Flow hydraulics in an ephemeral gully system under different slope gradients, rainfall intensities and inflow conditions

Authors: XU, XIMENG - Institute Of Geographic Sciences And Natural Resources; ZHENG, FENLI - Northwest Agricultural & Forestry University; Wilson, Glenn

Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2021
Publication Date: 4/22/2021
Citation: Xu, X., Zheng, F., Wilson, G.V. 2021. Flow hydraulics in an ephemeral gully system under different slope gradients, rainfall intensities and inflow conditions. Catena. 203:105359. https://doi.org/10.1016/j.catena.2021.105359.
DOI: https://doi.org/10.1016/j.catena.2021.105359

Interpretive Summary: Flow in an ephemeral gully (EG) system includes flow through rills and shallow flows from areas between rills which are all connected by a drainage network into the main EG channel that is called an EG system. There has been a lot of research on the water flow properties of rill flow and sheet flow between rills, as well as their relationships with soil detachment and sediment transport. Few studies have been conducted to measure the flow properties of EG systems. Thus, experiments were conducted with two rainfall intensities (50 and 100 mm h-1) and two slope gradients (15° and 20°) under different inflow sources to study the flow hydraulics of an EG system. The results showed that flow velocity, shear stress, stream power and energy of the flows in the EG channel generally increased from the top to bottom of the hillslope The magnitudes of these variables increased as rainfall intensity and slope increased, as well as inflow from upslope of the EG channel and from the channel sideslopes increased. The flow parameters of the rill flows and sheet flows increased as rainfall intensity and slope increased. Lateral inflow from sideslopes had a larger impact on flow hydraulics under the lower slope gradient and lower rainfall intensity conditions. The type flow condition of each branch of the EG system was determined by each component’s flow properties called the Reynolds number and the Froude number. EG channel flows were classified as transitional to turbulent flow, rill flows were transitional flow, while sheet flows on interrill areas were laminar. Good relationships between the total sediment loss and the water flow properties were observed with the flow shear stress being the best property related to soil erosion of the EG system (R2=0.92). This study quantitively mapped the flow properties of the EG system and showed the relationships between the flow properties and the soil erosion features. This work is leading to the development of process-based soil erosion prediction models for EG systems.

Technical Abstract: Overland flow in an ephemeral gully (EG) system includes the EG channel, rill and interrill sheet flows which comprise an interconnected drainage network. There are abundant researches on the hydraulics of rill flow and interrill sheet flow, as well as their relationships with soil detachment rates and sediment loads. Few studies have been conducted to qualitatively assess the flow hydraulics of EG systems. Thus, two erosive rainfall intensities (50 and 100 mm h-1) and two slope gradients (15° and 20°) under different inflow boundary conditions were employed to study the flow hydraulics of an EG system using simulated rainfall and inflow experiments. The results showed that flow velocity, shear stress, stream power and cross section energy in the EG channel generally increased from the top to bottom of the slope with different trends and fluctuations under various inflow conditions. These magnitudes of these variables increased as rainfall intensity and slope increased, as well as inflow increased. The flow hydraulic parameters of rill channel flow and interrill sheet flow increased as rainfall intensity and slope increased. Lateral inflow had a larger impact on flow hydraulics under the lower slope gradient and lower rainfall intensity conditions. Flow regimes in the EG system were divided based upon the Reynolds number and Froude number. EG channel flows were classified as transitional to turbulent flow, rill flows were transitional flow, while sheet flows on interrill areas were laminar. Good relationships between sediment load and flow hydraulic parameters were observed and flow shear stress was best correlated with soil erosion of the EG system (R2=0.92). This study quantitively mapped the flow hydraulics in the EG system and showed their correlations with erosion features, contributing to the development of process-based soil erosion prediction models for EG systems.