Discussion. Effects of Use on the Hydraulic Resistance of Drainage Conduits.

Abstract

wrote that now the Authors had shown that the extent of slime has a significant effect on the flow-carrying capacity of sewers, it seemed necessary to differentiate between the types of sewer with which the designer had to deal. Presumably the relative amount of slime to be allowed for should be greater for foul sewers on the separate system than for combined sewers. Perhaps slime could be neglected in designing surface-water sewers, though if velocities were low, some allowance should be made for silting. Could storm-water overflow pipes be treated in the same way as surfacewater sewers or was slime likely to form in them?44. If there was likely to be a significant difference between foul and combined sewers, it would be useful if the Authors could give the type of each sewer which was tested. Since 'foul' was a somewhat relative term, though, the best course might be to give the depth corresponding to dry-weather flow in each case.45. Most of the tests reported were carried out at low relative depths of flow. In the cases where the test flow was of the same order as dry-weather flow, was it likely that the pipes would have lower equivalent values of k, when flowing nearly full?46. Further investigations would seem to be desirable to find to what extent the incidence of sliming differed in foul, combined, surface-water, and storm-water overflow sewers and also in rising mains and inverted syphons. Even visual inspection of a number of pipes in each category would be of value.Mr L. B. Escritt (Assistant Senior Engineer, London County Council) wrote that the factors which influenced turbulent flow in pipes were too complex to be expressed accurately and adequately by any existing formula that satisfied theorists. They included local changes of diameter and gradient, joints and excrescences in addition to the 'roughness' usually mentioned in the literature. However, hydraulic phenomena, when plotted on logarithmic ordinates and abscissae, almost invariably gave straight lines and this was true of the flow of water in pipes. Hydraulic radius, hydraulic gradient and velocity were accurately related by the empiric formula: V = Km"P where V=velocity in ft/s, m= hydraulic radius in ft, i=fall divided by length, and K = a constant.48.