عنوان مقاله [English]
Surface sediment's particles diameter is closely related to dominant processes of developing surface morphology of landforms. This study investigates the relation between sediments texture and gullies development in southern plains of eastern Alborz using statistical analysis and measuring the particles' diameter in four pilots in Garmsar-Seyyed Abad, Iran. The pilot sites were selected based on differences and similarities of surface morphology using satellite imageries and field studies. Samples were collected from bottom, middle, and upper part of gullies. After measuring the weight and doing sieve analysis of representative samples, the results were analyzed using GRADISTAT software in the form of graphs and tables. In addition the morphometric index was measured collecting 800 samples from 5*5 m boxes. The results showed that the surface sediments are derived from multiple sources which indicates the difference of effective processes over time (sites 1, 2 and 4, covered by gravel pavement called sandy gravel, sited 3 covered by gravely sand. Site 1 and 4 are derived from two sources, site 2 is three sources, and site 3 is single source). Poorly sorted sediments indicate that the size and type of gully's surface and deep sediments have not been able to fully affect on gullies development in all sites. Therefore, gullies development is not only related to sediments texture and diameter, but also consideration of more important factors such as tectonics processes is required in these sites.
Keywords: Garmsar, Granulometry, Morphometry, Gully erosion.
Gully erosion is one of the important and threatening factors of environmental resources balance and its sustainability. This type of erosion not only limited to abnormal changes in landscape, land degradation, soil loss, impossibility agricultural activities, and economic utilization of natural resources, but also will lead to increase flood hazards, sedimentation of farmlands and make them unsuitable.
In most cases, gullies are considered as environmental and surface change indicators, which are of great importance due to their rapid development. Sometimes, gully development is so quick that can be controlled by any means.
This research has been carried out in several steps: (a) at first, after data collection, digital images processing, and preparation of different maps using different softwares, the scope of the study area was divided into 4 sites. The sites were selected based on differences and similarities of surface morphology on satellite images and field studies. Total number of gullies is 35 in these sites, which one gully was selected and considered in each site. The analysis method is mainly based on comparing effective variables and analyzing their relationships. Sediment data collected during field studies were used to determine the percentage of soil particles (morphometry) in gullies. In this regard, three samples were taken from the bottom, middle, and upper sides across gullies' length in each site. In order to investigate gullies' depth changes, three samples were taken from each gully's wall beginning 25%, middle 50%, and ending 75%. Totally, 24 samples were taken and put in special pockets with given title and number, which transferred to laboratory.
Results and discussion
Site 1: It consists of two sources of well sorted sediments, with sandy gravel texture, fine-grained, and much stretched particles.
Site 2: It consists of three sources of sediments with moderate sorting, sandy gravel texture, very fine-grained, and much stretched particles.
Site 3: It consists of one source of poorly sorted sediments, with gravely sand texture, fine-grained, and moderate stretched particles.
Site 4: It consists of two sources of poorly sorted sediments, with sandy gravel texture, very fine-grained, and stretched particles.
Site 1 and 4 have two modes, such as site 1 has more coarse-grained sediments in surface layer and mixed sediments in sub-surface layer. Site 4 also includes coarse-grained sediments in surface layer and fine-grained sediments in sub-surface layer. On site 2, sediments are clearly derived from two different basins: a) a small and close basin with medium-sized sediments, and b) a large and far basin with very coarse-grained sediments. Fine-grained sediments can also be seen under the above two layers, which refers to availability of third mode in this site. As seen in the area, site 3 consists of very fine-grained pebbles, which derived from one source. All of these differences and inconsistencies in sorting, skewness, and kurtosis can be related to irregular, dispersed, energetic, and low energy flows during different periods on the lower playa, and the effect of tectonics and cross-sectional or permanent upliftig, which have been seen in many different sites of the region.
The type and size of surface and deep sedimentation of gullies have not been able to fully affect on gullies development in all sites. Perhaps, the sediment diameter of the region is the only reason of gully development in site 4, and non-development of it in site 2. In sites 1 and 3, gully development can not only be depended on sediment's texture and diameter of the region, but also some of other more important factors including tectonics should also be considered in these sites in further researches.