longshore currents simulation using mike 21 numerical model case study: kiashahr zone

introduction: determination of waves and the pattern of coastal currents is the first step in finding the effective physical factors on marine environments and coastal regions. moreover, wave induced currents play important roles in the determination of geometry and shape of coasts. in this research, patterns of longshore currents are simulated for situations before and after construction of jetties on the mouth of the kiashahr lagoon. construction of jetties may lead to change in the pattern of coastal currents not only near the entrance of the lagoon, but also in the adjacent areas. therefore, a precise study is necessary to understand and prevent the possible impact of the jetties on the study area. for this purpose, mike 21 software package was used to simulate patterns of longshore currents before and after construction of the jetties. the field measurements in the kiashahr coastal area are also used for calibration of the model parameters. besides, the results of this study can be used for other related research works on sediment transport, water quality, etc. in the study area. material and methods: kiashahr fishing harbor is located in the northwestern part of the iranian coast of the caspian sea at geographical coordinates on the 39° 57? e and 37° 26? n. kiashahr harbor and kiashahr lagoon are situated on the east side of sefid rud river, which is the most important river in the southern part of the caspian sea (figure 1). the model: the applied modeling system mike 21 consists of several separate modules. in this study, the hydrodynamic module (mike 21 hd) is used to calculate the water movements in the kiashahr coastal area. mike 21 hd is the basic computational hydrodynamic module of the entire mike 21 system, providing the hydrodynamic basis for other mike 21 modules such as for advection-dispersion (ad), particle tracking (pa) and sediment transport (st, mt). hd module simulates the water level variations and flows in response to a variety of forcing functions in lakes, estuaries, bays and coastal areas. the water levels and flows are resolved on a rectangular grid covering the area of interest. mike 21 hd includes formulations for the effects of; convective and cross momentum, bottom shear stress, wind shear stress at the surface, barometric pressure gradients, coriolis forces, momentum dispersion, wave-induced currents, sources and sinks (mass and momentum), evaporation, flooding and drying. the equations for the conservation of mass and momentum (x and y directions) are integrated over depth to describe the flow and water level variations. the equations are solved by implicit finite difference techniques with the variables defined on a space staggered rectangular grid. run of the model: the first step in the simulation of longshore currents is selection of the run extents in the hydrodynamic module (set up of the regional model). dimension and extents of regional model depend on boundary situation and model boundary conditions. in addition, accuracy of available hydrographic maps is effective on determination of regional model extents. it is necessary to firstly run nearshore spectral waves module (nsw) and calculate the radiation stress data to use as input in the hd model. for each section (before and after construction of the jetties) 50 regional models in the kiashahr marine area were set up to simulate littoral current velocities using the mud transport module (table 1). results: figures 1 and 2 show the simulation results obtained for the representative cases before and after the construction of jetties. in these figures, vectors indicate direction and size of longshore currents. based on the model results, construction of jetties has not considerably influenced the general pattern of littoral currents in the kiashahr zone. for the purpose of comparison, some stations are considered in the model area and variations in littoral current velocities are investigated for conditions of before and after the construction of jetties (figures 3 and 4). the results presented in this study show that the currents due to easterly waves have more influence on the mouth of lagoon. in addition, comparison of patterns for both situations indicated that after the construction of jetties, current speed around the lagoon entrance (stations p6, p8 and p9) was reduced about 35 percent so that the harbor entrance has become a calm place.