'Elevation of buildings' and ' Land raising' are two separated measures with the aim to elevate exposed elements.
Elevation of building
In order to protect an existing building from flooding, elevation of the building is one retrofitting method. The two major types of elevating living spaces above the expected flood level are:
1) lifting up a building on a new or extended foundation; and (2) extending a building upward by elevating the existing floor or adding a new upper story utilizing an existing foundation (FEMA, 2009). The first method separates the building from its foundation, raises it on a hydraulic jack, and constructs a new or extended foundation below it. The new and extended foundation can be continuous walls, or separate piers, posts, columns or piles and can be exposed to flooding. The second method removes the roof, extends the building walls, and constructs a raised floor. The abandoned lower area can then be used for parking, building access or storage.
The height of elevation is determined by the expected flood level, that is, the lowest floor of the living space must be above the flood level, including freeboard. As with a wetproofing measure, the foundation of the elevated building must be able to withstand hydrostatic pressure, hydrodynamic pressure, debris impact, and erosion by flooding. Design experts should be consulted for these elevation projects to evaluate whether the existing foundations can support an increased load to the building. If the project site is subject to high winds, earthquakes, or other hazards, such horizontal and vertical forces must be also considered. More detailed elevation techniques are explained in “Homeowner’s Guide to Retrofitting” (FEMA, 2009).
In response to Hurricane Katrina, FEMA evaluated building damage from the hurricane and provided recommendations on building structures in “Summary Report on Building Performance - Hurricane Katrina 2005” (FEMA, 2006). The assessment found that the buildings that survived the hurricane event have some elements in common, such as high first floor elevations, a well. embedded deep pile foundation, and structurally connected foundation and building frame.
In the United States, National Flood Insurance Program (NFIP) regulations only allow landfill, encroachment, and other developments within a floodway if they are proven through standard hydrologic and hydraulic analyses not to increase flood levels in the community during the base flood discharge (FEMA, 1993). New developments and significant improvements are, in general, required to not cause negative impacts, not only to increasing flood heights, but also in creating additional threats to public safety, inducing extra public expenditure, creating nuisances, or conflicting with existing local regulations.
Land raising (or placement of fill) requires an understanding of local site conditions, soil characteristics, methods of placing and compacting the land, etc. (FEMA, 2001). The permeability of soils affects water infiltration on the site, which in turn influences the safety of the foundations or basement structure. The higher the lowest floor of building is elevated in comparison with the expected flood level, the safer the building becomes. If the elevation of the building is not high enough compared to the expected flood level or if it includes a basement below the flood level, additional measures of dryproofing and elevating the building should also be considered.
In order to combat hydrostatic force and buoyancy force, appropriate buffer zones around a building should be installed with a setback distance from the edge of the flood hazard area. The fill soil should be homogeneous and of a low permeability. A drainage system installed around the building foundations with a sump pump can lower the level of seepage and make the structure safer. FEMA provides the method of calculating such seepage flow.
Raising houses, tube wells, and latrines above the expected flood level is an effective flood mitigation measure in developing countries. In India, there are examples of raised platforms in flood shelters constructed for local people and their cattle (WMO, 2005). In rural Bangladesh, homestead plinths of local people were raised to reduce vulnerability to flood disaster (Practical Action Bangladesh, 2010). In order to reduce water-borne diseases, especially during periods of inundation, tube-well platforms were raised above the highest ever recorded flood level with freeboard.
Major sanitation problems in flood.prone areas of developing countries are surface water contamination and difficult access to latrines during floods (Kazi and Rahman, 1999). Because overflow of a pit latrine poses serious health and environmental risks, the top of the latrine is extended above the expected flood level to avoid flood water intrusion into the pit which would expand its volume. One effective measure of preventing groundwater contamination by latrines is to surround the pit latrine with a sand filter and make the bottom of the pit impermeable. These measures are very simple and easily implemented by local people; however raising awareness about sanitary conditions and motivating people is the key to success of these projects.