The west of Hanoi is the flat area with less natural ponds and the Nhue river starting from Lien Mac to Ha Dong culvert is considered as the main drainage system of this area. Currently, the west of Hanoi has high speed of urbanization. As more agricultural land has been converted to urban area, the demand for drainage work is increasing that leads to the overuse of drainage systems and standing water. The approach is to use detention ponds, an important stormwater control structures
however, the effectiveness of detention pond highly depends on location where the pond meets the channel and scale of pond. Therefore, this study aims to apply a method to determine location and appropriate scale of detention pond but miminize the construction cost of a mixed urban-agricultural stormwater drainage system. The resutls for a case study: the drainate system in western Hanoi are listed as follows: For the drainage system is the west of Hanoi: the suitable ratio is 2.5 percent for each of three detension ponds when they centralized at the headworks. The area of detention pond is 2.91 percent of total drainage areas in which Lien Mac, Yen Thai and Yen Nghia account for 2.5 percent, 2.5 percent and 2.5 percent of its own drainge area, respectively when the sub-drainage areas are not equal. The ratio is 3.62 percent if the detention basins were distributed along the main channels
whereas, the ratio is 3.82 percent in case the dentention basins were distributed along main and first-order tributaries (PT2). Joe results of this study can be used in urban stormwater drainage system planning, or policy making to force new urban land or transition land to have proper areas for building detention basin.