都會區排水系統進水口堵塞與下水道淤積之淹水影響評估

Abstract

都會區的排水系統主要由地表逕流收集系統收集降雨產生之逕流水體，再透過排水系統之排放口排至河川或海域，而在極端氣候的影響下，短延時強降雨事件的增加，也造成排水系統無法負荷短延時內過量之逕流量，又在都會區內因為落葉、泥沙、垃圾等廢棄物影響排水系統之效率，其中包括地表進水口堵塞以及雨水下水道系統的淤積，皆增加了都會區積淹水發生的機率。 本研究採用二維快速地表漫地流模式與一維雨水下水道模式交互運算方法，並於模式銜接方法加入地表逕流收集系統之側溝進水口的集流效應以及地表逕流進入雨水下水道系統的連接管效應，再對都會區排水系統阻塞現象設計了四種情境，分別為不考慮阻塞現象、單一考慮進水口堵塞、單一考慮下水道淤積，以及同時考慮進水口堵塞以及下水道淤積，其中堵塞現象統一於鄰近路樹之進水口設置20%的堵塞率，而淤積現象則透過預估淤泥量回填至易淤積之線段，並使用準確度、偵測率與NSE值進行分析，以探討進水口堵塞與下水道淤積對淹水模擬之影響。 本研究以臺北市圓山雙連一帶作為研究區域，並透過三場歷史降雨事件進行評估，研究結果顯示，在都會區排水系統阻塞現象同時考慮進水口堵塞及下水道淤積於地表模擬淹水範圍以及模擬下水道水位皆較貼近調查狀況，其中又以同時考慮堵塞及淤積現象之情境最符合實際淹水以及下水道水位情形。

Urban flooding caused by the exceeded capacity of the urban drainage system could result from the short-term heavy rainfall events, which is the most important impact of climate change on urban drainage systems. An urban drainage system is generally defined as a runoff collection and transportation system, which is responsible for quickly removing stormwater runoff to storm drains and sewers from urban areas to prevent any flooding. In fact, clogging and blocking of urban drainage system due to the accumulation of garbage, fallen leaves or sediment is a common occurrence in urban area. In these cases, clogging and blockage effect should be considered as a reduction in terms of hydraulic efficiency. In this research, a new urban flooding simulation model that couples the two-dimensional rapid overland flow model (National Taiwan University Cellular Automata Flood Inundation Model, NTU-CAFIM) with the one-dimensional storm sewer model (Storm Water Management Model, SWMM) is adopted. This research also proposes a novel dynamic flow interaction method that considers not only the drainage effect of surface runoff to road storm drains but also the limited capacity of pipes connecting gutters and sewer system. Four scenarios are designed to evaluate the effects of clogging road storm drains and blocking sewer pipes on flood inundation simulation, i.e., (1) no clogging inlets and no blocking sewer pipes, (2) clogging inlets only, (3) blocking sewer pipes only, (4) clogging inlets and blocking sewer pipes. The clogging factors of clogging inlets locating near trees are all set as 20%. The method to translate the sediment deposition into the urban storm sewer system to simulate the blockage effect is also introduced. For analyzing the modelling result, three indicators are used, included Nash-Sutcliffe efficiency coefficient (NSE) for evaluating the sewer water level; both Accuracy (ACC) and Probability of Detection (POD) for evaluating the surface flooding areas. This study selects the area between Yuanshan and Tatung district as the research area, and three historical rainfall events are used to evaluate the scenarios mentioned above. In all the scenarios, the best flooding simulation result is inlets with clogging factor 20% around the trees and the sediment deposition in the pipes tend to be deposited.