F.J. Gracia

Empirical model of morphodynamic beachface behaviour for low-energy mesotidal environments

Abstract Four years of monthly monitoring were carried out on a South Atlantic beach in Spain, in a low-energy mesotidal environment where beaches change slowly, from reflective to dissipative states, following a typical seasonal behaviour. In this work a two-dimensional study of beach morphological variations has been made, related to the incident wave variations. After applying several morphodynamic parameters to the field data, poor or null representative results were obtained. It was then necessary to design other type of characterisation of the incident wave energy. In this sense, the intertidal normalised beach slope was compared with the erosive potential of the incident waves, expressed as a combination of the dimensionless grain fall velocity parameter ( Ω ) and the energy density of waves. Median grain size did not vary significantly during the surveys. For this reason, a new parameter, named wave erosivity factor , was introduced by considering the fall velocity of grains as a constant in the Ω parameter. The resulting ratio between normalised beach slope and wave erosivity expresses the equilibrium state of the beach for any given energy level. The departure from the equilibrium curve is largest in the intermediate situations, while at the extremes the points are better adjusted to asymptotic tendencies towards equilibrium: on reflective states, small increases in the wave erosivity will produce important beach changes; on dissipative beaches, important increases in the wave erosivity will not produce significant morphological modifications. The resulting equilibrium curve is presented as a function of the natural range of morphological variation of this beach.

Morphodynamic Characteristics and Short-Term Evolution of a Coastal Sector in SW Spain: Implications for Coastal Erosion Management

Abstract In SW Spain, numerous tourist beaches have been nourished over recent years to counteract coastal retreat. Nourishment projects have not produced great endurance, mainly because of the lack of basic information on coastal dynamics and longshore sediment transport paths. In order to cover these aspects in a sandy coastal sector south of the Guadalquivir River mouth, a beach monitoring program was carried out between 1996 and 1998, with a secondary program between 2000 and 2002. Four main beach types were identified, each one with different volumetric trends during the monitoring periods. Most beaches recorded accretion because of the prevalence of fair weather conditions, which emphasized differences among beaches. In a broad sense, beaches located on intertidal rock platforms recorded erosion or no change. Those located updrift of rock platforms or human-made constructions (groins) recorded accretion. These structures divide the coast into several littoral cells, which control longshore sediment transport between beaches. All this information is very useful for planning replenishment projects on eroding beaches. The distribution of cells, the sediment transport paths associated with them, and the volumetric trends obtained for every beach type were used for evaluating nourishment procedures, sand volume needs, and expected life time of the resulting artificial beaches.

Utility of Morphodynamic Characterisation in the Prediction of Beach Damage by Storms

ABSTRACT This work deals with the estimation of beach susceptibility to storminess in the Bay of Cadiz (SW Spain), and the concomitant environmental implications. For this purpose a comparison was made between the natural behaviour of beaches and the type and amount of coastal damage made by energetic waves in every beach type. Knowledge on the morphodynamic behaviour of beaches was obtained from a monthly topographic monitoring of 32 beach transepts normal to the coastline, performed during 30 consecutive months. Widely used morphodynamic parameters, like the Surf Similarity and the Surf Scaling parameters, were applied to the data, resulting in a general morphodynamic characterization of beaches, represented in a map of beach type distribution. Maximum coastal damage by storms was estimated by visual observations. Clear relationships between morphodynamic beach trends and amount of coastal damage have been obtained, and are presented in the form of a Beach Susceptibility Matrix. The matrix permits predictions of expected coastal damage associated with storms in other nearby beaches by means of a simple beach monitoring programme.

Morphodynamic responses of nourished beaches in SW Spain

Coastal erosion in SW Spain is affecting man-made structures and beaches that represent an important economic resource in the area. In the last decade the Spanish government carried out several nourishment works that have limited durability. Most of the artificial beach fills consist of a spill of natural dredged sand on the visible beach, leading to a flat, artificial berm with an important seaward slope and narrow foreshore. As a result, the initial dissipative profile was transformed into a fully reflective one. A beach monitoring program was carried out to record morphological evolution after the nourishment works. Several field assessments of disturbance depth were also made to characterize beach morphodynamics of a nourished beach (Rota) and a natural dissipative one (Tres Piedras), whose slope was similar to the pre-nourishment gradient of Rota beach. Natural dissipative beaches were characterized by spilling breakers that did not significantly affect bottom sand. The severe erosion recorded in the nourished zones was related to the new morphodynamic regime acting on these beaches, which was controlled by high erosive plunging breakers associated with high foreshore slopes. In conclusion, other nourishment practices should be used, better adapted to the natural beach morphodynamics of the zone, taking also into account the original grain size and density of the beach sands, in order to obtain more durable artificial beaches.

Long-shore distribution of morphodynamic beach states along an apparently homogeneous coast in SW Spain

We present a morphodynamic study of an apparently homogeneous rectilinear coast in SW Spain. The study area covers 14 km of mesotidal sandy beaches, interrupted in some places by rocky-shore platforms. The method used consisted of a monthly monitoring of 12 beach profiles during two years. According to the results obtained from the study, which also include granulometric analyses andin situ determination of the beach disturbance depth, three main beach classes have been differentiated: low-reflective beaches, dissipative beaches and rocky-shore platform beaches. Their longitudinal distribution is not linked to their distance to the main source of sediments in the area (mouth of the river Guadalquivir). Instead, a very irregular long-shore variation of morphodynamic beach states appears. It is deduced that this long-shore variation is mainly linked to local contouring conditions (e.g. the presence of rocky shoals which affect wave-breaking processes), and not to the regional long-shore currents prevailing in the zone.

Anthropogenic Influence on Spit Dynamics at Various Timescales: Case Study in the Bay of Cadiz (Spain)

Human interventions are one of the main drivers of coastal change in many areas, often generating undesired impacts like shoreline retreat. Sandspits are especially sensitive to anthropogenically-induced changes, especially those related to sediment supply. This work presents a case study of Valdelagrana spit in SW Spain, a sandbody where anthropogenic influence has been evident since Roman times. A variety of methods were applied to assess geomorphological and morphodynamic changes in the area at various timescales. Historical interventions involve mainly river course diversion, which caused important changes in sediment supply. More recently, coastal engineering structures and land reclamation deeply modified wave and current patterns in the area, triggering massive coastal erosion. As a consequence of this, the system has evolved from a drift-aligned spit to a swash-aligned barrier. This study provides insights into the consequences of human interventions at similar coastal settings.

A novelty coastal susceptibility assessment method: application to Valdelagrana area (SW Spain)

The main aim of this paper is to present a new methodology to determine coastal susceptibility to erosion and flooding processes by means of an index-based method. The proposed indices take into account physical parameters, such as dune and beach geomorphologic characteristics, shoreline evolution, local significant wave height and relative run-up. The coastal susceptibility has been estimated by elaboration of spatial input data into a GIS environment. The method has been tested in Valdelagrana area, a sandy spit located in SW Spain. The spit includes several morpho-sedimentary environments: sandy beach, discontinuous embryo dunes and foredunes, mud flats and wide areas of vegetated salt marshes. The Northernmost sector is densely urbanized whilst the rest is part of a natural protected area belonging to the Bahía de Cádiz Natural Park. The results obtained showed how the Southern part of the spit presented a high susceptibility due to an elevated erosion rate and the presence of low and discontinuous dune ridges.