The drive behind the Critical Drainage Area (CDA) allocation is to reduce downstream flooding by controlling the accumulative impact of surface water runoff from multiple development sites in sensitive catchment areas.
This means that any site, discharging surface water to a watercourse or public sewer, must attenuate the flow to mimic the green field runoff for a 1:10 year rain fall event. Where the surface water can be managed within the site for the "1:100+40%" condition, there is no change to the standard surface water drainage requirement.
The drainage assessment will normally form part of the Flood Risk Assessment for development proposals which are over 1ha within Flood Zone 1, where the main consideration will be appropriate management of surface water and should be appropriate to the scale and nature of the proposed development.
"All off site surface water discharges from development should mimic "Greenfield" performance up to a maximum 1 in 10 year discharge. On-site all surface water should be safely managed up to the "1 in 100+climate change" conditions. To satisfy the above will require additional water storage areas to be created within the site compared to the normal SUDS design thereby contributing to a reduction in flooding downstream."
Maps for these areas can be downloaded from the right of this page.
Or, you can use Devon County Council's map to search for CDAs. Click on the link, scroll down and accept the disclaimer, then tick Critical Drainage Areas in the 'flood risk' drop down menu. You can then add your address to see if your development is in a CDA.