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The scour depth at square and circular piers, posts, and piles can be calculated as follows: S = d [ 2, 2 (~t 65 X (; ) 0, 4 3) Where: S = depth of scour hole in feet d = depth of flow upstream of structure in feet a = diameter of post, pier, or pile in feet V = velocity of flow approaching the structure in feet per second g = acceleration of gravity-32,2 feet per second squared Table 4,5 has been prepared for numerous velocity and depth values and typical vertical member diameters and applies to average soil conditions (2,000-3,000 psf bearing capacity), For loose sand and hard clay, the values may be increased and decreased respectively, However, the assistance of a soils engineer should be sought where highly erodable soil conditions exist (e,g, loose sand) or where scour depth values lower than those provided in Table 4,5 are used, In reviewing the scour depths in Table 4,5, note their magnitude and potential impact on piers which are placed on grade. Table 4.5 DEPTH OF SCOUR (feet) Vertical Member Diameter Velocity in. (fps) 4 6 8 10 12 .75 I. 00 I. 25 I. 50 I. 75 2 1.00 I. 25 I. 50 I. 75 2.00 4 I. 50 I. 75 2.00 2.25 2.75 6 1.75 2.00 2.50 2.75 3.25 8 2.00 2.25 2.75 3.25 3.50 10 2.25 2.50 3.00 3.50 4. 00 15 2.50 3.00 3.50 4.00 4.50 The determination of maximum potential scour is critical in designing an elevated foundation system to ensure that failure during and after flooding does not occur due to any loss in bear- ing capacity or anchoring resistance around the posts, piles, or piers elevating the manufactured home, The anticipated scour depth must, therefore, be added to the preflood height above grade in determining the necessary post, pile, or pier design as shown in Figure 4,15, Without the inclusion of scour depth in elevated foundation design, failure and collapse of the foundation system is more likely after a flood, Before Scour A (tel" Seoue Figure 4.15 Addition of Scour Depth In coastal areas scour depths can be significant due to both localized scour as calculated above and area erosion resulting from the overall effects of storm surge, Since area erosion is difficult to predict, local regulatory officials should be con- tacted for further information and historical perspectives, Based upon the water velocity and type of soil, scour and erosion would increase the height above grade of the vertical member since the grade level wouid be reduced due to scour and erosion, As this occurs, the depth of burial of the vertical member also decreases an identical distance. This can result in an elevated foundation failure because the loss of natural supporting soils would change the conditions under which the elevated foundation system was designed, 40