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 5
∑ f x Φi
SD =
iu
i= 1
where:
fi = the fraction of particle size i entering the strip and is the particle index (dimensionless)
xu* = source area length of flow divided by total length (total length = source area length of flow + VFS
length of flow) (dimensionless)
φi = measure of depositability for each particle size i (dimensionless) is given in Table 4.2.6
The quantity fi is determined by soil type and the slope of the source area and VFS (these slopes are
assumed to be the same) . For a silt loam soil the computed fraction of primary clay, primary silt, and
small aggregates is given in Table 4.2.4.
Flanagan et al. (1989) provides example SD calculations for a field with silt loam soil in a moderate
slope of 2.5% with slope length of 100 m. For the example, the desired
VFS effectiveness is a SD of 0.5 and reduce sediment losses from 10 t/ha to 5 t/ha. The particle size
fractions reaching the strip are:
Fclay = 0.05
F+silt = 0.24
Fsm.agg = 0.36
If the VFS is 2 meters wide then xu* = 0.98 and o the values for primary clay, primary silt and
small aggregates are 0.11, 2.84, and 13.5, the SD would be estimates as 0.55 as in the sample
calculations Table 4.2.6.
Dillaha (1989) proposes that additional site selection, installation, and maintenance, and criteria
be incorporated into VFS implementation.
Site Selection
Soil drainage and depth to water table must be sufficient to allow adequate vegetative growth
and prevent extended periods of saturated soil
VFS site should be devoid of hillside seeps of other continuous discharges
4.2-20
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