Large Wood Inside Reservoirs: The Influence of Stem Characteristics on the Blocking Probability at an Ogee Crest Spillway

Large wood increases the hydraulic complexity of rivers. However, it may block and modify the flood discharge capacity of hydraulic structures such as bridges, weirs and spillways. The blockage of spillways with large wood has led to dangerous situations in several dams due to rising water levels in the reservoirs and extra loads on the structures. Still, little is known regarding how the characteristics of large wood influence the blocking probability of spillways and this may be essential information for safety evaluations of dams. Consequently, an experimental campaign was executed to estimate the blocking probability of large wood at an ogee crested spillway equipped with piers for a reservoir approach flow type. The present paper summarizes the influences of large wood characteristics (i.e. stem length, diameter and density) and hydraulic conditions on the blocking probability. A statistical analysis performed enabled to define 30 experimental repetitions, when doing blocking probability estimations of individual stems, for attaining estimation errors equal or lower than 0.10. With a systematic methodology, for the first time, the influence of the stem density on the blocking probability was evaluated. Results showed that an increasing stem density increases the blocking probability of individual stems when the stem length is greater than the bay width. It was noted that stems with a density close to water density have the highest blocking probability. Furthermore, the influence of the head was quantified. It was concluded that an increasing head can decrease the blocking probability. However, the head influence is not linearly related to the blocking probability and changes according to the stem characteristics and number of open bays. The majority of results indicated that five open bays has a lower blocking probability than one central open bay.