Vahid khorsandifar - Master of coastal and marine engineering, College of Engineering, University of Qom,Qom,Iran,
Reza kamalian - Assistant professor of coastal and marine engineering, College of Engineering, University of Qom,Qom-Iran

Monsoon wave is the main factor of littoral drift at Makran beaches. Summer and Winter Monsoons are caused by annual temperature difference between sea and land. The summer monsoon waves usually start from middle of June until the end of September. The most part of annual sedimentation happens during this times frame. In order to calculate harbor sedimentation in this area, usually a fixed averaged monsoon wave direction is used. Recently (2015) waves have been measured for several months at seven locations in front of the Makran shoreline which include directional wave energy spectrum. These instruments show that the monsoon wave direction is not constant. This paper focuses on the oscillation of the monsoon wave direction, its reason and its effect on long shore sediment transport. There are very limited research works in this regard in the literature. A research in California recently studied on the longshore sediment transport sensitivity to deep water wave direction variability (Adams, Inman, & Lovering, 2011). Ignoring monsoon wave direction undulation when calculating littoral drift, may cause inaccuracy in Qnet and error in sediment transport of the secondary direction (here; west to east). If a fixed wave direction is taken, the long shore sediment transport always remains in one direction. When the wave angle is more or less perpendicular to the coast line, sometimes sed. transport from the other direction also happens. This study compares the difference between fixed and variable wave direction (as in the time-series) effect on littoral drift calculation.