Simulation of Retention Effects on Dams

[Anonymous]

Hi,

I am momentartly modeling a Dam (h=15m, dam cest length = 290m) with a spillway to attenuate design floods (HQ100).

When flood waves arrive at spillways, there is a so called retention effect present, which attenuate the peak of the inflow wave up to 40 % (depending on the spillway). Because since the flood wave arrived at the spillway, the spillway starts spilling the water downstream and reduces the overflow water level caused by the wave. If you would see it in integral steps, the storage volume changes for each subsequent step, as shown in the relation. S (or V)= (Qin – Qout)dt

While simulating with HEC-RAS, I only find the results for the inflow wave and the volume that is spilled downstream by the spillway, but not for the combined wave including this retention effect.
But in fact it would be important to know for reducing the dam height and the extent of the spillway.

Does anybody know how to simulate and how to see the results for that effect. It might not be possible, but I am not an expert in HEC-RAS, and maybe there is a solution.

Apprechiating your help !!

Greets, Philipp in Rwanda

[Chris G.]

Hi Phillip-

It is indeed possible, and a great application fo rHEC-RAS. First this needs to be an unsteady flow model (I think it is, but you don’t specifically state this so I want to make sure).

To model the attenuation through a reservoir you can use a storage area to simulate the reservoir (easier, less accurate) or you can model it dynamically with cross sections (a little harder, more accurate). To see attenuation, you can view the stage and flow hydrographs and look at the flow entering the reservoir compared to the flow leaving the reservoir over the spillway. While in the stage and flow hydrograph viewer, make sure to go to the “Type” menu item to switch between different node types. If you do model the reservoir dynamically, you can tell RAS to produce stage and flow hydrographs for all cross sections by going to Options…Stage and Flow Hydrograph Locations, in the Unsteady Flow Analysis window. By default, RAS only produces S&F hydrographs for nodes the bound internal and external boundaries.

Good Luck

[Anonymous]

Hi Chris,

yes I did it as an unsteady flow model. …

I recalculated the retention as well in Excel “by hand” and had the same results, which means, HEC-Ras is doing it properly :). The storage area function is well suitable for my approach, so I don’t have to go the complicated way by defining cross sections.

Thanks for your advisory mate !

Philipp

[Author]

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