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]

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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

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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. The most important application of concentration time is to measure flood volume, the hydrographic shape of surface streams and the delay and peak time of the flood which is calculated through the empirical and logical methods. Various methods have been proposed for calculating the concentration time of the watersheds. However, we have chosen to use Kirpich method in this study as it is frequently used for small watersheds. It is calculated by the means of following equation:

77.0 − 385.0 TC = .0 0003L *S (4) Where Tc= concentration time (hour) S= the mean slope of the main waterway (m/m) L= the length of the main waterway

2.4 Determination of design rain To determine design rain, we used the intensity, duration and frequency tables of ZarrinShahr where the intensity was represented in millimeter per hour and the return period was represented in the year. As the concentration time of the watershed was set to 25.5 minutes, the duration of total rainfall will equal 0.75 hours (45/5/60). Rainfall time stage is 10 minutes and total rainfall is 0.9 inches. Rainfall distribution was set to constant intensity mode.

2.5 Determination of curve number (CN) In this study, we have used land use map and hydrologic groups in the environment of GIS software to determine CN for the studied area. For this purpose, the studied area was divided into the permeable and impermeable areas. Considering the land use map, poor vegetation, meadows and irrigated agriculture were among the permeable areas while the covered pool and metropolitan areas were grouped as the impermeable areas. In the impermeable areas, the areas are represented in terms of their impregnability according to curve number table.

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Fig. 3. CN map of Zarrin Shahr area

According to the results, the highest and lowest CN were 62 and 98, respectively. Furthermore, other positions were occupied by the CN of 75, 78, 83, 93, 95 and 98, respectively (Fig. 3). According to the results, the curve number of 95 has occupied the greatest area of the lands and other positions are respectively occupied by 78, 98, 83, 62 and 75 while the CN of 93 has occupied the smallest area. The area of curve numbers for Zarrin Shahr and their frequencies are represented in table 1.

Tab.1. Area distribution of curve numbers for ZarrinShahr Curve number Area (km) Frequency 62 599.61 4.13 75 251.4 1.73 78 2392.3 16.46 83 1285.82 8.85 93 117.96 0.81 95 8329.8 57.32 98 1556.4 10.71 Total 14533.29 100.00

Soil hydrologic groups' areas for ZarrinShahr regarding each land use and its corresponding curve number are represented in table 1. As the table has already shown, the largest area of the soil hydrologic group is for group D which is stone outcrops. The curve number corresponding to this group is 95. The smallest area of the soil hydrologic group is observed in group D which corresponds to the poor vegetation meadow. The curve number for this group is 83.

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Tab.2. Distribution of frequency of soil hydrologic groups' area for each land use and curve number Use Soil hydrologic groups Area (km) CN II Irrigated agriculture A 5.73 62 B 0.26 62 C 23.94 78 D 12.76 83 Fish pool A - - B - - C 1.18 93 D - - Poor vegetation meadow A - - B - - C - - D 0.15 83 Stone outcrops A 1.45 95 B 6.24 95 C 22.37 95 D 53.21 95 Metropolitan area A 0.69 98 B - - C 4.27 98 D 11.03 98 Meadow and shrubbery A - - B - - C 1.21 75 D 1.23 75 Total area - 145.74 -

In this study, the curve number of 95 has covered 57.3 percent of case study (more than half of the area) and 69 percent of the area is covered by the impermeable lands. Studying the discharge level of the runoffs indicates the increase of discharge in the impermeable areas. Considering this principle that increasing the percent of the impermeable areas increases the CN and also other principles indicating that an increase in CN reduces the surface capacity as well as the fact that the level of runoff goes up as less rain penetrates the soil, we can conclude that surface storage capacity in the case study is trivial and there is a great possibility for flood in this area.

2.6 Flood hydrographic simulation of SMADA software To prepare the corresponding hydrography, information concerning rainfall and the specification of the watershed is regarded as the inputs to the software. The specifications of a watershed are as follows: • Total city area (acre ) • Area of impermeable zone • Percent of the impermeable areas directly linked to the waterways • Concentration time (minute) • CN for the impermeable area • Initial loss of the impermeable area (inch) • CN for permeable area • Initial loss of permeable area (inch) In this paper, the area has been divided into permeable and impermeable zones and then, studied. An impermeable surface is a surface in which we observe the absorption, not penetration such as asphalt roads, roofs, etc. A permeable area which is directly linked to the waterway is a section of the permeable area where rainfall is directly directed towards the exit. To enter the rainfall information, we require rainfall height and duration and temporal distribution type of rainfall. The rainfall duration was assumed to equal the concentration time of the area and the duration of each rainfall was given as 10 minutes. As the distribution type of rainfall in the studied area was heterogeneous, we utilized a constant rain distribution. Finally, rainfall height was calculated in the form of intensity-duration-frequency graph in various return periods and the hydrography corresponding to that return period was drawn.

