بررسی تغییرات زمانی و مکانی توزیع اندازه ذرات رسوب های معلق در رودخانه پسیخان استان گیلان

بررسی ویژگی های فیزیکی ذرات معلق از موضوعات مهم در مطالعه رودخانه ها است. یکی از این ویژگی ها، توزیع اندازه ذرات رسوب معلق است، که بیان گر ارتباط بین منابع رسوب و فرآیندهای رسوب گذاری می باشد. هدف این پژوهش، بررسی تغییرات زمانی و مکانی توزیع اندازه ذرات رسوب معلق رودخانه پسیخان در استان گیلان می باشد. طی دوره زمانی هفت ماهه از مهر 1392 تا فروردین 1393، از دو ایستگاه هیدرومتری (مبارک آباد در بالادست و نوخاله در پایین دست) نمونه برداری صورت گرفت. به این منظور تعدادی نمونه رسوب معلق با فواصل زمانی 15 روزه به روش انتگراسیون عمقی در شرایط عادی و سیلابی دبی رودخانه برداشت و پس از آماده سازی، توزیع اندازه ذرات رسوب به روش پیپت اندازه گیری شد. میانگین وزنی قطر ذرات رسوب (mwd) و قطر ذرات با فراوانی 50 درصد (d50) در هر ایستگاه در تاریخ های نمونه برداری تعیین و منحنی توزیع اندازه ذرات رسم گردید. نتایج نشان داد که مقدار متوسط mwd و d50به ترتیب 0.062 و 0.052 میلی متر در ایستگاه مبارک آباد و 0.052 و 0.047 میلی متر در ایستگاه نوخاله بود. حداکثر mwd و d50به ترتیب 0.07 و 0.061 میلی متر در ایستگاه مبارک آباد مشاهده شد. هم چنین مطالعه توزیع اندازه ذرات رسوب نشان داد که ذرات کوچک تر از دو میلی متر در ایستگاه نوخاله بین 99-87 درصد و در ایستگاه مبارک آباد بین 94-83 درصد است. علاوه بر این در اکثر نمونه‌برداری ها، بین دبی جریان و اندازه ذرات رسوب معلق ارتباطی وجود نداشت که نشان دهنده طبیعت غیرهیدرولیکی و تاثیر متغیرهای متفاوت دیگر مثل منشاء رسوبات، تاریخ و انرژی انتقال، توانایی فرسایندگی، فرسایش پذیری و ته نشینی در اندازه ذرات رسوبات معلق در جریان رودخانه ای است. هم‌چنین براساس نتایج اندازه ذرات رسوب در ایستگاه مبارک آباد درشت تر از ایستگاه نوخاله می باشد که این موضوع را می توان به نوع فرسایش موجود در منطقه و برداشت شن از رودخانه نسبت داد. در نواحی بالادست به دلیل کوهستانی بودن احتمال وقوع فرسایش خندقی و زمین لغزش بیش تر، در حالی‌که در پایین دست فرسایش عمدتا پاشمانی و ورقه ای است. ضمن اینکه با کاهش شیب عمومی رودخانه، ذرات درشت تر قبل از رسیدن به ایستگاه نوخاله ته نشین می شوند.

The Study of Temporal and Spatial Changes of Suspended Sediment Particles’ Size Distribution in Pasikhan River in Guilan Province

Introduction: The study of physical properties of suspended sediments is one of the main topics in river studies. Sediment size distribution is one of the sediment physical properties which indicate the relation between the sediment source and its sedimentation process in watersheds. It is also important for prediction of the load of nonpoint source pollution, and for planning sediment trap structures. The Anzali Wetland, located on the southern coast of the Caspian Sea in northern Iran, is a large complex of freshwater lagoons with extensive reedbeds, shallow impoundments and seasonal flooded meadows. Environmental conditions in the Anzali Wetland have been degraded due to the increased inflow of sewerage, wastewater and solid waste from the industry, agriculture and urban area, and sediment from the upper stream mountainous area. The lagoon has decreased in size since the 1930s to less than a quarter of its former extent. The aim of the present study was to assess the changes in size distribution of suspended sediment in Pasikhan River as the most important river interring to Anzali Wetland.;Material and Methods: Pasikhan River originates from the South Mountains, has two branches namely Siahmezgi and Imamzadeh Ebrahim. The sampling carried out during a seven month time period (October 2013 to April 2014) at two hydrometric stations; Mobarakabad (upstream) and Nokhaleh (downstream). The samples were collected in 15 days intervals by depthintegration technique at normal condition. Particle size distribution was measured by Pipette method based on Stocks law. The mean weight diameter (MWD) of sediment particles was calculated, the sediment size distribution curve was drawn and the median grain size (d50) was calculated. According to the European classification, the particles size distribution was divided into four groups of fine sand (0.2 mm), coarse silts (0.06 mm), medium silt (0.02 mm), and fine silt and clay (equal to and less than 0.006 mm). The data were compared for each sampling time for both Stations. Flow discharge and suspended sediment load were also determined at each sampling date.;Results and Discussion: At the Nokhaleh station, the maximum observed flow discharge and sediment concentration were 51.4 m3/s and 4.162 g/L, occurred in February 4, 2014 and   November 3, 2013, respectively. The highest flow discharge and sediment concentration of the Mobarakabad were 9.8 m3/s and 2.633 g/L which were observed on February 19, 2014 and April 4, 2014, respectively. These changes and differences were partly due to topography and land use differences between upland and lowland and partly due to rainfall pattern. Results showed that the MWD and d50 were 0.062 and 0.052 mm on average, respectively at Mobarakabad station, and 0.055 and 0.051 mm, respectively at Nowkhaleh station. The maximum values of MWD and d50 were observed to be 0.07 and 0.061 mm, respectively at normal condition at Mobarakabad station. The study of sediment size distribution indicated that the particles smaller than 2 mm comprised 8394 percent of the suspended sediment at Mobarakabad station, and 8799 percent at Nokhaleh station. The percentage of particles smaller than 0.02 mm were observed to be 1233 and 1064 at Mobarakabad and Noukhaleh stations, respectively. Also the amount of fine silt and clay in suspended sediment were 316 and 524 percent at these stations. There was not any correlation between flow discharge and sediment concentration or sediment size distribution characteristics.;Conclusion: In most of the samples, there was not any relationship between the flow discharge and particle size distribution of suspended sediment which emphasize on the nonhydraulic nature of sediment transport and the effects of different factors including sediment sources, the season, transport energy, rainfall erosivity, soil erodibility and deposition process. Generally, the size of sediment particles at Mobarakabad station was coarser than Nokhaleh station. This could be due to the type of soil erosion which is different at upstream and downstream. In upstream regions, mainly because of severity of topography and vegetation cover including forest and rangeland, the occurrence of gully erosion and landslide is higher in comparison with surface soil erosion. But in downstream especially in paddy fields, the soil erosion type is mainly splash and sheet erosion. Also the cultivation practices including plowing and  paddling  of the field usually provides fine particles entering to the river.   In addition, the river profile is very gentle at the plain before the Nokhaleh station which resulted in deposition of coarser particles.