This study was initiated to examine shoreline changes behind offshore breakwaters to evaluate the design of a detached breakwater system. Curves for different wave conditions were constructed using the numerical model "Genesis". The model was used with limited data sets and calibration parameters to give reasonably accurate results. A prototype was used at Marabella Beach, Egypt (Mediterranean Sea) for calibration, verification, and data gathering. The calibrated model was compared with shoreline changes to determine proper values for sediment transport parameters before being verified. Afterward, field observation data recording Marabella shoreline changes from July 2004 – July 2005 was chosen. Better results were found when the average deep water wave direction was shifted to the down drift side and the wave height was decreased. The model was verified during the period of February – May 2004 with the existence of a single detached breakwater. Using the limited application of the model at Marabella, "Genesis" clearly is reasonably effective for simulating the influence of waves and coastal structures like detached breakwaters on the Northern Coast of Egypt's sandy beaches. The constructed curves provide useful information for practical purposes.