The development and testing of a shear cell for the purpose of measuring swash zone bed shear stress is presented. Direct measurements of bed shear stress were subsequently obtained using the shear plate in small, medium, and large-scale laboratory facilities. Measurements from both dam- break and bore-driven swash experiments are considered, covering a wide range of hydrodynamics and bed roughness. The dam-break problem is of interest here due to the theoretical analogy with the run-up of a solitary bore on a beach. Estimates of the flow velocities through the full swash cycle were obtained through numerical modelling and verified against measured velocity data. In conjunction, these data are used to calculate skin friction coefficients. The measurements indicate strong temporal and spatial variation in bed shear stress throughout the swash cycle, and a clear distinction between the uprush and backwash phase. For a single swash event, the maximum uprush bed shear stresses occur in the lower swash zone, within the range 0<x/Rx<0.3. The maximum backwash bed shear stresses also occur in the lower swash zone, and extend seaward of the initial bore collapse location. For a given cross-shore location the peak uprush bed shear stress is typically greater than the peak backwash bed shear stress by at least a factor two and up to a factor four. Local skin friction coefficients also indicate strong temporal and spatial variation. Furthermore, the behaviour of the local skin friction coefficient (back calculated from the measured bed shear stress using predicted, depth-averaged, flow velocities) over the swash cycle is inconsistent with the classical behaviour that is expected on the basis of the low Reynolds number flow. Smooth bed dam break and swash uprush friction coefficients appear to follow the general behaviour observed for smooth, turbulent open channel flow for an increasing Reynolds number. However, for a decreasing Reynolds number the behaviour of Cf differs from the steady flow relation. This is attributed the unsteady swash flow regime and flow history effects. It is expected that differences in flow history between the uprush and backwash have implications in terms of swash boundary layer growth and the resulting bed shear stress. A Lagrangian model for the swash boundary layer development is presented to consider these flow history effects. The model is based on the momentum integral approach for steady, turbulent, flat-plate boundary layers, with appropriate modifications to account for the unsteady flow regime. Fluid particle trajectories and velocity are computed and the boundary layer growth across the entire swash zone is estimated. Predictions of the bed shear stress agree well with the direct bed shear stress measurements and show a bias toward uprush sediment transport which has consistently been observed in measurements.
| Identifer | oai:union.ndltd.org:ADTP/254239 |
| Creators | Matthew BARNES |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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