The Great Stour Project - River and Coastal Flooding

Drainage

Drainage density is a measure of the extent of surface drainage, recorded as total stream length per square kilometre.

Where the rock type is chalk, there are very few or no surface streams and dry valleys are common. On occasions, after a period of exceptionally wet weather, some surface streams or bournes, such as the Nailbourne, may appear for a short period.

Where the rock type is impermeable clay, there are numerous surface streams. However, in dry periods many smaller streams, not sustained by groundwater, will dry out reducing drainage density.

Where land is low lying and, in this case only a few metres above sea level, the local water table is near the surface. Here, the former Wantsum marshes have been drained by ditches since medieval times to produce farmland.