Temporal scour evolution at wood bundles under clear water condition

In the current scenario, there is a rapid degradation of the various water resources due to a host of climatic and anthropogenic factors. Natural river channels are particularly destabilized, leading to recurrent siltation and flood problems. Low head eco friendly hydraulic structures are a viable option to recreate the natural flowing conditions in a river which consequently leads to a stabilized channel morphology [1]. In this regard, low head structures such as log frames [2], block ramps [3], and wood b undles [4] are typically used to control the sediment load in rivers as well as to create localized scour holes in the stilling basin, also promoting biodiversity in the channel. Among the above mentioned structure typology, wood bundles are placed both in straight and curved channels to provide perching sites for various invertebrates and to enhance bank safety. In channel curves, they redirect strong velocity gradients away from the outer bank towards the channel center thereby protecting the riverbank from failure due to excessive erosion. On the other hand, the scour holes in the stilling basin of wood bundles serve as an ideal spot for growth of various aquatic organisms includ ing fishes. The equilibrium scour characteristics associated to these structures have been recently investigated by [4], however the temporal evolution of the scour hole still poses a serious gap in knowledge. Therefore, this paper focuses on the temporal evolution of the scour hole at wood bundles in straight channels and presents some preliminary findings on the topic. A set of experiments were performed on wood bundles for a large range of hydraulic conditions and structure geometry under clear water conditions. Several factors like discharge and tailwater, structure height, number of structures in series combination an d spacing between consecutive structures in series were varied. Data analysis showed that the scour depth evolution is considerably dependent on the inflow conditions which can be modelled by a novel densimetric Froude number. Moreover, the results also in dicate that the scour development is usually faster for higher densimetric Froude number. Therefore, this paper provides insights on the evolution of scour hole at wood bundles in various stages of its growth which can facilitate the design of such structu re typology in future. To aid in comparison, Fig. 1a shows the flow characteristics and eddy formation at a wood bundle during the scour evolution process whereas Fig. 1b shows the equilibrium scour morphology for the same structure after test completion