The Effects of Impermeable Surfaces on the Flooding Possibility in Zarrin- Shahr, Isfahan Municipal Watershed

J. Appl. Environ. Biol. Sci. , 5(1)28-38, 2015 ISSN: 2090-4274 Journal of Applied Environmental © 2015, TextRoad Publication and Biological Sciences www.textroad.com The Effects of Impermeable Surfaces on the Flooding Possibility in Zarrin- Shahr, Isfahan Municipal Watershed Leyla Parsasyrat 1, Ali Akbar Jamali 2* 1M.Sc graguated, Department of Watershed Management, Maybod Branch, Islamic Azad University, Maybod, Iran 2Assistant professor, Department of Watershed Management, Maybod Branch, Islamic Azad University, Maybod, Iran Received: September 21, 2014 Accepted: December 4, 2014 ABSTRACT Flood is one of the most destructive natural disasters causing a great damage to natural resources. Changing the land uses, converting the urban lands and rising the impregnability of surfaces have changed the hydrological regime of urban watersheds in such a way that most of the rainfall turns into the runoff. This research seeks to use Santa Barbara urban hydrography in the environment of Storm water Management and Design Aid (SMADA) to study the effects of land use changes and the impermeable surfaces' arise in the flooding possibility of municipal watershed Zarrin Shahr in Isfahan province in Iran. To analyze the achievement data, the required maps of the area were first prepared by Arc-GIS software. Then, the preparation process of annual and maximally 24-hour rainfall data was undertaken by the help of the synoptic situation of Zarrin Shahr. All the parameters required for the above mentioned model such as concentration time, design rain and curve number (CN) were studied and determined. Then, the hydrographic simulation of the flood was done in the environment of SMADA. Two stations, namely Pol Kalleh in the west and Lenj in the east of the watershed were studied. As the results indicate, changing the green urban environment in the residential areas causes a considerable increase in maximum floodwater discharge. The impacts of this land use change are greater on the flooding possibility of the area studied in low return periods. KEYWORDS: Flood Hydrograph, Land Use Change, Santa Barbara Urban Hydrograph, SMADA, 1- INTRODUCTION According to the initial analysis, flood is a function of climatic events including the volume, intensity and temporal and local distribution of rainfalls, but various features of the watershed such as vegetation, land use type and human interventions have major effects on flooding. Due to the presence of residential areas and human congestion, the urban areas are greatly vulnerable to natural dangers, especially in flood. On the other hand, more cases of flood are reported every year and their damages threaten the economic and social sections of countries [1]. Land use changes in the watershed related to the urbanization are always reflected in the flow regime. Flood management seeks to coordinate human activities and flood dangers using suitable studies for improving water regimes and adjust human activities in such a way that these threats are likely to be reduced. Many Iranian and foreign researchers have conducted several detailed studies relying on these factors. We can refer to [2-16]. Vang et al. (2008) believe that increasing the urban impermeable surfaces leads to the reduction of natural feeding of underground aquifers which in turn causes the ecological damages due to the reduction of flow rate in the level of the watercourse. Za’eri and Sofianian [17] who had studied the effects of impermeable surfaces on the urban runoffs have concluded that in a 36-year period, Isfahan has undergone a 2-percent growth in her residential areas and a 22-percent increase in her roads and streets rendering much larger impermeable surfaces. This increase in the impermeable surfaces has contributed to the production of higher amounts of polluted runoffs and also more erosion in the downstream of the watershed. A study conducted by Noorazuan et al.,[18] in the watershed of the Langat River in Malaysia showed that the level of runoff increases as the urbanization grows while assessing the relationships between the changing land uses, vegetation and their effects on the hydrological regime using GIS technique and this is due to the undeniable increase of impermeable surfaces of land uses related to cities which has led to some changes in the level of hydrographs. *Corresponding Author: Ali Akbar Jamali, Assistant professor, (A.A. Jamali) Department of Watershed Management, Maybod Branch, Islamic Azad University, Maybod, Iran, E-mail: [email protected] 28 Parsasyrat and Jamali, 2015 In a study done on the economic benefits of rain water and runoff reduction through the urban green and garden land uses in Beijing, Zhang et al.,[19]claimed that replacing the vegetation by the impermeable surfaces reduces the impermeable surface and as a result, the rate and level of the runoff caused by rainfall are more likely to be increased. Studying the effects of urbanization on the flooding possibility of Tirsolam watershed, Suriya and Mudgal[20]concluded that increasing the impermeable surfaces disturbs the balance of natural water. Modeling the urban development dynamics and their effects on the properties of surface run-off, Kumar et al.,[21]showed a linear relationship between maximum depletion and the peak time of the studied watershed. With the goal of attaining sufficient information and knowledge about the effects of impermeable surfaces and runoffs caused by the rainfall in a section of Zayanderood watershed, Zarrinshahr and her northern outskirts located in Isfahan province were studied and analyzed. 