An experiment was designed to eliminate the free surface from simulated naturally roughened open channel beds from which results were compared to data with a free surface from another study. All other pertinent variables were held constant. From this comparison, a relationship was established for the additional energy loss due to the presence of a free surface in the flow over these channel beds.P = 0.23 - 0.28 D/D25where P is the proportion that the channel conductance coefficient (C/g1/2) is reduced due to presence of a free surface, D is the flow depth, K25 is a measurement of roughness height and D/K25is the relative roughness and was varied from 1 to 7.
The channel conductance coefficient was found to be non-dependent upon Reynolds number.
A parameter describing bed element spacing was identified as the ratio of vertical projected area of all bed elements to the total bed area, and was found to be constant for a particular channel bed. Roughness spacing had only a minor effect on the channel conductance parameter.
The channel conductance coefficient was related to the relative roughness by a power function and the following prediction equation was established relating the channel conductance coefficient to the relative roughness and spacing parameter:C/g1/2 = 3.0 (D/K16)0.317 exp(0.007/θ)where D/K16 is the relative roughness and θ is the spacing parameter
