Hydraulic Modeling to Quantify Benefits of Floodplain Restoration

Abstract

In rural areas, flood management is often achieved with large civic projects such as dams and levees. In contrast, the restoration of floodplain processes is done as a remediation or seen as a separate end. Restoring floodplain processes into the flood management toolbox will be essential for the health and safety of people living in flood prone areas. The Chehalis watershed of Southwestern Washington is indicative of the changing philosophy behind water management. This case study examines how restoration is being studied as a viable alternative to continued levee and dam construction. Previous research has focused on small-scale interventions or broad watershed-wide statistical aggregation of sites. There exists a gap in research towards the mid-scale, reach-based, restoration. In this work, I quantify the reach-scale benefits of three floodplain restoration techniques: side channel creation, in-channel modifications, and floodable basins. I use HEC-RAS modeling software to simulate the flood attenuation benefits of these interventions compared to a control landscape that emulates the South Fork of the Chehalis River. The resulting simulations show that the floodplain restoration strategies increase floodplain area inundation by 38% and delay the passage of floodwater by 22-44%. These findings demonstrate that floodplain restoration techniques can be effective in floodplain management. They can not only hold water on the landscape but encourage processes such as groundwater recharge associated with inundated soils. When the tertiary benefits are considered, such as habitat creation and increased tourism, restorative floodplain management should be viewed as a viable alternative to traditional flood mitigation strategies.