The Potential of Green Infrastructure (GI) to Reduce Urban Runoff in Phnom Penh City

Abstract

Land-use changes and the development of urbanization intensify the urban environment including stormwater runoff. An excess runoff overflow impervious surfaces, provokes local air and water pollution, soil erosion, and especially create floods. Despite the gray infrastructure which relies on the big size of drainage pipes, many cities around the globe are adapted to be resilient by integrating green infrastructure (GI) in either pre-development or as a retrofit to deal with these problems. Recently, the development of GI’s concept is potentially coping with a highly urbanized area to resolve the disappearance of open spaces and green spaces on source control by using a variety of elements. Phnom Penh, a typical rapid urban development with a huge proportion of impervious urban surfaces, has been vulnerable to urban flood during the rainy season for almost two decades. The purposes of this research were to: 1) to investigate the performance of GI elements for reducing peak runoff rate in the urban area of Phnom Penh which has the tropical climate and 2) to examine the types of GI elements that are prominent for reducing runoff in different land uses. Three typical urban land uses in a center of Phnom Penh were investigated: residential housing, commercial, and mixed residential and commercial land-use. Two scenarios were designed: scenario 1 referred to non-GI and scenario 2 referred to the integration of GI elements. Due to the characteristic of these three typical urban land-uses, four GI elements include trees, bioswales, permeable pavements, and green roofs were implemented. The classification of land-use/land cover and the implementation of GI elements have proceeded on QGIS version 3.4.10 with support of the Google Satellite overlay-image. The Rational Method was used to estimate the peak runoff rate (Q) in both scneanrio2 and their overall outcomes were computed and analyzed in Ms. Excel.

The result demonstrates that the implementation of GI elements in these three urban land-uses is varied due to the different characteristics of land-use and the share of the existing land cover. Accordingly, the application of the various types of GI element and their potential of the runoff reduction is also different. Trees and permeable pavements are the best performance while bioswales show the least effective in these three land-uses. Green roofs have high performance in residential land-use and mixed-land-use, and at medium performance in a commercial land-use. The effectiveness of combined GI elements between the three areas is also obtained. The combined GI elements in mixed land-use had the most effective to reduce runoff, compared to the other two land-uses. In commercial and mixed land-use, the combined GI element is at high performance while it is low in residential land-use. The runoff reduction in entire central Phnom Penh consisted of three typical land-use, which approximately reduced by forth-ten (39.40%) of the total area when a half (49.39%) of GI is applied in the entire central Phnom Penh.

The study reveals that green infrastructure (GI) significantly reduces flood problems by alleviating the peak runoff rates in the typical urban land-use in a tropical city. It is efficiently used in Phnom Penh to cope with urban floods at high performance.

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