• Kwan Tun Lee

      Articles written in Journal of Earth System Science

    • A simple depression-filling method for raster and irregular elevation datasets

      Pin-Chun Huang Kwan Tun Lee

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      A variety of methods have been developed to process digital elevation models (DEMs) in order to extract morphological properties of land surfaces. Almost all rely on depression filling to facilitate drainage analysis. This study proposes an intuitive and relatively simple depression-filling algorithm, which is readily applicable to raster and irregular datasets. In this study, topographic data of two watershed areas were provided to test the capability of this new algorithm. The results show that the proposed algorithm can accurately identify the locations of depressions and efficiently assign pseudo flow directions across the filled depressions.

    • Influence of storm magnitude and watershed size on runoff nonlinearity

      Kwan Tun Lee Jen-Kuo Huang

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      The inherent nonlinear characteristics of the watershed runoff process related to storm magnitude andwatershed size are discussed in detail in this study. The first type of nonlinearity is referred to rainfallrunoffdynamic process and the second type is with respect to a Power-law relation between peakdischarge and upstream drainage area. The dynamic nonlinearity induced by storm magnitude was firstdemonstrated by inspecting rainfall-runoff records at three watersheds in Taiwan. Then the derivation ofthe watershed unit hydrograph (UH) using two linear hydrological models shows that the peak dischargeand time to peak discharge that characterize the shape of UH vary event-to-event. Hence, the intentionof deriving a unique and universal UH for all rainfall-runoff simulation cases is questionable. In contrast,the UHs by the other two adopted nonlinear hydrological models were responsive to rainfall intensitywithout relying on linear proportion principle, and are excellent in presenting dynamic nonlinearity.Based on the two-segment regression, the scaling nonlinearity between peak discharge and drainagearea was investigated by analyzing the variation of Power-law exponent. The results demonstrate thatthe scaling nonlinearity is particularly significant for a watershed having larger area and subjecting toa small-size of storm. For three study watersheds, a large tributary that contributes relatively greatdrainage area or inflow is found to cause a transition break in scaling relationship and convert the scalingrelationship from linearity to nonlinearity.

    • Assessment of flood mitigation through riparian detention in response to a changing climate–a case study

      Kwan Tun Lee Pin-Chun Huang

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      Considering that urban areas may suffer more substantial losses than riparian farmlands during floods, diverting floodwater into riparian areas for temporal detention is expected to mitigate flood damage in downstream urban areas. In this study, an assessment has been conducted to evaluate the effect of floodmitigation through riparian detention in response to a changing climate in the Tou-Chien River basin of Taiwan. An integrated 1D–2D flow model was used to simulate the movement of flood wave in the main stream and the overbank flow inundating into the nearby lowlands. Based on the numerical simulation results, the flooding extents in the basin corresponding to different return periods of flood using existing flood prevention infrastructures were investigated. A detention strategy by lowering the levee along the riparian farmlands was proposed to avoid severe flooding in the densely populated urban areas of the basin. Research findings showed that the proposed detention measure can completely protect thedownstream areas from overbank flooding when a flood having 20-yr period occurs, and can effectively alleviate the downstream flooding area from 27.4 to $\rm{7.6 km^{2}}$ for a flood possessing 200-yr period.

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