Short and medium-term evolution of a coastal sector in Cadiz, SW Spain

Abstract

The present paper is a combined study on the medium and short-term evolution of the littoral between Chipiona and Rota (SW Spain). The analysis of coastal evolution over a period of 24 years (i.e. medium-term), was carried out using six, well temporally spaced, photogrammetric flights. Cliff top and dune toe were used as shoreline indicators to solve problems related to the use of watermark in tidal environments. Short-term littoral variations were monitored monthly by means of an electronic theodolite over a period of 2 years. The width variations of the dry beach were surveyed by always using a constant, average value for the mean sea level position derived from the numerous surveys carried out. The obtained data was representative of both seasonal and annual variations.

Over medium-term, most of the littoral recorded erosion, while accretion has been recorded over recent years in the southern part. Coastal erosion was related to the impact of several storms and dune accretion was linked to the action of eastern winds.

Over the short-term time frame, about the same percentage of coastal erosion and accretion was recorded. Most of the important accretion trends were observed at the central and southern parts of the littoral, the largest amount of erosion being recorded in the southern end of the littoral, in a nourished beach. All the studied beaches (except Punta Candor dunes) showed more short-term changes than medium-term ones, because of different factors, essentially the great seasonal variability of the studied beaches.

The partial discrepancy between the medium and the short-term trends was related to the applied methodology, which demonstrates the problem of comparing different coastal features (dunes and cliffs on one hand, and beaches on the other) and to the implications of 2–3 years of lasting fair weather conditions that favoured beach and dune accretion, within an historical retreat tendency related to storm actions.

Introduction

Shoreline position fluctuates in a variety of time scales, and this behaviour introduces many difficulties when reconstructing medium-term coastal trends (Carter, 1988, Crowell et al., 1993, Jiménez et al., 1997, Short, 1999). Variability in coastline position may be the response to a single factor or a combination of them (Bird, 1993, Forbes et al., 2004). Principal causes of coastal erosion or accretion are individual large storm events (Stone et al., 1996, Stone et al., 1997, Donnelly et al., 2001), seasonal variability in wave energy and/or circulation in the nearshore zone (Sonu and James, 1973, Masselink and Pattiaratchi, 2001), multiyear to decadal-scale variations in storminess, wave energy and coastal morphodynamics (Thom and Hall, 1991, Forbes et al., 1997, Zhang et al., 1997, Shand et al., 2001) and long-term variations in the relationship between climate and sediment supply (Orford et al., 2001, Orford et al., 2002).

Coastal changes are surveyed using a wide variety of methods and data sets according to the study time spans (Smith and Zarillo, 1990, Dolan et al., 1991, Crowell et al., 1991, Crowell et al., 1993, Jiménez and Sánchez-Arcilla, 1993, Gorman et al., 1998, Jiménez et al., 1997, El-Asmar, 2002). Studies on short-term shoreline dynamics are usually carried out at small spatial scales, during a time span of less than 10 years (Crowell et al., 1993). The most common technique used is beach topographical profiling or 3D survey, repeated at regular intervals, in order to measure daily to annual variations in shoreline position and beach volume (Morton, 1979, Carter, 1988, Komar, 1998, Corbau et al., 1999, Short, 1999).

Vertical aerial photographs, satellite images, maps and charts are very useful tools for the reconstruction of coastline changes at long (> 60 years) and medium (between 60 and 10 years) temporal scales and spatial scales (Crowell et al., 1993) and, further, they display coastal type distribution, land uses and dune field evolution (Taney, 1961, Dolan et al., 1980, Leatherman, 1983, McBride et al., 1991, Fisher and Overton, 1994, Jiménez et al., 1997, Forbes et al., 2004). The precision and accuracy of aerial photogrammetric measurements depend on their own characteristics (Dolan et al., 1980, Crowell et al., 1991, Andres and Byrnes, 1991, Moore, 2000) and on the difficulties of locating shoreline position, typically taken as the high water line (especially in microtidal environments, Dolan et al., 1979, Dolan et al., 1980, Leatherman, 1983, Pajak and Leatherman, 2002), or identified in mesotidal environments as the seaward vegetation limit, dune foot or cliff top (Crowell et al., 1993, Fisher and Overton, 1994).

The prediction of the future coastline trend at short and medium terms (years–decades) must be based on the study of coastal changes which have occurred in the recent past, taking into account a comparable time scale. However, this approach requires the combination of data obtained from the two main methods cited above. Morton (1979) analysed coastal evolution at geological and recent time scales. The former was related to natural processes and the latter was principally linked to human activities. Smith and Zarillo (1990) studied medium and short-term beach variations, by using two vertical aerial photographs to reconstruct the medium-term coastline trend, and the Emery (1961) methodology of beach profiling to measure short-term shoreline position changes over a period of 13 months. They concluded that short-term changes in shoreline position can be quite important and such coastline variations may be the single largest source of error in quantitative calculations of medium and long-term shoreline position changes.

In Spain, Jiménez and Sánchez-Arcilla (1993) and Jiménez et al. (1997) analysed the Ebro delta evolution. Jiménez and Sánchez-Arcilla (1993) obtained similar trends using shoreline position data from three sets of aerial photographs, covering a period of 32 years, and from beach profiling techniques, carried out with a seasonal periodicity over 4 years. Jiménez et al. (1997) studied the short-term evolution (over 2 years) of the Ebro delta, with seven sets of aerial photos with a time span of four months, and beach profiling techniques. They obtained a perfect agreement from a qualitative point of view, i.e. erosion and accretion zones always coincided, but recorded small differences in measured values, especially in very flat coastal sectors.

The quantitative comparison of data obtained from these different techniques requires: i) the availability of data which is representative enough for the studied coast; ii) to measure variables and parameters that represent similar natural processes acting at different time scales, and iii) the reduction of all the data and parameters to a common, comparative, time scale. The results obtained by this general procedure can then be used for establishing predictions of short and medium-term coastline trends, provided that no human intervention will alter the prevailing natural coastal processes in the near future.

The current work presents the results of a combined study on the short and medium-term evolution of the littoral between Chipiona and Rota (SW Spain). It is an interesting sector showing a variety of coastal environments distributed along a quite homogeneously oriented coast. The littoral is not greatly affected by human structures, which consist only of two small groynes at either end of the studied littoral, some rip-rap revetments and seawalls and a small number of beach nourishment works.

In this study, short-term variations have been monitored with a monthly periodicity during a period of 2 years, the obtained data being representative of both seasonal and annual variations. The analysis of coastal evolution for a period of 24 years (medium-term trend, according to Crowell et al., 1993), was carried out using several, well temporally spaced, photogrammetric flights.