1- yazd
Abstract: (36 Views)
Introduction and Objective: Floods and their resulting destruction are among the major environmental challenges in various parts of the country, particularly in arid and semi-arid regions, causing extensive damage each year to infrastructure, natural resources, and the livelihoods of local communities. Among the key factors exacerbating river instability and altering their hydrological regime are human activities such as unregulated construction within riverbeds and riparian zones, excessive extraction of materials from riverbeds, removal of riparian vegetation, and failure to adhere to technical standards in development projects. The main river of the Pishkouh-Yazd watershed, which is perennial and arterial in nature, has been classified as highly unstable and sensitive according to Rosgen’s classification, due to its unique geomorphological conditions and unrestrained human interventions. In recent years, this river has experienced severe erosion, and in some sections, gullies over 6 meters deep have formed—clear indicators of extensive degradation of the riverbed and banks. These unstable conditions not only intensify the risk of destructive floods but also pose a serious threat to the region’s qanats, especially those whose shafts or galleries intersect the river’s width or course. The blockage, collapse, and sedimentation of these qanats due to erosion-induced deposits have inflicted significant economic and ecological damage on local residents, underscoring the urgent need for targeted management and protective interventions.
Material and Methods: In response to the repeated concerns and demands of local residents regarding flood damage and riverbed erosion, the assessment of various river regulation options was prioritized. Following the evaluation of technical, economic, and environmental aspects, the construction of stone-and-concrete check dams was selected as an effective and feasible solution. These structures, as one of the widely used methods in river engineering, play a crucial role in reducing flow energy, preventing bed and bank erosion, and controlling sediment transport. During the design process, efforts were made to develop structures that are both durable and efficient by maintaining a balance between risk tolerance and construction costs. To this end, a maximum acceptable risk level of 10% and a minimum service life of 50 years were considered to ensure the long-term stability of the structures. For hydraulic calculations, the design was based on a flood discharge with a 500-year return period, estimated at approximately 150 cubic meters per second. Based on this discharge, the flood backwater height was estimated at 1.2 meters, and the flow action depth was approximately 50 centimeters. In addition, the natural riverbed width and the allowable width for structural implementation were found to be nearly equal—around 100 meters. This alignment enabled the project to be executed effectively without the need for major alterations to the natural river course. These measures are critically important for reducing flood risk, protecting infrastructure, and improving the region’s environmental conditions.
Results: Three years after the construction of stone and concrete bed sills in the river, and the occurrence of at least five flood events with a discharge exceeding 50 cubic meters per second during this period, field investigations and monitoring indicate the successful performance of these structures in improving river conditions. The results demonstrate that these structures have played a highly effective role in erosion control, bed stabilization, and flow regulation. One of the most significant achievements of implementing these bed sills was the natural restoration of deep and dangerous gullies that had formed in previous years due to severe erosion along the riverbed. Additionally, by redirecting flood flow from the banks toward the center (or thalweg) of the river, these structures have reduced hydraulic pressure on the riverbanks. As a result, the risk of bank erosion and lateral flood expansion has been significantly minimized. Furthermore, the reduction in scouring and stabilization of the riverbed have enhanced the morphological stability of the river in critical reaches, creating favorable conditions for the recovery of riparian vegetation and the improvement of natural habitats. The findings of these assessments indicate that the use of bed sills in rivers across arid and semi-arid regions can be considered an effective and sustainable solution for erosion control, flood risk mitigation, and ecological balance preservation.
Conclusion: To enhance the safety and stability of hydraulic structures and reduce risk to below 10%, it is recommended that particular attention be paid to the selection of design discharge in projects with a service life of less than 20 years—especially in the fields of watershed management and water resource planning. For small-scale corrective structures such as low dry-stone dams, stone-mortar structures, channel widening, and the delineation of the bed boundaries of waterways with Strahler orders 1 to 3, it is advised to use flood discharges with a minimum return period of 100 years as the design basis. This helps prevent structural failure during extreme flood events. Furthermore, for the construction of larger and more sensitive structures such as check dams, protective walls, levees (lateral embankments), as well as bridges and culverts built across rivers and streams of order 4 and higher, it is essential to base the design on flood discharges with a minimum return period of 500 years. This approach not only increases the structural resilience and longevity of such installations but also significantly reduces potential damages during severe and unexpected flood events. Additionally, it facilitates better water resource management and the protection of critical infrastructure.
Material and Methods: In response to the repeated concerns and demands of local residents regarding flood damage and riverbed erosion, the assessment of various river regulation options was prioritized. Following the evaluation of technical, economic, and environmental aspects, the construction of stone-and-concrete check dams was selected as an effective and feasible solution. These structures, as one of the widely used methods in river engineering, play a crucial role in reducing flow energy, preventing bed and bank erosion, and controlling sediment transport. During the design process, efforts were made to develop structures that are both durable and efficient by maintaining a balance between risk tolerance and construction costs. To this end, a maximum acceptable risk level of 10% and a minimum service life of 50 years were considered to ensure the long-term stability of the structures. For hydraulic calculations, the design was based on a flood discharge with a 500-year return period, estimated at approximately 150 cubic meters per second. Based on this discharge, the flood backwater height was estimated at 1.2 meters, and the flow action depth was approximately 50 centimeters. In addition, the natural riverbed width and the allowable width for structural implementation were found to be nearly equal—around 100 meters. This alignment enabled the project to be executed effectively without the need for major alterations to the natural river course. These measures are critically important for reducing flood risk, protecting infrastructure, and improving the region’s environmental conditions.
Results: Three years after the construction of stone and concrete bed sills in the river, and the occurrence of at least five flood events with a discharge exceeding 50 cubic meters per second during this period, field investigations and monitoring indicate the successful performance of these structures in improving river conditions. The results demonstrate that these structures have played a highly effective role in erosion control, bed stabilization, and flow regulation. One of the most significant achievements of implementing these bed sills was the natural restoration of deep and dangerous gullies that had formed in previous years due to severe erosion along the riverbed. Additionally, by redirecting flood flow from the banks toward the center (or thalweg) of the river, these structures have reduced hydraulic pressure on the riverbanks. As a result, the risk of bank erosion and lateral flood expansion has been significantly minimized. Furthermore, the reduction in scouring and stabilization of the riverbed have enhanced the morphological stability of the river in critical reaches, creating favorable conditions for the recovery of riparian vegetation and the improvement of natural habitats. The findings of these assessments indicate that the use of bed sills in rivers across arid and semi-arid regions can be considered an effective and sustainable solution for erosion control, flood risk mitigation, and ecological balance preservation.
Conclusion: To enhance the safety and stability of hydraulic structures and reduce risk to below 10%, it is recommended that particular attention be paid to the selection of design discharge in projects with a service life of less than 20 years—especially in the fields of watershed management and water resource planning. For small-scale corrective structures such as low dry-stone dams, stone-mortar structures, channel widening, and the delineation of the bed boundaries of waterways with Strahler orders 1 to 3, it is advised to use flood discharges with a minimum return period of 100 years as the design basis. This helps prevent structural failure during extreme flood events. Furthermore, for the construction of larger and more sensitive structures such as check dams, protective walls, levees (lateral embankments), as well as bridges and culverts built across rivers and streams of order 4 and higher, it is essential to base the design on flood discharges with a minimum return period of 500 years. This approach not only increases the structural resilience and longevity of such installations but also significantly reduces potential damages during severe and unexpected flood events. Additionally, it facilitates better water resource management and the protection of critical infrastructure.
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