بررسي همبستگي مورفومتري ترك‌هاي حاصل از فرونشست با تغييرپذيري ادافيك در دشت نيشابور

Morphometric correlation of land subsidence related fissures and edaphic variability over Neyshabour Plain

در دهه اخير، پديده فرونشست به‌عنوان يك مخاطره ژئومورفيك، در بخش وسيعي از دشت‌هاي ايران در حال وقوع است. اين پديده با بروز ترك‌ها و شكاف‌هايي در روي زمين و آسيب زدن به سازه‌هاي سطح زمين شناخته مي‌شود. هدف اين مقاله بررسي ارتباط آماري بين تغييرپذيري ادافيك با تغييرات مورفولوژي ترك‌هاي حاصل از فرونشست زمين در دشت نيشابور مي‌باشد. در ابتدا چهار منطقه در سطح دشت نيشابور شامل روستاهاي بازوبند، كاريزك، بشرو و كلاته‌قنبر كه شكاف‌هاي مرتبط با فرونشست در آن‌ها به لحاظ ويژگي‌هاي مورفومتري مانند عرض، طول و ژرفا برجسته و متفاوت هستند، شناسايي و انتخاب شد. در طول شكاف اين مناطق نمونه‌برداري خاك به‌صورت سيستماتيك در دو حالت سطحي (30-0 سانتيمتر) و عمقي (100-30 سانتيمتر) صورت گرفت و اندازه‌گيري شاخص‌هاي مورفولوژي شامل عرض، طول و عمق شكاف انجام شد. به‌منظور بررسي ويژگي‌هاي فيزيكي – شيميايي خاك، پنج مشخصه بافت، دانه‌بندي، درصد رطوبت، pH و هدايت الكتريكي نمونه‌ها تعيين شد. سپس با بهره‌گيري از آزمون‌هاي آماري ناپارامتريك، ارتباط بين ويژگي‌هاي فيزيكي- شيميايي خاك با شاخص ­هاي مورفولوژي شكاف­ها بررسي و تحليل شد. ضريب همبستگي بين متغيرها نشان داد كه رابطه بين شاخص طول شكاف و درصد رطوبت در سطح اطمينان 95 درصد، با ضريب همبستگي 0/677 معني‌دار مي‌باشد. همچنين نتايج نشان داد كه همبستگي بين شاخص عمق شكاف با درصد رطوبت، درصد شن و درصد رس در سطح عدم اطمينان 5 درصد معني‌دار مي‌باشد. نتايج همبستگي آماري تأييد كرد كه رطوبت خاك عاملي جهت گسترش ابعاد شكاف (عمق و طول) است. همچنين افزايش مقادير هدايت الكتريكي نيز باعث افزايش ابعاد شكاف (عمق) در منطقه مطالعاتي شده است.

Introduction Land subsidence related fissures caused by tectonic movements and human activities are developing along many plains of Iran considering arid and semi-arid areas. Two approaches in relation of extension of subsidence related fissures are dominant. One of them, considers tectonic as a main factor and other focus on ground-water depletion. The Nishabour plain is a critical plain in Khorasan-Razavi and Iran as well as the subsidence-related fissures are extending along recent decades and treat civilization of this region. The ground aquifer depletion rate is estimated 8 m/y in the region. In this study, has been assessed the correlation between fissures morphology and edaphic variability in four points with different fissures regarding fissures morphology dimensions. Methodology At the first, the basic data including maps of geology, topography, and geomorphology were prepared. The meteorological data were gathered between 1365-1395, GPS station data and groundwater level data during 1366-1394 were provided as well. The sampling was applied over four regions with different morphology of fissures, where a systematic sampling was used for each regions in two depths including surface (0-30cm) and deep (100cm). So 11 points in each region were collected and ultimately 88 soil samples were taken. Indices of soil texture, moisture, electrical conductivity, and pH were measured for each sample. The SPSS (version 22) was used for statistical analysis. A parametric and non-parametric methods was used based on normal distributions. Finally, to investigate the correlation between the morphology of the fissures with soil edaphic characteristics, a non-parametric Spearman correlation coefficient was used. Results and discussion: The statistical results for confidence coefficient 95% showed a significant amount between clay and sand in studied areas. There were significant differences between EC amounts in four regions. Soil pH values were not significantly different between the four regions. There was no significant difference between the amount of moisture content in four areas. Based on soil particle size distribution curve, the soil of studied regions are fine and coarse particles are in less. The results of Spearman correlation test indicated that coefficient of correlation between the fissures and moisture content is 0.677 with a significant relationship at the 95% confidence level. Also between the width of the fissures and variable moisture content, clay, and sand percent showed a significant correlation. The correlation coefficient between the depth of the fissures with moisture, sand, and clay are respectively, -0.703, 0.220 ,-0.378 with a significant at the 5% level of uncertainty. The results showed development of depth and length of fissures depend on soil moisture oscillations. Therefore, soil moisture is a main factor to dynamic of fissures morphology. The result also shows that depth of fissure is increasing with levels of electrical conductivity, thus electrical conductivity amount, disperse the soil particles and decrease the adhesion properties, and ultimately extend the fissures. With the increase in the percentage of sand, according to the adhesion less coarse-grained soils in fine soils, the size of the fissure intensified. So following increasing fine particles due to high water capacity holding, which increase the soil moisture and so the width of the fissure is less developed. Conclusion There are fissures in west, central and eastern Nishabour palin and the results obtained from GPS stations on the plains confirms the occurrence of land subsidence. The subsidence rate shows more than 10 centimeters per year. Active tectonic in the studied areas and over-exploitation of ground-water in recent years with drought conditions due to develop of big fissures in last decade. Unfortunately this phenomena treat agriculture and settlements life in the region with historical civilization. This research investigated correlation between edaphic and soil characteristics variability and subsidence-related fissures in Nishabour plain. The results of Spearman correlation showed that soil moisture content is a main factor in relation to fissures expansion. To control fissures expansion, the methods to avoid over-pumping is recommended in the region. Also modification of crop pattern and using new technologies in agriculture can be useful tool for decreasing damages to the lands.