2.7 Flood hydrography in various return periods in the case of converting green and garden landuse to residential areas The highest level of runoff discharge in various return periods was observed in the concentration time of 1 hour. As the return period increased, the discharge level went up and the highest discharge was recorded in a return

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period of 100 years while a 2-year return period exhibited the lowest one. As the return period increased, the concentration time for the peak discharge of the flood decreased.

2.8 Index stations Lenj and Pol Kalleh stations situated at the downstream of the watershed between the eastern longitude of 51 34' and northern latitude of 32 24' with the height of 1672 and in the upstream between the eastern longitude of 51 13' and northern latitude of 32 22' with the height of 1750 in Isfahan province were used as index stations, respectively.

3- RESULTS AND DISCUSSION

The discharge level is presented in terms of m 3/s. In the permeable areas, the highest level of discharge in a 2- year return period was 2m 3s-1 which took place in a concentrated time less than one hour. The highest level of discharge in a 5-year return period was 5.83m 3h-1 taking place in a concentrated time of 1 hour and in a return period of 10 years. The highest discharge level was 7.96 m 3 in a concentrated time of 1 hour. In a return period of 25 years, the highest discharge level was 10.45 m 3 in a concentrated time of 0.83 hours. In a return period of 50 years, the highest discharge level was 12.25 m 3 in a concentrated time of 0.83 hours. Finally, in a return period of 100 years, the highest discharge level was 14.03 m 3 in a concentrated time of 0.83 hours. The flood hydrography in various return periods in the presence of the green and garden land use is represented. The highest level of runoff discharge in various return periods was observed in a concentrated time of 1 hour in a way that increasing the return period time increased the discharges. The highest and lowest discharge levels were respectively observed in the return periods of 100 and 2 years. As the return period increases, the concentration time of flood peak discharge reduces. In a study of vegetation management effects on the level of runoff in Golestan province, vegetation absorbs rain and evaporates some part of that. Vegetation helps to transfer water to the surface of the earth slowly and the impediment it makes against the flow of water increases the absorption level of the soil. Thus, vegetation has a significant impact on the runoff of the watersheds. Changing the type or density of vegetation can reduce or increase destructive floods or runoffs. The hydrography shape of flood in various return periods in case of changing green and garden land uses in the residential areas has been shown in figure 4.

16 14 12 TR=2

/s) TR=5

3 10 8 TR=10 6 TR=25 4 TR=50 2 0

Discharge (m Discharge 0.17 1 2time (hr ) 3 4 5

Fig. 4. Flood hydrography in various return periods when there is garden land use in the city

Discharge level is expressed in terms of m 3s-1. In the permeable areas, the highest level of discharge during a return period of 2 years is 2.43m 3s-1 at a concentration time of 1 hour. In a return period of 5 years, the highest discharge level is 7.07m 3s-1 in a concentrated period of 1 hour. In a return period of 10 years, the highest discharge level is 9.66m 3s-1 in a concentrated period of 1 hour. In a return period of 25 years, the highest discharge level is 12.68m 3s-1 in a concentrated period of 1 hour. In a return period of 50 years, the highest discharge level is 14.87m 3s-1 in a concentrated period of 0.83 hours. Finally, the highest discharge level is 17.03m 3s-1 in a concentrated period of 0.83 hours in a return period of 100 years. Studying the effects of urban development on the waterways, [13] stated that developing the urban installations distorts the natural morphology of the hydrologic balanced ecosystem. The hydrological imbalance of watersheds leads to huge floods and base stream reduction. • Figure 5 shows the hydrography of flood in various return periods in the case of converting green and garden landuse to the residential space. According to figure 5, the highest level of runoff discharge in various return periods was observed in a concentrated time of 1 hour and increasing the time of return period yielded a greater discharge. The highest and lowest levels of discharge were respectively observed

34 Parsasyrat and Jamali, 2015

in the return periods of 100 and 2 years. Increasing the return period reduced the concentration time for flood peak discharge. In a study of the effects of forestation and green and garden landuse creation on flood, Farazjoo and Khalilzadeh[1] stated that converting the current green and garden landuses to the residential areas would turn the whole city into a vast impermeable area. The resultant CN will equal 83 which results in the reduction of initial losses and the increase of runoff volume. Studying the effects of urbanization on the flooding possibility of Tirsolam watershed, Suriya and Mudgal [20] stated that increasing the impermeable areas distorts the natural balance of water penetration. Lack of permeability increases the runoffs and leads to the peak of flood even in short and low intensity rainfalls. Changing the land uses related to the urbanization is always reflected in a stream regime. Flood management seeks to coordinate human activities and flood dangers using appropriate studies for improving the water regime. Thus, the urbanization hydrological impacts must be taken into consideration in the urban planning.

