پژوهشنامه مدیریت حوزه آبخیز (Jul 2024)
Estimation of Manning\'s Roughness Coefficient by the Inverse Solving Method using Observational Data (Sanij River-Yazd, Iran)
Abstract
Extended Abstract Background: The coefficient of hydraulic roughness of rivers is one of the most important factors in the planning, design, operation, and maintenance of water resources projects in river engineering studies. The value of the hydraulic roughness coefficient varies in diverse and complex conditions of rivers and is affected by various factors, usually in hydraulic models the roughness coefficient shows the most sensitivity compared to other parameters. A correct estimate of the roughness coefficient improves the understanding of flow hydraulics and river conditions. Despite many efforts, the inability to accurately estimate the roughness coefficient and the use of Manning's constant value (n) are the main error factors in flood simulation and flow depth calculation. The flow roughness coefficient is typically not constant and changes dynamically as the flow depth changes. The best way to determine the roughness is to measure the flow rate and calculate Manning's n through the inverse solving of Manning's equation. The present study mainly aims to determine more precisely the roughness coefficient of the Sanij River upstream of the Faizabad hydrometric station. Methods: The studied area in the current research is the Sanij watershed, 30 km from Yazd city in Taft city, Yazd province. This basin has an area of 153,173 square kilometers. To achieve the objectives of the research, after conducting field studies, the Faizabad hydrometric station at the outlet of the Sanij watershed was used to collect the required flood discharge and ash-flow data in the study area; Therefore, the hydraulic radius and the value of Manning's roughness coefficient were estimated through the inverse solving of the corresponding equations and the determination of other hydraulic parameters such as velocity and slope. The slope was measured with an inclinometer and a leveler. Results: The lowest value of the n is equal to 0.034, corresponding to a discharge of 180 m3s-1 , while the highest value of the Manningʼs roughness coefficient is equal to 0.119, corresponding to a discharge of 2.083 m3s-1 . As the discharge decreases, the roughness coefficient increases. The function of the roughness coefficient in relation to the discharge (with R2 = 0.80) indicates their inverse and significant relationship. The function of the hydraulic radius in relation to the discharge (with R2 = 0.944) indicates that the discharge and hydraulic radius have a direct and significant relationship. The roughness coefficient has an inverse lower less-significant relationship with the hydraulic radius. Every flood creates different roughness levels with different sedimentation rates; therefore, Manning's roughness coefficient will vary depending on variations in particle diameter. Usually, the channels or rivers of dry areas are temporary, in the descending branch of the hydrograph and at the end of the flood, larger parts remain on the surface of the bed, and it causes errors in the estimation of Manning's roughness coefficient based on experiments and local visits. The apparent error is caused by the larger diameter particles remaining on the surface of the bed and the carrying of fine particles by the flow. On the other hand, during the flow, the orientation of the sediments is generally in line with the direction of the flood, which creates the least hydraulic roughness; But with the reduction of flood intensity and the remaining of coarse-grained particles, in addition to the increase of hydraulic roughness, the random roughness of particles, which plays an effective role in Manning's roughness coefficient, increases. In flows where the flow depth is lower than D90, or smaller than the diameter of large pebbles in the bed, Manning's roughness coefficient reaches its highest value under static-hydraulic conditions, and inflows with greater depth From D90 due to special hydrodynamic conditions, the lowest value of Manning's roughness coefficient was observed. In other words, at high flow rates, the relationship of the roughness coefficient on the entire flow is reduced; therefore, with the increase of the flow depth, the value of n decreases and the hydraulic radius increases. This phenomenon is mostly seen in rocky channels and rivers. The current research showed that in the rivers of dry and rocky areas, we usually encounter overestimated values of Manning's roughness coefficient due to the presence of large pebbles in the bottom of the dry bed. This phenomenon can be seen especially in floods with lower discharge and the cause of this can be related to the reduction of hydraulic roughness in the bed during the flood flow. This phenomenon can be effective in overestimating or underestimating the roughness coefficient. Conclusion: The results show that the roughness coefficient changes from 0.034 to 0.119 in the study range and has an inverse and significant relationship with discharge (R2 = 0.8). Moreover, the roughness coefficient has an inverse relationship with the hydraulic radius (R2 = 0.59). The roughness coefficient is not constant and changes in different flood events. Discharges with a depth less than D90 are those in which Manning's roughness coefficient reaches its maximum value of 0.11. In this study, the lowest Manning's roughness coefficient was observed in discharges with a depth greater than D90 and a flood height greater than 50 cm, as in the discharge of 115m3s-1 , the roughness coefficient “n” decreased to a limit of 0.034. The roughness coefficient is unstable in different events, because during the passage of the current or in the ascending and descending branches of the flood hydrograph, depending on the speed and power of the flow, the bed granularity is dynamically changing, and Manning's roughness coefficient (n) is instantaneous. It changes. During the peak flow, it is difficult to understand the hydrodynamic roughness coefficient and it can lead to overestimation or underestimation of the roughness coefficient; Therefore, it is better to consider this case to achieve more accurate values of roughness coefficient in river engineering studies
