The Influence of Floodplain Restoration on Flow and Sediment Dynamics in an Urban River

A study of floodplain sedimentation on a recently restored floodplain is presented. This study uses a two-dimensional hydro-morphodynamic model for predicting flow and suspended sediment dynamics in the downstream of Johnson Creek, the East Lents reach, where the bank of the river has been reconfigured to reconnect to a restored floodplain on a 0.26 km2 (26-ha) site. The simulation scenarios include event-based (10, 50, 100, and 500 year floods) deposition modelling of flood events and long-term modelling using the 64 historical flood events between 1941 and 2014. Simulation results showed that the restored floodplain significantly attenuates the upstream flood peak by up to 25% at the downstream. Results also indicated that approximately 20-30% of sediment from the upstream is deposited on the East Lents floodplain. Further, deposited sediment over the simulated period (1941-2014) is approximately 0.1% of the basin’s flood storage capacity, however the reduction in the storage does not offset the overall flood resilience impact of the flood basin. The sediment conservation at the East Lents flood basin as predicted by the model reduces the annual sediment loading of the Johnson Creek by 1% at the confluence with Willamette River, providing both water quality and flood resilience benefits further downstream.