HEC-RAS Plan and Boundary Condition Setup

The modeling approach selected to calculate FDR determines how many HEC-RAS plans and unsteady flow files are required. The boundary conditions at the reservoirs also differ depending on the features in the watershed and the modeling approach. This section discusses what to consider when creating new unsteady flow files and plans in HEC-RAS for different modeling approaches, particularly for the unregulated alternative.

Unsteady Flow Files and Boundary Conditions

In most cases, there are two unsteady flow files—one for the regulated condition and one for the unregulated condition—needed for FDR computations. The regulated unsteady flow file uses local flows from HEC-HMS, reservoir outflows from HEC-ResSim for all of the reservoirs in the system, and pool elevations for any downstream reservoir(s) from HEC-ResSim.

Setup of the boundary conditions for the unregulated alternative can vary depending upon the available HEC-RAS geometries.

If the watershed contains only one headwater reservoir, and observed inflows and outflows are of good quality, observed data can be utilized for these runs, and HEC-ResSim is not required. In this case, the unregulated alternative would use the observed inflow hydrograph as the upstream boundary condition instead of the outflow from the reservoir.

For all other watersheds with multiple dams and/or large levees, the inflows and outflows calculated in HEC-ResSim need to be set as the boundary conditions for the regulated and unregulated model runs. The model linking for the regulated condition is the same as the typical real-time setup. Depending on the geometry, layout of the unregulated condition can be handled in a couple of different ways. HEC-ResSim calculates the unregulated flow which can be linked to the HEC-RAS model.

In the case of tandem reservoirs with reaches broken at the dam, the outflow for the downstream reservoir(s) should be the Flow-Unreg time series from HEC-ResSim versus the HEC-HMS inflows. The Flow-Unreg simulated time series includes the unregulated flows from the upstream reservoir(s) whereas the HEC-HMS inflow only includes the regulated outflows and local flows between the upstream reservoir and the reservoir in question.

The unregulated boundary conditions for HEC-RAS geometries with continuous reaches through downstream tandem reservoirs are nearly identical to that of the regulated condition. The only difference is setting the outflow from the headwater reservoir(s) to reservoir inflows from HEC-HMS. Any downstream tandem reservoir(s) do not use pool elevation or outflows from HEC-ResSim. In other words, the HEC-RAS model should receive inflows and local flows from HEC-HMS entirely.

Unsteady Flow Plans

The number of required HEC-RAS plans depends on whether there are levees in the system and if they are considered large or small levees.

If using the HEC-FIA Holdouts Approach, only two HEC-RAS plans are required - the regulated and the unregulated conditions for the watershed. In this case, the only additional set up required is the unregulated plan with the newly created unregulated unsteady flow file. Additionally, this approach can be utilized in watersheds with no levees. The table below lists the HEC-RAS plans needed for the HEC-FIA Holdouts Approach.

Required HEC-RAS Plans for HEC-FIA Holdouts Approach

If using the Detailed Approach, two unsteady flow files (regulated and unregulated) are assigned to the four specified plans in HEC-RAS. The results of these four alternatives are passed to HEC-FIA where four individual damage computes are made. The table below lists the HEC-RAS plans needed for the Detailed Approach.

Required HEC-RAS Plans for Detailed Approach