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<br />. <br /> <br />As shown in the following table, hydrostatic pressure, whether it affects a building laterally or <br />vertically, increases rapidly as floodwater depths increase: e <br /> <br />Height, R (in feet) 1 2 3 4 5 6 7 8 <br />Pressure'Ph (in pounds per square foot) 62 125 187 250 312 374 437 499 <br /> <br />Hydrodynamic pressure is exerted on all vertical surfaces of obstructions, such as building <br />foundations, by the impact of velocity water and debris. Depending upon site-specific flood <br />characteristics and the strength of the foundation, hydrodynamic pressure can overload and <br />destroy a building's foundation. The openings criteria are intended to equalize hydrostatic <br />pressure and are!lQl intended to minimize hydrodynamic pressure on the foundation. Hydrody- <br />namic pressure must be considered in the design of any foundation system where velocity waters <br />or the potential for debris flow exists. If flood velocities are excessive (greater than 5 feet per <br />second), foundation systems other than solid foundation walls should be considered, so that <br />obstructions to damaging flood flows are minimized. Safe foundations in such locations include <br />pile, post, column, and pier foundations. These types of foundation systems are appropriate for <br />A zones in coastal environs subject to waves and velocity floodwaters, as well as in riverine <br />floodplains subject to velocity floodwaters (velocities greater than 5 feet per second) and areas <br />subject to debris flows and ice floes. In areas with high-velocity floodwaters, it is advisable to <br />construct any enclosures below the BFE using the breakaway wall specifications described in <br />FEMA's "Coastal Construction Manual.'" As stated previously, breakaway walls in A zones e <br />must have openings compliant with NFlP regulatory requirements so that hydrostatic pressures <br />are equalized during low-level flood events. <br /> <br />Design Criteria for Engineered Openings <br /> <br />Engineered openings that allow floodwaters into an enclosure for the purpose of equalizing <br />hydrostatic pressures shall be designed using the following criteria: <br /> <br />. The difference between the exterior and interior floodwater levels should not exceed 1 foot at <br />any time during the flood event. Greater differences can result in excessive hydrostatic <br />pressures and structural damage to the enclosure walls. <br /> <br />. The arrangement of the openings must be capable of equalizing the hydrostatic pressures <br />associated with the "worst -case" rate of rise of floodwaters. Historical flooding information <br />should be used to determine rate of rise. A rate of rise of 5 feet per hour was assumed in the <br />development of the NFlP non-engineered openings design criteria. <br /> <br />. Because of the large amount of debris associated with flooding and because openings will <br />often be equipped with some form of vermin screen to meet applicable building codes, there <br />is a high probability that openings may be obstructed during a flood. For this reason, a <br />substantial safety factor is needed. Standard engineering practice is to use a safety factor of 5 <br />in similar life/safety situations involving potential structural failure; therefore, a safety factor <br /> <br />e <br /> <br />8 <br />