Flood Risk & Drainage

Drainage Design Principles

Surface water drainage features will intercept and store rainwater runoff in new sustainable drainage systems (SuDS).

The proposed SuDS within the site will include dry basins with opportunities for additional interspersed features e.g. swales, permeable paving, filter drains, rain gardens and tree pits to further slow and direct flow while improving water quality and providing amenity and biodiversity benefits.

Water will be subject to a controlled discharge to the existing ditch at significantly reduced rates compared to existing (more than 70%), before continuing off-site to the east.

The drainage strategy will provide for large storm events (1% probability events) with an increased rainfall intensity to allow for climate change (45%) as well as 10% allowance for urban creep (e.g. residents building extensions or paving over areas), while reducing discharge rates, thus providing significant betterment over existing and expected future conditions.

Additional capacity and control measures of upstream flow, are proposed within the northern catchment to further reduce flood risk downstream in Southill to the east.

This approach will protect the development from flooding throughout its lifetime whilst also offering a reduced flood risk to Southill.

Flood Risk Improvement Opportunity

Specialist flood modelling has been undertaken to assess existing conditions and opportunities through future development.

The implementation of the proposed surface water strategy would reduce the peak rate of discharge entering Southill whilst also mitigating the effects of any increased runoff volume and adverse impacts due to climate change. The proposals provide an opportunity to reduce the frequency and risk of flooding within Southill, which would otherwise continue to increase due to climate change impacts.

Flood Risk Improvement Measures

Localised channel restriction and an impoundment bund within site would engage the enlarged northern basin with controlled flow from the existing ditch prior to discharging downstream. This would deliver:

Safe vehicular and pedestrian links within the site. The existing route of the Public Right of Way across the ditch would be improved.

Onset of flooding to be less frequent event
- Current onset: 2%AEP (1 in 50yr) storm event
- Post dev. Onset: 1%AEP (1 in 100yr) storm event
- Shown to prevent flooding within Southill in up to the 1.3%AEP event (1 in 77 year)

Volume of overland flow reduced
- Circa 2,000m3 additional storage volume provided on site for managing upstream runoff passing through the site
- Equivalent of approx. 20 double decker busses

Flood Risk and Drainage

Drainage Design Principles

Example CG Fry & Son homes fronting SuDS basin

Flood Risk Improvement Opportunity

Flood Risk Improvement Measures

Flood Risk and Drainage

Drainage Design Principles

Example CG Fry & Son homes fronting SuDS basin

Flood Risk Improvement Opportunity

Flood Risk Improvement Measures

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