Modeling a plunge pool in HEC-RAS 2

[RICEOWL]

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?

 

[gbam]

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.

 

[francesca]

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

 

[Drew08]

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.