Steady versus Unsteady Flow

[Doug H.]

My existing condition HEC-RAS model consists of 3-7′ CMP culverts. I am proposing replacement with a 3-sided concrete “box” that is 25′ x 8′. The new structure drops the 50-year WSEL upstream of the structure by about 5′. I am running a steady flow analysis, therefore the model doesn’t show any increase in the water surface elevation downstream of the culvert. Is this realistic, or is Unsteady Flow actually occurring during the existing condition? Any guidance documents I can refer to for this? Any help would be greatly appreciated. thanks!

[Chris G.]

Doug-
Unless you manually enter a flow change below the culverts, in steady flow, RAS will use the same discharge for your entire reach. Since there were no changes downstream of the culvert, you will show no changes in results downstream of the culvert. Steady flow will only measure the increased/decreased stage upstream of the culvert. If you believe the existing or proposed structure creates enough of a backwater effect to significantly attenuate your discharge as it approaches and then passes through the culvert, then you should measure this effect by running HEC-RAS in unsteady flow.

Unsteady flow is always occuring in natural streams. The degree to which it affects your design/analysis efforts dictates which option you use-steady of unsteady. If you’re not sure, you should try both ways.

Good Luck-
Chris
@RASModel

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