Investigating the performance of check dams in granularity of sedimentation in a watershed affected by debris flow (Nanor, Baneh)

Background and Objectives: Mass erosion is the movement of rock and soil down the slope, mainly due to gravity. One of the most significant mass movements is debris flow. These are actually fast moving landslides and are a very dangerous natural disaster in mountainous areas. To control erosion and prevent sediment movement downstream, there are various biological and engineering methods available. Debris flow is most commonly controlled by check dams. The main purpose of building these dams is to control and reduce the amount of sediment entering the rivers. The backs of most dams may be filled with coarse-grained sediments and in areas with debris flows, so it is imperative to monitor how well these structures maintain different sediments. In some parts of the Nanor Baneh watershed and in the adjacent basins, there has been debris flow that has caused damage to the forests downstream. Consequently, there has also been a filling of the waterways and reservoirs of check dams. Therefore, the purpose of this study was to investigate the sedimentation granularity of check dams built in various parts of the watershed.
Materials and Methods: Based on available sources, information, and maps of the basin, 58 points were identified, and their surface soil was sampled. To determine soil PSD, a hydrometry method using a 24-hour reading and sieve series was used. There are 88 gabions and masonry check dams evaluated in this research. Sediment samples were collected from each dam at two points and at two depths (a total of 57 sediment samples). After air-drying, the sediment samples were passed through a 2 mm sieve and the size distribution of particles smaller than 2 mm was determined using the hydrometric method (24-hour reading) and the same sieves as the soil samples. To determine the size distribution of particles larger than 2 mm, the sieve method (sieves with openings larger than 2 mm) was used. A comparison of the particle size distribution of soil samples upstream of the dams and the sediments behind the dams was conducted to investigate the effectiveness of these dams in trapping fine-grained and coarse-grained sediments. Finally, the gradation curves and D50 of the sediments behind the dams were examined.
Results: Based on the size distribution of particles smaller than 2 mm in the soil and sediment samples, the ratio of sand, silt, and clay in sediment samples (with mean values of 90.53, 5.45, and 4.02, respectively) varied significantly from those in the soil samples upstream of the dams. The results of D50 ratio (< 2 mm) of sediments to the soil sample upstream of check dams (average 25.92) revealed that most check dams were effective in trapping sand and larger particles, but they were less effective in keeping smaller particles in suspended sediments. Furthermore, the high D50 values for the total sediments (average 3.80) indicate that most sediments are coarse-grained. A number of check dams located in the path of the debris flow were also completely filled in one or two rainfall events, according to local evidence and field studies.
Conclusion: Most check dams were highly effective at keeping coarse-grained sediments and preventing their movement downstream, according to sediment analysis behind them. But, these dams have little effect on trapping fine-grained sediments, and complementary methods for controlling and trapping them downstream of check dams may be needed. However, the role of these dams in reducing runoff intensity, reducing the peak flow of floods, protecting roads and residential areas downstream and many other effects cannot be ignored.