Sustainable Development and Planning II, Vol. 1 423 The effects of urbanization on the natural drainage patterns and the increase of urban floods: case study Metropolis of Mashhad-Iran S. R. Hosseinzadeh Department of Geography, Ferdowsi University of Mashhad Abstract Alluvial fans create a special type of flash flood threat, especially in arid and semi-arid areas where the fans support urban development. Most of the cities in Iran have developed in such environments, and are facing flash and urban floods because of the rapid growth of the cities, and the deficiency of urban planning in the last four decades. The present study aims at analyzing the urbanization effect on natural drainage patterns and their immediate result, namely the increase of vulnerability of the metropolitan city of Mashhad. Mashhad, as the second religious metropolis of the world, and second metropolis of Iran has an annual 12 million pilgrims, and 3 million population. The results of this study indicate that the rapid growth of the city from 1966 to 2002 has had excessive destructive effects on the catchments area, and has caused the decrease of impervious surfaces, and the increase peak discharge of the urban floods. Keywords: urbanization, flash flood, urban flood, drainage pattern. 1 Introduction The effects of urbanization on the hydrology and morphology of watersheds have been studied by Leopold [16, 17], Hammer [8], Hollis [9], Dunne and Leopold [5], Klein [15], Arnold [2], Gordon et al. [7], Arnold and Gibbons [3], Christopher [4], Paul and Meyer [21] and White and Greer [23]. According to previous studies, development of cities in the watershed increases the area of impervious surfaces which decreases infiltration of precipitation and in creases runoff. Runoff increases in proportion to the cover impervious surface in a watershed and the increased storm runoff increases peak discharge and flood
424 Sustainable Development and Planning II, Vol. 1 Magnitudes. Reduced infiltration of precipitation to groundwater aquifers may reduce groundwater recharge, and stream base flow [13]. Although the increases of flood and runoff have been mentioned in the previous studies, their effects have not been noticed in relation with the city itself, and urban population. The present study analyzes the effects of urban development on flash and urban floods and the deficiency of urban planning in its increase and seeks a way to decrease the environmental effects of these floods on metropolitan cities in Iran as most of these cities are located on alluvial fans where many small watersheds join them from the mountain sides. Urbanization disrupts the hill slope profiles, ruptures of vegetation, compacts soils and disrupts the stream long profiles. It also changes the natural drainage patterns and increases the urban flood hazard [5]. 2 The study area and methods The Metropolis of Mashhad in the Khorasan province is located in the Northeastern part of Iran and in the Kashaf Rood basin. To study the effects of the development of this city in urban floods, an area of 576 sq. kilometers between 36 0 13 to 36 0 25 N and 56 0 28 to 56 0 44 E were chosen. This area includes three geomorphologic features: mountain, pediment, and flood plain. The expansion of Mashhad starts from the flood plain toward the mountain and covers the whole Alluvial fans. To study the role of the city in inundation, the analysis was mainly carried out based on the historical geomorphologic method along with and its combination with the experimental methods as follows: The mountain landscape was subdivided into 19 stream catchments and the basic studies and the effects of urbanization were carried out through topographical maps, Air photographs, field observations and peak of the flood in each watershed have been calculated according to the rence of 2 to 100 years. The aim of this calculation was to determine the maximum discharge of flood entrance to the urban area from the catchments. On the pediment, alluvial fans and flood plain landscapes, the boundary of the city were determined by the air photographs of 1966, and the geomorphologic terrains, and the natural drainage networks were extracted and drawn on the basis map. Thereafter, the effects of the expansion of the urban area on the previous geomorphologic terrains were indicated, using satellite images. The changes that were caused by the expansion of the city in the natural drainage network, especially in the main channels, were measured and analyzed through air photographs and field observations. By the zonation of area into 8 impermeable regions according to the coverage area of each main urban canal, the maximum of flash flood in each level and the flood peak of each watershed in the mountain area were calculated and the discharge surplus in each canal was estimated. Finally, after a careful consideration of the original conditions of the canals, especially their width before the expansion of the city, corrective solutions for each component area were presented.
