This investigation focuses on the prediction of sediment transport and beach
evolution in coarse-grained beaches. This includes observed morphological
changes on both gravel and mixed beaches from experimental investigations at
the Large Wave Flume (GWK) in Hanover. Germany. The recorded measurements
show that the majority of morphology change took place adjacent to the zone of
wave-breaking, close to the shoreline in both cases. Based on these observations,
the discussions are carried out with psirticular regard to the observed tendency
for onshore transport axid profile steepening in the swash zone. The aim is to
identify the cross-shore hydrodynamics and sediment transport mechanisms
involved, to advance understanding of this type of beach and to improve our
qucintitative capabilities for predicting shoreline and morphological changes in
this zone.
With this in mind, this thesis includes a discussion of the physical processes
related to swash hydrodynamics and sediment transport. It also introduces the
description of the mathematical framework used to study wave hydrodynamics in
the swash zone. Emphasis is given to the Boussinesq equations which have been
found to be a suitable approach. For these equations an evaluation of the two
available shoreline boundary conditions is carried out and it is shown that the
moving shoreline accurately reproduces the velocity field in the swash zone.
The profile evolution investigation is carried out evaluating the transport rates
from a bed-load sediment transport formulation coupled with velocities
calculated from a set of Boussinesq equations [Lynett et al 2002). Then the
equation for conservation of sediment is solved to estimate the morphological
changes as proposed by (Rakha et al 1997). It is shown that such an approach is
useful to investigate the processes that control this evolution.
A discussion on the influence of bottom friction on the predicted profiles is
presented. Numerical results in both beaches show that the use of a higher
friction factor f during uprush improves the simulations of morphological
changes. However, the variation of friction by itself was not able to reproduce the
measured profiles. A plausible reason to explain this is that further mechanisms
other than friction play an important role in the overall response of coarsegrained
beaches.
For both beaches it is established that, if the efficiency factor (C) in the sediment
transport equation and bottom friction are kept the same in the uprush and
backwash, accurate representation of profile evolution is not possible. Indeed,
the features of the predicted profiles are reversed.
When the C parsimeter is set larger during the uprush than during the
backwash, the predicted profiles are closer to the observations. Differences
between the predicted profiles from setting non-identical C-values and friction
factors for the swash phase, are believed to be linked to both the infiltration
effects on the flow above the beachface, the bore collapse picking up sediment
from the bed, and the accelerated flow in the uprush. The discussion is made
with reference to main physical processes acting over the beachface for both the
mixed and gravel beach.
Date of Award | 2005 |
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Original language | English |
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Awarding Institution | |
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Concerning Swash On Steep Beaches
Acuna, A. P. (Author). 2005
Student thesis: PhD