Contributing areas

It is important for the designer to demonstrate what the contributing areas are and how they are accounted for in the calculations. Catchment and contributing area plans are useful ways to show this.

We need to understand overland flow, both before and after the construction of a proposed development, to ensure that flood risk is not increased. Catchment plans can be created by interrogating topographical surveys and LIDAR data.

These catchment plans can then be used to establish what areas are likely to generate runoff which can contribute to the SuDS.

We expect all impervious surfaces to use a runoff coefficient (CV value) of 1 in the supporting models.

UKSuDS suggests 2 methods for determining the runoff coefficient from vegetated surfaces. We will accept either of these methods but highlight that the second presents challenges in demonstrating compliance and assessment.

Our preferred approach is that designers use a fixed percentage equal to SPR (Standard Percentage Runoff coefficient for the soil). This is the simplest approach where the percentage of runoff is fixed for all rainfall events regardless of return period or duration.

At the time of writing, it was unclear how the most commonly used software would be able to account for 2 different runoff coefficients in their inputs which are normally only varied for summer and winter.

We suggest that a simplified approach is used, whereby the simulation and design setting CV values are 1, however the contributing area for pervious surfaces is factored by the SPR value. This allows pervious surfaces to be included in a network model and enables a designer to demonstrate compliance with CV values for impervious surfaces being 1.

A designer is free to demonstrate compliance in alternative ways, but their approach must be clearly described in the drainage statement and shown in the supporting models and plans.

It is important that contributing area plans are submitted to support an application.  The plans must show the areas that are used for the greenfield runoff calculations. Individual areas should be specified in metres squared or hectares.

It is suggested that a network contributing area plan is also submitted. This is particularly important where network models are used and if a designer has applied an SPR factor to the pervious contributing areas. This plan should show impermeable areas that drain to each node to align with the network model. Pervious areas must show their full contributing area, with an additional label to describe their SPR factored area. Only the SPR factored area will be seen in the model, but the full contributing area must be shown to ensure that the total contributing area for the site matches the one used for the greenfield runoff calculations.

We need to check the SPR value for the site.  This should be included with and match your greenfield runoff calculations. To check those, we will normally ask you submit your FEH point descriptors. 

FEH 22 point data files should be submitted in accordance with clause 5.1.7 of the FEH Web Service terms of use.  These must be submitted directly to land.drainage@arun.gov.uk rather than to the planning portal (via the planning officer), this is to ensure that the data remains confidential in accordance with those terms.