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ABSTRACT Connecting real-time measurements of current–bed interactions to the temporal evolution of submarine channels can be extremely challenging in natural settings. We present a suite of physical experiments that offer insight into the spectrum of interactions between turbidity currents and their channels, from i) detachment-limited erosion to ii) transport-limited erosion to iii) pure deposition. In all three cases channel sinuosity influenced patterns of erosion and deposition; the outsides of bends displayed the highest erosion rates in the first two cases but showed the highest deposition rates in the third. We connect the evolution of these channels to the turbulence of the near-bed boundary layer. In the erosional experiments the beds of both channels roughened through time, developing erosional bedforms or trains of ripples. Reynolds estimates of boundary-layer roughness indicate that, in both erosional cases, the near-bed boundary layer roughened from smooth or transitionally rough to rough, whereas the depositional channel appears to have remained consistently smooth. Our results suggest that, in the absence of any changes from upstream, erosion in submarine channels is a self-reinforcing mechanism whereby developing bed roughness increases turbulence at the boundary layer, thereby inhibiting deposition, promoting sediment entrainment, and enhancing channel relief; deposition occurs in submarine channels when the boundary layer remains smooth, promoting aggradation and loss of channel relief. 

Abstract Hydrologic connectivity controls the lateral exchange of water, solids, and solutes between rivers and floodplains, and is critical to ecosystem function, water treatment, flood attenuation, and geomorphic processes. This connectivity has been well‐studied, typically through the lens of fluvial flooding. In regions prone to heavy rainfall, the timing and magnitude of lateral exchange may be altered by pluvial flooding on the floodplain. We collected measurements of flow depth and velocity in the Trinity River floodplain in coastal Texas (USA) during Tropical Storm Imelda (2019), which produced up to 75 cm of rainfall locally. We developed a two‐dimensional hydrodynamic model at high resolution for a section of the Trinity River floodplain inspired by the compound flooding of Imelda. We then employed Lagrangian particle routing to quantify how residence times and particle velocities changed as flooding shifted from rainfall‐driven to river‐driven. Results show that heavy rainfall initiated lateral exchange before river discharge reached flood levels. The presence of rainwater also reduced floodplain storage, causing river water to be confined to a narrow corridor on the floodplain, while rainwater residence times were increased from the effect of high river flow. Finally, we analyzed the role of floodplain channels in facilitating surface‐water connectivity by varying model resolution in the floodplain. While the resolution of floodplain channels was important locally, it did not affect as much the overall floodplain behavior. This study demonstrates the complexity of floodplain hydrodynamics under conditions of heavy rainfall, with implications for sediment deposition and nutrient removal during floods.