Hamidreza Rabiefar - Ph.D. Department of Civil Engineering, Islamic Azad University-South Tehran
Mahyar Pourlak - MahyarPourlak, M.Sc.Member of Young Researchers Club, Islamic Azad University-South Tehran

Wave diffraction is a sea phenomenon occurs around breakwaters and islands that is an important factor in designing a suitable port basin. The portion of the Wave passing the end of the barrier will have a lateral transfer of wave energy along the wave crest. The diffracted wave crests in the lee of the borderwill form approximately concentric circular arcs with the wave height decreasing exponentially along the crests that we consider it as Diffraction . By the means of powerful Softwares such as DHI Mike21- BW Model (based on Boussinesq Wave theory) that would easily be described.Warm up period is a one of basic Mike21-BW Module items that should be considered thoughtfully in each model, either the wave generated randomly or regularly. To evaluate the changes on warm-up period on these models in coastal regions, using a verified model for manipulate the value of this parameterseems to be necessary. The MIKE user manual considered a variant for this parameter that is 5T to 10T, which make this parameter T related function. In this paper by using MIKE21-BW model, the affection of the warm-up period change on the diffractionwill be discussed. It is concluded by changing the warm-up periodin model characteristics mentioned in, the changes on the Diffraction Coefficient (Cd) is not tangible, but may have much more affection such as wave velocity in different model nodes

Diffraction, Sensitive Analysis, Warm-up Period, MIKE 21, Boussinesq Wave Theory,Numerical Model