تحليل فضايي مخاطرات ژئومورفولوژيكي توسعه‌ي شهر در حريم رودخانه‌ي خشك كلان‌شهر شيراز

Spatial Analysis of Geomorphological Hazards of Urban Development in the Banks of Khoshk River in Shiraz, Iran

توسعه‌ي شهرنشيني و گسترش مناطق اسكان جمعيت در حريم رودها مستلزم مطالعات دقيق درباره‌ي شرايط سيل‌گيري محدوده‌ي مورد نظر براي توسعه است. بيشتر خسارات وارده‌شده به شهرها ناشي از وقوع سيل به دلايلي نظير تخطي در بستر و حريم رودخانه‌ها، پايين بودن ظرفيت پل‌ها و معابر حركت آب، ناپايداري سيل‌بندها، ساخت و ساز در مسير سيل‌ها و كاهش پوشش گياهي اراضي مشرف به شهرهاست. بر اثر اين اقدام‌ها هر ساله صدمات جبران‌ناپذير مالي و انساني به مناطق گوناگون كشور وارد مي‌آيد. در اين پژوهش كلان‌شهر شيراز از لحاظ مخاطرات ناشي از فرايندهاي ژئومورفيك رودخانه‌اي بررسي شده است. مطالعه‌ي اين مخاطرات در دو قسمت صورت گرفت. در يك قمست مخاطرات ناشي از فرسايش رودخانه و آبراهه‌ها (تخريب، حمل و رسوب‌گذاري) و مخاطرات ناشي از طغيان آب و سيلاب مطالعه شد و در قسمت ديگر تحليل و بررسي نقش عوامل انساني تشديد‌كننده‌ي مخاطرات رودخا‌نه‌‌اي انجام گرفت. روش تحقيق در اين پژوهش كتاب‌خانه‌اي ـ ميداني است. نخست از عكس‌هاي هوايي و تصاوير ماهواره‌اي براي بررسي حوضه‌ي‌ آبريز رودخانه‌ي خشك و پهنه‌هاي سيل‌خيز استفاده گرديد. سپس، بازديدهاي ميداني و برداشت نقاط زميني با استفاده از GPS، تفسير عكس هوايي و كاربرد GIS نقشه‌ي مناطق در معرض خطر حاشيه‌ي رودخانه و فاصله از رودخانه ترسيم شد. در نهايت مشخص شد كه از يك طرف بخش شرقي كلان‌شهر شيراز به دليل قرارگيري بر روي دشت سيلابي از طرف آبراهه‌ها و رودخانه‌ي خشك و از طرف ديگر با تعرض به حريم رودخانه، ابعاد و ارتفاع نامناسب پل‌ها، احداث كنارگذرها و... در معرض مخاطرات محيطي ناشي از فرآيندهاي رودخانه‌اي و عوامل انسان‌ساخت قرار دارد.

The process of urbanization and development in high-risk areas such as river banks has increased the vulnerability of urban communities to environmental hazards. The banks of Khoshk River in Shiraz is one of these areas. These hazards are two parts : hazards resulted from river and waterways erosion (destruction, transportation and sedimentation) and the hazards resulted from floodings over the surrounding urban areas. In order to prepare the literature review for this study, the various books, theses and articles were applied. Also, in order to determine the spatial position of this section, the Satellite Images and Google Earth pictures were used. The Global Positioning System (GPS) was also applied for the field observations such as collecting spatial data, extracting the kind of formations, Geological structures and faults. ArcGIS and Global Mapper 16.2 were also used for data processing and mapping. The geomorphological hazards in Khoshk River bank were evaluated in two parts: The evaluation and analysis of the role of river and flooding processes in creating the environmental hazards for Shiraz. The evaluation and analysis of the role of humans as the intensive factors of riverine and floods hazards in city. The evaluation of longitudinal profile in the river indicates that when the stream is entered to plain, the water moves with more speed because of faults and high steepness over the Drake alluvial fan. One indication of this process is the presence of coarse sands and angled gravels. In this part, the erosion of riverbank is much higher than the erosion of river bed. In this section, the longitudinal profile of the river has a regular trend of concave and convex sections due to the erosion in convex parts and sedimentation in concave parts. In addition, there is a balance between deposition and digging process. The erosion is very intensive in regions where arc meander is close to the flooding plain of the bank and causing the destruction of all facilities. The longitudinal profile in the river indicates that the height and slope of river has been reduced from North West to Maharloo River. The average slope of the river is 2.40%. In order to determine the role of flooding in creating risks for Shiraz, the floodwater discharge data were collected from Regional Water Organization. Furthermore, in order to understand the role of maximum discharge values, various experimental relations were used in the basin. The un- ordered development of urban areas especially in the north west, destruction of natural areas intensified the amount of runoff and reduction of vegetation cover. The pick values of maximum discharges in Khoshk river with the return periods of 50 and 100 years waere estimated 115m3/s to 131.4m3/s respectively which may result in overflowing of water on the streets. The human factors include the construction of bridges on the river, fencing river with stones and construction of beach, construction of bypasses for public transportation and reducing the traffic in the riverbed and trespassing to the river bed in Shiraz caused the overflowing of water from the river. The last floods in Shiraz occurred in 1987 and 2002 that caused major losses to the houses and commercial places close to Khoshk River. In order to analyze parts of river that are close to the town and have more important influences on the hazards and disasters, the satellite images of khoshk river basin in the town were taken and the river was classified in three sections with regard to risks level, river morphology and river classification based on its hazards for close areas as high risk (Maali abad Bridge limits to Fazilat Bridge and Sardkhaneh Bridge to Maharloo River), low risk(Tange sorkh to Maali abad Bridge) and medium risk (Fazilat Bridge to Sardkhaneh Bridge).