Morphologic properties derived from a simple cross‐shore sediment transport model

Abstract

Abstract. This paper builds on the now classical discussions by Bowen [1980] and Bailard [1981] on the applicability and implications of Bagnold's [1963] sediment transport model to nearshore profile modeling. We focus on the morphologic implications of both the strengths and weaknesses of Bagnold's model, isolating the transport terms that are well predicted (i.e., mean flow terms) from those that are not well predicted (i.e., transport due to correlations between flow and sediment load). We factor Bagnold's model into a dimensional transport magnitude and a nondimensional term. The nondimensional term describes the relative importance of transport due to undertow, gravity, and correlations between flow and sediment load. The transport magnitude largely determines the response time of nearshore profiles. For typical nearshore environments this response time was estimated to vary as a function of incident rms wave height (Hrms) from ----500 years (Hrm s "• 0.5 m) to 2 years (Hrm s '-" 3 m). The relative importance of competing transport mechanisms is shown to depend strongly on the relative wave height (defined as the ratio of the rms wave height to the local depth). Simplified nearshore transport parameterizations that are a function of this variable were derived and were interrogated for the existence and form of equilibrium profiles. Several differences from previously computed equilibrium profiles were noted. First, because the relative wave height saturates in natural surf zones, equilibrium profiles converge to a relatively flat profile near the shoreline. Second, under some situations a seaward sloping equilibrium profile may not exist. Third, the long response times combined with unknown stability of an equilibrium profile make it difficult to assess the physical connection between theoretical equilibrium profiles and profiles observed in nature. IntroductionAt present, accurate prediction of nearshore bathymetric change at all relevant scales is impossible. Part of the difficulty is that the relevant scales span a very broad range, from millimeters (individual sand grains) to kilometers (the cross-shore width of the surfzone) and tens of kilometers (alongshore extent of littoral cells). The largest spatial scales are particularly important because they contain the majority of the spatial and temporal variability of nearshore bathymetric change [Lippmann and Holman, 1990; Plant et al., 1999]. These are also the spatial and temporal scales that characterize human interactions with the coast. Unfortunately, the difficulty in modeling and prediction is acute at the largest scales, since evolution at this scale requires the integration over all smaller scales [Roelvink and BrOker, 1993].Ideally, the interaction between the large-scale morphology (e.g., surf zone sandbars or the cross-shore profile as a whole) and small-scale processes (e.g., wave-driven hydrodynamics and sediment transport) can be described in terms of param- The advantage of a purely large-scale model is its transparency, which allows di...

Influence of Sediment Transport Formulation on the Linear Stability of Planar and Barred Coasts

“…This result confirms the validity of the implicit assumption often made in linear stability analyses that the 2DV morphological development of coastal systems is typically slower than the 2DH or 3D morphology, which is studied in this paper. This assumption is also validated by observations, see Ruessink et al (2000) and Plant et al (2001).…”

Self-organization mechanisms for the formation of nearshore crescentic and transverse sand bars

“…Even though this simplified profile does not correspond exactly with any of the equilibrium profiles found in field observations or used in modelling (Dean 1991;Plant, Ruessink & Wijnberg 2000), it shares with them the main qualitative characteristic which is its concavity. In view of the fact that the investigation of equilibrium beach profiles is still under way (Plant et al 2000) and that we are interested in alongshore non-uniform features, this seems a reasonable choice. The vertical wall at the shoreline is introduced in order to avoid the complications of a moving shoreline and the dynamics of the swash zone that are expected to have little effect on the surf zone bars.…”

Recurrent neural network modeling of nearshore sandbar behavior

“…6 that the effect of memory on the cross-shore migration of the outer Gold Coast bar is negligible may advocate the use of MLPs for our future work on sandbar behavior. However, whether this conclusion also holds for the cross-shore migration of the inner bar (which is sheltered from the offshore waves by wave breaking on the outer bar and thus may respond differently to offshore wave forcing than the outer bar (Plant et al, 2001;Southgate & Möller, 2000)) and for the evolution of the abovementioned crescentic patterns remains to be investigated.…”

“…An example of modeling such temporal dependencies can be found in Wright, May, Short, and Green (1985), who use hydrodynamic forcings weighted over several days to provide for a measure of relaxation time in predicting the evolution of sandbar variability. Based on cross-shore sediment transport modeling, Plant, Ruessink, and Wijnberg (2001) also suggested that the response time of beach profiles is exceedingly long compared to the timescale of the variability of the offshore wave forcing. O'Hare and Huntley (2006) later on debated this suggestion, indicating that, while it may be true for quiescent wave conditions, the morphological timescale under storm conditions might be of the same order as the nearshore hydrodynamics.…”