Integrated H&H modeling with HEC-RAS

[Michael Getachew Tadesse]

Coming from a different H&H model to HEC-RAS, I have some conceptual questions as to how HEC-RAS works under the hood:

1. Does HEC-RAS completely ignore the groundwater surface water interaction in the basin we are trying to model? For instance in Florida where the groundwater is relatively close to the surface, surface water flows and water levels are affected by groundwwater and vice-versa. Does it mean in these instances, HEC-RAS is not the best tool to use? If HEC-RAS considers groundwater flow, how is the quantified in the model?

2. If I am interested in an operational flood-forecasting, I will need real time simulation of flows coming in to the basin/canal/river before routing it in the hydraulic model (HEC-RAS). Are there capabilities to simulate rainfall-runoff and hydraulic models simultaneously? OR do we always need to run the hyrological model first and get the flows and use HEC-RAS to calculate flows/water levels?

[Chris Goodell]

HEC-RAS does not implicitly consider groundwater flow. You can add a groundwater input to your RAS model as a uniformly distributed inflow hydrograph, but you have to figure out what that inflow is and enter it as a boundary condition. In the current version of RAS, you can include spatial and temporal distrubution of rainfall on a 2D mesh (called “rain-on-grid” modeling). That would allow you to simulate the rainfall-runoff process directly in RAS, however be careful with this, as results will be highly sensitive to the n values and infiltration rates you use and there’s a lot of uncertainty with these parameters when doing rain-on-grid modeling in RAS. Suggest reading the latest manual carefully on this.

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