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62.~ psf ~' . .~.:..:' .;-. -I. .....Jj.-.... .~:~<~::-: .._-""... , . '-~.::": ;}:~~> ~ %' 'fir:;';::..:: .:.1.-,- ~~~."",",.. Figure 4.7 Buoyancy Force Based on a manufactured home weight of 20 pounds per square foot of floor area, it is possible to determine the net buoyancy forces associated with differences in floodwater height. Table 4,2 is the gross buoyancy force (Fb) ex- erted per foot length of the manufactured home due to a particular difference in anticipated water depth (h) inside and outside the manufactured home, '>" The Table includes up to 48 inches of water height difference for the purposes of highlighting the magnitude of forces, Based on the floor design capacity, any buoyancy above a nine to 12 inch water height differential can cause the floor deck- ing to pull off the floor joists, thereby causing water inundation into the home, Those areas in the tables are shaded to highlight the practical limits in relation to buoyancy and the need to consider additional elevation, .. The risk of flotation is largely dependent on the rate of water rise, With a fast rise, there is insuffj, cient time for water to seep into the manufactured home, resulting in flotation from increased buoy- ancy forces, In slow rising flood conditions, floodwater can enter the manufactured home at a rate more equal to the water rise, thereby counteracting any buoyancy forces but causing substantial inundation damage, Elevation above anticipated flood levels is again highly important to minimize damage to the manufactured home, HydrodynamiC Forces Hydrodynamic forces are those imposed on any object which restricts a moving fluid, In the case of a manufactured home, hydrodynamic forces are caused by floodwater impact against the manufac' tured home and foundation, Drag forces, as shown in Figure 4,8, are one type of hydrodynamic force and are dependent upon the velocity of the floodwater, the drag coefficient of the surface restricting the flow and the depth to which the sur- face is submerged. h .~, ;~~' %~'" ~"'" Figure 4.8 Drag Forces Table 4.2 BUOYANCY FORCES IFb) Hare Width (ft.) . 3 4 5 6 . Difference in water height Inside and outside the mlnufactured hane (in.) 12 15 18 21 .. 30 36 42 48 12 62.4 124.8 187.2 249.6 312.0 14 92.S 145.6 21S.4 291.2 364.0 24 124.S 249.6 374.4 499.2 624.0 28 145.6 291.2 436.8 582.4 728.0 Fb = h x v x W (lb./lin. Ct. of hare) \'lbere: h = (hl-h2) Ct.; v = 62.4 Ib./cu. Ct.; W = width (Ct.) 36