Quantitative comparison of morphometric indices and alluvial fan toes in arid regions with emphasis on Central Iran

Extended Abstract
Introduction
Alluvial fans are important geomorphic landforms due to their advantages and their hazards. Many researchers in different aspects have investigated alluvial fans. A sudden change in the topographic slope at the mountain front and a decrease in stream power are proposed as the main factors of alluvial fan formation. However, the relationship between alluvial fans and active depositional processes on the surface of alluvial fans. the relationship between the area and the slope of the alluvial fans with geomorphic and geologic characteristics of their basins and investigating the morphology of alluvial fans using quantitative characteristics have been widely studied by many researchers, few studies have focused on the topographic characteristics of alluvial fan toes. The slope is a useful morphometric indicator to distinguish alluvial fans from other depositional landforms distributed on the pediment. The aim of this study is to quantitatively investigate the morphometry and slope changes on the alluvial fan toes of an arid region in order to distinguish them from other depositional landforms on the pediments. Therefore, we analyzed the slope of 40 alluvial fans and their morphometric characteristics in central Iran.
 
Materials and methods
To investigate alluvial fans' morphometric characteristics, we first selected 40 alluvial fans in central Iran using satellite images. We chose a minimum length of 2000 meter for the alluvial fan selction. Next, six morphometric parameters including the overall slope of the alluvial fan (SO), mean slope of the area above the fan toe (SA), mean slope of the area below the fan toe (SB), the ratio of SA to SB (RS), the total length of the alluvial fan (L) and the sweep angle (AF) were measured. The slope of the alluvial fans was calculated using the study area SRTM (Shuttle Radar Topography Mission) Digital Elevation Model (DEM) with a 30-meter spatial resolution. The digital elevation model was converted to the UTM (Universal Transverse Mercator) coordinate system since we used the metric measurements in this study. The overall slope of the alluvial fans, alluvial fan length, and alluvial fan sweep angle were measured using the digital elevation model and QGIS software. To calculate the slope of the area above and the area below the alluvial fan toe line, we first created a buffer with 250 meters distance from the toe line. This distance was applied to avoid unwanted errors. Each buffer has 1000 meters distance. Generally, we used Google Earth pro, ArcMap, QGIS, and the digital elevation model of the study area to measure the morphometric characteristics of the fans and SPSS to apply the statistical calculations.
 
Result and discussion
To calculate the morphometric parameters and to analyze the slope changes of the alluvial fan toes, we first selected 40 alluvial fans in Central Iran. Afterward, the sweep angle, alluvial fan length, overall slope of the alluvial fan, the average slope of the area above the fan toe, and the average slope of the area below the fan toe were calculated. According to Table 1, the most considerable sweep angle equals 156.12 degrees on the alluvial fan number 31. The smallest sweep angle belongs to the alluvial fan number 1 with the value of 14.7 degrees. Among the studied alluvial fans, the alluvial fan No. 17 has the shortest length (2076.679 m), and the alluvial fan No. 26, with a length of 44569.45 m, is the most elongated alluvial fan. In terms of overall slope, fan No. 17 has the highest value in slope (3.38 degrees), and the alluvial fan No. 16 has the lowest (1.64 degrees). Most of the studied alluvial fan have a slope of 2 to 2.5 degrees in terms of the slope of the area above and below the fan toe. The mean overall slope for the studied fans is 2.27 degrees. In terms of the RS factor, most of the fans are distributed in the range of 1 to 1.5. Generally, the slope decreases from the apex to the toe in an alluvial fan. The most important factors for the slope changes on the surface of the alluvial fans include flow velocity reduction, reducing the flow power, and reducing the channel width to depth ratio. Tectonic activity is also one of the essential factors in determining and changing the slope of alluvial fans. Sometimes due to severe tectonic uplift, new sediments deposit on the young and elevated surfaces of the alluvial fan, leading to steep slopes on the surface of the alluvial fan. Based on the results, with increasing the alluvial fan length, the slope of the area above the fan toe decreases. The same correlation can be seen between the length of the fan and the overall slope. The negative correlation between alluvial fan overall slope and alluvial fan length in this study is consistent with the results of other studies.
 
Conclusion
In an alluvial fan system, the apex of the fan is the steepest part, and the slope decreases to the fan toe. Different factors such as tectonics, stream discharge, sediment materials, etc., affect the slope of the alluvial fan. This study showed that most of the studied alluvial fans have straight profiles. The alluvial fan's straight profile indicates that the materials from the catchments area have been transported during the catastrophic flooding events and preserved on the alluvial fan surface for an extended period. The similarity between the fan's overall slope and the slope of the area above the fan toe is a sign of inactivity of erosional processes. When SA is smaller than SO, the alluvial fan will have a concave profile. The results of this study are also consistent with other studies showing that the arid region alluvial fans have a greater slope than humid regions fans. The RS values in the studied alluvial fans reflect the effect of fluvial processes on the slope changes on fan toes. It also shows that erosional processes have been inactive for a long time in the studied fans.