Sustainable Development and Planning II, Vol. 1 425 3 The discussion and the result 3.1 Catchments and the indirect effects of urbanization The watersheds leading to Mashhad are named with abbreviations: A1 to A19 and their physical parameters are presented (table 1). All of the catchments contain high slope and have short concentration time. Nearly all of the watersheds are formed in the metamorphic and ultrabasic Permian rocks with extensive stone outcrops, little infiltration and dispersed thin soils. Due to low precipitation (an annual of 280mm) the vegetation in these areas is disperse and poor. Table 1: Physiographic characteristics of catchments. Catchments Area (km 2 ) Circus (km) Gravels coefficient Elevation (m) Min. Max. Mean. Stream length (km) A1 1.22 4.94 1.25 1067 1473 1192 1.98 0.25 A2 0.92 4.67 1.36 1080 1479 1215 2.01 0.23 A3 2.21 7.08 1.33 1092 1509 1261 2.62 0.31 A4 1.48 5.92 1.36 1090 1367 1194 2.99 0.40 A5 0.40 2.98 1.32 1039 1240 1111 0.99 0.15 A6 0.91 4 1.17 1050 1260 1133 1.36 0.18 A7 1.93 6.81 1.37 1100 1430 1235 2.75 0.35 A8 2.47 6.99 1.24 1120 1423 1253 2.58 0.33 A9 1.30 5.84 1.43 1140 1443 1269 2.39 0.30 A10 0.64 3.82 1.33 1100 1256 1147 1.59 0.23 A11 3.46 13.31 1.55 1090 1510 1277 5.33 0.65 A12 3.46 8.33 1.25 1120 1534 1289 3.38 0.40 A13 1.59 6.17 1.37 1119 1521 1220 2.65 0.30 A14 1.59 6.98 1.27 1160 1526 1280 2 0.21 A15 0.42 2.69 1.15 1159 1310 1124 1.01 0.15 A16 3.23 8.57 1.33 1170 1500 1312 2.67 0.33 A17 0.89 4.2 1.26 1160 1350 1220 1.66 0.21 A18 0.42 3 1.3 1120 1265 1160 1.02 0.13 A19 0.35 2.6 1.23 1120 1230 1112 0.92 0.13 Time of cons (hour) The development of the city of Mashhad in the last four decades, in addition to the change in Microclimatic conditions [7] has also caused extreme changes in the morphology of the watersheds. These changes have increased the flood velocity and the volume of Flash flood sediments. The changes include the following: The disruption of the slope stability: The urban need for stone resources has caused the disruption of the stability extraction from the 100 mines in the watersheds and has developed the escarpments. It has also formed holes with a diameter of 50 to 100 meters in the location of each mine and has left a high volume of loose rubbles on the sides and the bottom of valleys.
426 Sustainable Development and Planning II, Vol. 1 The disruption of the stream long profiles: the need for sand and gravel resources for the construction of highways and streets of the city has caused a great volume of sand and gravel to be taken away from the main river-beds. This withdrawal has changed the river beds greatly and has made them unstable. Furthermore, the disposal of building wastage in the valleys has completely blocked some stream channels and formed anthropogenic landforms. As these instances are very varied concerning their forms and material, they are very sensitive to eroding hence capable of making sheet floods during heavy rains. Soil compaction and the destruction of Natural vegetation: More than 60 kilometers of the roads are constructed in the catchments which in addition to destroying the vegetation also disrupt the steep slopes, reduce of penetrability and the produce of unconsolidated materials or loose rubble layer. 