HEC-RAS Plan and Boundary Condition Setup

The method used to compute AFDR determines how many HEC-RAS plans and unsteady flow files are needed. Reservoir boundary conditions also vary based on the watershed's characteristics and the selected modeling approach. This section outlines key considerations when setting up unsteady flow files and plans in HEC-RAS, particularly for modeling the unregulated condition.

Unsteady Flow Files and Boundary Conditions

In most cases, two unsteady flow files are required - one for the regulated condition and one for the unregulated condition. The regulated flow file incorporates local inflows from HEC-HMS, reservoir outflows from HEC-ResSim, and pool elevations from HEC-ResSim for any downstream reservoirs.

Unregulated Conditions for Watersheds with a Single Headwater Reservoir

The setup for the unregulated condition depends on the watershed’s layout and the geometry available in HEC-RAS. In simpler cases where there is only one headwater reservoir and reliable observed inflow and outflow data, observed inflows can be used directly as the upstream boundary condition. In such situations, there is no need to run HEC-ResSim. However, using observed data means the HEC-FIA Holdout method cannot be applied, as it requires output from HEC-ResSim to compute damages. Still, it is acceptable to calculate FDR estimates using gridded output, since only one reservoir is contributing to downstream damages, and all economic benefits will be attributed to that reservoir.

Unregulated Conditions for Watersheds with Multiple Dams and/or Large Levees

For more complex watersheds containing multiple dams and/or significant levees, both inflow and outflow time series from HEC-ResSim must be used to define boundary conditions in both regulated and unregulated model runs. For the regulated scenario, model linking typically follows the same structure as a real-time configuration. The unregulated setup, however, depends on how the geometry is defined.

Unregulated Setup for Tandem Reservoirs with Discontinuous Reaches

For tandem reservoir systems where reaches are segmented at each dam, the outflow from the downstream reservoir should use the Flow-Unreg time series from HEC-ResSim. This is important because the Flow-Unreg series captures the upstream reservoir's contribution, whereas the HEC-HMS inflows only reflect regulated outflows and local flows between reservoirs—an important distinction for tandem systems. Additionally, pool elevations must be assigned differently in unregulated scenarios. Instead of using HEC-ResSim outputs, a constant elevation below conservation pool, a pre-project rating curve, or normal depth should be used to represent the no-project condition. Since HEC-ResSim is not used for unregulated pool elevation in this case, it will not appear in model linking within CAVI.

Unregulated Setup for Tandem Reservoirs with Continuous Reaches

In geometries where the reach flows continuously through downstream tandem reservoirs without being segmented, the unregulated boundary condition setup closely mirrors that of the regulated case. The primary difference is at the headwater reservoir, where the outflow should be set to HEC-HMS-calculated inflows. No pool elevations or outflows from HEC-ResSim should be used for downstream reservoirs in these cases; instead, the entire model should rely on inflows and local flows generated by HEC-HMS. Additionally, any boundary conditions associated with inline structures or storage area connections that previously represented dams should be removed if those dams have been removed from the geometry. If the outlet works are still in place and capable of passing the inflows, gates should be modeled as open for the unregulated scenario.

Unsteady Flow Plans

The number of required HEC-RAS plans depends on the presence and type of levees in the watershed.

Watersheds with No Levees or Only Small Levees

If the watershed contains no levees or only small ones (for which HEC-FIA is used to estimate damages), then only two HEC-RAS plans are necessary: one for the regulated condition and one for the unregulated condition. The unregulated plan must be paired with the newly created unsteady flow file described above. It’s important to note that the simplified HEC-FIA Holdout method, which uses only two HEC-RAS plans, does not include calculations for agricultural damages. If estimating agricultural damages is required, the more detailed four-plan setup must be used, although only two distinct unsteady flow files (regulated and unregulated) are still needed.

Watersheds with Large Levees (Detailed Approach)

In contrast, if the watershed includes large levees and a detailed with/without levee analysis is desired, then four HEC-RAS plans are required. These include regulated and unregulated scenarios for both the existing condition (with levees) and a modified geometry without levees. The same two unsteady flow files (regulated and unregulated) are applied across all four plans. The results from these runs are then input into HEC-FIA, either using the four-plan Holdout method or four individual gridded computes.

Summary of Required HEC-RAS Plans

For watersheds without levees or with only small levees, the two required plans are:

For watersheds with large levees using the detailed approach, the four required plans are: