HEC‑RAS Culvert FHWA Full Flow Computations

Culvert flowing full

For culverts flowing full, the total head loss (or energy loss) through the culvert is measured in feet (or meters). The head loss, HL, is computed using the following formula:

The head loss (1)

The friction loss in the culvert is computed using Manning’s formula, which is expressed as follows:

Manning's formula (2)

friction loss

The exit energy loss is computed as a coefficient times the change in velocity head from just inside the culvert, at the downstream end, to outside of the culvert at the downstream end. The entrance loss is computed as a coefficient times the absolute velocity head of the flow inside the culvert at the upstream end.

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