Figure 2.9. Typical cross section used in analytical method.
Input Requirements
Required input data are sediment inflow concentration, side slope, bank
water discharge. If sediment inflow is to be calculated, which is the
recommended procedure, then additional data are required for the supply reach.
These are: base width, side slope, bank roughness coefficient, bed-material
median grain size, geometric gradation coefficient, average slope, and discharge.
It is important that the base width be representative of the total movable-bed
width of the channel. The bank roughness should serve as a composite of all
additional roughness factors, i.e., channel irregularities, variations of channel
cross-section shape, and the relative effect of obstructions, vegetation, and
sinuosity. Only flow that is vertical above the bed is considered capable of
transporting the bed material sediment load. If the gravel bed calculations are to
be made, the user has the option of inputting the added roughness and meander
multiplier for the Cowan option.
Water Discharge
The design discharge is critical in determining appropriate dimensions for the
channel. Investigators have proposed different methods for estimating that
design discharge. The 2-year frequency flood is sometimes used for perennial
streams. For ephemeral streams the 10-year frequency is sometimes used. The
"bankfull" discharge is sometimes suggested. Others prefer using the "effective"
discharge, which is the discharge that transports the most bed material sediment.
Currently, there is no generally accepted method for determining the channel-
forming discharge. It is recommended that a range of discharges be used in the
analysis to test sensitivity of the solution.
42
Chapter 2
Theoretical Basis for SAM.hyd Calculations
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