Floodplain Land Cover and Flow Hydrodynamic Control of Overbank Sedimentation in Compound Channel Flows

Abstract

Overbank sedimentation is predominantly due to fine sediments transported under suspension that become trapped and settle in floodplains when high‐flow conditions occur in rivers. In a compound channel, the processes of exchanging water and fine sediments between the main channel and floodplains regulate the geomorphological evolution and are crucial for the maintenance of the ecosystem functions of the floodplains. These hydrodynamic and morphodynamic processes depend on variables such as the flow‐depth ratio between the water depth in the main channel and the water depth in the floodplain, the width ratio between the width of the main channel and the width of the floodplain, and the floodplain land cover characterized by the type of roughness. This paper examines, by means of laboratory experiments, how these variables are interlinked and how the deposition of sediments in the compound channel is jointly determined by them. The combination of these compound channel characteristics modulates the production of vertically axised large turbulent vortical structures in the mixing interface. Such vortical structures determine the water mass exchange between the main channel and the floodplain, conditioning in turn the transport of sediment particles conveyed in the water, and, therefore, the resulting overbank sedimentation. The existence and pattern of sedimentation are conditioned by both the hydrodynamic variables (the flow‐depth ratio and the width ratio) and the floodplain land cover simulated in terms of smooth walls, meadow‐type roughness, sparse‐wood‐type roughness, and dense‐wood‐type roughness.

Does Channel Narrowing by Floodplain Growth Necessarily Indicate Sediment Surplus? Lessons From Sediment Transport Analyses in the Green and Colorado Rivers, Canyonlands, Utah

“…It is well established that riparian vegetation can substantially modify fluvial processes (Dean & Schmidt, 2011; Dean & Topping, 2019; Diehl et al, 2017; Griffin et al, 2005; Manners et al, 2015; Perignon et al, 2013; Tal et al, 2004; Vincent et al, 2009). Vegetation reduces channel margin flow velocities (Griffin et al, 2005; Manners et al, 2015), roots bind fluvial deposits thereby increasing the threshold required for erosion (Abernethy & Rutherford, 2001; Gran et al, 2015; Pollen, 2007), plant stems increase the form drag on fluvial deposits thereby reducing the boundary shear stress (Manners et al, 2015; Nepf, 1999) and promoting deposition (Butterfield et al, 2020; Diehl et al, 2017; Le Bouteiller & Venditti, 2014; Zong & Nepf, 2010), and sufficient vegetative roughness can disrupt flood conveyance (Burkham, 1976a, 1976b; Dean & Topping, 2019; Gellis et al, 2017; Juez et al, 2019).…”

Causes of Variations in Sediment Yield in the Jinghe River Basin, China

“…Table 5. Predicted annual sediment discharge in the Jinghe River (Sources: Yao et al 47 ; Liu et al 48 ; Yellow River Conservancy Commission 49,50 www.nature.com/scientificreports/ due to improper considerations 50. To reduce this error, the authors proposed the adoption of a variety of methods to compare and verify the calculation results. For example, statistical analysis was conducted.…”

“…The mechanistic relationships among the climate, underlying surfaces, runoff, and sediment discharge are complicated. Any method may cause errors due to improper considerations 50 . To reduce this error, the authors proposed the adoption of a variety of methods to compare and verify the calculation results.…”

“…For example, recent reduced heavy rainfall has led to a decreased rainfall-induced sediment yield and consequently a decreased sediment discharge. However, according to forecasts by the Intergovernmental Panel on Climate Change (2014) 50 , extreme weather and heavy rainfall events are likely to increase in the future. The reduction in sediment due to variations in rainfall was calculated as follows: where WS r is the future sediment reduction caused by reservoir works, i.e., the sum of the sediment retention potential of the remaining capacity of the existing reservoirs and that of planned future reservoirs; WS d is the sediment reduction caused by future water diversion works, which can be obtained by multiplying the water diversion in the basin forecasted according to the social and economic development by the average sediment concentration in the water diversion period; WS sc is the future sediment reduction caused by soil and water conservation, obtained from areas subject to existing and planned soil and water conservation works and the corresponding sediment reduction rates; and WS d is the forecasted value of sediment reduction in the basin.…”

“…For example, recent reduced heavy rainfall has led to a decreased rainfall-induced sediment yield and consequently a decreased sediment discharge. However, according to forecasts by the Intergovernmental Panel on Climate Change (2014) 50 , extreme weather and heavy rainfall events are likely to increase in the future. The reduction in sediment due to variations in rainfall was calculated as follows:…”