This model allows design of stormwater drainage channels used to convey design flood discharges associated with storm runoff from the catchment. It calculates flow depths within the drainage channels using the Manning equation for uniform flow. Manning’s flow resistance coefficient values for various liner materials and the relationship between the Manning’s coefficient and flow depth were obtained from hydraulic engineering literature.
Hydraulic shear stress (reflecting both flow velocities and flow depth) is related to the erosion potential within the proposed drainage channels. A lining material is specified to resist the hydraulic shear stress and to prevent erosion according to Table 1 (in the model file). Often flow velocity is used as the only measure of erosion potential. Selecting a liner material according to hydraulic shear stress is more robust than selecting a liner material according to flow velocity only.
Channel roughness is affected by the relative height of the roughness compared to the flow depth. As a result, Manning’s roughness generally decreases with increasing flow depth. Roughness values provided in design charts such as Chow (1959) may need to be adjusted for actual flow depths. Table 2 (in the model file) provides typical roughness values for various drainage liner material types under various flow depths. Manning’s n was interpolated based on the flow depth and therefore varies along the length of the drain in line with changes in flow depths.
The lining material affects the resistance to flow quantified using Manning’s coefficient. Calculations of flow depth, velocity and Manning’s coefficient were therefore performed iteratively to allow convergence to the correct solution.