Document Type : Research Paper
Authors
1 Assistant Professor, Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran
2 PhD Candidate, Department of Civil Engineering (Water Engineering), Islamic Azad University, Najafabad Branch, Isfahan, Iran
3 Associate Professor, Soil Conservation and Watershed Management Research Institute (SCWMRI), Agricultural Research, Education and Extension Organization (AREEO), Tehran, Iran
Abstract
Introduction
Seasonal floods in mountainous regions of Iran are a major challenge for water resources management and the protection of rural communities. This study evaluates the effectiveness of check dams in reducing seasonal flood impacts in the Sijan watershed, Alborz Province, Iran. The watershed was selected because of its high flood potential, the occurrence of destructive debris floods (including the 2015 event), and the importance of Sijan village as a popular tourist destination. In 2018, the Forests, Rangelands and Watershed Management Organization of Iran constructed one masonry check dam and four gabion check dams along the main channel upstream of the village to reduce debris flood hazards. The main objective of this research was to simulate the historical flood event and evaluate the effectiveness of these structures in modifying flood hydrographs. Because the stream is seasonal and hydrometric data are limited, a physical model was used as the main research tool. This paper presents the results of the first stage of the study, which focuses on evaluating the effects of the check dams on runoff flood hydrographs (clear water flow without sediment).
Materials and methods
A direct physical modelling approach was used to investigate the influence of check dams on flood peak reduction and peak delay. Field investigations were carried out along a 700 m reach of the Sijan stream after the construction of the check dams. A detailed topographic survey at a scale of 1:1000 was then conducted. Due to laboratory space limitations, a 168 m section containing three 1 m high check dams was selected for modelling. A 1:10 scale physical model was built at the Soil Conservation and Watershed Management Research Institute following geometric and dynamic similarity principles. To eliminate infiltration effects, the channel bed and banks were made impermeable. The experiments considered two main factors: channel condition and inflow hydrograph characteristics. Three channel conditions were tested: (1) no check dams, (2) empty check dams, and (3) sediment-filled check dams. Flow conditions included both steady flow for stage-discharge calibration and unsteady triangular hydrographs for flood simulation. The time to peak was examined under three conditions: shorter than, equal to, and longer than the watershed concentration time. Peak discharges were selected below the estimated 10-year flood because of laboratory limitations. Hydraulic variables were measured using standard weirs, and all experiments were video recorded to improve measurement accuracy. The analysis focused on three hydraulic indicators: peak discharge reduction, delay in peak arrival time, and changes in flood base duration. In total, 90 experiments were conducted, and the outlet hydrographs were recorded using a sharp-crested rectangular weir.
Results and discussion
The results demonstrate that 1 m high check dams significantly modify flood hydrographs under different hydrological conditions using clear water over a rigid, non-erodible bed. In the natural channel without check dams, increasing inflow discharge reduced both peak attenuation and peak delay. Empty check dams showed the highest efficiency for floods with peak discharges below 4.74 m³/s (approximately corresponding to return periods of less than five years). Under these conditions, the dams temporarily stored runoff and effectively attenuated the flood hydrograph. However, as flood magnitude increased or the dams became filled with sediment, their storage capacity decreased, resulting in lower flood mitigation performance. The experiments showed that empty check dams reduced peak discharge by up to 28% and increased the time to peak by up to 36% during small and moderate floods. In addition, the ratio between watershed concentration time (tc) and flood time to peak (Tp) was identified as a key parameter controlling dam performance. For floods with short hydrograph duration (Tp < tc), representing intense short-duration storms, the dams achieved their highest efficiency, reducing peak discharge by up to 22.5% and increasing peak delay by up to 43%. Their effectiveness gradually decreased as flood return period and hydrograph duration increased. The results indicate that the design of check dam systems should consider watershed hydrological characteristics, including flood return period, hydrograph shape, time to peak, and sediment accumulation, to maximize flood reduction efficiency. Three empirical equations based on dimensional analysis were developed to quantify the effects of check dams on flood hydrograph characteristics. These equations showed good predictive performance, with coefficients of determination (R²) ranging from 0.81 to 0.92. However, their application to other watersheds requires local calibration.
Conclusions
The physical model results also identified the ratio tc/Tp as a practical indicator for evaluating check dam performance. When tc/Tp > 1, floods have a short time to peak and a narrow hydrograph. Under these conditions, check dams provide temporary storage, increase backwater effects, enhance local turbulence and energy dissipation, and significantly reduce peak discharge while delaying flood propagation. When tc/Tp ≈ 1, the inflow hydrograph is well balanced with the storage capacity of the dams, resulting in effective but moderate flood attenuation. When tc/Tp < 1, floods have a longer duration, and the available storage behind the dams gradually becomes full. Consequently, the proportion of dissipated flow energy decreases, leading to lower reductions in peak discharge and smaller delays in flood timing. Overall, the findings demonstrate that check dam performance depends not only on the structural characteristics of the dams but also on watershed hydrology, particularly the tc/Tp ratio. Therefore, in watersheds with short concentration times, check dams are particularly effective for mitigating flash floods and should be designed to maximize temporary storage and energy dissipation. In contrast, in watersheds with longer concentration times, combining check dams with larger storage facilities and other watershed management measures is likely to provide better flood mitigation. The study also highlights the importance of considering long-term sediment deposition during the design and maintenance of check dams, as sediment accumulation significantly reduces their storage capacity and hydraulic performance over time.
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