Another barrier option is to create levees around your historic building. Levees are created by digging a trench around the building and then filling the trench with a mound of compacted fill to a height at least one foot over the 500-year flood level. Because levees are made of fill, and the slope of the fill has to be shallow enough to ensure slope stability, this flood protection option requires a lot of extra land around the building. It also requires access to a lot of fill, which can be challenging to acquire and expensive to transport. Typically, levees are not suitable for flood levels over six feet deep.
Other important considerations include how constructing levees close to property lines can negatively impact local drainage patterns, which may increase flooding for your neighbors. Levees can also fail or be overtopped during large or long flood events, and the levees, as a new part of the site’s landscape, must be maintained. Finally, as a conspicuous addition to the landscape, levees can adversely affect the historic building’s relationship to certain landscape features.
ELEVATE ON CLOSED FOUNDATION
Similar to elevating a building on columns or piers, historic buildings can also be raised on solid perimeter foundation walls. Typically, the building is raised in place and then the foundation walls are extended from the current foundation using concrete masonry units (CMU) or cast-in-place concrete. Consequently, the existing foundation walls must be analyzed to confirm that they have sufficient capacity to accept the additional load, and, if the existing foundation system does not, it should be strengthened. The existing foundation system may also need to be strengthened to accommodate the lateral loads expected on the extended foundation walls. To reduce the lateral pressure on the extended foundation walls, openings, such as flood vents, can be installed to allow water to enter the space beneath the structure, which allows the hydrostatic pressure to equalize on either side of the foundation walls. Elevation on a closed foundation is most suited for floodwaters that are expected to be low to moderate in depth and velocity.
ELEVATE BY RAISING LOWEST LEVEL INSIDE THE BUILDING
There are a couple of different ways to raise the lowest level of the building. One method is to construct a new floor several feet above the existing floor. This method is most appropriate for buildings constructed out of masonry or concrete masonry units with floodwater depths less than four or five feet. The construction process involves removing the roof, raising the window and door openings, adding new rows of masonry units to extend the height of the building, and constructing a new floor system. The floor system, which should be elevated above the 500-year flood level, can either be a new wood frame system or a new concrete slab on grade. For the new concrete slab on grade, fill must be placed and compacted on the existing slab before the new slab is poured. Another method is to abandon the lowest level of the building as a habitable space and construct another story on top of the building. The abandoned level should be wet floodproofed to prevent the failure of the existing walls. Again, this method is most appropriate for buildings constructed of masonry or concrete masonry units.