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46 &.3 Hydrodynamic Loads &,3.1 General As water moves around a structure, it creates what is known as hydrodynamic loading, This loading or force is created by the moving water impinging on the structure and thus is highly dependent on the velocity of flow, &.3.2 Application The most frequently considered hydrodynamic loads are those that occur above ground where velocities may be relatively high. However, this kind of load can occur below the ground level if openings or conduits exist which allow free flow of the flood water. The important consideration for hydrodynamic loads during floods is velocity, For low velocities, especially below 5 fps, the effect is often relatively insignificant. Water velocities usually dec- rease with distance from the main channel of a stream. Because of this, the location of a structure within a flood plain may help determine whether to design for hydrodynamic load. &.3.3 Methodology The equation for dynamic pressure Pd is: v' P d = CdP '2 where mass density p is normally equal to approximately 1.94 slugs/f\3for water. Velocity V is in feet per second and Pd will be in pounds per square foot. The drag coefficient Cd is dimensionless and depends on the shape of the object around which the water is flowing, Studies have shown the maximum Cd for mobile homes Is 1.31. Unless a detailed analysis is performed for a particular structural shape, a value of 1,25 should be the minimum Cd used. From research related to wind resistance the value of Cd can be determined from the width to height ratio, b/h, Where the width is the structure side perpendicular to the flow and the height is the depth of water above the bottom floor of the structure, Table &.4 gives Cd values for different b/h ratios. Once the hydrodynamic pressure is known, it must be applied over the area upon which the water is impacting, Table 6,1, Effective Equivalent Fluid Weights, Effective weifht of Yeft, Equivalent Saturated Soi , Ysat F uid Weight 30 pet 92 35 97 45 107 100 162 120 182 Soil Type" Clean sand and- gravel: GW, GP, SW SP Dirty sand and gravel of restricted permeability: GM, GM.GP, SM, SM-SP Stiff residual silts and clays, silty fine sands, clayey sands and gravels: el, Ml, CH, MH, SM, Sc. GC Very soft to soft clay, silty clay, organic silt and clay: CL, ML, OL, CH, MH, OH Medium to stiff clay deposited in chunks and protected from infiltration: eL, CH , See Table &.2 for SOil type definitions