Design flood estimation methods for rivers with extensive tidal interaction zones.

摘要

Flooding problems may never be solved as long as people live in proximity of riverbanks with high inundation risk. A rational approach well embraced by civil engineers is to derive flood estimates and adopt mitigation measures corresponding to an acceptable risk level. However similar approaches have not been well addressed in many rivers with extensive tidal interaction zones (TIZ), such as those found in Sarawak, Malaysia.; The primary objective of this research is to streamline and develop a set of design flood estimation methods applicable to the TIZ with varying degrees of sophistication. Methods in use such as the direct frequency analysis method, the regression-like rating-curve method, the joint probability method and the hydrodynamic modelling method are evaluated. Although these methods have their merits, they have many inherent limitations and constraints. Two new methods that are statistically based yet do not compromise the physical meanings of the processes involved are developed. The first is a low-pass filter method involving Fourier transforms resulting in filtering of low frequency river level series. A second new method is based on wavelet de-noising techniques that considers tidal contributions as noise in a smooth function. An algorithm ensuring a consistent separation technique is developed. Various wavelet families are investigated together with different decomposition levels and thresholding options. Conventional flood frequency analysis techniques are subsequently applied directly to the separated flow series.; The secondary objective of this research is to establish a comprehensive flood estimation procedure for ungauged catchments in the study area based on regional flood frequency analysis. The need arises because inflow floods deriving from the major ungauged tributaries, which affect flows in the TIZ, have to be estimated in some of the methods.; After comparing all the methods, it is recommended that an appropriate number of water-level observation stations should be established at strategic locations in the TIZ on a continuous long-term basis. With these data, the effects of climatic change on sea level rise and the subsequent effects on flood estimations in the TIZ can be assessed accurately.; In conclusion, the method to be chosen for design flood estimations in the TIZ is very much dependent on data availability. If long-term water level data in the TIZ is available, the new fundamental method using wavelet de-noising gives direct and reliable results. By considering the interaction effects, it is superior to the conventional direct frequency analysis. Since wavelet methodology is still evolving, there are further potential applications for this particular method.