The site was close to the Canons Brook, and shown to be in Flood Zone 2 (medium risk) and close to Flood Zone 3 (high risk) on the EA Flood Map for Planning.
As no modelled flood levels were available to assess the risk to the development, it was necessary to build a simple 1D hydraulic model using Flood Modeller software. A channel cross section survey was carried out so that accurate cross section data could be included in the model. Hydrological analysis was undertaken using gauged flow data from the Canons Brook, to determine suitable inflows to the model. The model was run for critical fluvial events to provide flood levels to assess the development against, and to inform any necessary mitigation measures.
The EA reviewed the model and agreed it was acceptable to assess the development. The model demonstrated the site was located in the equivalent of Flood Zone 1, not Flood Zone 2 as shown on the EA Flood Map for Planning, and therefore was at low risk of flooding. This meant that no specific flood mitigation measures would be required.
Sustainable drainage in the form of lined permeable paving was included to attenuate the surface water runoff onsite. There was a limited discharge to the Canons Brook, which provided a significant reduction in runoff when compared to the existing situation.
Planning permission was granted in June 2020.
Flood Risk Assessment, Hydraulic Modelling and Sustainable Drainage Design
Fold Croft, Harlow, Essex