While values of 0.6mm and 1.5mm are used as standard, in some cases a variation from these values may be justified. BS EN 16933-2 recommendations for foul and combined sewers is shown in the below table īS EN 16933-2 also recommends that these roughness values include some allowance for local headlosses such as those from junctions and pipe joints. This is also the recommendation in Sewers for Adoption for design of new drainage systems. These values have typically been used for design in the past and therefore these values are covered in the published charts and tables. While the true roughness coefficient can be affected by a number of conditions including pipe material, condition and sedimentation conditions, for general design purposes it is recommended that roughness coefficient values of 0.6mm and 1.5mm are used for general surface water and foul water drainage pipes respectively. For this reason manufacturer’s roughness coefficients should not be used for general drainage design. While hydraulic tests can confirm the roughness coefficient of the pipe in laboratory conditions, in practice the roughness of the pipe is dictated by other factors as described below. ![]() The value of the coefficient must be determined from hydraulic tests. ![]() ![]() While this coefficient may have the units of length it cannot be measured directly from the pipe. It is used in the Colebrook White Equation. The Colebrook White Roughness Coefficient or equivalent sand roughness coefficient is a coefficient describing the internal roughness of the drainage pipe.
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