Racks represent an efficient measure to maintain the discharge capacity and the operational safety of spillways on dams for a pronounced accumulation of driftwood. Previous research projects have dealt with these structures, in particular with regard to the reservoir level rise, various measures to limit the latter as well as with rack forces. In the case of run-on-river weirs, the problem of driftwood is amplified, since the wood clogs more compact on the rack due to the increased flow velocity.
The rack-bar force upstream of a river weir for an "extreme" accumulation of driftwood was investigated in the context of this project using a physical model. The boundary conditions were systematically varied, namely the discharge, the flow velocity and the rack bar spacing. An “extreme” accumulation of driftwood was assumed, as well as a partial and a full rack.
The driftwood-induced rack force could again be expressed in terms of the drag coefficient. The reference surface had to be adjusted since the driftwood was pushed in several layers against the rack at higher flow velocities. Over all, the drag coefficient had to be increased to 27, in contrast to 20 for the tests on spillways in reservoirs.
As expected, the reservoir level rise for the clogged weir depended on the presence of the rack and the approach flow head, which increase in rivers as compared to reservoirs. The effects of both parameters could be quantified. The positive effect of the rack on the discharge capacity was once more proven. With the combination of clogged rack and the smallest weir heights, the blockage took control of the rating curve, the weir was no longer decisive.