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United States Department of Agriculture

Agricultural Research Service

Research Project: INTEGRATION OF CLIMATE VARIABILITY AND FORECASTS INTO RISK-BASED MANAGEMENT TOOLS FOR AGRICULTURE PRODUCTION AND RESOURCE CONSERVATION

Location: Great Plains Agroclimate and Natural Resources Research Unit

Title: Impact of sediment load on manning coefficient in supercritical shallow flow on steep slopes

Authors
item Zhang, Guang-Hui -
item Luo, Rong-Ting -
item Cao, Ying -
item Shen, Rui-Chang -
item Zhang, Xunchang

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 6, 2010
Publication Date: November 23, 2010
Citation: Zhang, G., Luo, R., Cao, Y., Shen, R., Zhang, X.J. 2010. Impact of sediment load on manning coefficient in supercritical shallow flow on steep slopes. Hydrological Processes. 24:3909-3914.

Interpretive Summary: The Manning’s friction coefficient (n) is a widely used and extremely important parameter for use in water flow computation including flow velocity and depth. Manning’s n is affected by many factors such as bed roughness, size of bed materials, and vegetation cover, which have been extensively studied. Few studies have been conducted to quantify the effects of sediment load on the Manning’s n on steep slopes. using fixed-bed flume, the study was conducted to investigate the potential effects of sediment load and slope on Mannings n. Slope gradient varied from 8.7 to 34.2%, unit flow rate from 0.00066 to 0.00526 m2/s, and sediment load from 0 to 6.95 kg/m/s. The results showed that Manning’s n varied in both sediment-free and sediment-laden flows within a range of 0.012 to 0.047. The apparent Manning’s coefficients of sediment-laden flow were much greater than those of sediment-free flow. The effect of slope gradient on Manning’s n decreased as sediment load increased. This probably was caused by an increase in friction between water-bed interface (roughness of the flume bed - increase in deposition leads to increased roughness) with increasing sediment load. For sediment-laden flow, Manning’s n could be estimated with a power function of unit flow discharge and sediment content. This result indicates that the estimation of Manning’s n may be improved by considering sediment load in the flow. It would be useful to hydrologists and hydraulic engineers to get a better grasp on estimating Manning’s n.

Technical Abstract: The Manning equation is one of the most widely used formulas for calculating velocity of overland flow in hydrological and erosion models. Precise estimation of the Manning’s friction coefficient (n) is critical to simulate the processes of overland flow and soil erosion. Few studies were conducted to quantify the effects of sediment load on the Manning’s n on steep slopes. This study was conducted to investigate the potential effects of sediment load on Manning’s n in a flume with fixed bed under wide ranges of hydraulic and. Slope gradient varied from 8.7 to 34.2%, unit flow rate from 0.00066 to 0.00526 m2/s, and sediment load from 0 to 6.95 kg/m/s. The Reynolds number ranged from 664 to 8372. The results showed that Manning’s n varied in both sediment-free and sediment-laden flows within a range of 0.012 to 0.047. The apparent Manning’s coefficients of sediment-laden flow were much greater than those of sediment-free flow. The effect of slope gradient on Manning’s n decreased as sediment load increased. This probably was caused by an increase in friction between water-bed interface (roughness of the flume bed, it takes deposition to increase roughness) with increasing sediment load. For sediment-laden flow, Manning’s n could be estimated with a power function of unit flow discharge and sediment content. Further studies are needed to quantify the potential effects of sediment load on Manning’s n on erodible beds and in fields.

Last Modified: 9/21/2014
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