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Modeling a plunge pool in HEC-RAS
2

Modeling a plunge pool in HEC-RAS

Modeling a plunge pool in HEC-RAS

(OP)
I am modeling a relatively steep channel(3% slope).  Channel bottom is shale with quite a few plunge pools.  I'm wondering is anyone knows of a reference for how to model the plunge pools in HEC-RAS?

RE: Modeling a plunge pool in HEC-RAS

HECRAS will model in a mixed flow regime, therefore, one can cut xsecs to match the physical characteristics of the plunge pools.  I have modeled plunge basins as energy dissipators in HECRAS and obtained very close results when compared to the design for energy dissipators.  On the other hand, I have ignored the basins and filled in the holes using a much reduced manning's roughness to represent water.  I would recommend that you model it and look at the results to see if they make sense.  If you can defend your model then you're done.  

RE: Modeling a plunge pool in HEC-RAS

Pick your Manning's coefficient accordingly and use a regression analysis to calculate the slope.

RE: Modeling a plunge pool in HEC-RAS

2
Cut cross-sections at riffles and choose a higher "n" to represent looses in the riffle/step-pools sequence.  Capturing every riffle/step-pool sequence in a long reach can get way too tedious and adds nothing to the accuracy of the model.  

Jarrett's equation is a good spot to start for Manning's "n" on steep riffle/step-pool reaches with low flows.  During higher flows, the step-pools tend to have less influence and Manning's reverts back to that obtained with traditional methods considering bed material and vegetation.

Jarrett's:
n = 0.32 x [slope]^0.38 x [hydraulic radius]^(-0.16)

Jarrett, Robert. 1984. Hydraulics of high-gradient streams. Journal of Hydraulic Engineering. 110(11): 1519-1539.   

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