There are several methods for calculating AFDR using CWMS. These methods offer flexibility, allowing you to choose the best approach for your specific watershed. All methods rely on the CWMS software suite, which computes event-based hydrologic and hydraulic outputs using HEC-HMS, HEC-ResSim, and HEC-RAS. These outputs are then fed into the HEC-FIA model. The HEC-FIA model integrates the latest economic data, including updated structure inventories and agricultural grids, improving on older depth-damage curves traditionally used for flood damage estimation.

To determine the most suitable approach for your watershed, consider the following questions:

  1. Does your watershed have multiple reservoirs involved in flood damage reduction calculations?
  2. Are the reservoirs operated in tandem, or do they function independently as headwaters projects?
  3. Does your watershed include levees?
    1. Are the levees large or small?
    2. Can the flood damage behind these levees be assessed using level pool routing, or is more complex 2D modeling required?
  4. How complex is your HEC-RAS model? Does it take a long time to run?

The answers to these questions will help guide the selection of the most appropriate method for your watershed.

Common Approach: Single Forecast Run in CWMS

The most commonly used method involves a single forecast run within CWMS, where HEC-FIA uses internal logic to compute flood damages and damage reduction results for each project in the watershed. This approach is ideal for relatively simple watersheds, those with parallel-operated reservoirs, and watersheds with or without small levee systems. It requires only one HEC-RAS geometry, which represents the "with-levee" condition.

More detailed guidance on this approach can be found in the HEC-FIA Holdout Grids Compute section.

Alternative Approach: Multiple HEC-RAS Simulations for Complex Watersheds

For more complex watersheds, such as those with large levee systems or tandem reservoirs, an alternative approach may be necessary. In this case, a single forecast run within CWMS is still used, but instead of HEC-FIA calculating the "without-levee" inundation, you must include four separate HEC-RAS simulations. Even if you don't have large levees, but still need to compute agricultural damages using the HDF methodology in HEC-FIA, this approach can be adapted to calculate those damages without requiring four separate HEC-RAS models.

For more details on this method, refer to the HEC-FIA Holdout RAS HDF Compute section.

Other Options: Multiple Forecast Runs

While the preferred method is a single forecast run in CWMS, alternative setups can involve multiple forecast runs. In this case, you would post-process the results from multiple standard HEC-FIA damage alternatives to calculate the final AFDR results. Although this method is still valid, it's less commonly used today due to improvements in CWMS and HEC-FIA. For more information, refer to the HEC-FIA Individual Computes section.