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Estimation of Manning's roughness coefficient by inverse solving method using observational data (Sanij River-Yazd, Iran)
Mahtab Alimoradi , Mohammad Reza Ekhtesasi , Arash Malekian
Yazd University
Abstract:   (313 Views)
 Extended Abstract
Introduction and Objectives: The hydraulic roughness coefficient of rivers is one of the necessary factors in river engineering studies. In hydraulic models, the roughness coefficient typically exhibits the greatest sensitivity when compared with the other parameters. The correct estimate of the roughness coefficient improve the understandings the hydraulics of the flow and the conditions of the river. Despite of many efforts, the inability to accurately estimate the roughness coefficient and the use of Manning's constant value (n) is the main error factor in flood simulation and flow depth calculation. The flow's roughness coefficient is typically not constant and changes dynamically as the flow's 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 main purpose of present study is determine more precisely the roughness coefficient of the Sanij River upstream of the Faizabad hydrometric station.
Material and Methods:  The studied area is the Sanij watershed, which has an area of 149.9
Km2  and is located in Taft city in Yazd province, Iran. To achieve the goals of the research, field studies and stage-discharge data of Faiz Abad hydrometric station were used. Therefore, through the inverse solving of the corresponding equations and determination of other hydraulic parameters such as velocity, slope, hydraulic radius, the value of Manning's roughness coefficient was estimated. The slope measurement was done with an inclinometer as well as a leveler.
Results:  The lowest value of the Manningʼs roughness coefficient (n) is equal to 0.034, corresponding to the discharge of 180 m3s-1 , while the highest value of the Manningʼs roughness coefficient corresponding to the discharge of 2.083 m3s-1  is equal to 0.119. As the discharge decreases, the roughness coefficient increases. The function of the roughness coefficient in relation to the discharge  whit R2 = 0.80, indicates the inverse and significant relationship them and the function of the hydraulic radius in relation to the discharge whit R2  = 0.944, indicates, that discharge and hydraulic radius have a direct and significant relationship. Also, the roughness coefficient with the hydraulic radius has a inverse relationship by lower significance. Every flood creates a different roughness with its different sedimentation; therefore, depending on variations in particle diameter, Manning's roughness coefficient will vary. Usually, rivers in arid regions are temporary, in the descending limb of the hydrograph, leavs coarser material in the bed which causes an error in estimating Manning's roughness coefficient. In discharges where the flow depth is lower than D90 , Manning's roughness coefficient reaches its maximum value and due to the unique hydrodynamic circumstances, the Manning roughness coefficient is at its lowest value in discharges with a depth greater than D90 . In other words, in high discharges, the relation of the roughness coefficient to the total flow is reduced and by the increase of the flow depth, the nʼvalue decreases.
Conclusion: The results showed that the roughness coefficient changes from 0.034 to 0.119 in the study range and it has an inverse and significant relationship with discharge with R2  0.8. Also, the roughness coefficient with the hydraulic radius has an inverse relationship with R2  equal to 0.59. The roughness coefficient is not constant and changes in different flood events. Discharges whose depth is less than D90  are those in which Manning's roughness coefficient reaches its maximum value of 0.11 and we observed the lowest value of Manning's roughness coefficient in discharges with a depth greater than D90  and a flood height greater than 50 cm, as in the discharge of 115m3s-1 , the value of the roughness coefficient “n” decreases to the limit of 0.034. During peak discharge is difficult to understand the hydrodynamic roughness coefficient and it can lead to underestimation of the roughness coefficient; therefore, to obtain accurate roughness coefficient values in river engineering research, this case study should be considered as a good example.
 
Keywords: Stage-discharge, Hydraulic radius, Manning's roughness coefficient, D90. Faizabadabad
Full-Text [PDF 980 kb]   (45 Downloads)    
Type of Study: Research | Subject: هيدرولوژی
Received: 2023/09/23 | Revised: 2024/03/16 | Accepted: 2024/01/6
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