Potomac Waterfront Flood Mitigation Study - City of Alexandria

FLOOD
ELEVATION
BASE FLOW
Overview of Flood Mitigation Measures
Figure 3-4: LAG Elevated to Protect Building from Flooding
A raised patio is a patio that would be rebuilt at a new, higher elevation to prevent the flow of
floodwaters into the structure. Other localized flood barriers can be designed to accomplish the
same effect if a patio is not an option, including localized yard berms and small privacy walls
that are designed to be floodwalls. The raised patio or other localized flood barrier must be
designed by a registered architect, engineer, or other certified professional who is responsible for
ensuring that the design prevents flooding.
Two main differences need to be considered when evaluating dry floodproofing for residential
structures verses for commercial structures. The first difference is that dry floodproofing a
residential structure does not remove it from the FEMA floodplain and, thus, does not alleviate
the requirement for flood insurance. However, dry floodproofing can be used to remove
commercial structures from the FEMA floodplain. In addition, for a floodproofed structure to be
removed from the FEMA floodplain, it is important that any construction that is below the BFE
meet the FEMA criteria for flood damage resistance. For more information, refer to FEMA
Technical Bulletin 2 (August 2008).
Most wall materials, including brick, will leak unless constructed or modified using special
waterproofing techniques. Care should be taken when applying a sealant to the outside of a brick
wall. Waterproofing compounds can deteriorate or fail if exposed to floodwater for extended
periods of time. In addition, sealants may also be subject to damage, particularly in areas that
experience high velocity floodwaters, or waters containing debris or ice.
Floodproof membranes or coatings can also affect the aesthetic quality of a building. Clear
coatings, such as epoxies and polyurethanes, are generally not as effective as cement or asphalt
based coating. Therefore, the aesthetic appeal of a brick wall is lost with the use of higher quality
sealants. One way to solve this problem is to add an additional layer of brick to the structure with
the sealant located between it and the original brick surface. However, this is not considered an
appropriate technique for historic structures.
Floodproofing the walls of a structure by applying a membrane or coating to the surface could be
considered for almost all of the floodprone buildings in the study area. However, without careful
care and maintenance, these sealants may still leak. In addition, it is difficult to use this
floodproofing method without compromising the building aesthetics. Modifications would
require review and approval by the City’s Board of Architectural Review, since the focus area is
within the designated historic district. Therefore, floodproof membranes or coatings have not
been considered as an option for this study.
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