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 Grassed Waterways. Grassed waterways, practice 412, are used primarily to prevent rill and
gully formation, to stabilize an active gully, as a stable outlet, or to convey runoff from terraces,
diversions, or other outlets without causing erosion or flooding and to improve water quality. Grassed
waterways must used in conjunction with upland soil conservation practices (Baker and Johnson 1985).
A secondary function may be to trap sediment and sediment-attached nutrients in runoff.
Depending upon the rate of deposition, the capacity of the grassed waterway may be exceeded, and
can become buried and rendered ineffective (Lake et al. 1977). The Black Creek, Indiana watershed
study found that grassed waterways were more effective in reducing sedimentation if used with other
BMPs such conservation tillage, diversions and terraces. DelVecchio and Knisel (1982) found minimal
treatment by a grassed waterway compared to contouring and crop residue management. They found
little effect on dissolved nutrient concentrations due to grassed waterways. Davenport et al. (1984)
simulated the effect of grassed waterways with several tillage scenarios using the model CREAMS.
They found that where overland flow sediment yields were high as in the case of conventional tillage
systems, grassed waterways could remove approximately half of the overland flow sediment yields.
However, Davenport (1984) found that high rates of deposition in grassed waterways could cause
long-term maintenance problems. Hamlet et al. 1987 report lower trapping effectiveness during extreme
rainfall events and resuspension of previously deposited sediment from a grassed waterway.
Most studies using grassed waterways report the effectiveness of a system of BMPs that
include a grassed waterway. Since flow is channelized, only minimal long-term sediment-attached
nutrient removal can be expected from grassed waterways. Effectiveness is a function of maintaining
channel shape, vegetation and upland soil erosion control. Asmussen et al. (1977) report 2-6%
effectiveness for removing sediment from a plot study. Using CREAMS to simulate effectiveness
Davenport (1984) found a 1% sediment removal and a similar removal rate for nutrients for a grassed
waterway below a no-till system.
Filter Strips . Filter strips are designed to remove runoff-borne nutrients and sediment in
combination with other source area BMPs. Interest in the use of filter strips has been increasing due to
their relative low cost and their effectiveness in removing pollutants in carefully controlled short-term
research studies. However, there is a great deal of uncertainty about the long-term (10-20 years)
effectiveness of most filter strips now in use. Design is an important concern. The minimum standards
of VFS design are provided by the SCS (1987). This section focuses on cropland filter strips referred
to as vegetative filter strips (VFS) .
To estimate the effectiveness of VFS each site must be inspected before installation and
periodically throughout its life to evaluate functional attributes. Dillaha (1989) identifies the processes in
flow hydraulics thought to control pollutant removal in VFS. Pollutant removal is
likely a function or infiltration of runoff and pollutants, deposition of suspended sediments and sediment-
attached nutrients, filtration of suspended pollutants by vegetation, adsorption on soil and plant surfaces,
4.2-18
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