Resumen
In tidal rivers design return period is multiplication of return periods of floods and tidal flows. This research considers the down stream of the Karun River in southwest of Iran as case study. For calibration of Fluvial-12 model, the deformations of river cross sections from 1996 to 2013 were measured by surveying. Using of the Ackers ? White (1973) equation eventuates the best fitness between calculated deformations by Fluvial-12 model and observed deformations. This equation is an appropriate sediment transport equation for river beds and banks that their materials are silt or fine sand such as the most of tidal rivers in the world. For design return period N years, the highest erosion occurs at flood return period N years and tidal flow return period 1 year. At cross sections near the mouth of the river the highest sedimentation occurs at flood return period 1 year and tidal flow return period N years but at cross sections far from the mouth of the river the highest sedimentation occurs at flood return period N years and tidal flow return period 1 year. Because sedimentation occurs at flood cycle of tide at near the mouth of the river, tidal flow that its velocity is low causes sedimentation. On the other hand, erosion occurs at ebb cycle of tide and fluvial flow intensifies ebb velocity toward the mouth of the river therefore fluvial flow causes erosion at the whole length of the river.