Backwater rise due to driftwood accumulations

Lukas Schmocker, Dieter Brändli, Volker Weitbrecht, Robert M. Boes

Tuesday 30 june 2015

14:20 - 14:35h at South America (level 0)

Themes: (T) Flood risk management and adaptation, (ST) Flood risk assessment

Parallel session: 6J. Floodrisk - Assessment

During flood events mountain torrents and rivers may transport significant amounts of driftwood or large woody debris (LWD). This driftwood can accumulate and block spillways at hydropower plants as well as bridges and weirs along rivers. Blocking of a river cross section will always lead to a backwater rise and possible overtopping of the flood embankment. Furthermore, also manmade driftwood retention structures such as debris racks result in a backwater rise, once they start to retain driftwood. For both cases, it is of primary importance to quantify the resulting backwater rise. The present paper summarizes a series of laboratory tests, where the backwater rise due to a driftwood accumulation at a rack or bridge was investigated. The backwater rise was determined depending on various flow conditions and for different driftwood characteristics. The results highlight the decisive effect of the Froude number and the porosity of the driftwood accumulation on the backwater rise.