آسيب‌پذيري ناشي از سيلاب شهري (مطالعه موردي: تهران، دركه تا كن)

Vulnerability of Urban Flooding Case Study: Tehran, Darake to Kan

تسطیح زمین، تجاوز به حریم رودخانه‌ها و مسیل‌ها، سبب رخداد تغییرات الگوی زهكشی طبیعی و جاری شدن سیل در نواحی شهری، آب‌گرفتگی معابر و افزایش هزینه‌های نگه‌داری شهر می‌شود. شهر تهران مستقر در دامنۀ جنوبی كوه‌های البرز مركزی، در پایین‌دست حوضه‌های آبریز متعددی قرار دارد. گسترش شهر تهران تا ارتفاع 2200 متری، باعث تفاوت ساختاری در بافت فیزیكی این شهر و درنتیجه مستعد مخاطرات طبیعی ازجمله سیلاب شده است. مناطق 2 و 5 شهرداری تهران در شمال غرب تهران به دلیل مجاورت با حوضه¬های دركه، فرحزاد، و كن، بالا بودن سهم نسبی ساخت‌وسازها، بالا بودن نسبی تراكم مسكن و جمعیت، تغییر كاربری و وارد شدن به حریم رودخانه و بهره‌برداری نادرست از مسیل¬ها در معرض ناپایداری ناشی از سیلاب قرار دارد. این پژوهش با استفاده از نقشه‌های توپوگرافی 1:25000، داده‌های اقلیمی و هیدرومتری (2012 تا 1995) و داده‌های كاربری دریافت شده از سازمان شهرداری تهران انجام شده است. معیارهای انتخابی با روش مقایسه دوتایی وزن دهی شدند و بر اساس توابع فازی تلفیق گردیدند. برای تشخیص نقش عوامل مؤثر از روش‌های تهیه نیمرخ آبی و هیدروگراف استفاده شد و ضرایب اهمیت با استفاده از تحلیل شبكه عصبی چند لایه MLP استفاده گردید. نقشه آسیب‌پذیری حاصل از وزن‌های مقایسه‌ای و توابع تلفیقی فازی نشان داد كه بیشترین آسیب‌پذیری در خارج شهر پیرامون مسیل‌ها می‌باشد. به دلیل اهمیت حجم رواناب در حوضه‌های بالادست مناطق شهری، سهم حجم رواناب با احتساب مساحت نواحی تعیین گردید. نتایج، سهم رواناب حوضه كن در نواحی شهری را زیاد نشان داد. لذا برای ارزیابی ورودی مسیل كن به تهران، ایستگاه باران‌سنجی سولقان انتخاب شد و پروفیل طولی و هیدروگراف آن ترسیم گردید. نتایج مطالعات آسیب‌پذیری در نواحی شمال‌غرب تهران كه حوضه‌های دركه تا كن در بالادست آن قرار دارند و مناطق شهری 2 و 5 را شامل می‌شود نشان داد كه تركیبی از عوامل بالادست و شهری سبب تشدید آسیب‌پذیری این مناطق می‌شود. اگرچه در مناطق خارج از شهر حواشی رودخانه‌ها و مسیل‌ها بالاترین آسیب‌پذیری را نشان می‌دهند، لیكن عوامل دیگری ازجمله بافت فرسوده، سطوح شیب عمودی و ضریب انحناء در تعیین میزان آسیب‌پذیری نواحی شهری دخالت می‌كند. شواهد این ادعا در برداشت‌های میدانی آشكار است. جهش‌های دبی پیك در هیدروگراف ماه اكتبر سال 2012 بر لزوم توجه به رواناب خروجی حوضه كن دارد كه تهران به‌طور سالانه حوادثی در رابطه با حوضه كن را تجربه می‌كند.

