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Potomac Waterfront Flood Mitigation Study - City of Alexandria

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FLOOD<br />

ELEVATION<br />

BASE FLOW<br />

Overview <strong>of</strong> <strong>Flood</strong> <strong>Mitigation</strong> Measures<br />

Figure 3-4: LAG Elevated to Protect Building from <strong>Flood</strong>ing<br />

A raised patio is a patio that would be rebuilt at a new, higher elevation to prevent the flow <strong>of</strong><br />

floodwaters into the structure. Other localized flood barriers can be designed to accomplish the<br />

same effect if a patio is not an option, including localized yard berms and small privacy walls<br />

that are designed to be floodwalls. The raised patio or other localized flood barrier must be<br />

designed by a registered architect, engineer, or other certified pr<strong>of</strong>essional who is responsible for<br />

ensuring that the design prevents flooding.<br />

Two main differences need to be considered when evaluating dry floodpro<strong>of</strong>ing for residential<br />

structures verses for commercial structures. The first difference is that dry floodpro<strong>of</strong>ing a<br />

residential structure does not remove it from the FEMA floodplain and, thus, does not alleviate<br />

the requirement for flood insurance. However, dry floodpro<strong>of</strong>ing can be used to remove<br />

commercial structures from the FEMA floodplain. In addition, for a floodpro<strong>of</strong>ed structure to be<br />

removed from the FEMA floodplain, it is important that any construction that is below the BFE<br />

meet the FEMA criteria for flood damage resistance. For more information, refer to FEMA<br />

Technical Bulletin 2 (August 2008).<br />

Most wall materials, including brick, will leak unless constructed or modified using special<br />

waterpro<strong>of</strong>ing techniques. Care should be taken when applying a sealant to the outside <strong>of</strong> a brick<br />

wall. Waterpro<strong>of</strong>ing compounds can deteriorate or fail if exposed to floodwater for extended<br />

periods <strong>of</strong> time. In addition, sealants may also be subject to damage, particularly in areas that<br />

experience high velocity floodwaters, or waters containing debris or ice.<br />

<strong>Flood</strong>pro<strong>of</strong> membranes or coatings can also affect the aesthetic quality <strong>of</strong> a building. Clear<br />

coatings, such as epoxies and polyurethanes, are generally not as effective as cement or asphalt<br />

based coating. Therefore, the aesthetic appeal <strong>of</strong> a brick wall is lost with the use <strong>of</strong> higher quality<br />

sealants. One way to solve this problem is to add an additional layer <strong>of</strong> brick to the structure with<br />

the sealant located between it and the original brick surface. However, this is not considered an<br />

appropriate technique for historic structures.<br />

<strong>Flood</strong>pro<strong>of</strong>ing the walls <strong>of</strong> a structure by applying a membrane or coating to the surface could be<br />

considered for almost all <strong>of</strong> the floodprone buildings in the study area. However, without careful<br />

care and maintenance, these sealants may still leak. In addition, it is difficult to use this<br />

floodpro<strong>of</strong>ing method without compromising the building aesthetics. Modifications would<br />

require review and approval by the <strong>City</strong>’s Board <strong>of</strong> Architectural Review, since the focus area is<br />

within the designated historic district. Therefore, floodpro<strong>of</strong> membranes or coatings have not<br />

been considered as an option for this study.<br />

28-JUL-10\\ 3-5

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