Page 135 - Newham SuDS DESIGN & EVALUATION GUIDE

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Detailed Design Swale 4. Where swale vegetation is kept less than Filter drains 3. A lower perforated pipe will assist Detailed Design

Filter drains, sometimes called a French drain
Swales are shallow, flat bottomed vegetated
after Henry Flagg French (1813-1885), is an
100mm, the shoulders at the top of the
channels which can collect, treat, convey and
discharge and an upper perforated pipe
can act as an overflow. However, neither
swale can be ‘scalped’ leaving bare soil.
open stone filled trench.
store runoff.
may be necessary depending on the
The shoulders should therefore be
1. The basic profile is a 1 in 3 or 1 in 4 side
1. Runoff should ideally cross the long edge
rounded to prevent this happening.
design and location.
slope to a flat base falling at no more than
of the trench as a sheet. This may require
5. Where inlet flows are concentrated to
1 in 50 to prevent erosion.
Most filter drains are designed with geotextile
a temporary level timber board along the
2. Base width less than 1m wide will increase
erosion apron may be needed.
the risk of erosion and ditch forming,
unconsolidated soil.
blinding from fine materials in soils. An
conversely, base width wider than 3m a points through an upstand kerb an leading edge to prevent erosion of lining. Many geotextiles are susceptible to
alterative liner is the use of hessian which will
2. A sacrificial top layer may be considered
meandering channel can develop. biodegrade over time by the time soils
at the top of the drain to trap any silt for
3. 150mm clean topsoil over subsoil. Ripping simple removal. Alternatively, a grass filter around the filter drain will have stabilised.
or light harrowing will improve
strip placed in front of the filter drain will
establishment of the swale by providing a reduce potential for clogging.
key for the topsoil, encourage deep
rooting and assist infiltration.

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