طبقه‌بندی مخاطرات رودخانه‌های شهری (مطالعه موردی: استان مازندران، شهر نوشهر)

ژئومورفولوژی می‌تواند با مطالعه مخاطرات رودخانه‌ای در مدیریت سیستم‌های رودخانه‌ای مشارکت نماید. در این مطالعه، رودهای شهری نوشهر مورد بررسی و طبقه‌بندی قرار گرفتند. نوشهر در شمال ایران و استان مازندران واقع شده است. در این مقاله روشی برای طبقه‌بندی مخاطرات رودخانه‌ای در مناطق شهری پیشنهاد شده است. کار در سه مرحله صورت گرفته است: 1) بازه‌های کانال بر اساس ویژگی‌های ژئومورفیک و شاخص‌های تعدیل رود تفکیک شدند. حساسیت زئومورفیک هر بازه در سه گروه زیاد، متوسط و کم تقسیم‌بندی شدند. 2) در مرحله دوم، هر بازه رودخانه‌ای بر اساس فعالیت مستقیم انسان(مدیریت) و سازه های رودخانه‌ای تقسیم‌بندی شدند. بازه‌های رودخانه‌ای به چهار گروه a، b، c و d گروهبندی شدند. در مرحله سوم، با ترکیب مراحل قبلی (تعدیل ژئومورفیک و فعالیت انسانی) رودهای شهری به 12 طبقه تقسیم‌بندی شدند. شدت فعالیت فرایندهای ژئومورفیک هم با مقادیر 1، 2 و 3 رتبه‌بندی شدند. ظرفیت تعدیل ژئومورفیک همه بازه‌ها در گروه متوسط و کم قرار دارد. بازه‌های رودهای کورکورسر(41) و ماشلک (1412) تقریباً طبیعی بوده و در گروه md و mc قرار دارند. این کانال‌ها ظرفیت تعدیل طبیعی متوسط داشته و مداخلات حفاظتی در کمتر از 10 درصد بازه وجود دارد. بازه‌های رودخانه گردکل در طبقه la و lb طبقه‌بندی شدند. این کانال‌ها ظرفیت تعدیل طبیعی کمی داشته و کناره‌های آن‌ها با پوشش‌های محافظ حفاظت می‌شود. پس از طبقه‌بندی رود، مخاطرات کانال‌های رودخانه‌ای شامل فرسایش، رسوب‌گذاری، فعالیت انسانی و هیدرولوژی در هر بازه مورد بررسی قرار گرفتند. در طبقه md فرسایش غلبه داشته و در طی سیلاب فرایند تعدیل طبیعی اتفاق می‌افتد. دخالت‌های انسانی مشاهده‌شده در این بازه‌ها شامل اشغال دشت سیلابی، انباشت نخاله‌های شهری و برداشت رسوب از بستر رود می‌شود. در طبقات la و lb در همه بازه‌ها اشکال رسوب‌گذاری دیده می‌شود. رسوب‌گذاری موجب کاهش ظرفیت کانال و خروج سیل از بالای تراس های آبرفتی شده که موجب ایجاد خسارت به مناطق شهری شده است.

The classification of Channel Stream Hazards in Urban Areas (Case Study: Mazandaran Province, Noshar)

Introduction;;Geomorphology can contribute to the management of fluvial systems through study of channel stream hazards. Hazards have been identified for geomorphic systems, being defined as any landform change, natural or otherwise, that adversely affects the geomorphic stability of a place (Chin & Gregory, 2005). River hazards include flood, erosion, and deposition. Among human modifications, urbanization has the most irreversible impacts on river systems. Urban development can have two direct or indirect effects on the rivers. Direct modifications of river channels, such as straightening and channelization along with indirect modification changes the Earth’s surface by replacing the natural land cover with impervious surfaces (Chin et al., 2013).;In this research, the city of Noshahr has been studied. Noshahr is located in north of Iran in Mazandaran Province. The city is situated in south of the Caspian Sea and western coastal plain of Mazandaran. In this area, the width of coastal plain is less than 5 Km and it is geologically bound by Alborz Mountain Ranges. Urban geology is composited with marine and alluvial sediments. The climate of the region is temperate with an average annual rainfall of 1273 mm and the mean of temperature is 16.2C. Three major streams, Mashlak, Gerdekal, and Korkorsar cross the urban area.;;Materials and Methods;;The paper has proposed a classification method for channel stream hazards in urban regions. The work is done in three steps: a) The channel reach was identified based on the geomorphic characteristics and the stream adjustment indicators. The step assesses the river sensitivity of each reach based on its capacity for adjustment and classifies the reach scale adjustment based on the magnitude and rate of the response observed in aerial photographs (Reid & Brierley, 2015). Geomorphic sensitivity of each reach was ranked as high (H), moderate (M) and low (L); b) In the second step, each stream reach was divided based on direct human activity (management) and stream structures. The stream reach has categorized into four classes: A, B, C and D. Class A shows the most interventions, while class D is at least interventions in each stream reach. c) In this step, channel stream was classified into twelve classes by combining the two previous steps (geomorphic adjustment and human activity). The intensity of geomorphic process activity are marked with values 1, 2 and 3.;;Result and Discussion;;In this study, 14 reach of urban streams were investigated. Dimensions of three main streams of Mashlak, Gerdekal and Kurkorsar were measured using aerial photos (19992016) and the field work. All stream reach in study area are limited between alluvial terraces. The average height of these terraces varies from 2 to 4 meters and has been created in the last phase of the retreat of the Caspian Sea. The capacity of geomorphic adjustment is moderate (M) and low (L) in all stream reaches. Channel types are classified according to natural adjustment capacity and human interventions. According to this, the reaches of Korkorsar (14) and Mashlak streams (1214) have near naturally channels and fall into classes of MD and MC. These channels have a moderate natural adjustment capacity, and the human interventions to protect of rivers are less than 10%. Channel reaches in the GerdeKal were categorized into the classes of LA and LB. These channels have a low adjustment capacity and their banks have protected by bank revetments such as concrete and boulder more than 60 percent. After classification, the river channel hazards including erosion, sedimentation, human activity, and hydrology were investigated at each reach of channel. In the class of MD, erosion is dominant and natural adjustment occurs during floods. The observed human interference is occupying the flood plain, urban debris accumulation and the quarrying of channel sediments. In LB and LA classes, sedimentation forms are observed in all reaches. Sedimentation has reduced the channel capacity; hence, flood has exited from the alluvial terraces and has damaged urban areas. Also, removing the arch of the rivers (creating a direct channel), the construction of transverse walls in the channel bed and the reduction of the channel slope are effective factors in the outflow of the channel.  ;;Conclusions;;Channel stream classification based on river hazards can accommodate channel reaches with similar processes and forms in a group, and it provides an analysis of the available and probable hazards in stream channels. In addition, the effects of upstream river can be studied in the downstream channel. Such a spatial view provides a useful framework for management strategies.