2- MATERIALS AND METHODS 2.1 Case study The urban area of ZarrinShahr with the total area of 14574 acres is located between the eastern longitude of 51 ° 21’ to 51 ° 33’ and the northern latitude of 32 ° 18’ to 32 ° 30’ in Isfahan province. Minimum and maximum heights of the studied area are 1646 and 2295 meters, respectively. This watershed is restricted to Najaf Abad and Falaverjan in the north, Mobarakeh in the east, Chahar Mahal Bakhtiari province in the west and Mobarakeh and Chahar Mahal Bakhtiari province in the south. Zarrin Shahr is a part of the Zayandeh Roud valley (Fig. 1) located at the foot of the mountains and its soil is made by the erosion of the surrounding mountains as a result of seasonal streams and sediments of the Zayandeh Roud which is very fertile. The climate of this area is under the impacts of the mountainous areas of Shar-e-Kord and the dampness of the Zayandeh Roud on the one hand, and the hot climate of the central regions on the other hand. Thus, this place has hot summers and cold winters. The climate of the desired area is dry (according to Emberger method) and is distributed throughout the area. Mean precipitation and temperature in the area are 116 mm and 14°C, respectively. Study stages was shown in flow chart (Fig. 2). Fig. 1. ZarrinShahr Watershed and Hydrometric Station in Isfahan province, Iran 29 J. Appl. Environ. Biol. Sci. , 5( 1)28-38, 2015 Urban Flood urban Tools and Two stations Parameters Study watersheds Method Pol Kalleh in the converting the use Santa Barbara land uses Change west urban lands urban hydrography Stormwater impermeable Lenj in the east Management and surfaces Design Aid (SMADA) hydrological Arc-GIS hydrographic simulation regime flood annual and maximally 24-hour rainfall changing the green urban landuse design rain increase in maximum Analyze Analyze floodwater discharge. concentration time greater effect on Floods rainfall turns into with low return periods the runoff Fig. 2. Study flow chart 2.2 Introducing Santa Barbara urban hydrograph method (SBUH) This method was first introduced by Stabcher concerning water protection and floodwater control division of the Santa Barbara region (California) in the national congregation of Urban Hydrology and Sediment Control in University of Kentucky (1975). Independence from peak discharge level is one of the benefits of this method. Calculations of this method can be done either manually or by computer. Following equations are used in this method. a) The height of runoff for each period is calculated by the following equations. R(I)=dp( ∆tʛ (1) R(P)=(I-d) ʜpʚ∆tʛ Ǝ Fʚ∆Tʛʝ R( ∆tʛ Ɣ RʚIʛ ƍ Rʚpʛ Where R(I)= the runoff of impermeable areas R(P)= the runoff of permeable areas P( ∆tʛ= the height of rain at the collection time of ∆t F( ∆tʛ= the penetration over the collection time of ∆t d= the impermeable part of the watershed, which is directly linked to the waterway (in the form of a fraction) ∆t= the collection time period (hours) R( ∆tʛ= the total height of the runoff 30 Parsasyrat and Jamali, 2015 b) Momentary hydrography calculated by multiplying the total height of runoff and each time period of ∆ and then, dividing the resultant value by the collection time of ∆. I( ∆t)=R( ∆t)A/∆̴ (2) c) Hydrography of the exit stream is calculated through the momentary hydrography routing of I( ∆t) with a hypothetical tank and a delay time equaled to the concentration time of tc basin. Flood routing is conducted by the means of the following equations: Q(2)=Q(I)+Kr ʜIʚ1ʛ + Iʚ2ʛ − 2Q (I)ʝ (3) Kr= ∆t/(2tc + ∆t) Where I= discharge to the hypothetical tanker Tc= concentration time Kr= routing rate According to the above-mentioned equations, it is clear that we need to calculate the concentration time, design rain and rainfall loss rate using Santa Barbara urban hydrographic method. We use the curve number (CN) to calculate the rain losses. 2.3 Concentration time Concentration time is the time required for a surface stream to get to the exit of a watershed from its farthest point. On one hand, this time depends upon the physiographical characteristics of the watershed such as surface, the length and slope of waterway, terrain accidents, vegetation type and thickness and on the other hand, the rainfall levels and their temporal and spatial distributions can reduce or increase concentration time.

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