Performance assessment of check dams to controlling discharge and sediment load over their useful lifespan period (Case study: Kond watershed)

Evaluation of watershed operation performance is always done by comparing watershed responses before and after operations. This process causes the effect of the operation to spread over time and not to see its true effect and consequently, the weakness of that series of operations will remain unknown over time. Therefore, considering the importance of check dams in watershed management programs in Iran, the goal of this study is determination of real-time impact or life span of check dams in controlling discharge and sediment load and evaluating of their impacts in Kand’s watershed in Tehran province. .

At the first, life span of check dams has obtained based on Double-Mass curve, and field surveys . Then, the study period (1999-2015) is divided into three periods: before the construction of the check dams (first period), the life span of the check dams (second period) and the end of the life span of the check dams (third period) and the discharge and sediment load changes at the outlet of watershed are investigated and compared with the common approach. Sedimentation rate curve methods were also used to estimate the sediment load in each period.

The time points of occurrence of slope fractures in double mass curves as well as field observations indicate that the duration of impact (life span) of check dams in the Kond’s watershed was on average four years (from 1999 to early 2003). The results of the evaluation of the impacts of the check dams in the case study indicate that despite the constant precipitation conditions, the outflow discharge and the maximum, minimum and average characteristic discharges in the second period, compared to the first period, has decreased by 43, 49.66, 53.28 and 100, respectively. The amount of sediment load reduction in the second period is much more noticeable than the first period and is 92.47%. However, at the end of the life span of check dams (third period), the amount of discharge and sediment load has increased, returning to the level of the first. Comparison of the results of this evaluation approach (determine the life span) with the common approach (before and after project implementation) also shows that in the common approach, due to the distribution of the effects of operations over time, the performance of check dams will be measured at half their actual performance.

The results of this study show that considering the life span in the evaluation process will provide more realistic results. Actually the efficiency of check dams in Kond’s watershed is considerable during their life span, but their shorter life span is a significant weakness of this type of watershed operation. Actually relying on check dams to stabilize watersheds has not been a successful solution and only prevent to exit sediment production from the watershed and, for this reason, in the third period, the watershed response has returned to the pre-operation conditions (temporary effect). Therefore, the benefits of watershed management operations in the Kond’s watershed were off-site impacts and the on-site impacts were limited to reducing the slope of the main stream.In fact, in order to increase the life span of check dams, watershed management operations with on-site impacts should be prioritized, which this in itself requires erosion control at the source, ie the slopes