WATERSHEDS - Water Quality Engineering in Natural Systems - page 220

6.2.3.4 Collection System Pollution Control. Collec-
tion system pollution control practices include methods
and practices for removing pollutants from runoff after
they leave the source area. Such methods include
grassed waterways and channel stabilization, riprap and
gabions, and catch basins. These are described below.

and grassed waterways remove pollutants by
slowing down the flow, filtering by grasses, infiltra-
tion, and nutrient uptake by vegetation. however,
in contrast to grass filters, the grasses in swales are
submerged and flow in the waterways are turbu-
lent and hence less effective for removal of par-
ticulates than grass filters. Generally, the lower the
slope and velocity, the better the treatment perfor-
mance of swales. For effective pollutant removal
control, the depth in the swale should not be
greater than 30-45 cm (1-1.5 ft), and the velocity
should not exceed 0.9-1.8 m/s (3-6 ft/s). higher
slopes can be reduced by check dams and/or by
temporary barriers made of straw bales and cloth.
Riprap and Gabions. Riprap is a layer of loose rock
or concrete blocks placed over an erodible soil
surface, and gabions are blocks made of wire mesh
filled with smaller rocks or gravel. These are illus-
trated in Figure 6.13. Both riprap and gabions are
used primarily for channel stabilization in high-
erosion zones, such as sharp bends, channel drops,
and flow energy dissipators (stilling basins) below
the outlets of sewers, narrow bridges, and near
connections with lined high velocity channels.
Catch Basins. A catch basin is a chamber or well,
usually built at the curb line of a street, through
which stormwater is admitted into the sewer
system. A catch basin typically has as its base a
sump which should be large enough to provide
storage for trapped debris. A typical catch basin is
shown in Figure 6.14. The schematic diagram of
the catch basin in Figure 6.14a shows the hood that
is necessary to keep floatables out of the sewer
system, and the bottom drain that can be used in
permeable soils to remove stored water in the
catch basin. A typical system in Figure 6.14b shows

Grassed Waterways and Channel Stabilization. When
conveyance channels are used, channel erosion
can be controlled by designing hydraulically stable
channels. Grassed waterways are probably the
most inexpensive but effective means of convey-
ing and treating water. In a simple case, a grassed
roadside swale will perform as well or better than
a more expensive buried storm sewer. A typical
grassed swale, shown in Figure 6.12, is a shallow
channel with side slopes on the order of 3:1 (h :V)
and is used only for conveyance of surface runoff,
hence is mostly dry and contains water only during
and following rain. Similar to grass filters, swales

Figure 6.12. Typical grassed roadside swale.

(a) (b)

Figure 6.13. (a) Riprap; (b) gabions.