Physical Processes of Nature-based Solutions for River Restoration

The goal of river restoration is to restore the river's natural status and functionalities. Nature-based solutions (NBS) are becoming increasingly popular in practice. These restoration solutions reintroduce natural materials such as large woods into rivers. In contrast to traditional restoration structures that use impervious materials, NBS are complex in shape and mostly porous (leaky). The lack of general guidelines for quantifying river responses around porous NBSs leads to great uncertainties in engineering practice. This study investigates the physical transport processes associated with engineered log jams (ELJs) as a NBS. We conducted lab experiments to measure sediment and flow transport around scaled ELJs. We also performed computational fluid dynamics (CFD) simulations with the same setup and to complement the experiments. CFD results revealed the details of how porosity and bleeding flow affect the main flow features, such as downflow, horseshoe vortices, lee wake vortices, and flow contractions. Both experimental and numerical analyses show that, due to the bleeding flow through pore spaces within logs, turbulent flow and sediment transport processes around porous ELJs differ from those around solid structures.