Location: Watershed Physical Processes ResearchTitle: A laboratory study of channel sidewall expansion in upland concentrated flows
|QIN, CHAO - Northwest Agricultural & Forestry University|
|ZHENG, FENLI - Northwest Agricultural & Forestry University|
|Wells, Robert - Rob|
|XU, XIMENG - Northwest Agricultural & Forestry University|
|WANG, BIN - Beijing Forestry University|
|ZHONG, KEYUAN - Northwest Agricultural & Forestry University|
Submitted to: Soil and Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2017
Publication Date: 12/21/2017
Citation: Qin, C., Zheng, F., Wells, R.R., Xu, X., Wang, B., Zhong, K. 2017. A laboratory study of channel sidewall expansion in upland concentrated flows. Soil and Tillage Research. 178:22-31. https://doi.org/10.1016/j.still.2017.12.008.
Interpretive Summary: Soil erosion in the Loess Plateau of China has been significant. Locations within the Loess Plateau are referred to as the "hilly gully" and "gully" regions. Significant portions of the total sediment load can be attributed to concentrated flow and gully development. Farming within the region often takes place on the side slopes of the plateau (i.e. steep land). Similar to the US during the dust-bowl era, understanding the mechanics of erosion and management of this vital soil resource have created the need to conduct laboratory investigation of processes and counter measures. Here, channel expansion (widening) in the presence of a plow-pan is explored. Plow-pans are dense layers of soil that are created during the tillage operation at the depth of tillage, primarily the soil just below the plow is compacted and smeared, which gives rise to increased strength and erosion resistance. When concentrated flow erodes the surface material, the channel deepens until the plow-pan is encountered, then the channel begins to widen. This set of experiments specifically look at widening processes impacted by erosion of the channel toe (fluvial erosion), tension crack development along the channel bank and failure block size and shape. Similar to research conducted in streams and rivers, the progression from toe erosion, tension crack development, block failure and block transport highly impact the timing of sediment flux and width development.
Technical Abstract: Gully erosion contributes large amounts of sediment within watersheds and gullies incise landscape into fractured patches on the Loess Plateau of China. As one of the main processes of channel development, gully widening occupies as much as 80% of total soil loss, especially in the presence of a less erodible soil layer. Our knowledge on channel widening is still limited due to the fact that most theories on bank failure are based upon river bank expansion mechanisms. Simulated scouring experiments were designed to investigate how inflow rate, slope gradient and initial channel width affects channel widening processes. Mechanisms of channel bank failure were analyzed. Soil boxes (2.0 m-long, 0.3 m-wide and 0.5 m-deep) with two slope gradients (15° and 20°), four inflow rates (1.0, 2.0, 3.0 and 4.0 L min-1) and two initial channel widths (4 and 8 cm) were subjected to simulated rainstorms followed by clear-water overland flow. A down sprinkler rainfall simulator system and constant-head water tank were used to apply the pre-rain and scouring simulations, respectively. The results showed that sediment delivery and channel width increased with increased flow discharge, bed slope and initial channel width decrease. Toe scour, crack development, bank failure and block transport were the four main processes of channel widening. Except for basal scour depth, other indicators including basal scour depth, arc length, tension crack length, width and their rates increased with time, flow discharge, bed slope and initial channel width decrease. Normal, surface and reverse are three modes of basal scour arcs observed in this study. Crack lengthening rate was the best indicator to describe channel widening and sediment delivery, while basal scour arc length was the best indicator to predict unit sediment delivery.