Calculating capacity in ditch section with uphill slope

wesley_pipes · May 10, 2024

I am trying to analyze the capacity of an old ditch with some sections that have uphill slope (uphill = negative slope). Without using any modeling software like HEC-RAS, what are some ways I can estimate how much capacity we are losing in these sections? Assuming I have all the survey/cross-section data for the upstream and downstream sections. Is there a "back-of-the-envelope" way of estimating this?

Thanks

CarlB · May 12, 2024

Backwater analysis.
Assume a flowrate. Starting at downstream end, should be some controlling water level, or free-flow. Break ditch into sections of similar cross-section. For each, calculate HGL to achieve assumed Q, such as with Mannings equation. Carry elevations upstream. End result is starting water elevation for that Q.

Don’t know clear meaning of lost capacity for certain sections. If cross-section is enlarged, will get more Q through that portion for a given HGL.

FacEngrPE · May 12, 2024

The high points in the ditch are going to control the available section below the grade line, and the low points are going to set the maximum water level.

Using these constraints I would lay out grade line estimate a scaled profile drawing.

It likely would be less effort to model in USEPA Storm Water Management Model (SWMM) as open channel flow.

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Calculating capacity in ditch section with uphill slope

wesley_pipes

Agricultural

CarlB

Civil/Environmental

FacEngrPE

Mechanical

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