Page 135 - Bexley SuDS DESIGN & EVALUATION

Filter drains

Filter drains, sometimes called a French drain 3. A lower perforated pipe will assist
after Henry Flagg French (1813-1885), is an discharge and an upper perforated pipe
open stone filled trench. can act as an overflow. However, neither
may be necessary depending on the Detailed Design
1. Runoff should ideally cross the long edge
design and location.
of the trench as a sheet. This may require
a temporary level timber board along the Most filter drains are designed with geotextile
leading edge to prevent erosion of lining. Many geotextiles are susceptible to
unconsolidated soil. blinding from fine materials in soils. An
alterative liner is the use of hessian which will
2. A sacrificial top layer may be considered
biodegrade over time by the time soils
at the top of the drain to trap any silt for around the filter drain will have stabilised.
simple removal. Alternatively, a grass filter
strip placed in front of the filter drain will
reduce potential for clogging.

130
1

2

3

London Borough of Bexley SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates