I read a thesis which used the formula v = K(S^0.5) to determine the flow velocity in a channel. The value for conveyance K was adopted from Ward and Trimble (2004) and was mentioned to be equal to 5 for a river bed composed mainly of sand and gravel. Can anybody help me find the K value for a river bed which mostly consists of boulders with patches of vegetation?
Thank you.
It should be noted that in the formula you have indicated, S is the bed slope or the slope of the energy grade line (m/m). Moreover, the K parameter you specified is not the conveyance since when compared with the flow velocity relationship derived from Manning's Q equation, results in:
K = (1/n) Rh2/3 (1) in SI units
In equation (1) K has the dimension of a velocity, i.e. m/s, which does not correspond to the true definition of conveyance.
When you consider the well-known Manning formula expressing the discharge Q, it is easy to deduce that the conveyance of the channel (or river) is as :
K = (1/n) A Rh2/3 (2) in SI units
in cubic meters per second
This is the true definition of conveyance which is a measure of the carrying capacity of the channel section or river section.
where:
n is the Manning's Roughness coefficient (s/m1/3)
A is the flow area (m2) depending on the flow depth and the width of the considered channel or river.
Rh is the hydraulic radius (m) depending also on the flow depth and the width of the considered channel or river.
Considering K = 5 for a river bed composed mainly of sand and gravel is a rough approximation which can lead to an erroneous value of the flow velocity because the depth of the flow is an important parameter, as well as the Manning's roughness coefficient.
It is necessary to use relationship (2) to calculate the value of the conveyance. The value of the coefficient n is given by tables available in the literature (Refer to the following link). https://pubs.usgs.gov/wsp/2339/report.pdf
Regards
Channel Conveyance, K. K = (ϕ/n) A Rh^(2/3). For Manning's equation, K combines roughness. and geometric characteristics of the channel.
=> Find your Area (A) and Hydraulic Radius (Rh) based on channel's geometry
=> Find Manning's "n" roughness here:
https://pubs.usgs.gov/wsp/2339/report.pdf
Calculate the channel conveyance
From Manning's equation, the velocity of flow, V=1/n* R(2/3) S(1/2). Therefore, K=1/n* R(2/3) where: n is the Mannings roughness coefficient that depends on the type of river bed material say boulder with patchy vegetation (read the value from standard graphs) in your case, and R is the hydraulic radius which depends on the channel geometry (the ratio of crossectional Area (A) to the wetted perimeter of the channel (P)).
Thank you everyone for replying. I had an inkling that the equation referred to the Manning's equation. That is why the thesis author using a K value of 5 by just looking at the channel bed surface made no sense to me. To be honest, that thesis smells fishy to me now.
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