The mechanics and rate of headcut growth on floodplains: Implications for the formation of channel networks

Floodplain channel networks have important implications for sediment transport dynamics on floodplains, floodwave conveyance, and mechanics of the main river channel. Floodplain channels form independently of the main channel and can span multiple meander wavelengths. A potential mechanism for floodplain channel formation is the development of headcuts nucleating off topographic lows in the floodplain. In this study, we test this mechanism using Delft3D, a physics based morphodynamic model, by simulating flow over an oxbow lake on a generic floodplain. We build a generic floodplain using initial and boundary conditions consistent with floodplain hydrodynamics on the West Fork of the White River, Indiana, USA. Our experiments show that oxbows can serve as a nucleation site for the development of headcuts. The headcuts form from a water level gradient developing at the interface of the upstream edge of the oxbow and the floodplain. The lower water level in the oxbow drives flow acceleration over the lip of the oxbow initiating erosion. The water profile remains unchanged as the headcut migrates further up-valley. Headcuts branch into multiple channels forming a channel network. Additionally, we investigate how oxbow morphology, substrate composition, and flood discharge influence the rate of headcut migration on floodplains and the resulting channel network morphology. We find that increasing discharge and initial oxbow depth creates faster headcut migration rates, deeper floodplain channels, and a higher channel density in the network. The initial substrate appears have no influence on the ability to develop headcuts, however migrations rates slightly decrease with increasing non-cohesive sediment. The resulting channels are deeper and narrower for cohesive dominated substrates, where non-cohesive produces wider shallower channels. Results suggest that headcut migration rates, and channel network morphology and density scale with a characteristic flood discharge.