3.2 Flash flood frequencies and discharge Concerning the physical conditions of the basins and the destructive effects of urbanization and based on the magnitude and the frequency of rainfall curves of Mashhad synoptic station, the flood crest in the watersheds has been calculated for 2-100 years and the result has been presented in table 2. Table 2: Estimation of highest flood discharge (Peak Flood). Catchments 2 years 5 years 10 years 25 years 50 years 100 years A1 3.82 6.10 7.62 9.96 11.29 13.74 A2 2.88 4.60 5.74 7.51 8.51 10.35 A3 7.51 11.13 14.10 18.86 21.24 25.84 A4 3.69 5.44 6.92 9.43 10.63 12.94 A5 1.83 2.74 3.35 4.27 4.88 6.09 A6 3.73 5.96 7.44 9.73 11.03 13.42 A7 6.56 9.72 12.31 16.47 18.55 22.57 A8 8.42 12.47 15.80 21.13 23.80 28.96 A9 4.41 6.54 8.28 11.08 12.48 15.19 A10 1.72 2.76 3.44 4.50 5.10 6.20 A11 12.62 18.25 23.48 32.56 37.31 45.43 A12 8.63 12.74 16.20 22.05 24.86 30.27 A13 4.54 6.73 8.52 11.40 12.84 15.63 A14 7.33 11.73 14.64 19.14 21.70 26.40 A15 1.28 1.92 2.34 2.98 3.41 4.26 A16 11.00 16.30 20.64 27.61 31.10 37.84 A17 2.17 3.88 4.92 6.23 7.32 8.2 A18 1.07 1.96 2.45 3.08 3.62 4.05 A19 0.9 1.63 2.04 2.57 3.02 3.39 3.3 Urban development pattern and its direct effects The expansion of the city of Mashhad during the last four decades has been shown on the geomorphologic map from 1966 to 2002 respectively. (Figures 1
Sustainable Development and Planning II, Vol. 1 427 and 2) Based on figure 1, the area of the city in 1960s has been 29.1 sqkm. And has mainly been located on the flood plain. This area has increased to 228.1 sqkm in 2002 and has covered more varied geomorphologic features. (Table 3) The resulting changes from this process can be studied as follows: The increase in the impenetrable surfaces: Through field observations, it was evident that except of the flood plain, the other geomorphologic features had a very high rate of penetrability before the expansion of the city. After the expansion, 118.4 sq. kilometers of the plains had high penetrability in the Alluvial fans and 80.6 sq kilometers had low penetrability in the flood plains as more than 80% of urban areas were turned into asphalt areas. As the steepness of the Alluvial fans was more than the flood plains steepness (2 to 3%) the urban watersheds were capable of more runoff and floods. Table 3: Geomorphologic features and urban development. Landscape type Landscape Average City area on City area on area Km 2 slope (%) 1966 (Km 2 ) 2002 (Km 2 ) % Mountain, Incelberg, Buttes 42.63 7.4 20 0 0.6 Pediments 137.1 23.8 2 0.05 14 Alluvial Fans 137.4 23.6 2 0.975 104.4 Flood Plain 251.68 43.6 > 1 28.075 109.7 Total 576.6 100 -- 29.1 228.7 Table 4: Changes in drainage systems associated with urbanization. Watershed Area (Km 2 ) Urban Natural Total Eliminated or obstructed main channels Length Number Eliminated secondary channels Length Number (km) (km) Nu 1 9.2 5 14.2 5 12 32 17.6 2-9 Nu 2 7.62 0.42 8.04 4 11.13 69 29.2 5 Nu 3 4.87 7.39 12.26 7 16.11 21 15.6 6 Nu 4 6.8 -- 6.8 5 8 50 31.6 3 Nu 5 3.02 6.38 9.4 6 11 35 13.5 6 Nu 6 6.5 5.7 12.2 7 10.6 47 22.4 4 Nu 7 2.21 1.31 3.52 3 2.4 13 6 4 Nu 8 2.71 5.83 8.54 6 11.7 10 4.8 6 Total 42.93 32.03 74.96 277 140.7 43 82.9 -- Section of new canal Based on the urban land cover maps, an average 25% of the land uses have been appropriated to streets and green areas, 50% to houses, and 25% to the commercial and public units. The runoff of all streets; and the runoff of 25% of the roofs enter the urban drainage systems. In the field measurements, it became clear that the time of concentration of urban floods has doubled, and the flood frequencies have increased to 100%.
