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<br />of flooding is a very important consideration when
<br />elevating a manufactured home, A two foot depth
<br />of inundation can potentially result in damages up
<br />to about 80 percent of the value of the manufac-
<br />tured home,
<br />The conventional manufactured home installa-
<br />tion may provide adequate protection from inun-
<br />dation in some flooding situations, where depths
<br />of flooding do not exceed the 24- to 36-inch foun-
<br />dation height of the conventional installation,
<br />In deeper flooding situations the height of the
<br />conventional manufactured home installation is ex-
<br />ceeded, Once the floodwaters inundate any por-
<br />tion of the manufactured home, the probability of
<br />significant damage increases. In addition to
<br />damage resulting from inundation, differential
<br />water levels between the interior and exterior of
<br />the manufactured home result in uplift forces on
<br />the manufactured home, These forces can result
<br />in movement of the manufactured home off its_ _ _ _
<br />- .---fOundation supports and cause other structural
<br />-' '?E. damage to the manufactured home, This situation
<br />can be expected where there is a rapid rate,of-rise
<br />of floodwaters resulting in insufficient time for the
<br />water to enter the manufactured home and
<br />counteract the buoyancy forces,
<br />
<br />Velocity of flooding is the time rate of the linear
<br />motion of floodwaters usually measured in feet per
<br />second, Flood velocities vary from point to point
<br />in a floooplain and determine the magnitude of
<br />hydrodynamic forces affecting a manufactured
<br />home, Velocity floodwaters will result in significant
<br />lateral forces on the typical manufactured home
<br />installation, Lateral forces on the foundation sup-
<br />ports, along with erosion and scour of the foun-
<br />dation, can resuit in failure of the foundation if im-
<br />properly designed,
<br />When flood depths exceed the foundation
<br />height, lateral forces from velocity floodwaters on
<br />the walls can move the manufactured home off its
<br />foundation supports, resulting in inundation and
<br />damage to the home, Conventional installation
<br />techniques are inadequate to withstand the
<br />amount of lateral force which can be expected
<br />from velocity floodwaters,
<br />The potential for debris impact forces is also
<br />much greater in a velocity flooding situation,
<br />because the amount and size of debris will vary
<br />from location to location, The magnitude of the
<br />debris impact force upon a manufactured home
<br />or the foundation supporting the home is directiy
<br />proportional to the velocity of the floodwater, Basic
<br />assumptions concerning the size of debris and
<br />velocity of impact must be made when designing
<br />a foundation in a high velocity flood-prone area,
<br />The potential for scour and erosion of the foun-
<br />dation also increases with velocity, If scour and
<br />erosion are of concern; protection for the founda-
<br />
<br />Debris Impact Force-Floods also present an
<br />additional hazard to structures by creating the
<br />potential for debris impact as shown in Figure 2.4.
<br />Debris impact forces are produced by solid objects
<br />carried by the floodwaters striking the structure,
<br />These forces are extremely difficult to predict and
<br />quantify with a high degree of accuracy, yet their
<br />effect on structures is significant enough to war-
<br />rant consideration,
<br />
<br />
<br />Figure 2.4 Damage From Debris
<br />
<br />Debris impact forces include isolated objects of
<br />normally encountered sizes, such as logs; con-
<br />glomerates of floatable objects, such as ice floats;
<br />and extremely large masses, such as a collapsed
<br />structure, Forces are produced by objects both
<br />striking and resting against a structure, For the
<br />purpose of design, assumptions on object weight
<br />and impact speed must be made, Chapter IV
<br />presents specifics on debris impact forces,
<br />The magnitude of these three types of flood
<br />forces is dependent on several flood related
<br />parameters such as velocity, depth, duration, rate-
<br />of-rise, and frequency,
<br />Flood Hazard Parameters-Flood hazard
<br />parameters are used to determine the degree of
<br />hazard that can be expected at a site and provide
<br />necessary information for evaluating and choos-
<br />ing damage reduction and site design strategies,
<br />
<br />Depth cij flooding is the difference between the
<br />water surface elevation at the time of flooding and
<br />the normal grade elevation of the flooded area,
<br />Flood depths determine the magnitude of the
<br />hydrostatic forces that act on a manufactured
<br />home, There is a direct relationship between depth
<br />of water and amount of hydrostatic pressure,
<br />Greater depths of water exert greater pressure on
<br />a structure, A water depth of five feet, for exam-
<br />ple, exerts 312 pounds of pressure per square foot
<br />of surface, This pressure can cause buoyancy,
<br />lateral displacement, and overturning of a
<br />manufactured home as well as structural damage
<br />to the floor and wall assemblies, In addition, depth
<br />
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