The relationship between morphometric parameters of alluvial fans’ upland catchment basin and soil resistance properties

Morphometric properties of alluvial fans’ upland catchment basin (e.g. the area, length of the main flow channel, and slope) can affect particles’ features like roundedness, surface texture, and shear resistance (i.e. grains’ angle of internal friction and cohesion). To carry out the current study, first 21 alluvial fans were selected through examining satellite images and conducting field observations. The selected alluvial fans were similar in terms of lithological features and different with regard to their morphology. Samples were obtained from the top, middle, and bottom of alluvial fans. The results showed that, through geomorphological processes, alluvial fans’ area and length of the main flow channel increases and their slopes declines from the top to the bottom. These features make grains at the bottom more rounded and less rough. The highest degree of roundedness and smoothness were observed in particles with the size of 12.5-16 mm in the largest area (2.58 km2), the longest main flow channel (3.63 km), and the lowest average slope of catchment basin (31.7(˚. Depending on the morphometric parameters of the catchment basin, the angle of internal friction ranged from 44.3 ˚at the top to 25.6 ˚at the bottom of alluvial fans. Indeed, the highest angle of friction was detected in alluvial fans with the smallest catchment basin area (0.139 km2), shortest main flow channel (1000 m), and highest slope (43.8 ˚). Thus, the rise in catchment basins’ area and length of the main channel flow and the reduction in their slope will lead to smaller angle of internal friction.

Keywords alluvial fans. Morphometric. Geomorphological. Geotechnic. Soil.

1. Introduction

Alluvial fans constitute one of the main geomorphologic environments in humans’ lives. Given that they are widely scattered across the earth and provide suitable conditions for human life, alluvial fans are of great importance for human activities as well as urban and rural settlements. From an economic perspective, alluvial fans play a significant role in providing sedimentary materials (e.g., sand) used in road and building construction. They are also Important reservoirs of underground water (Bahrami et al., 2015). Various factors influence the properties of soil and materials accumulated in alluvial fans. One of these factors is the rock type of catchment basin feeding alluvial fans (Ritter et al., 2001). Moreover, the geomorphological properties of the catchment basin, such as area, slope, length of the main flow channel, and geomorphological processes, can impact soil properties like grain size, roundness, and surface texture (Bahrami et al., 2018). An alluvial fans’ morphology depends on morphometric properties of its catchment basin (Moscariello, 2017). The area of the catchment basin of an alluvial fan affects the volume of the sediment produced and transported in a flow (Tucker & Hacock, 2010; Blair, 2003; Snyder et al., 2003). As the area of the drainage basin goes up, so does the volume of transported degraded materials (Harvey, 2002; Ferrill et al., 1996). Thus, larger catchment basins give shape to larger alluvial fans and produce more sediment (Tomczyk, 2021). The area of alluvial fans’ catchment basin also influences the intensity of incoming flood flows (Bahrami et al., 2018) in that larger area of catchment basin results in longer and larger volume of flows in alluvial fan flooding (Talling, 2000; Synder et al., 2003; Tucker & Hacock, 2010). In fact, large flow volumes are capable of constructing big alluvial fans (Blair, 2003). Changes in grain morphology depend on the transport distance, grain type, surface roughness, and grain size (Bahrami et al., 2018). Alluvial fans whose catchment basins have longer and larger main flow channels have a smaller average slope in comparison with those that possess smaller catchments (Seif and Mokarram, 2013; Valkanou et al., 2013). The diameter of sedimentary grains often is smaller in larger catchment basins since the grains are likely to travel distances further away from the apex of alluvial fans, which also have more gentle slopes (Blair & Macpherson, 2009). If the grains travel longer distances, they will undergo more abrasion (Bahrami et al., 2018). During the abrasion process, the grains first lose their corners and edges, followed by becoming smoother (Blair, 2003). Transport distance, degree of abrasion, degree of roundness, and surface texture are influenced by the area of upland catchment basin. Therefore, larger alluvial fans have larger amounts of round grains with smooth surface texture (Bahrami et al., 2018). Sediments transported by glaciers and debris environments or under the influence of gravity are generally angular and have a rough texture. When grains are transported by wind or water, they are more likely to have a smooth and round texture (Bridge & Demicco, 2008). Grains’ type also affects their roundness. Calcareous and soft grains become rounder than silica and hard ones even in shorter distances (Blatt et al., 1980). On average, small alluvial fans with small catchment basins possess steeper slopes than bigger basins and their grains are typically bigger since they are transported over shorter distances (Lustig, 1965; Tomczyk, 2021). As the slope goes up and the area of alluvial fans declines, soil will have more angular corners with irregular shapes and hard texture. Higher porosity and angularity in soil increase its interlocking property in comparison with round grains. In fact, angularity and roughness influence shear strength (Barton, 1993; Santamarina & Cho, 2004). The interlock between angular grains enhances their cohesion and angle of internal friction (Sukumaran & Ashmawy, 2001). For example, angular sand grains have higher friction angle compared to round grains (Terzaghi, 1967; Santamarina & Cho, 2004). Thus, shear strength rises with increase in grain angularity (Mirghasemi et al., 2002). Bigger grains are less cohesive. Moreover, bigger grain size increases soil resistance, which is due to shear strength caused by friction. In grains with smaller size, the angle of internal friction goes down, leading to lower shear strength (Santamarina & Cho, 2004). Nonetheless, cohesiveness of smaller grains increases their shear strength (Zhao et al., 2014). Soil cohesiveness is also influenced by soil moisture because raising moisture results in lower resistance in clay soils (Mitarai and Nori, 2006). Moist clay has low resistance since it does not contain big grains, which reduces internal friction. As such, higher moisture drops cohesiveness. If the soil is dried in the air, it becomes more cohesive and, therefore, its resistance goes up (Fookes, 2007). From a quantitative perspective, there is no published comprehensive study examining the association between soils’ geomorphological processes and geotechnical properties. The existing studies have narrowly concentrated on the relationship between soils’ geomorphological parameters and physical properties as well as the association between soils’ physical and geotechnical properties. Consequently, little is known about the correlation between soils’ geomorphological and geotechnical features. To address this gap, the current study sought to explore the relationship between geomorphological parameters and geotechnical properties of soils accumulated in alluvial fans. Examining geomorphic properties of alluvial fans’ upland catchment basins and their relation with soils’ shear strength can deepen our understanding in this regard. The findings will be helpful in macro discussions of urban management, industrial towns, and development plans. They can also contribute to locating the best options for development plans.

