Hydraulic Loss Coefficients for Culverts (2012) / Chapter Skim
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50 6.1 Summary Quantifying hydraulic roughness coefficients is commonly required in order to calculate flow rate in open channel and closed conduit applications. Much of the theory of resistance on open channel flow is derived from studies on pressurized circular pipe, which features the Darcy-Weisbach roughness coefficient, f, which is dependent upon Re, Rh, and/or k.
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51 Turbulence level and the relative roughness of the pipe or channel can influence the flow resistance or hydraulic roughness. Much of the current theory regarding resistance is based on knowledge gained from the study of commercial pipes flowing full.
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52 similar to a smooth wall (See Figure 6-1)
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53 trapezoidal corrugations oriented normal to the flow direction (see Figures 6-6 and 6-7)
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54 (e.g., the block height, width, length, and spacing)
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55 expected that the predicted normal depths associated with the variable n values should also vary. Consequently, based on the good correlation between the uniform flow and GVF n data presented in Figure 6-8, yaverage was selected as the representative flow depth parameter for calculating Rh, Re, V, etc., rather than a predicted normal depth, for flow conditions where a uniform flow was not present.
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From page 56... ...
56 n Relationships If n were constant (as is often assumed) and solely dependent on k, four horizontal lines, one for each roughness material tested, should result when plotting n versus Re.
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57 materials (e.g., smooth acrylic sheeting) or for "rougher" boundary materials when a quasi-smooth boundary condition is present (e.g., metal lath and trapezoidal corrugation roughness material n versus Re data become constant)
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From page 58... ...
58 curves in Figure 6-11, however, is significantly reduced relative to the n versus Re data in Figure 6-10. It is also interesting to note that despite the fact that the block and trapezoidal corrugation roughness elements are the same height, the n versus Rh/k data trend differently in Figure 6-11.
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From page 59... ...
59 In developing Equation 6-2, Manning's (1889) primary objective was a simple open channel flow equation with a roughness coefficient (n)
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