Impact of roughness elements on reducing flow velocity at outlets of box culverts

For culverts on slopes greater than critical, rougher material causes greater depth of flow and less velocity in culverts of equal size. Velocity varies inversely with resistance thus roughness elements resistance is obviously an important factor in reducing velocity at the outlets of culverts on steep slopes to prevent scour in downstream of culverts on roads. The criteria for design directives of new energy dissipaters must provide for sufficient energy dissipation, characterized by simplicity of design, effectiveness of energy dissipation and low construction cost. In this study, two physical models were built with producing roughness elements at the end part of the culverts to evaluate the outlet velocities of high-energy culverts. The results showed that with a slope ranging from 5 % to 13 %, the proposed roughness elements reduce energy dissipation from 48.7 % to 52.9 % for roughness elements without gaps and from 51.6 % to 53.9 % for roughness elements with gaps. This result also indicated that the roughness elements in the sloping box culverts can be used to replace energy dissipater structures in the downstream of the box culverts in the Central region of Vietnam.