برآورد عمر مفید مخزن سد نگارستان با ترسیم منحنی هیپسومتریک به‌روش strahler اصلاح‌شده و تصاویر ماهواره‌ای

مخازن سدها از منابع حیاتی تامین آب جوامع بشری محسوب می شوند که آگاهی داشتن از مقدار حجم آب ذخیره شده در آن ها امری ضروری است. استفاده از اشل و منحنی حجم-سطح-ارتفاع مخزن که به کمک عملیات نقشه برداری تهیه می شود، روش معمول برای تخمین ظرفیت ذخیره سازی مخازن می باشد. با گذشت زمان و وقوع رسوب گذاری های متعدد، منحنی حجم-سطح-ارتفاع مخزن تغییر می کند که استفاده مجدد از آن نیازمند تصحیح این منحنی است. عملیات هیدروگرافی مخازن با استفاده از ابزاری مثل اکوساندر روشی مرسوم برای تصحیح این منحنی است که علاوه بر پرهزینه بودن، زمان بر نیز می باشد. در سال های اخیر مطالعات مختلفی برمبنای سنجش از دور با هدف تخمین حجم آب ذخیره شده در مخازن، به محاسبه سطوح آب برای ترسیم منحنی حجم-سطح-ارتفاع پرداخته اند. اساس کار این مطالعات تفکیک پهنه های آب وخشکی به کمک شاخص های طیفی، محاسبه سطوح آب و ترسیم منحنی های سطح-ارتفاع مخازن با استفاده از روابط خطی یا چندجمله ای ها می باشند. محدودیت این روش ها عدم دقت مناسب روابط خطی یا چندجمله ای در برازش منحنی سطح-ارتفاع مخزن برای نقاط ابتدایی و انتهایی بازه تغییرات عمق آب است. در این پژوهش با هدف رفع محدودیت روابط خطی و چند جمله ای برای پیش بینی دقیق نقاطی از منحنی سطح-ارتفاع مخزن که داده های مشاهداتی به دلیل عدم وقوع موجود نیستند، از ترسیم منحنی هیپسومتریک به روش استرالر اصلاح شده استفاده شده است. با استفاده از منحنی هیپسومتریک می توان ظرفیت ذخیره سازی مخزن بین سطوح آب متوالی را محاسبه کرد و حجم نهایی آب ذخیره شده در آن را به دست آورد. براین اساس با مقایسه حجم آب ذخیره شده در زمان فعلی و ظرفیت ذخیره سازی طراحی مخزن در زمان شروع بهره برداری، نرخ رسوب گذاری و عمر مفید مخزن سد نگارستان برآورد شد. نتایح نشان داد که در بازه زمانی 9 ساله، ظرفیت ذخیره مخزن در تراز سطح آب حدود 189.5 متر که معادل تراز تقریبی تاج سرریز می باشد، از حدود 24 به 20 میلیون متر مکعب کاهش یافته است که بر این اساس نرخ متوسط رسوبگذاری سالانه مخزن حدود 1.6 درصد برآورد شد. همچنین در این مدت، تراز کف مخزن سد نگارستان به دلیل انباشته شدن رسوبات به‌طور متوسط حدود 10 متر افزایش داشته است و کمترین تراز کف مخزن از 160 به حدود 170 متر رسیده است. مطابق نتایج به دست آمده از این پژوهش و با فرض ثابت بودن شرایط اقلیمی، عمرمفید مخزن سد نگارستان از ابتدای سال 1403 حدود 53 سال برآورد شد.

assessment of useful life of negarestan reservoir dam using hypsometric curve by modified strahler method and satellite imagery

