Critical deposit velocity in internally corrugated pipes

Internally corrugated pipes are frequently used in urban drainage for steep slopes, in virtue of their flow velocity reduction capacity due to their macro-roughness. A large number of experimental studies concern sediment transport in smooth and rough pipes, but the mechanism of sediment transport or deposition into internally corrugated pipes is still unknown. After a short review of the literature, this paper considers the definition of critical deposit velocity in internally corrugated pipes as the value of the velocity in correspondence to which the solid particles start to form stationary deposits on the channel bottom. It is shown that a fundamental role is played by the macro-roughness and, particularly, by the distance between two successive crests of the corrugation. Dimensional analysis permitted the identification of non-dimensional groups involved in the problem, which take into account the characteristics of flow and sediments (non-cohesive material). Experimental laboratory tests were performed, using a 17.40m-long corrugated pipe with a diameter of 200 mm, at different longitudinal slopes and with a uniform fine gravel. Results are presented giving the critical velocity as a function of the volumetric sediment concentration, the ratio between sediment diameter and hydraulic radius, and the friction factor.