Blockage of a Stem Batch at a Spillway with Piers and Head Increase Measurements

In-stream wood has positive effects on the morphological and hydraulic complexity of rivers but floods can transport large amounts of woody material towards reservoirs in forested basins. For dams in areas prone to produce and transport woody material it can be an exacerbating hazard factor. Accumulations of floating debris into reservoirs can have significant negative impacts on the operation and functioning of a dam, increasing the extent of flooded areas. Hence, an adequate spillway design in view of driftwood is of paramount importance to overcome potential impacts on dam safety. Experiments in a laboratory facility were executed to evaluate the process of blockage for a large number of artificial stems at a spillway inlet. Due to its broad use, an ogee crested spillway equipped with piers was designed for the experiments. By systematically supplying a batch of stems, the effect of different blocked volumes of artificial stems was qualitatively investigated. Once blockage occurred, it was investigated how the jam shape could affect the head upstream of the spillway. The process of blockage is highly variable and several different scenarios of blockage were obtained during the experimental repetitions. For a reservoir flow approach it was found that similar blocked volumes of stems can have different effects on the head at the reservoir. Generally, when stems longer than the bay width were in contact with the weir crest, a head increase was measured. It has been noted that for the case of small flow velocities, a floating carpet of stems upstream of the piers, was not significant in terms of head increase at the reservoir. Additionally, when the majority of the blocked stems were aligned with the flow direction, higher head increases were measured.