introductionreservoirs are crucial for water supply to human societies, making their proper and planned management essential. dams serve multiple purposes, including urban water supply, agricultural irrigation, flood control, and hydroelectric power generation. in order to properly manage and monitor the consumption of these important reserves, it is inevitable to know their capacities. using water stage and the reservoir's initial volume-area-elevation curve, which is prepared with the hydrographic operations, is a common method for estimating the storage capacity of reservoirs at different water levels. over time, the occurrence of numerous sedimentations, often due to factors such as floods, can alter the initial volume-area-elevation curve of a reservoir, requiring it to be updated. hydrographic operations, using tools like eco-sounders, are conventional methods for updating this curve; however, these methods are both expensive and time-consuming. in recent years, various studies have focused on remote sensing techniques aimed at estimating the volume of water stored in reservoirs, using water levels to establish the surface area-elevation curve. the basis of these studies is the separation of water-land masks using spectral indices, the calculation of water levels, and the development of reservoir surface area-elevation curves through linear or polynomial relationships. however, the main limitation of these methods is the inaccuracy of linear or polynomial relationships in fitting the surface area-elevation curves at the beginning and end points of the water stage change interval, which correspond to the empty or full states of the reservoir. this inaccuracy arises due to factors such as drought or flood events. in this research, the limitation of linear and polynomial relationships in accurately predicting the points of the reservoir surface area-elevation curves, where observational data are unavailable due to non-occurrence, was addressed by using the modified strahler method to draw the hypsometric curve. this method allows for the calculation of the storage capacity of the reservoir between successive water levels and the determination of the final volume of water stored in the reservoir. by comparing the volumes of water stored at the present and initial reservoir capacities, the sedimentation rate and the useful life of the negarestan dam reservoir were estimated. material and methodsnegarestan dam (kabudval) is located on the qarasu (zarin gol) river, 45 km east of gorgan in the golestan province. this dam is used for purposes such as supplying urban water to aliabad city and supplying water needed for the agricultural irrigation network of qarasu. in this study, landsat8 satellite images were used to estimate the useful life of the negarestan reservoir. the required images of the roi were downloaded through the usgs database and pre-processed in envi5.3 software. using visible and infrared spectral bands, water indices ndwimcfeeters, ndwigao, mndwi, aweish and tcwet were calculated to separate land-water masks. after evaluating the accuracy of the obtained water level results by comparing it with the initial volume-area-elevation curve of negarestan reservoir, the mndwi index was used as the most accurate index to calculate water levels. in this study, the modified strahler method was used to obtain the hypsometric curve of the surface area-elevation of the reservoir, which has high accuracy in extrapolating the beginning and end points of the curve. by using the hypsometric curve, water levels were extracted for arbitrary water levels, and with the help of the prismoidal method, the volume between consecutive water levels was calculated. the sum of these volumes equaled the current storage capacity of the reservoir. to estimate the sedimentation rate of the negarestan dam reservoir, the current storage capacity was compared with the initial storage capacity in 2015. based on this comparison, the useful life of the reservoir was accurately predicted. results and discussionvalidation results for calculating water surface areas using ndwimcfeeters, ndwigao, mndwi, aweish and tcwet water indices showed that the mndwi index with an average water surface areas calculation error equal to 5% is more accurate than other indices. therefore, the mndwi index was used in this study. additionally, the comparison of the volume of water stored in the negarestan reservoir with its initial storage capacity at the time of operation revealed that, over a period of 9 years, the storage capacity of the reservoir (at a water level of approximately 189.5 meters), which is close to the overflow crest level, had significantly decreased. it has decreased from about 24 to 20 million cubic meters, based on which the average annual sedimentation rate of the reservoir was estimated, to about 1.6%. the results showed that in a period of 9 years, the average level of the bathymetry of negarestan reservoir has increased by 10 meters due to the accumulation of sediments, and the minimum level of the batymetry has reached from 160 to about 170 meters. according to the statistics of the international commission on large reservoirs (icold), the average annual sedimentation rate of the world's reservoirs is reported to be about 0.95%, and the results show that this amount in the nagaristan dam reservoir is almost 2 times the average rate. it is universal. according to the results obtained from this research and assuming constant climatic conditions, the useful life of the nagarestan dam reservoir was estimated to be about 53 years from the beginning of 2024. conclusionconsidering the increasing importance of water resources management, including dam reservoirs, this study employed a fast and cost-effective method based on remote sensing to calculate the volume of water stored in dam reservoirs and estimate their useful life. in addition to providing appropriate accuracy, this method was able to overcome the limitations of previous approaches in estimating the volume of accumulated sediment in the deeper parts of the reservoir. as a result, it offers a reliable tool for the effective management of water resources.