Expect the Unexpected – Monitoring geomorphic changes and evaluating overall effectiveness in meeting project goals and objectives in highly dynamic alluvial environments – Part 1: Hydraulics and Sediment

The foundation for the design of the Hansen Creek Floodplain Restoration project was a holistic approach to restore floodplain connectivity and natural geomorphic processes. Of primary importance was to restore the alluvial fan’s ability to store sediment, form distributary channels into the floodplain wetland, and create naturally forming habitat without a “heavy-hand” engineering footprint. The design approach relied on natural hydraulic forces creating the distributary channels and complex habitat features rather than a structured and engineered floodplain and associated channels. Design features included passively activated distributary channel avulsion points and limited floodplain grading to provide a general flood flow direction. This generalized grading plan provided for spatially varying floodplain inundation levels to support the proposed revegetation mosaic, provided micro-topographic complexity, maximized potential shallow water edge habitat for salmonids, and focused the flood flows in certain “paths” to optimize the placement of LWD structures that could be afforded within the available project construction budget. But was it enough? Should the project have been phased differently? If so, how can large projects such as this with longer implementation schedules be effectively funded? Is the alluvial fan functioning as planned? Did we meet the project objectives? What metrics or measurements should you use to monitor “success” from a geomorphic perspective when the outcome is based on random hydrologic events and complex processes at alluvial fans? Sediment deposition at the Hansen Creek Floodplain Restoration site has been extensive since construction was completed, but not uniform across the site. This seriously limits the analysis of channel and floodplain topographic cross section data. Is more detailed topographic analysis required? How do you fund longer term monitoring that is controlled by random hydrology, without a set monitoring schedule? This presentation will discuss these questions relative to the design objectives, design features, hydrologic inputs, how the project has performed to date with regard to sediment storage and creation of distributary channels, and issues associated with monitoring restoration progress at this site to help guide future projects with similar challenges