How do I find stream depth given flow rate ?

[nova276]

I need to find out how high the water in a creek gets when a flow of 100 cfs flows through it. I've got some survey data with four cross sections of the stream over a 2000 foot reach length, so I can figure out the cross sectional areas and a rough estimate of the volume of the stream for this reach. Is there a way I can use that information to determine the water depth for a particular flow rate?

 

[RWF7437]

You could use the Manning Equation to solve fro AR^2/3 or you could model the stream using the HEC-RAS computer program if you are familiar with it. The toughest part may be to make a good estimate of the roughness coefficient "n".

good luck

 

[ngneer]

without thinking about involved hyrdaulic step method analysis, couldn't you simply use manning's eq to solve for depth by plugging slope, roughness, and hyraulic radius in terms of depth. For ex. rect channel R = (Width x Uknown Depth) / (width + 2depth).

 

[nova276]

Yep, Manning's equation will work. I knew there was a simple solution, I just couldn't come up with it! Having one of those days I guess. Thanks to both of you.

 

[LHA]

I would not approve calculations of stream depth based on Manning's. A swale, yes. A short, isolated, straight piece of pipe, maybe as long as I did not think full flow where possible, then no. A long stream, with grade changes, cross sectional changes, obstructions, differing embankment and bottom conditions and sudden added flows? Never. HEC-RAS is the only way I know to properly address the entire dynamic of a stream flow...velocity, Froudes, depth, critical/subcritical/mixed regimes all act on each other and change the each other continuously in stream flow.

If you've got that much survey data, the toughest part of HECRAS is done. The surveyor will hopefully remember the boundary conditions well enough for accurate Manning's "n" (which you need with Manning's or HECRAS).

 

[LeoSC]

I agree with Iha. Do you have photos of the reach? The Manning equation will give you an approximation, but would not be correct in a reach where the flow is gradually variable.