Potomac Waterfront Flood Mitigation Study - City of Alexandria

Refinement of Mitigation Measures
SECTION TWO: REFINEMENT OF MITIGATION MEASURES
2.1 SUMMARY OF INITIAL FLOODING ASSESSMENT
As part of this project, URS prepared the Initial Flooding Assessment Report, dated October
2007, which summarized flooding problems, identified their causes, and identified and
categorized potential flood mitigation solutions. In addition, this report identified three types of
flooding events to be considered.
The project considers three discrete flood events: nuisance flooding (elevation 4.0 feet North
American Vertical Datum [NAVD]); intermediate flooding (elevation 8.0 feet NAVD); and
extreme flooding (elevation 10.2 feet NAVD). The three flood events have return periods
associated with them. A return period or recurrence interval is the estimated period of time
between occurrences of equal-sized events. For example, the Federal Emergency Management
Agency (FEMA) Base Flood has a return period of 100 years; therefore, it is referred to as the
100-year flood or one percent annual flood. Figure 2-1 shows the inundation areas for selected
flood-level categories.
For the extreme and intermediate floods, the return period was interpolated from the City of
Alexandria and District of Columbia Flood Insurance Studies (FISs), specifically the Potomac
River flood profile. The 6-foot flood elevation event was analyzed for a specific flood mitigation
alternative that is discussed later in this report. A logarithmic equation was developed using all
four flood elevations and known return intervals. For the nuisance flood, the return period was
computed through a statistical regression analysis of a U.S. Geological Survey (USGS) tidal
stream gage located on the Potomac River at Wisconsin Avenue in Washington, D.C. The return
periods for the flood events are listed in Table 2-1.
Table 2-1: Return Periods of Studied Flood Events
Flood Event Elevation (feet NAVD) Return Period (years)
Nuisance 4.0 1.5
6-foot Flood (Elevated Walkway) 6.0 10
Intermediate 8.0 30
Extreme 10.2 100
Figure 2-1 shows the areas of inundation for the 4-foot, 8-foot and 10.2 foot flood elevations.
Figure 2-2 provides a graphical representation of Table 2-1, with additional reference points
including the peak elevation of flooding during Hurricane Isabel, mean high and low water
elevations and the relative height for a floodwall that would protect against the 100-year flood
level with three feet of freeboard.
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