Potomac Waterfront Flood Mitigation Study - City of Alexandria

5.2 CALCULATION OF PROJECT BENEFITS
Economic Valuation (Benefit-Cost Analysis)
The benefits for each mitigation alternative were analyzed at a planning level, so assumptions
were made to simplify the analysis. Instead of analyzing all flood depths at a structure for all
flood elevations, each of the analyzed discrete flood event’s (extreme, intermediate, and
nuisance) benefits were calculated then summed together. If an alternative did not provide any
mitigation at a specific flood event, then no benefits were calculated.
The following data were used to develop benefits as well as some cost data:
Assessed Value of 406 properties (land and building value)
United States Census Bureau Web site for annual business income
Surveyed FFEs
Business questionnaire regarding Hurricane Isabelle
The steps for calculating benefits are outlined below.
1. Determine the Flood Depth
Structure FFEs for all potential flood prone structures were compared to the previously
specified flood events (i.e., extreme, intermediate, and nuisance) for the different flood
frequencies.
Example: The home is a 2-story residential building located in the Jones Point study
area. The FFE is 8.47 feet (with basement). The flood depth at the extreme flood event
(10.2 feet NAVD88) in this case would be 1.73 feet.
The following assumptions were made because of data limitations:
All residential basements were considered finished.
All basements were assumed to be the first floor flooded.
All commercial buildings were assumed to not have basements.
A comparison between the FFE and the LAG was performed, to determine if the FFE
was the basement or a higher floor. The LAG was determined by known LAG data
(surveyed information) or using the City of Alexandria topography.
If the FFE was lower than the LAG, then the basement was determined to be the FFE;
otherwise, the FFE was reduced by 8 feet to reflect the basement elevation.
2. Calculate the Average Depth (only for residential structures)
For residential structures, the average depth was used for all structures within a study
area. A sensitivity analysis was performed for the Jones Point Area to determine the
disparity between the benefit calculated for each house using individual depth data and
using average depth data. In the final total benefit, only about a four percent difference
occurs. For commercial structures, too many variables occur to make similar
assumptions, so the flooding depth at each event was calculated on a structure-bystructure
basis.
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