332 CHAPTER TWELVE
PREDICTION OF SEDIMENT-DELIVERY RATE
Two methods of approach are available for predicting the rate of sediment accu-
mulation in a reservoir; both involve predicting the rate of sediment delivery.
One approach depends on historical records of the silting rate for existing
reservoirs and is purely empirical. The second general method of calculating the
sediment-deliveryrate involves determining the rate of sediment transport
as a function of stream discharge and density of suspended silt.
The quantity of bed load is considered a constant function of the discharge
because the sediment supply for the bed-load forces is always available in all
but lined channels. An accepted formula for the quantity of sediment transported
as bed load is the Schoklitsch formula:
(12.128)
whereGbtotal bed load, lb/s (kg/s)
Dgeffective grain diameter, in (mm)
Sslope of energy gradient
Qitotal instantaneous discharge, ft^3 /s (m^3 /s)
bwidth of river, ft (m)
qocritical discharge, ft^3 /s (m^3 /s) per ft (m), of riverwidth
(0.00532/S4/3)Dg
An approximate solution for bed load by the Schoklitsch formula can be
made by determining or assuming mean values of slope, discharge, and single
grain size representative of the bed-load sediment. A mean grain size of 0.04 in
(about 1 mm) in diameter is reasonable for a river with a slope of about 1.0 ft/mi
(0.189 m/km).
EVAPORATION AND TRANSPIRATION
TheMeyer equation, developed from Dalton’s law, is one of many evaporation
formulas and is popular for making evaporation-rate calculations:
(12.129)(12.130)where Eevaporation rate, in 30-day month
Cempirical coefficient, equal to 15 for small, shallowpools and 11 for
large, deep reservoirs
ewsaturation vapor pressure, in (mm), of mercury,corresponding to
monthly mean air temperature observed at nearby stations for small
bodies of shallow water or corresponding to water temperature
instead of air temperature for largebodies of deep water
# 1 0.1wEC (ewea)#Gb86.7
D1/ 2gS3/2(Qibqo)