應用洪災指標於二維HEC-RAS及3Di模式之評估

Abstract

地形資訊(DEM)的品質及精度，大斷面高程的分布以及一二維水理模式的應用皆是繪製淹水潛勢地圖的關鍵影響因子。高解析度地形資訊的日益普及，二三維水理模式以及高速計算機的開發將提升淹水潛勢地圖測繪技術的能力。本研究以台中市南屯區筏子溪作為研究區域，使用相同的地形資料（DEM）和兩種水理模式（二維 HEC-RAS和二維 3Di）分析了水理模式建模和淹水潛勢地圖對於地形資料，幾何配置，邊界條件和模型類型的敏感性。目的透過考慮洪災評估指標(FRI)來評估此兩種水理模式模擬淹水潛勢地圖的差異及能力，並以QGIS 做為模式輸入，執行模型和劃定洪水淹沒區域的預處理和後處理工具。研究結果表明，透過比較不同分析單元總淹沒面積和水文滯留足跡（HFR），使用高解析度的 DEM 資料可以幫助獲得準確的洪水潛勢圖以及判斷水理模式之間的侷限性差異，同時顯示了網格解析度在水理模式中的重要性以及其如何以更高的解析度精確判斷洪災影響程度一般來說在本研究中評估的大多數情況下，HEC-RAS 模式預測的洪災影響範圍更大, ，顯示其高估了該研究區域的洪災影響區域及程度接近現實的 15%。然而，3Di 水理模式的預測顯示出更準確的模擬結果及洪災影響區域接近現實的 40%，故3Di被證實比HEC-RAS更適合應用於本研究區域台中市等都市地區的洪災模擬。

Flood inundation mapping is influenced by many factors such as the quality of terrain data (Digital Elevation Model, DEM), the cross-sectional configuration, and the use of a one-dimensional or two-dimensional hydrodynamic model. The increasing availability of high-resolution topographic data, development of two and three-dimensional hydrodynamic models, and access to fast computing computers, are revolutionizing the flood inundation mapping process. The objective of this research is to evaluate both of the two-dimensional hydrodynamic models, such as HEC-RAS and 3Di on the flood inundation mapping process, by considering its flood hazard index. By using the same topographic data (DEM) and two types of models (2D HEC-RAS and 2D 3Di), on the study reach of the Fazih River, located on the Nantung District of Taichung City; the sensitivity of hydraulic modeling and flood inundation mapping to terrain data, geometric configuration, boundary conditions, and model type is analyzed. QGIS is used as pre- and post-processing tools to prepare model inputs, execute models, and delineated flood inundation areas. The result from this study shows that with a higher resolution DEM data is possible to have an accurate flooding inundation map, as well as the differences between the hydrodynamic models and their limitations or advantages by comparing their total inundation area and hydrological footprint residence (HFR) values for each case analyzed. The results also show the importance of the grid resolution in hydrodynamic models, and how mesh resolution is shown to change the inundation extent with a higher resolution. In general, HEC-RAS modeling predicts a larger inundation extent, in which most of the cases evaluated in this research, it proves to overestimate the flooding area or results (15% closer to reality), however, 3Di hydrodynamic modeling shows a more accurate inundation extent in each of the cases (40% closer to reality) and proves to be a more suitable and higher dimensional numerical model to use in urban areas such as Taichung City in Taiwan, rather than conventional modeling like HEC-RAS.