Wetproofing (or wet floodproofing) is different from dryproofing in that it allows flood water to enter a structure, though both floodproofing methods have the same purpose, that of preventing damage to the structure and its contents and creating no additional threats to public safety (FEMA, 1993).
Equalizing water levels on the inside and outside of a building by wetproofing can result in some advantages for the building structure. Firstly, because the difference in interior and exterior water levels causes hydrostatic pressure on the building walls, equalizing water levels by keeping the change at the same rate minimizes the hydrostatic pressure and thus the building does not require such a strong structure to withstand such pressure. Secondly, inside water reduces the buoyancy effect of hydrostatic uplift forces. Although a wetproofed building is relatively less affected by hydrostatic and hydrodynamic forces, the structure must be adequately anchored to prevent floatation, collapse, or lateral movement. Without engaging in major structural reform, wetproofing is often a more cost effective measure for buildings.
On the other hand, disadvantages of wetproofing are obviously caused by water entering the building. To prevent damage to the contents of a building, mechanical and utility facilities (such as electrical, heating, ventilation, plumbing, and air conditioning equipment) must be elevated above the expected flood level or must be protected from flood water entering or accumulating within the various components. Empty liquid containers, including the ones buried underground, should withstand a buoyant force by anchoring or even filling them with potable water before flooding (of course, after the flood quality control should be established and implemented before using the water stored in the tank). Because windows are especially vulnerable to flood waters and debris, protective screens, reinforced glass and impact.resistant plastic are recommended solutions.
Before adopting the wetproofing method, the site situation should be carefully considered. Wetproofing is not appropriate if the site experiences rapidly rising flood water, high.velocity flood waters, and a short flood warning time. In this case, elevation or relocation of buildings are more preferable solutions. If a wetproofed facility requires some preparation time for minimizing flood damage, for example shifting or elevating equipment or content, sufficient warning time is necessary. If the facility needs to be accessed during flooding, safe access routes should be secured for either escaping from, or gaining access to, the site.
Because most of the existing facilities do not expect to come into contact with water and often use permeable construction materials, retrofitting wetproofing techniques by applying flood resistant materials are necessary. The materials must be resistant to flood forces, deterioration caused by repeated inundation, and excessive moisture and humidity during and after flooding. Because flood water contains silt, chemicals, and organic materials, which can be hazardous to the structure and the residents, the structure and the materials need to be easily cleaned without leaving any contaminants. Concrete, hard brick, plastic, metal, and pressure.treated wood are possibly suitable materials for covering walls and floors. Cleaning up after a flood includes washing and disinfecting walls, floors, and other surfaces. Because flood.induced mould and contaminants are hazardous to human health, wetproofing is not suitable for living spaces (FEMA, 2009). After the cleaning process, the drying out process can take up to six weeks to remedy any structural damage and health problems.
Another category for which adopting wetproofing could be considered includes structures located near water and functionally dependent on water uses, such as docking, seafood processing, port facilities, and ship repair facilities. Moreover wetproofing may be suitable for agricultural structures used for production, harvesting, storage and drying, provided that agricultural commodities and livestock are raised and kept dryproof.
Wetproofed structures must meet the required technical standards, conducting site-specific evaluation by technical experts or designated government offices, if necessary. In addition to the standards, all kinds of local or national regulations, building codes, etc. should be met. It should be noted that some local regulations may exceed national regulations. Combining wetproofing with dryproofing and elevation may achieve optimal protection for the site. Before implementation, an economical assessment and evaluation is required to understand if the cost of business interruption and cleanup activities may make wetproofing less feasible in comparison with dryproofing.
FEMA (Federal Emergency Management Agency), 1993: Non-Residential Floodproofing - Requirements and Certification for Buildings Located in Special Flood Hazard Areas in accordance with the National Flood Insurance Program. FIA.TB.3. www.fema.gov/library/viewRecord.do?id=1716
FEMA (Federal Emergency Management Agency), 2009: Homeowner’s Guide to Retrofitting - Six Ways to Protect Your Home From Flooding. FEMA P.312, Second Edition. www.fema.gov/library/viewRecord.do?id=1420