Incipient motion of boulders in open channel flow

Abstract

The use of boulders to create habitat heterogeneity is important for aquatic diversity, and being able to predict the stability of a boulder that is placed in a river will aid in sizing the boulder. Identifying ways to increase the stability could save costs associated with over design or replacement due to the boulder washing away. Existing research on incipient motion centres on determining threshold conditions for bed material or protruding elements within a bed surface with relation to, among others: shape of particle, size of particle, relative depth of particle to flow depth, and impact of channel slope. The existing research is limited to bed material that is of a similar size and there is no research on the conditions for incipient motion elements that are relatively large compared to the bed material it is resting on. An idealised flume study was performed to identify trends that several factors have on boulder stability, as well as to verify the results obtained from a pivoting analysis model prediction for a spherical boulder. An additional study was performed to obtain drag coefficients that were suitable for use on spherical boulders that were either embedded into the bed material or simply resting on top of the bed material. The results of the drag experiments were varied; only the results for the non-embedded were suitable to integrate into the model predictions while drag coefficients for the embedded boulders need to be taken from previously published results. The results of the flume study provided good confirmation of the model predictions with the average absolute experimental error being 4%. The trends identified in the flume study show that the most effective method in improving a boulder’s stability is to embed it into the bed material with this being more effective than increasing the size of the boulder.