Investigation on the effectiveness of gabion check dams in amount and grain size of sedimentation in the Rezin watershed, west of Iran

Background and objectives: Soil conservation dams are constructed across waterways or gullies to reduce the intensity of water flow, sediment retention and monitoring, reduce the peak discharge of floods, increase concentration and lag time in a basin area, and finally correct channels width and length of channel. Gabion dams are porous structures, they are mostly used in many watersheds covering water reservoirs in Iran where soil erosion by water is occurred steadily in uplands. Various factors such as the location of the dam along waterway, waterway slope, the drainage area of each dam and watershed soil’s particles size affect the performance of these dams. Up to now, the effectiveness of the gabion dams has not been investigated in sediment deposition in semi-arid watersheds. The study was carried out to determine the performance of gabion dams and the role of different factors in trapping sediment and its grain size distribution in a semi-arid watershed in west of Iran.
Materials and methods: In this study, eleven gabion dams named G1 (in the lowest part of the waterway) to G11 (in the highest part of the waterway) in the resin watershed located in the north of Kermanshah were studied during the year 2018. Sediment sampling was performed from three locations behind each dam from a depth of 0 to 20 cm. Then the physical properties of sediments and watershed soil including the percentage of sand, silt and clay particles were determined. The granulation of sand particles was measured by separating the particles through appropriate sieves and washing the particles on each sieve and the particle size distribution of gravel and boulder was measured by separating the particles by weight method. The volume of sediment was calculated by measuring height and area of deposition area behind the dams. The drainage surface area for each dam was obtained by ground survey and google earth images. Slope gradient of drainage area of each dam was obtained through the slope of the waterway.
Results: The results indicated that the performance of the gabion dams in trapping of sediment is different in different sections of the waterway. The dams constructed upstream of waterway received more sand but less clay and silt than the downstream dams. Also, the amount of sand in the sediments of the dams was higher than the watershed soil. The particle size distribution of sand showed that the most sedimented particles in the back of the dams were related to particles with a size of 1 to 2 mm and the frequency of particles with a diameter of 0.053 to 0.075 mm was the lowest. Particles with a diameter from 2-4.75 mm had the highest share of coarse particles (pebbles) in sediment, while the share of larger particles with a diameter of 9.37 to 12.5 was the lowest. Also, among the various factors, waterway slope had a significant effect on amount of sediment behind the dams.
Conclusion: The distribution of sediment grain size behind the gabion dams varies due to the location of the dams along the waterway. The gabion dams, which are constructed upstream play an important role in trapping coarse-grained sediments. Sediment volume behind the dams is positively affected by drainage surface area and particularly slope gradient of waterway. Therefore, slope gradient of waterway and location of the dams along waterway could be taken into consideration for enhancing their effectiveness in amount of sediment trapping and type of sediment material. Construction of these dams with lower porosity such as masonry check dams in lower parts of waterways is necessary to trapping fine sediment particles as well as preventing the pollution of surface runoff.