2. Material and methods

In addition to the study of paper and scientific references and field-based works, experimental-inductive method was used for data collection and analysis. The results were obtained based on laboratory principles. In order to examine the relationship between the area, slope, and length of the main flow channel of upland catchment basin of alluvial fans with shear resistance properties (e.g., cohesion and angle of internal friction), 21 alluvial fans located in Direh Rural District in Gilan-e Gharb County in western Iran were selected. Given that discrepancies in rock type can influence soil properties, care was taken to select alluvial fans with similar sediment types. To this end, first geological maps, satellite images, and field observation of Sarpol-e Zahab and Gilan-e Gharb were carried out. As a result, the southern slope of Dane-Khoskh anticline was selected as the best area. The upland catchment basin of all alluvial fans in this region are made of thick Asmari formation. Upon selecting the alluvial fans and drawing their maps, their upland basins and flow networks were identified. Then, the area, length of the main flow channel, and slope of the upland catchments were gauged. This was followed by collecting samples from the top, middle, and bottom of the alluvial fans. Samples were obtained from five different spots of each of these three parts which about 10 meter apart. The samples were subsequently mixed and the average sample was gleaned through dividing the mixture into four parts. Following this procedure will make it more likely to have a sample with utmost similarity to the soil composition of the study area. The samples were then taken to the laboratory to conduct lithological, sedimentological, and geotechnical tests. Soil particle size analysis was conducted on the samples following ASTM-D421. To examine geotechnical properties of alluvial fans’ sediments, soil resistance parameters (including cohesion and angle of internal friction) were assessed via running direct shear test in line with ASTM-D3080. In order to minimize the impact of density and moisture on soil resistance parameters, the tested samples had the same degree of moisture (i.e. 18%) and the same weight per unit volume (i.e. 1.6 g/cm3). SPSS, Excel, and GIS software programs were exploited to analyze the data and explore the associations between geomorphological parameters and physical properties as well as the relationships between physical and geotechnical properties of soil. At the end, the findings were conflation to shed light on the correlation between soil resistance parameters and morphometric properties of upland catchment basins in alluvial fans.

3. The study area

Dane-Khoskh anticline is located in the western side of Kermanshah Province between Sarpol-e Zahab and Gilan-e Gharb, and is part of folded Zagros (Bahrami, 2013; Bahrami et al., 2018). The highest part of this anticline with a height of 1352 m is located in its central part, while the lowest point, which is 600 m high, is in its northwestern part. Direh plain and Qaleh Shahin plain are respectively located in its southwestern and northeastern sides. The anticline stretches from the northwest to the southeast. Moving from the center to the northwest of the anticline, one can observe slight changes in the western side. The width of the anticline in the southeastern, central, and northwestern parts respectively are 6400 m, 5000 m, and 1300 m. It is advancing in both northwestern and southeastern sides. In the western side of the anticline, the layers’ slope is steeper in the southwestern part than the northeastern one. In general, layer slope steepness goes up from the northwest to the southeast as one approaches the center of the anticline, which is its highest spot. In the central part of the anticline, there is a slight depression and deviation of flow channels parallel to the anticline axis. This is attributed to the operation of a reverse fault in the southwestern slope. The entire anticline is made of Asmari limestone formation. The occurrence of earthquakes, narrow valleys on the edge of the anticline, and changes in the direction of drainage networks, which are some geological and geotechnical evidence of the study region, show that the studied anticline is still rising and is tectonically active (Bahrami, 2013; Bahrami, 2019). Figure 1 showes the location of the study area. Figure 2 shows the location the the samples were obtained in the edge of Daneh-Khoshk anticline.

options for development plans.