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 provide P values for agricultural practices, the maximum value of which is 1.0. On construction sites,
however, values for P may be greater than 1.0. The New Jersey sediment control manual (NJ SSCC,
1982) provides P values for these conditions.
Sediment Delivery. The movement of sediment and sediment-attached nutrients for a given
runoff event, from source area to receiving water body is a complex process. Over the course of many
events pollutants may be deposited, resuspended, scrubbed from solution by adsorption, undergo
chemical reaction or biological uptake and redissolved by desorption or decay. In spite of sediment
yield process complexities, it is possible to focus on important factors and derive an approximate
estimate of pollutant delivery.
According to Mills et al. (1985), annual watershed sediment yield due to surface erosion as:
∑X A
Y = SD
(1.1.18)
k
k
k
Where:
Y = average annual sediment yield (tons/yr)
Xk = average annual erosion from source area k as given by the USLe (t/ha)
Ak = area of source area k (ha)
SD = watershed sediment delivery ration (dimensionless)
The sediment delivery ratio (SD) is the mass of sediment delivered to the point of measurement
divided by the mass of soil loss due to gross erosion. All of the soil that is eroded or transported may
not reach the point of measurement. Many sediment delivery models have been developed using a
variety of geomorphic terms to calculate the sediment delivery ratio. Criteria for model selection should
consider source area type (individual field, subwatershed or watershed), geographic location and
sediment traps in the watershed.
For individual fields, the distance from the source area to the watercourse may be used to
estimate sediment delivery. Reckhow et al., 1988 developed a distance-based SD from Texas,
Oklahoma and extreme southern Kansas). The distance-based SD for either an individual field or
watershed is:
1n (SD) = 1.10 - 0.34 1n (D)
(1.1.19)
where:
R2 = 0.8
S.E. = 0.356
n = 25
1.1-17
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