Beware the Roughness of Riprap

Nam Theun 2 is a major hydroelectric scheme recently commissioned in Laos. The scheme involves an inter-basin transfer from the Nam Theun River to generate approximately 1000MW of hydro power. Discharge from the power station to the downstream Xe Bang Fai River is via a 27km long diversion channel, 8km of which is riprap lined. In order to satisfy design constraints on both velocity and water level, the channel design considered a wide range of roughness parameters estimated from standard references and previous similar experience. During commissioning however, the hydraulic roughness of the riprap was found to be much smoother than expected, lower than lower bound estimates from standard references. This paper examines the data from the project and in particular the construction methodology in order to draw conclusions in relation to riprap design and roughness prediction. The outcomes will be of great interest to hydraulic design practitioners.