Zoning of risk of creating Gypsum Sinkholes using Fuzzy modelCase study: Jabber Badre plain, Ilam

The existence of formations that encompass gypsum layers provides the basis for the generation of gypsum karst forms. The thickness and quality of gypsum layers affect the generation of these forms. Jaber plain is located in the northwest of Ilam, Ilam Province within the Zagros folded belt. Given the existence of Gachsaran formation in this plain, gypsum karstic phenomena may have a highly destructive impact on roads, facilities, agricultural lands, villages and residential units in the plain, among which karstic sinkholes are the most important ones. Despite the issues described above, the phenomenon is unfortunately less addressed and investigated. In this study, it is attempted to prepare a karst development zoning map using a fuzzy model. Ten environmental factors are employed as independent variables to prepare the map, namely dip, dip face, geology, distance from the conic line, concentration points of hillslope streams, distance from river, elevation, land use, climate map and vegetation (normalized difference vegetation index (NDVI)). The results of this research suggest that 36.9% of the study area is located in high-risk zone, 49% of the study area is located in medium-risk zone and 14.1% of the study area is located in low-risk zone. According to the field surveys and investigations into the zoning map, the dip and location of intersection of stream lines and gypsum layers on the hillslope are introduced as the main factors affecting the karst development in the area.IntroductionThe word ''karst'' refers to a set of geological processes and phenomena resulting from the dissolution of soluble rocks. The decomposition and destruction of the structure in these rocks form a unique hydrological regime which results in the formation of specific landforms (Ministry of Energy, TAMAB, 1994). The effective and necessary factors for the creation and development of karst are divided into three groups: physical, chemical and hydrogeological factors. The water penetrates into the fissures of soluble rocks, such as calcium carbonate, causes the development of fissures, and creates karstic forms (Mull et al., 1988). Despite the widespread occurrence (Klimchouk et al., 1996) and relevant importance in terms of function (risks, effects, and sources), gypsum karst has attracted less interest. The features of gypsum karsts can be the source of many problems such as the subsidence and sudden formation of sinkholes. Therefore, it is very important to identify and study the gypsum karsts for the construction and retention of various structures, especially the hydraulic ones, and also for the sustainable water management.MethodThis is an applied-development research based on the field and library methods. For this purpose, the dip, dip direction, vegetation, land use, geology, climate, distance from river, flow concentration points of hillslope streams, and distance from the conic line were used to prepare the layers. Also, the field studies were carried out to prepare and identify the dispersion map of sinkholes.In this study, the fuzzy model was used to provide the zoning map for the evolution of karst. Fuzzy logic replaces and largely completes the conventional methods for the design and modeling of a system which requires advanced and relatively complex mathematics using the verbal values and conditions or, in other words, the expert knowledge, aiming to streamline the system design (Salami, 2010: 20). The degree of membership accepts the values between zero and one or these two limits (Van Alphen and Stoorvogel, 2000: 1710). In the rough set theory, if we consider a set, each member of the reference set is either in the set or not, and the following function can be defined for a given set A (Equation 1) (Momeni, 2010: 197).█(μ_A (x)={█(1 ,If x∈A@0 ,If x∉A)┤#(1) )This function assigns 1 to each member of set A and 0 to each member outside set A.After delineating the plain boundary using the 1:50000 topographic map and determining the geological formations using the 1:100000 geological map and also preparing a vegetation map using the Landsat8 images, the maps of dip, dip face and elevation are also extracted from the DEM of area by the ARCMAP software. Finally, a score was assigned to each of the layers by examining a number of sinkholes in the area which were identified after the field studies using Google Earth software. From this map, 9 effective factors in the form of raster were multiplied by these scores, and finally, after collecting 9 maps by Raster Calculator tool, the final map was obtained.Results and discussionGypsum sinkholes have attracted a little interest from researchers, although they can have a huge impact on facilities and roads. In this research, it was tried to identify the factors greatly contributing to the formation of gypsum sinkholes so that they could be effective in planning and managing the structures and in the effects this phenomenon can have on them. The placement of the Gachsaran formation in the synclines, due to its plastic nature, makes it practically possible to place gypsum karstic forms at lower elevations. The proximity and placement of river bed on Gachsaran formation causes the gypsum layers of this formation to be placed adjacent to the river flow, which facilitates the formation of sinkholes. Another important factor in the formation of sinkholes in this study area is the placement of gypsum layers adjacent to the conic line, which causes the hillslope water flow from the rainfall in the range of the conic line to easily perform the dissolution process. Also, the field visits carried out in the region indicate the formation of these forms in the stream lines in the hillslopes of the Gachsaran formation, which is the intersection point of the concentrated hillslope flow with the formation gypsum layer in the formation point of these sinkholes. It should be noted that the horizontal dip of the gypsum layer can cause the gypsum to be isolated by the marl, making the formation of the sinkholes in the hillslope impossible.ConclusionThe factors such as low dip, geology of area, low elevation and intersection of streams with gypsum layers of the Gachsaran formation in the hillslopes have had the greatest effect on the formation of these phenomena. The results from the zoning map for the evolution of gypsum karst show that out of total 1114.1 square kilometers of Jaber plain, 36.9% is located in a high-risk zone, containing 85% of the sinkholes, and 49% in the low-risk zone, containing 15% of the sinkholes, and 14.1% of the study area is in the medium-risk zone.