Ziba Kazemi - Assistant Professor, Department of Civil Engineering, Ahar Branch, Islamic Azad University, Ahar,

The residence time is helpful in understanding the basic mechanisms controlling the flow of substances through an estuary. In addition, these variables are useful indicators of thesensitivity of estuarine eutrophication. In this study, the calculation of the residence time is determined for the Western part of the Johor Strait, Malaysia, a relatively narrow strait system of carrying freshwater contributions from Skudai River, Pendas River and Pulai Riverand their tributaries. As a first step, the model configuration included the construction of the orthogonal curvilinear grid for the water bodies in the Western part of Johor Strait, interpolation of bathymetric data to the grid and construction of hydrodynamic input files. A three-dimensional hydrodynamic model was selected in this study. Numerical simulations were completed under 13 different steady freshwater inflow conditions, ranging from anaverage annual flow to peak flow with return periods 100 year. The modeling results indicated that during low flow conditions the average residence time of the estuary is approximately 134.96 days while under high flow conditions this value can be as low as 11.83 days. Under low flow conditions, the transport of materials in the estuary is dominated by dispersive action of the tide while under high flow conditions the advection from thefreshwater inflows controls the export of substances in the estuary of Western part of Johor Strait. The analysis suggested considerable spatial variation in residence time under both low and high flow conditions. It is recommended that in the future water quality model in this region.

water age, residence time, hydrodynamic, EFDC, Johor Strait, threedimensional