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United States Department of Agriculture

Agricultural Research Service

Research Project: ASSESSING CONSERVATION EFFECTS ON WATER QUANTITY AND QUALITY AT FIELD AND WATERSHED SCALES

Location: National Soil Erosion Research Lab

Title: Flume experimental evaluation of the effect of rill flow path tortuosity on rill roughness based on the Manning–Strickler equation

Authors
item Strohmeier, Stefan -
item Nouwakpo, Sayjro -
item Huang, Chi Hua
item Klik, Andreas -

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 28, 2014
Publication Date: February 18, 2014
Citation: Strohmeier, S., Nouwakpo, S.K., Huang, C., Klik, A. 2014. Flume experimental evaluation of the effect of rill flow path tortuosity on rill roughness based on the Manning–Strickler equation. Catena. DOI:10.1016/j.catena.2014.01.11.

Interpretive Summary: In most soil erosion models, down cutting by surface flows on hillslope rills are normally quantified by a simplified equation that assumes the flow channels are straight and well-defined in its cross sections. This simplified equation accounts for the friction from the channel bottom. Nevertheless, we have observed that most natural rills and channels tend to meander. Hence, there is a question on whether the rill erosion equation correctly describes the rill flow processes. In a laboratory soil box, we observed the down cutting and change in rill morphology in two types of rills: preformed straight rills and free developing rills. Our results showed that there is a need to include a rill morphology term called tortuosity to account for the shape (or turning) of the rill channels. The combined rill bottom friction and tortuosity improved the prediction of rill erosion. Soil erosion scientists can benefit from these research findings in order to develop better erosion prediction models.

Technical Abstract: Numerous soil erosion models compute concentrated flow hydraulics based on the Manning–Strickler equation (v = kSt R2/3 I1/2) even though the range of the application on rill flow is unclear. Unconfined rill morphologies generate local friction effects and consequently spatially variable rill roughness which is in conflict with the assumptions of (sectional) uniform channel flow and constant channel roughness of the Manning–Strickler equation. The objective of this study is to evaluate the effect of rill morphology on roughness and hence to assess the Manning–Strickler roughness coefficient (kSt) by rill morphological data. A laboratory experiment was set up to analyse rill hydraulics and roughness of I.) Free Developed Rill (FDR) flows and II.) Straight Constrained Rill (SCR) flows in the flume. The flume experiment generated Manning–Strickler roughness coefficients (kSt) between 22 m1/3 s-1 and 44 m1/3 s-1 reflecting a potential area of the roughness parameter uncertainty. It was found that FDR experiments generated significantly lower kSt values compared to SCR experiments, because skin and local friction effects in the FDR experiments were more efficient reducing flow velocity probably due to higher energy dissipation. Rill flow path tortuosity (Tort) was used to describe the rill morphology of the experiments and correlation statistics between Tort and kSt identified considerable explanatory capacity of rill flow path tortuosity on rill roughness. The flume study demonstrated that a regression model between Tort and kSt can be used to assess local friction effects of unconfined rill morphologies and hence to reduce the area of uncertainty of the Manning–Strickler roughness parameter.

Last Modified: 10/24/2014
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