Potomac Waterfront Flood Mitigation Study - City of Alexandria

Overview of Flood Mitigation Measures
Since floodproofing actions will be driven by individual property owners, it is not likely that this
measure will be fully implemented. Therefore, in general, floodproofing is recommended in
conjunction with other flood mitigation measures.
3.1.4 Elevate Structures
The goal in elevating structures is to raise the first finished floor above the 100-year flood
elevation (extreme flood event). Elevating structures can be accomplished in two ways. A home
or business may be elevated by being lifted off its existing foundation, building a new foundation
to an appropriate height, and resetting the home on the new foundation. The second way to
elevate a structure is to raise the floor inside the house while leaving the outside of the house in
its original position. This is only an option for structures with relatively high ceilings or where
the elevation required is small. It may also necessitate abandoning a floor that is below the 100
year flood elevation and moving personal property to a higher floor. This is referred to as
“internal elevation.” Internal elevation is described in Section 3.2.2.
The most suitable structures for elevation are one- or two-story wood frame buildings. Data
obtained from the City of Alexandria showed that most structures in the study area are brick or
masonry buildings that are attached to other structures. Furthermore, the entire study area is
within the Old and Historic Alexandria District. Also, most of the study area is within the
National Register District. The nature of this alternative includes an element of risk for historic
buildings that may be unacceptable. There is a possibility of excessive cracking taking place
when elevating brick and masonry structures. Also, for those structures to be elevated, any
attached buildings would need to be elevated at the same time, which could be a very complex
process. A further complication to this process is the fact that the entire study area is within the
City’s Historic District, so any mitigation work would need to comply with historic preservation
guidelines. Therefore, due to the difficulty and complexity of elevating row homes and large
masonry buildings, elevation is not recommended as a flood mitigation alternative.
3.1.5 Increase Inlet and Road Elevations
During extreme tide events, Strand Street, within the Waterfront Commercial focus area, is
subject to frequent flooding. Storm sewers are typically designed to quickly convey stormwater
away from roadways. However, in instances where extreme tides back up into the municipal
storm sewer, the storm sewers cannot convey the flow from surface runoff. If the storm sewers
back up to an elevation equal to the road surface, the water overflows the catchbasin (inlet) and
the roadway floods. This causes traffic safety issues, which generally requires the City of
Alexandria officials to close the roadway. Closed roadways present further safety issues in
limiting access for emergency vehicles. Other access issues include limiting access to residences
and businesses. Note that this occurs at elevations lower than the nuisance flooding elevation of
4.0 feet as defined by this study. Areas where the inlet rim elevations (elevation of the inlet at the
top, where it intersects the road) were less than 4.0 feet were the focus of this alternative.
Flooding that occurs more often than the nuisance flood can sometimes be remedied by raising
the existing road elevation, as well as the associated inlets and manholes along the road. By
increasing the road and inlet rim elevation, the water back-flowing in the storm sewer must reach
a higher elevation to overtop the catchbasin and flood the road. Design constraints that need to
be analyzed are the elevation of the sidewalks and first floors of the buildings along the roads.
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