Hammeken Arana, AM; (2017) Wave run-up on beaches and coastal structures. Doctoral thesis , UCL (University College London).

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Abstract

Wave run-up is an important design criterion for coastal structures and beach nourishment projects. Coastal engineers commonly use empirical formulae to predict this parameter. These formulae generally include the effect of berms, roughness and angle of wave attack, but neglect the influence of parameters such as hydraulic conductivity and beach groundwater levels. This thesis presents a laboratory and numerical study aimed to improve the predictive capability of existing formulae as well as to enhance our understanding of the swash hydrodynamics and their interaction with permeable beaches. In particular, it investigates the influence of hydraulic conductivity, roughness and beach groundwater on wave run-up and swash flows. Most of the data presented in this study were obtained from wave flume experiments performed on smooth-impermeable, rough-impermeable and rough-permeable slopes. The influence of hydraulic conductivity on swash hydrodynamics was quantified by means of a novel experimental setup consisting of non-deformable permeable structures, in which the influence of the surface roughness was isolated. A procedure based on the development of time-stack images provided accurate measurement of run-up and swash depths, while pressure transducers were used to measure the water table elevations inside the permeable structures. Laser Doppler velocimetry, a technique that does not disturb the flow, was used to measure the velocity profile of the uprush and backwash flows. In addition to the laboratory experiments, simulations using a Volume-Averaged Reynolds-Averaged Navier-Stokes (VARANS) model, validated against experimental results, were used to investigate the influence of hydraulic conductivity on the near-bed flow velocities and to obtain larger datasets of run-up on impermeable slopes. Analysis indicated that existing formulae adequately predict run-up from breaking waves on impermeable slopes. However, no previous formulae gave reliable predictions of run-up from non-breaking waves. Therefore, new empirical formulae were derived for non-breaking waves on impermeable slopes. These give good predictions when compared with the present data and data available in literature. The beach groundwater levels were found to have negligible influence on wave run-up. In contrast, hydraulic conductivity was shown to have a significant effect on wave-structure interaction parameters such as wave run-up, wave-induced water table elevation, swash depths, and swash flow velocities. As a result, new prediction formulae for breaking and non-breaking waves on permeable slopes were developed; these formulae include the influence of surface roughness and hydraulic conductivity through a new non-dimensional parameter. Moreover, flow velocity measurements in the swash zone showed that infiltration enhances onshore flow and time asymmetries. This is expected to promote onshore sediment transport inside the swash zone. The near-bed velocity measurements were also used to estimate bed shear stresses using the log-law method. The results showed that infiltration directly increases the bed shear stresses during the uprush phase, mainly due to the change in the boundary layer thickness. However, infiltration was also shown to indirectly reduce the bed shear stresses during the backwash phase by significantly reducing the backwash flow depths and velocities (continuity effect). Video observations of the breaking processes showed that hydraulic conductivity alters the shape of waves breaking on the slope. However, the change in shape is small and in all cases, the breaker type remained the same. Hydraulic conductivity was also shown to decrease the breaking point distance of plunging waves. The video analysis was also used to validate a new criterion presented in this study to determine whether or not waves will break on the slope; this criterion was shown to give better predictions of the transition between breaking and non-breaking waves than existing breaking criteria. This is one of the first studies to include the influence of hydraulic conductivity on run-up prediction formulae. If the porosity or hydraulic conductivity of a coastal structure or beach is known, these formulae in combination with the reduction factors suggested by EurOtop (2007) can lead to more accurate predictions of wave run-up and wave overtopping on permeable slopes. The improved understanding of the influence of hydraulic conductivity on the wave-induced water table elevation and on the swash hydrodynamic processes will benefit the modelling and management of coastal aquifers as well as the prediction of sediment transport in the swash zone.

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