مدل‌ سازی عددی تخمین میانگین اندازه و غلظت رسوبات معلق مقطع رودخانه ها با استفاده از روش تیکه نگاری صوتی

امروزه با توجه به آمار بالای فرسایش در دنیا همچنان در اکثر ایستگاه های رسوب سنجی برای پایش رسوبات معلق از روش های سنتی استفاده می شود که هزینه و زمان بر بوده و با خطای انسانی همراه است. اخیرا فناوری های نوین نیمرخ جریان داپلر صوتی (adcp) و آکوستیک توموگرافی یا تیکه نگاری صوتی رودخانه (fat) رایج شده است. این فناوری ها بر پایه دانش آوصوتیات هستند که با ارسال و دریافت امواج صدا درون آب و تحلیل آن به تخمین میانگین غلظت و اندازه رسوبات معلق می پردازند. تخمین غلظت رسوبات معلق با استفاده از یک بسامد تنها با نمونه برداری از رودخانه امکان پذیر است. به همین جهت در این مطالعه برای اولین بار با توسعه روابط توموگرافی و استفاده هم زمان از دو بسامد امکان تخمین میانگین اندازه و غلظت رسوبات معلق رودخانه با استفاده از fat بدون نیاز به نمونه برداری ارائه شده است. در نهایت صحت عملکرد رابطه توسعه یافته با سه احتمال رخداد فرضی با مقادیر snr فرضی در هر بسامد مورد بررسی قرار گرفته است. نتایج تحلیلی با توجه به رابطه توسعه یافته نشان داد که روش تیکه نگاری صوتی با دو بسامد 10 و 30 کیلوهرتز قادر به پایش ذرات رسوبات معلق با میانگین اندازه شعاع حدودی بین 0.1 تا 10^5 میکرومتر و غلظت رسوبات معلق بین 0.1 تا 7.17 کیلوگرم بر مترمکعب است که در مقایسه با اِی دی سی پی قادر به تخمین مقادیر بیشتر (10^3 تا 10^5 میکرومتر) و (3.1 تا 7.17 کیلوگرم برمترمکعب) است.

numerical modeling for estimating the mean particle size and concentration of suspended sediments in rivers using the acoustic tomography method

introduction: today, due to the high statistics of erosion globally, the importance of continuous monitoring of rivers that carry most of the eroded rich soil in the form of suspended sediments is vital. in iran, almost one billion tons of eroded soil are destroyed annually. one hundred million cubic meters will reduce the volume of dams. in such a situation, most sediment measurement stations worldwide still use traditional methods to monitor suspended sediments, which are costly and time-consuming and are associated with human error. in recent decades, attention to new methods of monitoring suspended sediments in rivers has increased. one of these methods is hydroacoustic technologies such as acoustic doppler current profiler (adcp), which transmit sound waves with different frequencies underwater. the analysis of its speed and intensity and the number of sound waves attenuation due to collisions with suspended sediment particles can measure the mean suspended sediment concentration and particle size estimating the mean suspended sediment concentration using a single frequency is possible only by sampling the river. therefore, in this study, for the first time, with the development of tomographic relationships and the simultaneous dual frequencies, the possibility of estimating the mean particle size and suspended sediment concentration in the river using the fluvial acoustic tomography (fat) method without the need for sampling by using 10 and 30 khz are presented. material and methodsthe acoustic tomography consists of two sources of sound production and a sound receiver installed diagonally on both sides of the river; they monitor the river cross-section by sending and receiving sound waves. this system is considered an active measurement system due to its sound production source. the systems are synchronized using nanosecond satellites using gnu satellites and transmit sound waves at a single time. sound waves propagate throughout the water depth, and changes in water depth do not affect the measurement of this method, and there is no need to install several devices at different depths. these waves propagate forward in contact with the water surface and the bed of the aquatic environment. one of the disadvantages of the acoustic tomography method is trapping the foliage of plants around the transducers, which disrupts the measurement operation (bahreinimotlagh et al., 2019; bahreinimotlagh et al., 2018; bahreinimotlagh et al., 2015)result and discussion in this study, to test the ability of the proposed equation in acoustic tomography, three hypothetical probabilities (a1, a2, and a3) were considered with snr (36 and 23 db), (39 and 26 db), and (37 and 24 db) respectively. first, by using the proposed equation, estimate the sediment absorption coefficient by using frequencies (10 and 30 khz) as the hypothetical probability a1, a2, and, a3 are (0.013 and 0.025 db), (0.009 and 0.020 db) and, (0.011 and 0.023 decibels) respectively. finally, by solving two equations and two unknowns for each hypothetical occurrence probability (a1, a2, and a3), the average concentration and size of suspended sediments in the river section are equal to (1.43 kg / m3 and 24.89 μm), (0.54 kg / m3 and 9.64 μm) and (0.93 kg / m3 and 16.43 μm) were estimated respectively.in general, with increasing frequency, the measured snr values decrease. the noise increases with increasing frequency, and the snr decreases. similarly, by reducing the snr, the values of the sediment absorption coefficient also increase. it only could measure snr between 10 and 40 db.conclusionthis research showed that acoustic tomography could estimate ps and ssc with dual frequency using the proposed equation. the results showed that the acoustic tomography method could monitor suspended sediment particles with a radius between 105 to 0.1 μm and mean sediment concentrations between 7.17 to 0.1 kg/m3 at 10 and 30 khz.