Comments on “An approximation to short-term evolution and sediment transport pathways along the littoral of Cadiz Bay (SW Spain)” by Anfuso and et al. (Environ Geol 56:69–79)

Abstract

Anfuso et al. (2008) have presented a very extensive monitoring program which was conducted in part of the littoral of Cadiz (SW Spain). Topographic profiles were determined monthly from March 1996 to May 1998 inclusive at 24 points along a 28-km stretch of coast. Although the methodology presented is quite interesting, some of its aspects should be highlighted to avoid misunderstandings about its range of validity. Anfuso et al. stated in their introduction that ‘‘the obtained results allowed the characterization of the seasonal changes and morphodynamic behavior of the coast and enabled to address the evolutionary trend.’’ Nevertheless, according to Munoz-Perez and Medina (2006), a monthly monitoring frequency is not enough to detect changes occurring in a shorter term (e.g., during and after a storm). To filter these short-term changes, the authors followed the approach of Jimenez and Sanchez-Arcilla (1993) and used the widely used least-squares linear regression method. In this type of analysis, the value of the correlation coefficient shows the quality of the linear relationship between volume changes and time. Moreover, as the authors stated, extremely low correlation coefficients were observed for most of the profiles, which points out the high degree of variability of these beaches. Therefore, when Anfuso et al. claim that the rate of change for the beach in this region is usually slow, it could be pointed out that this assumption is true only for accretion periods. For instance, Victoria Beach, a nearby beach with similar sediment and coastline orientation, does not experience a remarkable regression during the winter except during storm events, when the sand volume removed may reach 68 m/m in just a few hours (Munoz-Perez and Medina 2006). Furthermore, the accretion period lasts approximately 3 or 4 months, from May to August, with an approximate rate of 1 m/m per day. The use of empirical orthogonal function (EOF) analysis also makes it possible to filter out changes (which are not at all small, involving elevations up to 0.60 m at the foreshore) related to fortnightly tidal variations, which could lead to erroneous interpretations when analyzing only the wave energy input (Munoz-Perez and Medina 2000). Profiles studied by Jimenez and Sanchez-Arcilla (1993) extended to the closure depth (approximately -6.5 m, referred to the Lowest Low Water Level), while the profiles surveyed by the authors are only topographic and therefore extend only to the LLWL. Moreover, the results were compared with trends observed in the past and with alongshore transport rates predicted by well-known transport formulae proposed by the Bailard (1981) and CERC (e.g., USACE 2002) models. Another improvement could have been achieved by a comparison with shoreline position data obtained from aerial photographs, as in the study performed by Munoz-Perez and Enriquez (1998) with pictures from 1946 to the present. The authors also assert that ‘‘the establishment of coastal trends is sometimes very difficult as shoreline position fluctuates in a variety of time scales because of seasonal J. J. Munoz-Perez (&) Applied Physics Department, CASEM, University of Cadiz, 11510 Puerto Real, Spain e-mail: juanjose.munoz@uca.es