7945

• UCREFRP Catalog Number: 7945
• Author: Bovee, K. D. and T. Cochnauer.
• Title: Development And Evaluation Of Weighted Criteria Probability-Of-Use Curves For Instream Flow Assessments
• USFW Year: 1977.
• USFW - Doc Type: Fisheries, Instream Flow Information Paper No. 3.
• Copyright Material: NO

Vs = 1n49 R2/s Si/2 (2)
where,
Vs = mean velocity of surface water
n = roughness coefficient, assumed to equal 0.035
for all calculations
R = Hydraulic radius, equal to the conveyance area
A (width X mean depth), divided by the wetted
perimeter, (width + 2 X mean depth). For wide,
shallow channels, the hydraulic radius approxi-
imates the mean depth.
S = Energy gradient; for uniform flow assumed parallel
to hydraulic gradient.
From equations (1) and (2) it is seen that,
Va _ V2 n2
S = K 2.22 R4~3 (3)
and
Va = Vs2 n2 K
2.2~T3
(4)
It may be shown that the limit of equation (4), as R approaches
zero, is K. At small depths and velocities, fine sediment may deposit
in the interstices of the gravel, lowering the value of K, the perme-
ability.
Therefore, for a given slope and required apparent velocity, the
critical factor at low flow is maintaining sufficient shear stress to
prevent deposition of finer particles on the gravel. This critical
shear stress is defined as:
Tc = wRS (5)
where,
w =-unit weight of water - 62.4 lb/ft3
R = Hydraulic radius
S = Energy gradient, assumed parallel to
hydraulic gradient
Survival rates of salmonid embryos, as a function of apparent
velocity, were found in Wickett (1958), Coble (1961), and Cloern (1976).
For a stipulated slope, K was calculated from equation (1) for each
apparent velocity listed. Terhune (1958) listed particle size distri-
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