τ c = Θ ( γ s - γ w ) d 50
Equation 2-40
where:
τc = critical shear stress
Θ = Shield's parameter
If this test shows that the actual bed shear stress exceeds the critical value then
the bed sediment is diagnosed as unstable. If the panel is designated as a panel
where riprap computations are desired, SAM will solve the riprap equation to
find the smallest riprap size that will be stable. If the panel is not so designated,
the program prints a message to the output file concerning the instability.
Considerable care must be employed when applying the cross-section shape
option because flow conditions may be outside the range of conditions used to
develop the riprap equations. When this option is used, it is important that the
side slope protection be defined as a single panel in the geometry input. SAM
will use 0.8 times the maximum depth in the panel for the local flow depth in the
riprap equation. When a bend radius is specified, SAM uses the average velocity
in the cross section for VAVE in the riprap equation. Therefore, it is important that
the input geometry include only channel geometry and discharge. When a bend
radius is not specified, SAM uses the calculated panel velocity for VAVE in the
riprap equation. This calculation may provide useful information for compound
channels in straight river reaches, but it is an extension of the procedure outlined
in EM 1110-2-1601. Careful study of the recommendations and guidelines
described in EM 1110-2-1601 should be considered essential.
In SAM, riprap computations begin with the smallest rock size. The hydraulic
roughness equation, in each panel designated as having riprap, is automatically
changed to the Strickler equation and the Strickler coefficient, is set equal to
0.034. Normal depth is calculated for the resulting n-values. The alpha method
is used to calculate normal depth and flow is distributed across the section. The
riprap size equation is solved for each panel. When the resulting size is stable in
each panel, riprap computations are finished. Otherwise, computations move to
the next larger riprap size and the procedure is repeated.
After the stable stone size is determined, a stage discharge curve is calculated
for the riprapped channel. The Strickler coefficient, which was 0.034 when
determining stone size, is increased to 0.038 in this calculation for flow capacity.
This calculation determines the rating curve with the selected riprap in place.
Blench Regime Equations
Stable channel dimensions may be calculated using the Blench (1970) regime
equations. These regime equations are also shown in ASCE Manual 54 (ASCE
1975). The equations were intended for design of canals with sand beds. The
basic three channel dimensions, width, depth and slope, are calculated as a
33
Chapter 2
Theoretical Basis for SAM.hyd Calculations
Previous Page
