Irrigation & Drainage Systems Engineering

Green Concept in Storm Water Management

Abstract

James C.Y. Guo

Storm runoff is considered one of important water resources for urban areas. Over a geologictime, streams and lakes are periodically refreshed with flood water and continually shaped with the flood flows. Urban development always results in increases of runoff peak rates, volumes, and frequency of higher flows. As a result, flood mitigation has become a major task in urban developments. Before 1970’s, storm water drainage systems in an urban area were designed to remove flood water from streets as quickly as possible. From 1970 to 1980, the US EPA conducted a nationwide stormwater data collection and reached a conclusion that stream stability is more related to frequent, small storm events rather than the extreme, large events. Since man-made stormwater systems were designed to pass extreme events, the large inlets and outlets release frequent events without any detention effect. As a result, urban pollutant and sediment solids are transported and deposited in the receiving water bodies. Under a US Congress mandate starting in 1980’s, a nationwide stormwater best-management-practices (BMPs) program was developed and implanted in major metropolitan areas. The tasks in BMPs include: (1) retrofitting the existing drainage facilities to achieve a full-spectrum control on peak flows, and (2) applications of Low-Impact -Development (LID) designs to reduce runoff volumes under the post-development condition. With the latest observations in climate change, the uncertainty in the design floodhas imposed unprecedented challenges in flood mitigation designs. The flexibility in freeboards and easements need to be refined in order to accommodate the changes in extreme rainfall events. This paper presents a summary of the Green approach in stormwater management and LID designs as the engineering measures to preserve the watershed regime.