Geomorphology
Mojgan Entezari; Hamide Gholamhaydari; Haji Karimi
Abstract
IntroductionMainly containing thin gypsum units, evaporite formations cover 8% of the Earth's surface and 25% of the Earth's continental surface (Ford and Williams, 2007). The term karst refers to a set of geological processes and the phenomena resulting from the dissolution of carbonate rocks. Decomposing ...
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IntroductionMainly containing thin gypsum units, evaporite formations cover 8% of the Earth's surface and 25% of the Earth's continental surface (Ford and Williams, 2007). The term karst refers to a set of geological processes and the phenomena resulting from the dissolution of carbonate rocks. Decomposing the structure of the rocks forms a unique hydrological regime, leading to the development of special landforms (Ministry of Power, TAMAB, 1994). In general, the eight elements needed to develop a karst are: precipitation, relief, lithology, thickness of carbonate layers, carbon, low temperature, pressure, and tectonic rocks (White, 1988). Sinkholes are the enclosed depressions that are known as the characteristic of carbonate and evaporite karst landscapes. In recent years, several genetic classifications have been published for sinkholes (Gutierrez et al., 2008).Materials and methodsTo conduct this research, a 160,000-m2 area of the Gachsaran formation in Maroon plain with a high density of sinkhole was examined and visited. The exact geographical location and specifications of 51 sinkholes were surveyed, and it was found that the sinkholes totally account for 25% of the surveyed area. Table 1 lists the specifications of the sinkholes. Also, in another field visit conducted in this study area, a sub-basin with an area of 6.2 hectares, average slope of 28%, and length of 2301.7 m was selected for the surface runoff sampling during the precipitation, which also included a number of previously surveyed sinkholes. The surface runoff samples taken included the first moment of runoff onset, which began 3 hours and 27 minutes after the start of precipitation. Then, 5:19, 5:45, 8:10, 9, and 9:20 after the start of precipitation, other samples of surface runoff were taken again to monitor the changes in the ions dissolved in the runoff in the spatial and temporal intervals. The minimum height of this sub-basin is 520 m, and its maximum height is 888 m above sea level. Also, in this regard, to estimate the average long-term rainfall of the region from the 20-year rainfall data of the meteorological stations in the study area, the daily rainfall data of the Maroon meteorological station in the vicinity of the study area, 1:100,000 geological map of Behbahan sheet, 1:50,000 topography, and 10-m DEM of the study area were used as the research data.Results and DiscussionThe obtained results showed that a total of 25% of the surveyed area is covered with the surface and visible sinkholes. It should also be noted that the dissolution in the Gachsaran formation is not the only erosion process that is taking place. In addition to the dissolution of gypsum, the marls of this formation are eroded by the surface runoff due to the precipitation and leave the basin as a suspended load, which accelerates the formation of sinkholes. Therefore, it can be stated that the time interval estimated in this study can be shorter, because the effect of the accelerating factor of the suspended load was not calculated, which requires the investigation and study to calculate the suspended load. Directing the runoff by sinkholes to downstream areas is likely to lead to the same expansion of the underground karst and tunnel, which is not visible. Also, it is not clear how many cavities and tunnels formed in the subsurface of this area, but this is not expected to be low based on the obtained results. There are roads, agricultural lands and, more importantly, residential areas on the gypsum zones, at least 25% of which are visible sinkholes, and the subsurface tunnels that are expanding every year at this dissolution rate. In addition to the effective rainfall, the presence of agricultural lands requiring continuous irrigation and intensifying the dissolution rate can lead to surface collapses and expansion of sinkholes, which ultimately threatens the life and financial security of the inhabitants in these areas.ConclusionBased on the field observations and the conducted studies, the sinkholes in the stream course have a generally elongated form and often have several ponors. Given that the sinkholes do not retain the aggregated runoff inside the sinkhole and the runoff exits through the same ponors, such diversity in the number of ponors results in a number of sinkholes in the stream course, which are connected together through a subsurface path. Unlike calcareous sinkholes that can direct the runoff current to karst aquifers, water is rapidly saturated due to the high solubility of gypsum. The dissolution is mainly limited to the opening area of the sinkhole, and less runoff finds the opportunity to expand the sinkhole in depth. It should be noted that based on the results, to increase the safety factor and reduce the risk of gypsum dissolution, it is necessary to monitor and study the subsurface area and to examine the subsurface cavities and tunnels that have formed so far. Finally, to reduce the risks of the dissolution rate, some arrangements such as sinkhole risk zoning should be made. This is because the cavities in the gypsum layers with the high dissolution rate can easily expand and prepare the conditions for the sudden collapse so that the whole plain suffers the collapse, incurring human and financial losses.
Geomorphology
Hamide Gholam Heidari; Mozhghan Entezari; Haji Karimi; Mohammadhosein Ramasht
Abstract
Introduction Maroon plain, located in Maroon basin is situated in Khuzestan, and Kohgiluyeh and Boyer-Ahmad provinces; and Jaber plain is positioned in northwest Ilam City in Ilam Province. The existence of Gachsaran Formation and gypsum karstic phenomena (the most important of which are sinkholes) in ...