1. Introduction Nowadays, population increase, progress in science and technology and development of industrial installations with the shortage of places in metropolitan areas have made considerable changes in the morphology of watersheds. Violation to territory of rivers, channels, and streams causes alterations in natural drainage pattern and overflowing due to the lower capacity of the changed channels and streams. This can increase the risk of flooding and inundation of the streets and also the costs of urban maintenance. Moreover, this will considerably increase the possible financial and life damages. 2. Study Area Tehran is a city located in the southern slopes of Central Alborz downstream many watersheds. Furthermore, the excess expansion of urban areas to 2200 m foothills as well as uneven development and structural difference in physical texture caused the areas to be subject to serious risks of flooding. Among the Tehran municipality regions, the regions 2 and 5 have be developed to absorb the overflow of population and immigrants from central and southern areas of the region. These areas are in the vicinity of the rivers of Darakeh, Kan, and Farahzad, on one hand, and have high rate of constructions, instable residential areas, relatively high density of population and houses, landuse changes, violation to the territory of rivers, and improper use of the channels, on the other hand. As a result, they are subject to flooding. Thus, the areas are highly important due to the possibility of huge flood occurrences. 3. Material and Methods The data of this study include elevation data extracted from topographic maps (1:25000), climate and hydrometric data (1995-2012), and landuse data from Tehran municipality. The effective or intensifying variables in vulnerability assessment of physical infrastructures in region 2 and 5 downstream the watersheds of Darakeh and Kan have been selected by field observations. As the variables of runoff, slope, aspect, landuse, elevation classes, curvature, old texture, surfaces perpendicular to slope and those parallel to the slope have different measuring units, they have been standardized by pairwise comparison method. For the lands upstream the study area, the runoff has been estimated by SCS (CN) model. Calculation of CN in urban areas has been based on landuse type and percentage of open spaces. The old texture is used in the method because of its influence in flood occurrence due to inadequate drainage, sound management, and permeability of urban structures. The integration of the data has been carried out by fuzzy functions using the weights obtained from the pairwise comparison. For vulnerability assessment, the gamma maps of 0.9, 0.85, and 0.7 have been prepared to compare their correlations with other variables. The gamma 8.5 had the highest correlation with the principal variables. To detect the role of effective factors, we have used water profile and hydrograph with the coefficients using the analysis of Multi-Layer Perceptron Network (ULP). 4. Results and Discussion The vulnerability map resulting from comparison weights and fuzzy integration functions has indicated that the most vulnerable areas are near the channels; particularly those increase the risk of flooding in urban areas through higher discharge. The comparison of the vulnerability map with the transportation networks indicates that most of the vulnerable areas are coincident with the west-east networks crossing the natural slope of the streams. It also indicates that the most vulnerable areas are intersections of the transportation networks of north-south and west-east. These intersections impede the flow of water over the natural north to south slope of the area. Due to the importance of the volume of runoff upstream, the proportions of the runoff were determined based on the area of each region. The results have also represented that the proportion of runoff from Kan watershed on urban areas was more than others. To assess the entry of Kan watercourse into the city, the Soleghan precipitation station has been selected. Three time series of precipitation has been drawn from 27th to 30th October 2000, 2007, and 2012. Based on the data, flood discharge was calculated in three different states and its long profile and hydrograph was depicted. According to field observations, the Kan valley is 3 meters wide and 1.5 meters high. According to long profiles, water flows are increased with an increase in rainfall; this is along with peak discharges and likely flooding on the hydrograph of Kan watershed. 5. Conclusion The results of vulnerability assessment in the study area indicated that a combination of upstream natural factors and urban agents make these areas vulnerable against the floods. Although outside the urban areas the lands near the channels and streams are more vulnerable, as also confirmed by field observations, there are some other factors affecting the vulnerability of urban areas. The old texture is greatly important in the vulnerability because it does not have a suitable drainage ability and the available drainage networks are also mainly in the ending part of the areas. Thus, the inundation can intensify the floods or even cause them. The vertical slope surfaces and the curvature in north and northwestern areas of Tehran also show conformity and non-conformity with the drainage and watercourses. The jumps of peak flow in the hydrograph of October 2012 make it necessary to pay more attention to the runoffs of Kan drainage basin as the city of Tehran is experiencing events related to the watershed.