ABSTRACT
Conversion of runoff into flood can cause irreparable damage to urban water gathering channels. A 20-fold increased of Urmia, new constructions development on the urban areas over the past five decades particularly on hills constructions and vegetation loss enormously and replacing with asphalt made this area prone to flooding and rainfall runoff especially during heavy rains at rainy seasons. According to the morphological specification, Urmia and its surroundings [about 200 Km[2]] were divided into different zones and investigated the torrents and vulnerable regions. Due to evaluation of more than 4000 data collected from Urmia Regional Water and Metrological Organization, about 27 million m3 rain and snow come to city annually. Primary evaluation shows that about 5 million m3 annually in spring seasons flow through the city. In this study, zones prone to runoff in Urmia city, margin and residential areas have been identified. The flood-prone areas were determined by using TOPSIS method and obtaining data from Meteorology Organization of West Azerbaijan during heavy rain for a five-year return period by combining existing data. According to this project, 17 separate regions are distinguished with respect to different morphological features. The studies showed that region No. 12 [about 757000 m[2]] with the ultimate point to ideal situation was the most flood-prone regions due to the cause and flow runoff and urban flooding; in a heavy spring rain about to 192 m[3]/min rainfall occurs, given the affecting coefficients much of it became runoff. Region No. 17 with about 1763000 m2, the closet point to ideal situation, was the most secure one on which heavy rain more than 275 m[3] was coming down. Weighting to the effective parameters involved in runoff showed that the connecting angle between natural draining systems and urban water gathering channels on the one hand, and the indicator of the length of channels and significant difference in height on the other hand had the maximum and minimum weight [effect] on the vulnerable criterion respectively. Accordingly, the regions 4, 12, 15 and 16 with about 3.4 Km[2] are the most vulnerable zones but on the other hand, the regions 17, 1, 2 and 3 are located on the low risk vulnerable zones. For this purpose, constructing the water gathering channels in urban areas, especially in the 4-6 and 8-16 regions is necessary to reduce the damages and hazards in these regions and also unpleasant sights in the heavy rains