Rouhangiz Akhtari; mohammad Rostami; Bahram Saghafian; Mohammad Elmi
Abstract
Introduction
The construction of check dams in the branches is one of the common methods of watershed management to control sedimentation, watercourse stability and reduce the flood hydrograph from the time of concentration and peak flow. In Iran, despite being 50 years old, in the wide implementation ...
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Introduction
The construction of check dams in the branches is one of the common methods of watershed management to control sedimentation, watercourse stability and reduce the flood hydrograph from the time of concentration and peak flow. In Iran, despite being 50 years old, in the wide implementation of this small-scale structure by the bodies affiliated to the country's Natural Resources and Watershed Management Organization as an executive body, a suitable quantitative and qualitative evaluation method has not been provided. In expressing the effectiveness of this structure, it is inevitable to simulate natural conditions in the presence and absence of this structure in hydrological and hydraulic models. Of course, field visits and measurement of the relevant parameters in the field are also considered to be primary measures in the verification of the simulation and the approximate expression of the effectiveness. The investigations showed that in many researches, the effect of correction dams on the runoff hydrograph has been considered. Simulation of the dams has been done hydrologically with changes in the slope of the waterway and basin time concentration or by using the method of routing in the reservoir that according to simplified hypotheses, estimates more than reality. Hydraulic simulation is more precise but has its own complexity and obstacles. Therefore, in this study, we tried to apply the effectiveness of improving check dams in runoff hydrograph by using both the accuracy of hydraulic simulation and the lack of complexity of hydrological relations. The effectiveness of check dams is computed by determining and applying coefficients in the waterway output hydrograph without improving check dams to obtain the waterway output hydrograph with check dams.
Material and methods
In this research, the effectiveness of successive check dams in reducing the output hydrograph of a triangular channel with three lengths of 1000, 2000 and 3000 meters in three longitudinal slopes of five, 10 and 15%, using the MIKE 11 hydrodynamic model, is considered. In this study, it is assumed that series check dams with a height of 2.5 meters will be constructed in each triangular canal, therefore, the number of check dams will vary from 20 to 180 based on their length and slope. In this study, the output hydrograph of the triangular channel was considered as the dependent variable, and the input hydrograph, channel length, and channel slope were considered as independent variables. Variations of outflow hydrograph peak discharge were investigated under two scenarios. The first scenario for the condition where the channel is without improving check dams and the second scenario for the case where the channel was studied with full of sediment series check dams in order to simulate the effectiveness of the dams in a waterway with hydrological parameters. Two criteria were defined to express effectiveness: the percentage of the intensity of hydrograph routing and the percentage of flow discharge change. The percentage of changes in peak discharge of the hydrograph is determined in relation to the peak discharge of the inlet hydrograph. In other words, "attenuation coefficient" was named based on the difference between inlet and outlet discharge for the scenario and for changes in length of waterway, slope and different amounts of inlet hydrograph. The percentage of change in peak flow discharge from the second scenario compared to the first scenario was also considered as the percentage of flow discharge change.
Results and discussion
Evaluation of the model results for hydrograph routing along the channel in exchange for changing independent variables in the form of two scenarios resulted in decreasing peak flow, increasing the base time of output hydrograph, and delayed time due to trending. The existence of check dams has doubled the change in the mentioned parameters. As the longitudinal slope of the waterway increases, the amount of storage in the canal decreases, and the output discharge and therefore the intensity of the routing (decrease in peak outflow relative to the inlet). Increasing the volume of inflow decreases the intensity of the routing. Routing intensity has an inverse relationship with longitudinal slope and has a direct relationship with channel length. Increasing the number of check dams increases the amount of storage in the canal and as a result, slope reduction occurs and the changes in output discharge are greater than inlet flow. Therefore, the intensity of routing increases. The main purpose of this study was to determine the effectiveness of improving check dams in reducing peak discharge of ouflow hydrograph from a triangular channel based on different conditions using a mathematical model. After performing various simulations and investigating different methods, it was observed that the effects of improving check dams on a outflow hydrougraph can be modeled as the effect of a linear reservoir with a lag time at the end of the channel. In other words, two linear reservoir function and a lag time function are applied to independent variables to obtain the dependent variable. For both output hydrographs obtained in the channel without and with improving dams, K values were estimated as linear reservoir function and TL as lag time function. The average storage coefficient (K) of the linear reservoir was estimated 500, 1100 and 1400 seconds respectively for lengths of 1000, 2000, and 3000 m and for three slopes. The mean lag time for the three mentioned lengths was 540, 1750, and 3700 seconds, respectively. As the length of the channel increases, the slope of the canal, as well as the inflow to the canal, as well as the inflow to the canal decreases, and the amount of the above parameters and therefore the attenuation coefficient increases.
Conclusion
If a stream is selected for the construction of improving check dams and the output hydrograph is available using empirical, hydraulic, and hydrological models in the absence of check dams, the outflow hydrograph from the stream will be simulated and modified for the existence of small-scale structures by applying the linear reservoir storage coefficients and the lag time obtained from this research. In this way, the effectiveness of the construction of improving check dams in flood control will be achieved in the mentioned waterway.