18 16

14 TR=2 /s) 3 12 10 TR=5 8 6 TR=10 4 Discharge(m 2 0 0.17 1 2 3 4 5 time (hr )

Fig. 5. Flood hydrography in various return periods in the case of converting gardens into residential areas

As it is evident from the graph in figure 6, Lenj station (as the index station) has had less discharge and water volume than the upstream station during 1981-1995. Since 1995, the pace of changes in discharge and water level has accelerated as a result of urban development and larger impermeable surfaces in such a way that Lenj station chart has been above Pol Kalleh station's one.

Pol Kalleh Lenj

350 300 /S) /S)

3 250 200 150 100

Discharge Discharge (m 50 0 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Year

Fig. 6. Rate of annual discharge changes in 2 stations (Pol Kalleh-Lenj)

Actual data have studied the hourly discharge for preparing flood hydrography in figures 7 and 8 two days are chosen from all flooding days and their corresponding hydrographs are drawn to the base water (Fig. 7) and without it (Fig. 8). The resulting hydrographs are then compared with Santa Barbara ones. As you can see in the picture, the

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base water for this hydrography is 3.15m 3s-1 and after subtracting base water from discharges, the smallest and largest discharges were 0 and 218.5m 3s-1, respectively.

300 250

250 200 /s) /s) 3 200 3 150 150 100 100 50

50 (m Discharge Discharge (m Discharge 0 0 0 10 20 30 40 0 10 20 30 40 time (hr) time (hr)

Fig. 7. Flood hydrography with the base water on Fig.8. Flood hydrography without the base water on February 9 th and 10 th , Pol Kalleh station February 9 th and 10 th , Pol Kalleh station

180 90 160 80

140 70 /s) 3 /s)

3 120 60 100 50 80 40 60 30

40 (m Discharge 20 Discharge (m Discharge 20 10 0 0 0 10 20 30 40 0 10 20 30 40

time (hr) time (hr)

Fig.9. Flood hydrography with the base water on Fig. 10. Flood hydrography without the base water on February 9 th , Lenj station February 9 th , Lenj station

Actual data have studied the hourly discharge for preparing flood hydrograph in figures 9 and 10. This day is chosen from all the flooding days and its corresponding hydrography is drawn to the base water (Fig. 9) and without it (Fig. 10). The resulting hydrography is then compared with Santa Barbara ones. As you can see in the corresponding hydrography, the base water for this hydrography is 71.5m 3s-1 and after subtracting base water from discharges, the smallest and largest discharges were 0 and 80.9m3s-1, respectively. As it can be seen, the peak discharge of urban hydrographs has been high in various return periods in the studied area in control stations. As the return period goes up, the peak discharge in both areas (control station and the studied area) increases. Changes have been significant in downstream stations; thus, it can be concluded that this station, which is near the ZarrinShahr station has faced the urbanized development and land use changes in the downstream of the watershed in recent years as the level of runoff and the discharge of desired area have increased. Results of a study conducted by Rahmani et al. [22] indicate that increasing the impermeable surfaces would trigger a rise in the level of runoff and flood which may be compliant with the results of this study.

36 Parsasyrat and Jamali, 2015

4- Conclusion Forestation and green and garden landuse creation increase the level of water penetration. As a result of greater levels of water absorption and larger permeable surfaces, the level of the runoff and flood may be reduced. As our study has shown by preparing various charts and also forming the hydrographs of the desired area as well as a discharge graph of February in 2 control stations located before and after the area, it was found that increasing the impermeable areas will lead to greater levels of runoff and flood. The presence of vegetation, including meadow and shrubbery is one of the factors that reduce flood due to their impacts on hydrologic cycle factors. Thus, green and garden landuse has increased the level of water penetration and contributed to control the runoff due to its great effects on these issues. The factors that directly influence the water penetration into the earth involve the differing of land uses and soil conditions which are the most important causes of flood in the area.

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