In estuary and coastal areas the storm surge will usually occur in typhoon season. When simulating the storm surge by numerical model, the wind speed at the height of 10m above the mean sea level will be usually used. Determining the wind drag stress on free surface reasonably plays an important role to simulate the storm surge accurately. In the past numerical models, the wind drag stress on free surface was calculated only considering the relative wind speed. There are many formulas about wind stress can be used, whereas these formulas are usually got in laboratory, where the water surface fluctuates in a small range, and the water elevation itself has not been considered in formula. Actually in some place, the astronomical tidal range is large, such as Yangtze estuary and Hangzhou bay in China, and during typhoon season, the water fluctuation range is much larger than ever there. In conventional method the wind stress will be underestimate in flood tide and be over-valuated in ebb tide without considering the fluctuation of water, so it is obviously unsuitable to take no account of the influence of tidal level on wind stress. Therefore in the intensive tide coastal area, the relationship of the relative wind speed, tidal level should be considered together. A new kind of wind stress formula has been established in this paper and been adopted in simulating the storm surge of typhoon Winnie in Yangtze estuary, and the results are better and satisfying.

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