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42 Chapter 2 Water Sources: Surface Water

Figure 2.9 A Watershed Dam in Northwest Iowa Is
Completely Silted In. The dam no longer functions
to store water, since its capacity has been lost.
(Courtesy of the National Resources Conservation
Service and USDA)

Silt accumulations cannot be removed economically from reservoirs by any means
so far devised. Dredging is expensive, and attempts to flush out deposited silt by open-
ing scour valves in dams are fruitless. Scour only produces gullies in the silt. In favor-
able circumstances, however, much of the heaviest load of suspended silt can be steered
through the reservoir by opening large sluices installed for this purpose. Flood flows are
thereby selected for storage in accordance with their quality as well as their volume.
Reduction of soil erosion is generally a long-range undertaking. Involved are proper
farming methods, such as contour plowing; terracing of hillsides; reforestation; cultivation
of permanent pastures; prevention of gully formation through construction of check dams
or debris barriers; and revetment of stream banks.
In the design of impounding reservoirs for silt-bearing streams, suitable allowance
must be made for loss of capacity by silting. Rates of deposition are especially high in im-
poundments on flashy streams draining easily eroded catchment areas. The proportion of
sediment retained is called its trap efficiency. A simple calculation will show that 2,000 mg/L
of suspended solids equals 8.3 ton/MG (1.989 tonne/ML) and that an acre-ft of silt weighs
3
3
almost 1,500 ton (1,360.8 tonne) if its unit weight is 70 lb/ft (1,211.4 kg/m ). In some parts
3
of the United States the volume of silt V in acre-ft (1 acre-ft 0.32585 MG 43,560 ft
s
3
1,233.5 m ) deposited annually can be approximated by the equation:
n
V cA (U.S. customary units) (2.7)
s
2
2
2
where A is the size of the drainage area in mi (note: 1 mi 2.59 km ), and c and n are co-
efficients with a value of n 0.77 for the U.S. southwestern streams and values of c varying
from 0.43 through 1.7 to 4.8 for low, average, and high deposition, respectively, the corre-
sponding values for southeastern streams being c 0.44 only and n 1.0. Understandably,
the magnitudes of c and n, here reported, apply only to the regions for which they were
developed.
The volume of silt deposited annually can also be approximated by the following
equation using metric units:
6 n
V 1,233.5c[A>(2.59 10 )] (SI units) (2.7a)
s
where

V s volume of silt deposited annually, m 3