Experimental study of fan delta evolution: Autogenic cycles of fully confined channelized flow and small secondary channelized flows

Abstract

Fan deltas grow largely through repeated cycles of lobe construction and channel erosion during the period of channelized flow. Although previous studies suggest that variable flood discharge may play a key role in strong erosion and related fully confined main channel formation in fan deltas, analog experiments with constant discharge and sediment supply showed that strong erosional events were cyclic. Understanding erosion cycles is important for land management and flood hazard mitigation. Here, we present flume experiments to investigate the mechanism and controlling factors of fully confined main channel-related erosion under constant discharge conditions. The results reveal that: (1) fully confined main channel-related erosion is triggered by turbulence within stratified fluid in hydraulic jump process. When the topography difference of the fan delta is sufficiently small, the flow direction changed from steeper flanks to the centerline. The flow velocity along centerline increase due to weak viscosity and steep slope of fan delta, changing the subcritical flow to supercritical flow. Supercritical flow transports sediment to the shoreline and is resisted by the standing body of water in the basin, causing the decrease of flow velocity accompanied by hydraulic jump. The turbulence formed in hydraulic jump process triggers the formation of scour hole which erodes the fan delta. (2) Input conditions control the range of the critical slope and frequency of erosion cycles. The critical slope is inversely related to the slope of the discharge and grain size of sediments, whereas it is directly related to the sediment/water ratio. The frequency of erosion cycles is directly related to the slope of substrate layer, discharge and sediment/water ratio. (3) The fan delta evolves under the interaction between the deposition process and topography. Moreover, dynamic equilibrium of experiments including main channel-related erosions and autogenic slope adjustments can be found in natural fan deltas.