428 Sustainable Development and Planning II, Vol. 1 Figure 1: Map of geomorphologic features of study area. The change in the natural drainage pattern: on the Alluvial fans, and in the flood plain, the expansion of the city has destroyed 277 natural secondary channels, about 140 kilometers long and 43 main channels measuring 82.9 kilometers long have been blocked or changed extremely. (Figure 4) The main natural channels including drained channels from the mountain and wide channels (wider than 10 m) were located on the Alluvial fans. The main channels, due to the urbanization limitations were blocked or their courses had been changed merged into one another. These 19 channels were finally connected to 9 main urban canals. The secondary channels had the same destiny as the main channels and vanished altogether. Urban drainage system includes streets, canals and water passages of the sidewalks which are eventually concentrated in the main urban canals. According to the coverage area of each canal we can divide the Alluvial fan area of the city that faces the serious problem of flood into 8 new urban watersheds and study the resulting changes according to table 4 in each one of them. 3.4 The weakness of urban drainage system Urban texture and its drainage system does not correspond very much to the natural drainage patterns, because in the urban development planning, rivers, as
Sustainable Development and Planning II, Vol. 1 429 one of the most important landscapes, have been omitted and canals with low capacity have replaced them. Measure weakness of urban drainage system, the maximum probable peak in each natural urban watershed is calculated. The results of the calculations have been presented in table 5. Figure 2: Table 5: Map of landforms and urban development. Peak flood in natural and urban drainage basins. Peak flood of urban main canal (m 3 Total of peak Water Urban Runoff /s) flood shed area (ha) coefficient 2 5 10 25 50 100 50 100 Nu 1 920 0.55 19.26 31.6 40.76 51.44 59.4 67.46 73 83 Nu 2 762 0.47 16.6 28.1 34.8 44.8 53.7 59.7 57.1 101.8 Nu 3 487 0.5 12.8 21 28.4 35.8 40.6 47.3 100 119.6 Nu 4 680 0.47 14.8 25.1 31.1 39.9 47.9 53.3 47.9 53.3 Nu 5 302 0.47 8.7 15.8 19.7 24.8 29.2 32.7 71.6 99.4 Nu 6 650 0.3 9.04 15.7 18.9 24.4 29.2 32.5 45.16 52.01 Nu 7 221 0.4 5.4 9.8 12.3 15.5 18.2 20.4 34.1 39.91 Nu 8 271 0.43 7.1 12.9 16.2 20.4 23.9 26.9 75.6 89.77 The comparison probable floods with 50 years, indicates that the existing urban canals are not capable of carrying the flash floods. The flood hazard will increase by the year 2015 when the asphalt covering will include all
430 Sustainable Development and Planning II, Vol. 1 the urban areas. Table 6 shows the features of the main urban canals and the surplus peak during the 50-year rence floods, while according to the field measurements and through air photographs, the width of none of the main natural canals before this development was more than 20 m, and the change in these conditions brings about the peak surplus. Figure 3: Map of urban development and change of the Natural drainage patterns. Table 6: The characteristics of urban canals and non capacity for flash floods. Number of canal Water shed Secondary Main Main canal section (m 2 ) Length of canal (km) Slope of canal (%) Nu 1 1 1 2-9 8.25 1-2 60 Nu 2 -- 1 5 2.87 3 69.9 Nu 3 2 1 6 3.75 3 78.5 Nu 4 1 1 3 3.7 2 45.5 Nu 5 1 1 6 2.43 3 48 Nu 6 -- 1 4 3.92 2 33.2 Nu 7 With Nu 6 4 0.625 2 39.9 Nu 8 1 1 6 0.4 4 60.3 Surplus discharge for 50 yearly floods ()
Sustainable Development and Planning II, Vol. 1 431 4 Conclusion The following results can be obtained from the sum of this study: The residential areas on the banks of the main channels of 8 watersheds are exposed to flash floods which will probably entail the destruction of houses and deaths of residents. The main exit water passage from the mountain in these areas enters to narrow streets and the water level rise to more than 2 meters in them. The neighboring areas around the main urban canals in Nu1, nu2, nu3, nu5, nu6 and nu8 measuring 6/1 sq. kilometers are exposed to urban floods and the damages are mainly done on residential units, and public land uses. As these areas have been built on unstable soils, they face a high risk. Nearly 80 sq. kilometers of the urban area in the flood plain which now is exposed to surface water covering leads to slow movement of traffic during the annual rainfall. The main reason for the increase of flood potential of the city is the defect in urban design and urban drainage system which have been done with no consideration to natural environmental conditions and applies a high density of land use in these areas. To reduce the danger of flood, a master plan for the reduction of this danger should be taken under consideration. This plan is an interdisciplinary investigation including the following items: The extraction of sand and gravel resources catchments should be forbidden and all the disposed waste material in the streambeds should be evacuated. The flood control plans within the watersheds should be put into effect. The main canal passages inside the urban area should be reconstructed, and the channels should be led to their original natural path. The urban drainage system and the original natural pattern should be matched with each other. The density of land use, in urban texture, should be reduced and permeable surfaces increased. References [1] Alizadeh. A. Applied Hydrology. Astaneghods Publisher 1989. Mashhad Iran. P 292. [2] Arnold.C. L. P. J. Boston and P.C. Patton. 1982. Sawmill Brook. An example of rapid geomorphic change related to urbanization. Journal of geology 90.pp155-166. [3] Arnold. C. L. and C. J. Gibbons. 1996.Impervious surface coverage, the emergence of key environmental indicator Journal of the American planning Association.62(2) pp243-258 [4] Christopher. Cumulative effects of urbanization on small stream in the Puget sound London Excoriation. Applied physics laboratory, collage of ocean and fishery sciences. University of Washington. pp 51-70.
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