Driftwood represents a potential threat to flood releasing systems, as it can block the flow control cross-section. Driftwood should therefore be kept away from the critical cross-section, for example by means of a driftwood rack.
The racks used so far are designed to hold back all driftwood. The following water course, however, then lacks large deadwood elements as a habitat-shaping element. We have therefore systematically examined a "partial" rack in the physical model, which usually aligns and guides individual pieces of driftwood through the spillway inlet. For driftwood groups (congested arrival) however, the rack behaves similar to conventional racks. At the same time, it ensures a much larger discharge capacity, almost comparable to that of the full rack, even under extreme driftwood appearances.
In addition, we measured driftwood-induced rack forces for different boundary conditions, as well as purely hydrodynamic forces. The former are a multiple of the latter. The tests have suggested that in principle the common flow resistance equation can be applied. This equation reflects the influence of the tested parameters relatively well, in particular the water surface flow velocity at the considered rack bar. Nevertheless, the surface area of the obstruction and the drag coefficient had to be adjusted. For both, the open distance between the rack bars plays a central role, as does the maximum wood length. With a pragmatic approach, an envelope resulting in a constant drag coefficient can also be assumed, regardless of the parameters examined.
