Development of a morphodynamic indicator for sub-regional integrated coastal area management

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The rapid development of infrastructure to cope with the expansion of the tourist industry along the coasts of the Mediterranean Sea has introduced numerous instabilities on the natural environment that are reflected by a long history of coastal planning and management efforts. The development of indicators has been established as a powerful approximation to characterizing certain measures of coastal state. Using the power and relative accuracy that wave propagation models have proved on non quantitative applications, a method is used in this paper that incorporates high resolution bathymetries and synthetic wave fields to produce characteristic energy settings in the nearshore region. The directional component of the wave forces are analyzed and a "cell" system depicted following basic gradients that should drive wave induced currents (and associated sediment transport) from a high momentum transfer area to a low momentum transfer area, corresponding to potential erosion and deposition events. Following this simple conceptual model, residual cells can be drawn and thus a cartographic version of potential erosion-deposition sites developed. This analysis provides repeatable and measurable ranked geographic data yielding an indicator of great potential use for decision makers and agencies monitoring coastal evolution. The analysis is performed along the eastern portion of the Costa del Sol in Andalusia, southern Spain, which environment is dominated by narrow beaches and occasional river deltas that influence sediment input as well as wave refraction and nearshore processes. The final maps, with the residual cells marked indicating points of potential erosion and deposition, has been presented and utilized as an aid to improve integrated coastal management by the Andalusian regional government allowing inventory and characterization for future reference.

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The morphosedimentary maintenance of a 225 km stretch of coast along the northeast Gulf of Mexico, from Grayton Beach, Florida, to Morgan Point, Alabama, has been interpreted previously within the framework of a unidirectional, integrated, monotonic longshore drift model, with a single headland source of sediment located east of Grayton Beach. Net longshore transport and the granulometry and composition of some 2000 foredune, beach and step samples indicate a cellular net drift system, supplied by three independent sources of sediment. One source is provided by the Pleistocene barrier island complex along Grayton-Mirimar Beach, the second at Pensacola Beach on Santa Rosa Island, and the third is onshore transport across the inner shelf between Pensacola, Florida, and Morgan Point, Alabama. The Pleistocene “headland” and Pensacola Beach supply two cells along Santa Rosa Island, whereas onshore transport from the low gradient inner shelf supplies sediment to three cells along the largely accretional beach-ridge-dominated coast from Pensacola Pass to Morgan Point. Drift cells along this coast experience negligible net sediment exchange. These findings have significant implications for both the late Holocene evolution and the morphodynamic maintenance of this coast.
The littoral-drift regime along the 230 km stretch of coast between Jacksonville Beach and Cape Canaveral, Florida, was investigated by beach- and dune-sand analyses and by computer simulation (WAVENRG program) of wave-generated longshore currents. The results of these two independent techniques contradict the classic view of a net littoral drift moving from north to south. At least six distinct coastal segments can be identified from the sediment textural and compositional data and the WAVENRG simulation of littoral currents. Each segment contains drift cells that are directed northward and southward. Adjacent segments experience minimal net sand exchange. These littoral-drift characteristics along the northeast Florida coast indicate a shoreline configuration that is well adjusted to the present ambient wave climate.
By using known results on the radiation stress associated with gravity waves, the total lateral thrust exerted by incoming waves on the beach and in the nearshore zone is rigorously shown to equal (E0/4) sin 2θ0 per unit distance parallel to the coastline, where E0 denotes the energy density of the waves in deep water and θ0 denotes the waves' angle of incidence. The local stress exerted on the surf zone in steady conditions is shown to be given by (D/c) sin θ per unit area, where D is the local rate of energy dissipation and c is the phase velocity. These relations are independent of the manner of the energy dissipation, but, because breaker height is related to local depth in shallow water, it is argued that ordinarily most of the dissipation is due to wave breaking, not to bottom friction. Under these conditions the local mean longshore stress in the surf zone will be given by (5/4)ρumax2s sin θ, where ρ is the density, umax is the maximum orbital velocity in the waves, s is the local beach slope, and θ is the angle of incidence. It is further shown that, if the friction coefficient C on the bottom is assumed constant and if horizontal mixing is neglected, the mean longshore component of velocity is given by (5π/8)(s/C) umax sin θ. This value is proportional to the longshore component of the orbital velocity. When the horizontal mixing is taken into account, the longshore currents observed in field observations and laboratory experiments are consistent with a friction coefficient of about 0.010.
The radiation stresses in water waves play an important role in a variety of oceanographic phenomena, for example in the change in mean sea level due to storm waves (wave “set-up”); the generation of “surf-beats”; the interaction of waves with steady currents; and the steepening of short gravity waves on the crests of longer waves. In previous papers these effects have been discussed rigorously by detailed perturbation analysis. In the present paper a simplified exposition is given of the radiation stresses and some of their consequencies. Physical reasoning, though less rigorous, is used wherever possible. The influence of capillarity on the radiation stresses is fully described for the first time.
A numerical model for the hindcasting of waves in shallow-water (hiswa) is described and comparisons are made between observations and model results in a realistic field situation. The model is based on a Eulerian presentation of the spectral action balance of the waves rather than on the more conventional (at least in coastal engineering) Lagrangian presentation. Wave propagation is correspondingly computed on a grid rather than along rays. The model accounts for refractive propagation of short-crested waves over arbitrary bottom topography and current fields. The effects of wave growth and dissipation due to wind generation, bottom dissipation and wave breaking (in deep and shallow water) are represented as source terms in the action balance equation. The computational efficiency of the model is enhanced by two simplifications of the basic balance equation. The first one is the removal of time as an independent variable to obtain a stationary model. This is justified by the relatively short travel time of waves in coastal regions. The second simplification is the parameterization of the basic balance equation in terms of a mean frequency and a frequency-integrated action density, both as function of the spectral wave direction. The discrete spectral representation of wave directionality is thus retained. An untuned version of hiswa has been tested in a closed branch of the Rhine estuary where measurements with buoys and a wave gauge are available. In this situation, where wave breaking and short-crestedness dominate, rms-errors in the significant wave height and mean wave period are about 10 and 13% respectively of the observed values.