The effect of design discharge and the performance of culverts on the natural flood crisis in selected watershed of Yazd province

Extended Abstract
Introduction
Culverts are among the most common and important transitional structures used to convey water, materials, or buried facilities beneath the ground. Therefore, the structures in question should be designed and constructed to maintain their safety, durability, and efficiency in a variety of environmental conditions. Several factors, including the flow pattern at the structure site, maximum instantaneous flood discharge, backflow due to blockage, morphological stability of the river, and erosion and scour effects, influence the hydraulic performance of crossing structures. Therefore, considering various influencing factors in the design and location of these structures to improve their performance is essential. On the other hand, hydrologic models are effective tools for simulating surface and subsurface hydrologic processes in watersheds and are widely used to enhance water resource management. Prediction of flood events and simulation of hydrological processes in watersheds are two fundamental applications of rainfall-runoff models that play a crucial role in water resource planning and management. Accordingly, this research was conducted using the HEC-HMS model to examine the impact of and performance of road-crossing water on flood crises in several selected watershed in the Yazd province.
 
Materials and Methods
The present study was conducted due to the presence of multiple intersecting structures and the history of damage during the floods of 2022 in six selected watersheds in the counties of Taft, Ashkazar, and Mehriz, including the Khamsian, Darbe Raz, Dashtak, Roobaz, Ghavam-Abad, and Konj-Kuh watersheds in Yazd province. The aim was to gather and obtain necessary information about corresponding rainfall-runoff events from relevant sources and extract precise details of intersecting structures such as culverts and small bridges to gain a comprehensive view of the physical, hydraulic, and structural conditions. This serves as the basis for evaluating capacity, analyzing hydraulic performance during floods, and assessing the efficiency of the studied route's drainage system. After identifying the number of culverts in the watersheds, the HEC-HMS software was used to determine the volume and amount of flood. To determine the compatibility of their flow capacity with the flood discharge using the HEC-HMS model, hydrological parameters were first extracted from the watershed and then, using numerical hydraulic models, the flow behavior at culvert sections was analyzed.
 
Results and Discussion
The results of comparing the hydraulic capacities of selected culverts with the design flood discharge at various return periods revealed that the hydraulic behavior of structures in response to increased return periods is non-linear and highly sensitive; in most cases, as intensity and frequency of rainfall increase, ratio (Qc/Qd) decreases rapidly, leading the structure into the critical. The watersheds of Khamsian, Dashtak, and Roobaz have minimum relative capacities and enter an unstable state from 5 to 10-year periods. A Qc/Qd <0.5 in these areas severe hydraulic section deficiencies, significant energy at the culvert outlet, and the risk of overflow from the culvert. Such conditions are mainly observed in structures with reduced hydraulic performance due toation, increased roughness, and geometric shapes. In the Darbe Raz and Dashtk watersheds, the structure's condition is acceptable up to 10-25-year return periods, but beyond 25 years, there is a likelihood of flooding and upstream reversal. The rate of capacity reduction to Qd decreasing from 0.09 to below 0.5 the 25-50- range indicates that the flood is entering unstable state and the outlet is starting saturate. In the Ghavam-Abad watershed, the ratio (Qc/Qd) above 1 in all return periods, making it the only structure evaluated as from a hydraulic design perspective. Ac/Qd value of 4.1 indicates a significant excess capacity resulting the larger outlet dimensions and the suitable longitudinal slope of the inlet channel. However, for extreme events above 500 years, this ratio decreases to around 1, indicating that the flow has reached the threshold of the final capacity. Given the potential for severe rainfall events with return periods exceeding 500 years in 2022, it necessary to reassess the design range for this structure as well. Conversely, the Konj-Kuh culvert exhibits the poorest performance, entering a critical state even at the 2-year return period, meaning that normal annual rainfall could trigger overflows.
 
Conclusion
In view of the flood rainfall in April 2022 in Yazd province, as one of the most intense recorded rainfall events in the contemporary statistical period; the analysis of ratios (Qc/Qd) showed that with an increase in the return period from 5 to 500 years, the average ratio of design capacity to flow rate from around 0.75 to less than 0.20 percent. Among the existing structures in the studied watersheds, only 1 structure was evaluated safe (Ghavam-Abad) and 8 structures are in critical conditions in one of the return periods less than an equal 25 years. This pattern that the initial design of culverts based on return periods lower than the of publication 415 (1 to 25 years).