Background

HEC-RTS 3.2.3 has the capability to compute flood damages reduced by reservoirs and levees from a CAVI forecast using HEC-FIA. This section will provide information on the basic setup within the CAVI and HEC-FIA to compute flood damages reduced. 

Each year USACE districts are required to report the Annual Flood Damages Reduced (AFDR) by each of their their flood risk reduction projects. AFDR is a monetary estimate that represents the amount of property damages that were averted by USACE reservoirs and levees. In the most basic sense, computing the flood damages reduced for an event involves comparing damages for the regulated (with project) condition to the unregulated (without project) condition.

The methodology to compute flood damages reduced can vary drastically and selection of the best suited method is dependent on the complexity of the system, the characteristics of the watershed, and level of detail desired. This type of analysis requires technical expertise in hydrology, hydraulics, and economics to produce and interpret results. The method outlined in this section is a basic approach for structure damage only that may or may not be appropriate for your system. 

Introduction

A forecast used to compute flood damages reduced will differ from a typical forecast used for operations decision making in a few key ways:

• Computing flood damages reduced in a CAVI forecast is typically completed after a flood event has occurred. This allows the hydrologic and hydraulic models to be calibrated to observed data, and allows for observed reservoir operations to be used instead of relying on a rule set. That said, the time window of a forecast used to compute flood damages should be almost entirely in the lookback period, and the models in the forecast should be calibrated to observed data wherever available.
  
  
• An additional HEC-RAS model alternative representing the unregulated event (without reservoirs) will be added to the default CAVI Program Order.
  
  
• The HEC-FIA alternative will use one of two inundation configurations: Holdouts Grids or Holdouts Cross Sections. These inundation configuration runs four separate computes to allow for the comparison of damage results for with and without projects.
  
  
• HEC-ResSim will be set to compute holdouts. Holdouts are simply a flow time series of what the reservoir held out or stored during the event. If there are multiple reservoirs in a system, holdout time series can be used by HEC-FIA to apportion the total damages reduced to each reservoir.