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Title: Numerical and experimental study of bore-driven swash on impermeable slopes
Author: Hondebrink, Luke Johan
ISNI:       0000 0001 3581 1186
Awarding Body: University of Aberdeen
Current Institution: University of Aberdeen
Date of Award: 2006
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Many experimental and field studies of swash have been conducted but swash hydrodynamics are still poorly understood. The aim of the research presented here is to improve understanding of swash hydrodynamics through large-scale experiments and numerical modelling. The focus has been on bore-driven swash on impermeable beach slopes. A swash numerical model has been successfully established to simulate bore-driven swash events on impermeable beach slopes. The model is 1-D and solves the Depth-Integrated, Reynolds Averaged Navier Stokes' equations using a 'shock-capturing' Weighted Average Flux scheme. A critical aspect in solving the equations is the approach used to parameterize stress terms in the momentum equation. The model uses a quadratic velocity parameterization (r xu2) with friction factor (/) appropriately defined. Detailed measurements of swash water depths, velocities and turbulence characteristics have been made for two swash events using a 'large-scale' swash rig capable of generating a single, repeatable, very large swash event. The two swash events differ only in beach roughness: a 'smooth' beach made of Perspex and a rough beach made of Perspex with a pebble (d50 = 5.7mm) layer glued on. The incoming bore of height 0.23m and velocity 3.2m/s creates swash events of about 7s duration with a run-up of 5.7m and 4.7m on the smooth and rough beach respectively. Results from these novel experiments are presented and show that bed-parallel velocities are depth-uniform on the smooth beach but have significant flow structure on the rough beach. The presence of a logarithmic layer in the depth profiles of velocity allows for bed shear stresses to be obtained. Bed shear stresses in the swash zone were seen to vary significantly both in space and time. Numerical model results are compared against experimental results. Two versions of the swash numerical model were used: one with a constant friction factor and the other with a friction factor that varies with local Reynolds number and relative roughness. The results show that when the friction factor or equivalent wall roughness is tuned to give the correct run-up, the water depth and depth-averaged velocity predicted by the model are in good overall agreement with the measured results. Further insight into the detailed swash processes is achieved by analysing the individual terms contributing to the Depth-Integrated, Reynolds Averaged Navier Stokes' momentum equation. This analysis was applied to the numerical model and experimental results. The analysis shows that large amounts of momentum, associated with bore arrival, are supplied during the initial stages of uprush. During uprush, momentum extracted due to shear stress is relatively small whilst the extraction of momentum due to the weight of the fluid acting down the beach is large. Overall this results in an uprush that is dominated by gravity effects. Hydrodynamics during backwash are different. Gravity effects during backwash act to supply momentum to the flow. The effect of bed shear stress on reducing momentum becomes more significant with time. The result is that during backwash, momentum extracted via bed shear stress becomes equivalent in magnitude to that supplied by gravity.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available