Near-bed flow behaviour in scour holes under laterally non-uniform flow conditions

Abstract

Recent research identified a novel scour-enhancing flow mechanism related to 3-D flow behaviour. Under specific conditions, vertical flow separation at the upstream crest of a scour hole can be suppressed as a result of lateral nonuniformities in the main flow. This so-called vertically attached flow state is associated with increased scour potential. The objective of this study was to evaluate the effects of lateral nonuniformities on scour-driving flow behaviour in setups with realistic width-to-depth ratios. Laboratory experiments form the backbone of this study. A measurement technique was developed that enables efficient and reliable determination of near-bed flow directions. The technique is based on tufts: small threads fixed to the bed that follow the local flow direction. An algorithm was created to convert video footage of tuft movement to numerical data on flow directions. Next, flow behaviour was studied through a set of experiments conducted in a simple laboratory setup that simulates a common scour situation in the field. Lateral nonuniformities were generated in a mixing layer downstream a lateral expansion of the inflow channel. Two distinct horizontal flow states were observed across the tested configurations: horizontal divergence and horizontal contraction. Horizontal contracting flow was more likely to occur in geometries with a short bed protection, a large horizontal expansion and a narrow inflow channel. The transition point between both flow states was narrow. Only a slight change in geometry could entirely turn around the flow state. The horizontally diverging flow state is accompanied with constant vertical flow separation at the slope's crest. The corresponding return current entails a near-bed flow in upward direction. The horizontally contracting flow state is related to an alternating vertical flow pattern in which vertical flow separation and vertical flow attachment take turns. An alternating vertical flow regime was never identified before in this context. The alternating flow state is not present in the jet's centre. Instead, vertical separation takes place here. Additionally, a consistent lateral near-bed flow from the mixing layer to the centre of the jet is present. The frequencies in which vertical attachment and separation alternate match the frequencies of two-dimensional coherent turbulence structures (2DCS) in the mixing layer. This similarity indicates a relation between mixing layer instabilities and suppression of vertical flow separation, which is a valuable new insight into the process of understanding nonuniformity-induced flow attachment. The presence of the horizontally contracting and vertically alternating flow state likely increases scour dimensions. As a scour hole develops, unidirectional lateral near-bed flow velocities stay relatively high, maintaining scour potential. Scour potential is largest in and around the mixing layer. This most likely results in a strongly 3-D scour hole with maximum scour depths in the areas directly affected by the mixing layer. Scour hole development in previous experiments on scour in similar setups confirms this scouring behaviour. The premise that excessive scour is caused by an alternative flow state provides the opportunity to further improve and extend the abilities of empirical scour prediction.