Nearshore hydrodynamics and sediment transport processes along the sandy coast of Boumerdes, Algeria

Abstract

The Boumerdes beach in the center of Zemmouri Bay, the northern part of the Algerian Mediterranean coast, is exposed to very energetic waves, inducing large sediment transport rates and shoreline changes. Estimation of sediment transport (ST) along the Boumerdes coast is challenging as the sediment transport pattern varies both spatially and temporally depending on the hydrodynamic agents, morphological factors, and presence of hard structures etc. This area is defined as being a dynamic sedimentary cell with a west-east-oriented shoreline and protected by hard coastal protection; these sites are experiencing degradation and accretion that continuously disrupt their hydro-sedimentary balance. This study aims to propose a methodology for the evaluation and quantification of the sediment transit along Boumerdes coast on an instantaneous to annual scale. This study is divided in two parts: (1) a short-term study, based on field measurements carried out during low to moderate energy swell conditions with Krauss sediment traps, and permitting to validate both empirical longshore sediment transport formulas (CERC 1984; Kamphuis in J Waterw Port Coast Ocean Eng 117(6):624–640, 1991, the modified Kamphuis 2013 and Van Rijn 2014) and a hydro-sedimentary numerical model and a wave propagation model, and (2) a long-term study, combining global optimization methods using offshore wave data provided from Infoplaza database and wave numerical modeling and SWAN (simulating waves nearshore) model: outputs to define the nearshore wave climate and then to compute the annual longshore transport. Finally, to better understand the longshore dynamic process on a larger scale, the Mike 21/3 FM coupled model simulations were performed for the swell states corresponding to the field conditions. Estimated annual gross longshore sediment transport rate (LSTR) for the study area is 22.02, 4, 3.5, and 11.3 × 105 m3/year based on the CERC (1984), Kamphuis in J Waterw Port Coast Ocean Eng 117(6):624–640 (1991), the Modified Kamphuis by Mil-Homens et al. (2013), and Van Rijn (2014). The LSTR estimate based on the modified Kamphuis formula (2013) was found to be a reliable estimate for the study region because of its realistic estimation, and an RMS (root mean square) error equal to 0.41 and which also includes the wave period, beach slope, and sediment grain size.