Dryproofing makes a building watertight and substantially impermeable to floodwaters (FEMA, 1993). Compared to wetproofing, dryproofing requires a more reinforced building structure to withstand floodwater pressures and impact forces caused by debris. Other important factors to be considered in dryproofing are watertight closures for doors and windows, prevention of floodwater seepage through walls, and check valves to prevent reverse flows from sewage.
Before selecting dryproofing as a viable floodplain management tool, numerous factors must be considered, such as flood warning time, purpose of building usages, mode of building entry and exit, flood depths, floodwater velocities, floating debris impact, flood frequency, etc. The flood proof function must work sufficiently for design flood level and additional freeboard is recommended because flood depth estimation includes a certain error and may be influenced by future development in the basin.
Sufficient warning time, which is calculated by the rate of floodwater rise and the existing flood warning system, is necessary for evacuation from a flood prone building, for installation of removable flood shields or gates, and for operation of sump pumps and check valves. If the warning time is limited, for example the structure is located in a flash flood area, flood proofed buildings should not be considered as the necessary operations to make it flood proof will require too much time. FEMA suggests flood velocity of 5 ft/s (1.5 m/s) and flood depth of 3 feet (90 cm) as thresholds for adopting dryproofing. If the flood exceeds these limits, the cost of dryproofing may become too significant and the dryproofing method is therefore not feasible. Any areas susceptible to severe debris flow, such as mountainous regions or areas facing ice flow in winter, are not suitable for flood proofed buildings in a cost.effective manner.
The building structure must be able to resist four types of flood.related forces: (1) hydrostatic flood force that freestanding water exerts on a submerged object; (2) buoyancy force that a building receives from surrounding floodwaters; (3) hydrodynamic force that vertical surfaces receive from moving floodwaters; and (4) debris impact force to withstand the flood.borne debris strikes on the side of building. FEMA provides an estimation formula for each force (Appendix 2). For more detailed standards of dryproofing structure design, FEMA has a comprehensive guidance and case study report “Engineering Principles and Practices for Retrofitting Flood.Prone Residential Structures” (FEMA, 2012).
Flood Shields for Openings
Doors, windows and air vents of buildings are potential flow paths where flood water runs into properties (DCLG, 2007). Raising the threshold of doors as high as possible without disturbing accessibility is a primary prevention measure. Sealed polyvinyl chloride (PVC) framed doors are a more preferable option than wooden doors and the doors should be properly fitted to their frames. Windows are also vulnerable to flood water and preventive measures of fitting and sealing similar to those for doors should be taken. The windows should adequately resist the pressure of flood water and prevent damage that could be caused by debris flows. Regarding ventilation vents, special designs of air vents that prevent water from entering into the premises are available on the market.
Temporary flood protection system
A temporary and removable flood protection system is provided in locations where permanent flood defences would not be suitable because they are not technically, economically or environmentally feasible (DEFRA, 2011). The temporary system includes a pre-installed system that requires operation; the system may be installed in a pre.constructed foundation, or it may also be a system where the whole of it is movable and needs to be installed. DEFRA defines the first two systems as “demountable systems” and the third one as a “temporary system” . These systems are further classified by their different structures, such as earth filled containers, air and water filled tubes, and panel type flood barriers.
In contrast to a permanent flood protection system, a temporary system brings an additional risk of operational failure. Taking this fact into consideration, a permanent system should be given priority if it is feasible and locally acceptable. In the event of a temporary system being adopted, it should be ensured that the movable parts of the system are at a minimum and that the reliability of all the operational processes including mobilization, installation and closure are at a maximum. If the temporary system requires significant preparation time, it is suitable for location at the downstream of a large river basin.
A temporary flood protection system can allow a dual function by ensuring effective flood control performance without obstructing the ordinary use of the building, for example access through a floodwall, or parking lot turning into a flood protection site. A temporary system also adds additional safety to a permanent system, which is often the case in critical disaster situations. There are several factors affecting the risk of operational failure, such as sufficient lead.in time, reliability of flood forecasting and warning, system maintenance, and training of operators. Because the flood warning system usually triggers the operational process of the temporary system, technical and human operational reliability is a pre-requisite for the temporary system. Regular training and emergency exercises together with flood operation manuals increase the reliability of the total system. Different temporary systems need different levels of installation skills and preparation time. Site.specific conditions, such as the location of the stockyard of the system parts, transportation means, and available resources of personnel and equipment, also affect the selection of an appropriate temporary system. Detailed advantages and disadvantages of different temporary systems and commercially available products are explained in “Temporary and Demountable Flood Defences” (DEFRA, 2011).