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Introduction Maroon plain, located in Maroon basin is situated in Khuzestan, and Kohgiluyeh and Boyer-Ahmad provinces; and Jaber plain is positioned in northwest Ilam City in Ilam Province. The existence of Gachsaran Formation and gypsum karstic phenomena (the most important of which are sinkholes) in the mentioned plains, can have a very devastating effect on man-made structures. The morphometric study of these phenomena can allow comparison of various parameters of the sinkholes and lead to some hypotheses about their evolution. In this research, the sinkholes of both plains were classified according to Basu and Switch methods; based on Basu Model, they are categorized in elongated and bowl-shaped sinkholes, and on the basis of Switch method, they can be classified as bowl-shaped, pit-shaped and funnel-shaped groups. The results of the univariate linear regression analysis between the morphometric components of the sinkholes showed that the components of the area with large diameter, area with small diameter, depth with area, and depth with large diameter, indicate the highest values of significant correlation with determination coefficients of 0.922, 0.873, 0.699, 0.581, respectively. The maximum significant relationship at the error probability level, less than 0.1 is between the components of the area and large diameter for 2nd-degree and 3rd-degree relationships with determination coefficients of 0.997 and 0.998 and the estimation error of 0.07 and 0.08. The results of stepwise multiple linear regression analysis between the morphometric components of the sinkholes indicated that the highest value of determination coefficient is related to the area, small diameter, large diameter, and depth with determination coefficient of 0.997, and estimation error of 0.024, while the least values of determination coefficient is related to slope with depth, and large and small diameters with determination coefficient of 0.584 and the estimation error of 0.091. Therefore, the area, large and small diameters, and depth can be suitable parameters for modeling. Karst is a geomorphic and hydrological system that is formed by the dissolution of soluble rocks such as limestone, dolomite, and gypsum (Eziot et al., 2014). Gypsum and anhydrite are among the rock types which can be karstified easily and quickly (White, 1988). Given the development of evaporate formations such as Gachsaran, Kalhor, Hith, salt domes, etc. in Iran and particularly Zagros basin, and as well, the presence of gypsum and anhydrite as their main constituents, their importance in engineering geological issues including dams, tunnels, roads and railways, structures, mines and so on is evident. There is a close relationship between karst geomorphology and its evolution, so that karst morphology phenomena are abundant and developed in the area (Karimi 2010, Ghobadi 2009). Generally speaking, four different mechanisms are characterized for the formation of sinkholes; they are as follows: solution from above, collapse from the bottom, soil transport, and removal of buoyant support. Sometimes the formation of the sinkholes is due to a combination of different mechanisms. It seems that the above-mentioned mechanisms can also be applied to gypsum sinkholes; the only difference is the formation speed of the landform (Gunay, 2002). In the present study, it is attempted to classify the various sinkhole types of Jaber and Maroon plains based on their morphology, and as well to determine the most suitable parameters for modeling. Methodology To survey the morphometry of gypsum sinkholes, after field studies and measuring the sinkhole characteristics of Maroon and Jaber plains, the required parameters were prepared by SPSS software (Table 1). Initially, using the obtained data, the sinkholes of two study areas were categorized by Basu and Switch methods, then the regression analysis of the morphometric parameters of the sinkholes was performed in SPSS software. For this purpose, univariate linear, 2nd-degree, 3rd-degree, and multivariate methods were used, and consequently, the most suitable models with higher preference values were presented. Results and discussion The results demonstrate that according to Basu method, the sinkhole types of the study area are circular and elongated. As well, based on the Switch method, and the diameter to depth ratio, the sinkholes of these areas are of bowl-shaped, funnel-shaped, and pit-shaped types; the number of pit-shaped sinkholes is greater than the other two types. The results of the univariate linear regression analysis between the morphometric components of the sinkholes show that the components of area with large diameter, area with small diameter, depth with area, and depth with large diameter, indicate the highest values of significant correlation by determination coefficients of 0.922, 0.873, 0.699, 0.581, respectively. The maximum significant relationship at the error probability level, less than 0.1 is between the components of area and large diameter for 2nd and 3rd degree relationships with coefficients of determination of 0.997 and 0.998 and the estimation error of 0.07 and 0.08. Also, the results show that the least significant 2nd-degree and 3rd-degree relationships are between environment and slope with determination coefficients of 0.025 and 0.081 and estimation error of 0.022 and 0.022. Besides, the results of stepwise multiple linear regression analysis between the morphometric components of the sinkholes demonstrated that the highest value of determination coefficient is related to the area, small diameter, large diameter, and depth with determination coefficient of 0.997, and estimation error of 0.024, while the least values of determination coefficient is related to the slope with depth, and large and small diameters with determination coefficient of 0.584 and the estimation error of 0.091. Conclusion According to the obtained results of the Basu method, the sinkholes of the Jaber plain are circular in shape and the Maroon plain sinkholes are classified among the elongated ones; certainly, the mechanism of formation of the sinkholes has directly influenced their shape and morphometry. The presence of creeks on the formations and gypsum layers has resulted in elongated sinkholes; in the Jaber plain where most of the sinkholes have been formed on the Knick line, the intersection point of a centralized slope flow and the gypsum layer has formed circular sinkholes. The results of field studies show that the depths of the Jaber plain are deeper; the reason can be the higher thickness of the gypsum layers of the Jaber plain compared with those of the Maroon plain and more importantly is that these layers are situated in Knick line and exposed to a powerful centralized slope flow. But the sinkholes of the Maroon plain are steeper, and the sinkholes of the Jaber plain enclose more area. After field observations as well as quantitative calculations of the ratio of the small and large diameters of the elongated sinkholes, the proportion of these two parameters is roughly calculated as 1/3. Quantitative results of univariate linear analysis between the morphometric components the sinkholes as well as the significant relationship at the error probability level, less than 0.1, for 2nd-degree and 3rd-degree relationships, and the results of stepwise multiple linear regression analysis between the morphometric components of the sinkholes express that the area, large and small diameters, and depth can be appropriate parameters for modeling.
