Sediment transport model of the southeastern Mediterranean coast

Abstract

The relationship between: (a) the gradual change of the shoreline orientation in the southeastern Mediterranean from east—west to north—south, (b) the longest fetch direction of the Mediterranean (east—west), and (c) the dominant source of sediments being the Nile Delta to the west, provides a unique opportunity for construction of a model of the longshore sediment transport patterns. The input for this model consisted of: (a) two depth grids totaling 33,795 depths, and (b) nine selected wave conditions. The nine selected wave conditions were based on an analysis of the existing wave observer data, fetch considerations and storm records. The input conditions were waves from the northwest and west with wave periods of 5, 9 and 11 sec, southwest waves of limited fetch with wave periods of 5 and 9 sec, and 13-sec waves from 282.5° azimuth, potentially the largest fetch (2,400 km) direction. The 13-sec waves are both the longest waves theoretically possible and also the longest waves observed. The results show that the rate of longshore transport is larger for short-period (5 sec) than for long-period (9, 11 sec) waves, for all wave approach directions, given the same input wave heights. This is because of the larger shoreline angles of the short-period waves, due to lesser wave refraction. The results of these computations also illustrate the presence of four distinct zones of longshore sediment transport (from southwest to northeast): 1. (1) Western Zone (El Arish to Rafah), where all wave approach directions cause transport to the northeast, and therefore net transport equals gross transport. 2. (2) Central Zone (Rafah to Haifa), which is characterized by converging longshore transport nodal points that shift along the coast in response to changes in wave direction and to a lesser extent, wave period (i.e., north of the nodal point longshore transport is directed to the south, and south of the nodal point transport is directed to the north). For waves from 282.5°, the nodal point is at Netanya and for 315° at Rafah, the southern border of the central zone. 3. (3) Haifa Bay, a 20-km shoreline segment having significantly lower wave energy than the other coastal zones for all wave conditions except 5-sec waves from the northwest which cause southern transport. Even for southwest 9-sec waves, the sediment transport in this area is essentially zero due to wave refraction, and this area may be considered a sediment sink. 4. (4) Northern Zone (Akko to Rosh Haniqra) which is subject to intense wave refraction over the nearshore Akhziv Canyon, resulting in great variably of local longshore transport for most wave conditions. This model of longshore sediment transport of the southeastern Mediterranean coast explains many observed shoreline phenomena. For example, the lack of quartz sediment north of Akko, derived from the Nile River and Kurkar cliffs to the south, appears to be due to Haifa Bay which acts as a sediment sink for northward moving sand. The large differences in sediment transport between the Bardawil Lagoon (Western Zone) and Gaza (Central Zone), as evidenced by the differing sand accumulations on the southern sides of coastal constructions in these two zones, is attributed to the increasing importance of waves from the northwest in causing southwestern transport along the coast northeast of Rafah. The lack of strong indicators of transport direction (such as sediment accumulations differentially on one side of the structures) along the coast of Tel-Aviv and to the north is due to the continuously changing directions of longshore transport with changing wave conditions in the Central Zone, and to intense refraction in the Northern Zone.