HEC-RAS obstruction ing

[James165]

I understand the concept of bridges in HEC-RAS in which the cross sections upstream and downstream of a bridge need to be situated at a distance that reflects the contraction and expansion lengths of flow. (The software cites "Flow Transitions in Bridge Backwater Analysis" [RD-42, HEC, 1995] as the source for the methodology) Should the same methodology be applied for determining contraction/expansion lengths as it relates to flow around an isolated obstruction such as a building, or is there a better methodology out there for such a case? The reason why I am asking is to determine the approximate footprint of a "hydraulic shadow" around a building, particularly on the upstream side.

 

[RWF7437]

This question has been asked before. You may be able to find those answers by searching this site, but I've never had much luck doing that.

Meanwhile, Appendix B of the HEC-RAS Hydraulic Reference Manual has a good discussion of [RD-42, HEC, 1995]. It appears you can treat your proposed or existing building just as you would treat a bridge pier and estimate the "shadow" lengths up and downstream. The Manual cautions you to observe such cases in the field during high flow conditions if it is possible to do so.

good luck

 

[gbam]

I have used that methodology as long as I can remember for obstructions. If the floodplain I am evaluating has a large number of smaller obstructions, I increase Manning's "n" based on the blockage. It really depends how big the obstructions are compared to the floodplain width. Also the expansion/contraction lines sometimes do not extend to the next US or DS XSEC. Just need to look at it and use your judgement.