ALTERNATIVE DESIGN FLOOD ESTIMATION METHODS IN TIDAL RIVERS

摘要

The estimation of a flood stage associated with a certain recurrent interval in a tidalinteraction zone is a pertinent issue that needs to be addressed explicitly by planners, insurers and civilengineers. Traditional flood frequency analysis based on some long period of observed river dischargedata is not applicable due to the complexity of the interaction of tides and river discharges. Hydrodynamicmodels have been used to assist the derivation of extreme flood levels, albeit many assumptions aremade with respect to the boundaries conditions. To meet probabilistic specifications, continuoussimulations of boundary variables such as rainfall and tides are used. There are many problemsassociated with this approach, including the demand for simultaneous observation data in differentlocations, the apparent lack of accuracy due to poor flow model calibration, and the large resourcesdemanded in model set-up. Bearing that in mind, an alternative approach is proposed based on thepremise that the tides, a well-behaved and high frequency component of the interaction phenomena, canbe separated out from the stage height series observed in the tidal interaction zones. New techniquesdeveloped using Fourier and wavelet methods are employed in analyzing the stage-height series resultingin separate filtered series, which are later subjected to conventional time series analysis and probabilisticanalysis. Data from rivers of Sarawak, Malaysia, and the St. Lawrence River, Canada were used toillustrate the approach. This approach has the advantage, among others, of being direct and explicit to thepoint of interest, such as a city or river port.