اثر ابعاد سلول نقشه دیجیتالی ارتفاعی (DEM) بر دقت مدلسازی جریان واریزه ای در نرم افزار RAMMS (مقاله علمی وزارت علوم)
درجه علمی: نشریه علمی (وزارت علوم)
آرشیو
چکیده
طرح مساله: جریان واریزه ای یکی از پدیده های مخرب طبیعی و زیست محیطی است که بدلیل ماهیت پیچیده فیزیکی آن، حل معادلات حاکم بر آن بصورت تحلیلی بسیار دشوار و در صورت عدم ساده سازی، در شرایط واقعی تقریبا غیر ممکن است. نرم افزار (RAMMS) یکی از مدل های عددی جهت شبیه سازی حرکت جریان واریزه ای است. در این تحقیق، اثر ابعاد سلولی نقشه DEM بر روی دقت نتایج این مدل بررسی شده است.روش شناسی: یک نقشه DEM با ابعاد سلولی 1 متر در یک منطقه مستعد جریان واریزه ای انتخاب شده و با استفاده از RAMMS، مدلسازی در آن صورت گرفته است. سپس با استفاده از نرم افزار ArcGIS، نقشه هایی با ابعاد سلولی 2، 3، 4، 5، 10، 15 و 20 متر از روی نقشه اصلی و به روش Bilinear ساخته شده و مجددا در مدلسازی مورد استفاده قرار گرفته است. یافته های تحقیق: نتایج نهایی نشان داد که مدل RAMMS حساسیت بسیار شدیدی به اندازه سلول نقشه DEM دارد بطوریکه با ثابت در نظر گرفتن سایر پارامترها، فقط تغییر سلول نقشه از 1 متر به 20 متر باعث ایجاد خطای 548+ درصدی در پهنه کلی جریان، 67- درصدی در حداکثر عمق جریان و 112+ درصدی در جابجایی کلی جریان واریزه ای گردید. همچنین مشخص شد که نرخ تغییر میزان خطا در پارامترهای جریان واریزه ای، برای نقشه های با اندازه سلولی کمتر از 5 متر تقریبا رشد کمی دارد اما برای نقشه های بزرگتر از 5 متر، این میزان خطا رشد بسیار زیادی دارد. نتیجه نهایی: به نظر می رسد که نرم افزار RAMMS فقط برای نقشه های DEM با ابعاد سلولی خیلی کوچک (کمتر از 5 متر) کارایی مناسبی داشته باشد و با افزایش ابعاد سلول، میزان خطا به مقدار بسیار زیادی افزایش می یابد بطوریکه دیگر خروجی مدل قابل اطمینان نیست.Impact of DEM Cell Size on the Accuracy of Debris Flow Modeling With RAMMS
IntroductionDebris flow, which is known as the most destructive natural hazard, is a complex environmental phenomenon in which a large volume of moving mass including mud, sand, rock, soil, water, and air travels down a slope under the impact of gravitational force. Due to this phenomenon's complex physical nature, the governing equations' analytical solution is complicated and almost impossible in real situations.In the last two decades, with the advancement in computer technologies and numerical methods, some robust software has been developed to simulate debris flow and particle movement. The Rapid Mass Movement Simulation (RAMMS) is a two-dimensional model to calculate the motion of geophysical mass movements (snow avalanches, rockslides, debris flows, and shallow landslides) from onset to runout in three-dimensional terrain. Since the numerical analysis in this software is based on the terrain parameters which are extracted from a Digital Elevation Model (DEM), the accuracy of the model might be very dependent on the resolution of the DEM. For this reason, this study's main aim is quantitively evaluating errors and uncertainties in RAMMS-Debris flow model outputs induced by the impact of DEM cell size. MethodologyRAMMS extracts all topographical parameters (i.e., slope angle, flow directions, streams, altitudes, inundation areas) from a DEM (ASCII or GeoTIFF format). Since the resolution of the DEM and the resampling process can significantly affect the physical terrain parameters of a given watershed such as the mean slope, altitudes, the cross-section of rivers, etc., therefore the RAMMS outputs may be altered with the changing of the DEM resolution which causes the propagation of errors in modeling and calculations. To investigate the impact of DEM cell size on the accuracy of RAMMS-Debris flow modeling, a watershed with high-resolution DEM (1 m), located in northeastern Iran, is considered. Then, this basic DEM is resampled using the bilinear method in ArcGIS to build DEM maps in various cell sizes such as 2, 3, 4, 5, 10, 15, and 20 m. Since the modeling process is based on the Voellmy-fluid friction model, the viscous-turbulent and the Dry-Coulomb type friction coefficients are considered constant at 200 and 0.2 respectively. Also, all other input data such as simulation time, inflow hydrograph, fluid density, etc. are considered constant for all scenarios.Results and discussionThe analysis showed that by increasing the DEM cell size from 1 meter to 20 meters, the mean slope of the computational domain decreased up to 4% such that, changing in DEM cell size from 1 meter to 10 meters decreases the mean slope by only 1% but with increasing the cell size to more than 10 m, the mean slope reduces significantly.In addition, the final results showed that the RAMMS model had an extreme sensitivity to the Dem cell size such that by keeping other parameters constant, only changing the Dem cell size from 1 meter to 20 meters caused an error of +548% in the overall inundation area, -67% in the overall maximum flow depth, and +112% in the run-out distance. It was also found that the magnitude of errors in the modeled debris flow parameters using DEMs with a cell size of less than 5 meters was almost small whereas for cell sizes larger than 5 meters, it was very large. Therefore, it seems that the RAMMS software generally works well only for DEMs with tiny cell sizes (less than 5 meters).ConclusionIn this research, the effect of the cell size of the Digital Elevation Model (DEM) on the accuracy of the results of the RAMMS model has been investigated. For this purpose, a DEM with a cell size of 1 meter was selected in an area that is prone to debris flow, then using ArcGIS software, some coarser maps with cell sizes of 2, 3, 4, 5, 10, 15, and 20 meters were made using the bilinear method. based on the results obtained from the modeling of various scenarios in this study, it is found that the RAMMS model has an extreme sensitivity to the cell size of the DEM map. Although the smaller the cell size of the DEM map, the higher the accuracy of the model, the increase in the DEM cell size (considering other parameters being fixed) causes a drastic increase in modeling error. This amount of error in DEM-20 m is almost 5.5 times that of DEM-1 m. Therefore, the important point in debris flow modeling with RAMMS is to use a suitable DEM cell size to increase the accuracy of the model. According to the results, if the DEM cell size is less than 5 meters, the magnitude of error is not significant and can be ignored.
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