WATERSHEDS - Water Quality Engineering in Natural Systems - page 218

(a) (b)

Figure 6.8. Infiltration trench. (a) During construction. (b) After construction.

clogged during construction, the entire area
contributing to the infiltration structure should
be stabilized before and during construction of
the trench. Oil and grease should be removed
before they enter infiltration trenches since
these contaminants are difficult to remove and
present a threat to groundwater.

Both infiltration basins and trenches are prone to clog-
ging by deposited solids, and to increase their lifespan
when sediment-laden stormwater flows are anticipated,
sediment traps prior to inflow are recommended. The
best pretreatment device is a grassed filter or buffer
strip along the periphery of the basin or trench, where
it is widely recommended that filter strips be at least
6.5 m (20 ft) wide to be effective.

Figure 6.9. Filter strip.

6.2.3.3 Attenuation of Pollutants. The most common
practices used to attenuate and reduce the transport of
pollutants from sources to receiving water bodies are
filter strips and buffer zones. These are described in
more detail below.

(NRCS) Type A or Type B soils, filter strips can
facilitate infiltration without underdrains. Filter
strips cannot treat high-velocity flows and there-
fore are generally used in small drainage areas.
Grass filter strips provide higher pollutant removal
rates than do grass swales. The difference between
filter strips and swales is in the type of flow. Flow
depths in filter strips are less than the heights of
the grasses, thus creating laminar flows that
enhance settling and filtering. Flow in swales is
concentrated and flow depths are greater than
the heights of the grasses, which usually results in

Filter Strips. Filter strips are vegetated sections of
land designed to accept runoff as overland sheet
flow from upstream developments or flow from a
highway or parking lot. Filter strips remove pol-
lutants from runoff by filtering, provide some infil-
tration, and slow down the runoff flow to promote
sedimentation. An example of a filter strip between
a roadway and an infiltration trench (filled with
stones) is shown in Figure 6.9. In areas with