and absorption or uptake of nutrients by plants. The basic design criteria to improve the performance of
VFS are slow uniform overland flow over a sufficient area and sufficient contact time.
Bingham et al. (1980) found reductions in nutrient concentrations from a grassed buffer were
attributable to dilution in the buffer area in addition to functional mechanisms .
Dillaha et al. (1985) showed nutrient concentrations leaving the filter strip exceeded the nutrient
concentrations leaving the strip. They attributed the increase to the flushing of accumulated nutrients or
to nutrients leached from vegetative cover.
Lanier (1990) found filter length, uniformity of flow in the field and along the filter length, slope
of field, type and density of vegetation, sediment size distribution and the amount of dilution runoff from
the contributing area are primary variables affecting VFS effectiveness. Source area management and
meteorological factors that affect influent pollutants, also determine VFS performance. The cropland
source area and the VFS must be viewed together, the entire system is interactive (Lanier 1990).
Management of the source area is key to filter strip effectiveness. Where cropland hydrologic
condition is poor, and erosion rates are high, long term performance of the VFS would be expected to
be low. Dillaha et al. (1989) found that sediment deposition occurred upslope or in the first few meters
of the VFS. After the vegetation in the upper part of the VFS was buried, deposition advanced down
slope reaching the end of the VFS (Dillaha et al. 1989). Niebling and Alberts (1979) and Tollner et al.
Dillaha (1989) found that where surface runoff accumulates in natural drainageways channelized
flow results and the VFS can be become ineffective. Topographic factors may limit VFS performance
and channelized flow may be more of a problem in hilly areas compared to flat area (e.g. coastal plain) .
Where shallow overland flow can be achieved, the effectiveness of VFS may be estimated in a
relatively straightforward manner by considering sedimentation and filtration as the primary removal
mechanisms. The link to nutrient removal is based on nutrient losses closely associated with sediment.
Soil particle size distribution affects removal and nutrient removal as well. Walter et al. (1979) found
that soils with the highest adsorption capacity have small particles with low density. Most of the
pollutant associated with sediment will be transported on smaller, dense particles (Dean 1983).
Flanagan et al. (1989) presented a simplified model to estimate VFS effectiveness for removing
sediment. Sediment-attached nutrient removals are expected to closely match sediment removal
efficiencies. The methods to estimate effectiveness are based on the CREAMS and its equations that
consider influent particle size distribution, strip width, and slope length. Also for a desired trapping
efficiency, the method determines VFS width.
For sediment of nonuniform particle size, the sediment delivery ratio (SD) is: