低衝擊開發配置對高密度都市的減洪效率之分析

Abstract

都市化改變城市的地貌且增加地表逕流量，排水管線的興建改變雨水的流動路徑也增加逕流速度，再加上氣候變遷所導致的極端降雨越來越頻繁，台灣都市地區發生洪水的頻率與受災程度增加，因此採取高效措施來減緩內水氾濫已成為都市雨洪管理首要的任務。低影響開發（LID）就是現代城市雨水管理的手段，傳統的LID元件藉由增加集水區的入滲及保水能力來延長集流時間，然而LID對內水的減少效率並不顯著，特別是在高重現期的狀況下。目前來說，滯洪池是減少溢淹最有效的設施，但在都市中找出最具有成本效益的滯洪池設計方案是非常困難的。本文提出以層級分析法（AHP）串聯暴雨管理模式（SWMM），以最大化的減洪量及最低的建造成本為目標，來進行都會區的滯洪池配置最佳化之研究。 研究區域為高密度開發的新北市中永和地區，降雨條件為50年重現期。以洪水深度和持續時間為指標，對所有研究區域內的氾濫節點進行評分及排序，藉此找出最佳的滯洪池數量和減洪效率，接著以此為初步方案作2種模擬情境和成本估算，最後評估不同情境下的減洪效率，來提出都市洪水緩解的規劃原則與策略。本研究結果顯示，12個滯洪池對中永和地區是最具成本效益的設計，可以減少約38%的都市內水量，在不同降雨條件下也依然能維持框架的穩健性。傳統的LID元件由於建造昂貴且減洪效率低，雖然設置滯洪池可以有效提升減洪量，但若考慮土地成本的情況下，修改管線的瓶頸段才是舒緩城市洪水最經濟的策略，而且研究指出降低洪峰流量與都市內水總量的減少並沒有直接的關聯性。最後，本研究提出的都市雨洪管理原則為：(1)修改設計錯誤的排水管線；(2)使用層級分析法找出對洪水最敏感的地方並直接放置滯洪池；(3)設計LID結構，通過增加逕流滲透的機會來協助減少洪水。

The urbanization has changed the geography of a city and increased surface runoff volume. And urban storm sewer infrastructure systems alter natural storm-water flow paths and accelerate runoff velocities. Because of climate change, as extreme rainfalls becoming more frequent, as do the number of flooding more intensive in urban areas in Taiwan. Consequently, flood hazards mitigation with effective measures has become the most important task of storm water management to deal with. The low impact development (LID) is a new concept of modern urban storm water management. The general LID lengthens the time of concentration by increasing the infiltration and rainwater storage of the catchment area, but the efficiency of reduction of inner water is not obvious, especially for the high return period. As for the present, storage is the most useful and direct method for flood mitigation in heavy rainfall events. Looking for the cost-effective design scheme of storage tanks, however, is very difficult in the highly concentrated city. Thus, this paper presents the analytic hierarchy process (AHP) to find an optimal scheme for storage tanks using storm water management model (SWMM). And the goal is to minimize flooding and storage cost. The case study was conducted in a highly concentrated region, Chung Ho and Yong He District, New Taipei City, under 50-year return period rainfall. By evaluating and ranking the flooding nodes with the AHP using two indicators (flood depth and flood duration), the optimal number of storages was obtained to have the best flood reduction efficiency. Designed two scenarios and cost estimate based on the preliminary scheme and indicated a general principle and strategy for the urban flood mitigation by comparing the performance of the plans in flood reduction efficiency. The results showed that 12 detention ponds are the highest cost-effective design with 38 percentage of flooding reduction on 50-year return period rainfall. It can still obtain the desired effects under different rainfall conditions. The general LID has the lowest benefit and his construction costs are enormous. Despite the efficiency of improving conduits are worse than placing detention ponds, it is the most appropriate strategy to slow down urban flooding if land expropriation and maintenance expenses considered. And the research has pointed out dropping peak flow may not be necessarily related to mitigating urban flooding. Finally, we present a general principle for storm water management in urban area: (1) Modify drainage networks where have incorrect design apparently. (2) Use AHP to find out where is the most sensitive to flooding and place storage tanks directly. (3) Design a few LID constructions to assist flooding reduction by increasing the chances for runoff to infiltrate.