate the surface water and rainfall. The mitigation measures had to ensure that the water would be moved away from the homes and reduce the risk of flooding elsewhere.
As infiltration SuDs, also known as soakaways, wouldn’t be appropriate given the low permeability of the area and high groundwater table, Ambiental created a surface water drainage design that included attenuation storage in order to limit the runoff rate of the site. It also had the added benefit of providing amenity and biodiversity benefits where possible and hold a greater volume of water on the site before discharging the water off site.
In addition to the strategy our team was able to provide a maintenance plan which was put in place, as well as a cleaning schedule, to make sure the Sustainable Urban Drainage Solutions continued to function as intended throughout the lifetime of the development.
The proposed development was demonstrated to have a low risk of flooding from all sources, including groundwater, surface water, river, and sewer. The SFRA had not identified the site to have experienced historical flooding from any sources, and so it was classified as having a low probability of flooding.
Ambiental designed a Surface Water Drainage Strategy which was approved. The strategy created met the standards required by the national policy to avoid any flood risk in a 1 in 100 year rainfall event using a variety of attenuation storage solutions.
Ambiental for this project incorporated:
- Permeable Paving
- A series of cascading attenuation Ponds to mitigate the surface water across the site’s varied topography
- Flow Restrictor (HydroBrake) to manage the flow of surface water through the various attenuation ponds and one at the final pond before discharging of site.