New Frontier for Alluvial Fans

Year: 2022
Presenter/s: Cygnia Rapp
Symposium Session: 2022 - 04 Stream Restoration from the (WSDOT) Transportation Perspective
Topics covered: fish passage, fish-salmon, floodplain, hydraulics, instream structure (culvert/bridge/dam), risk, sediment transport, stream, and wood


ABSTRACT

Implementing a region-wide fish passage restoration program offers opportunities for innovating floodplain restoration design, particularly where fish passage projects are co-listed as Chronic Environmental Deficiency (CED) sites. As an example, Childs Creek is a tributary of the Skagit River along State Route 20 near Lyman, WA. The existing stream crossing represents a frequent dilemma for WSDOT of how to restore sediment transport processes through a stream crossing within a highway alignment that crosses an alluvial fan. The Childs Creek stream crossing is both a fish passage barrier and recurring maintenance problem due to the loss of the natural sediment storage capacity of the now impaired alluvial fan. Historically, Childs Creek was relocated west of its original alluvial fan floodplain around 1905 with the construction of a logging railroad that truncated the toe of the fan. The current alignment of SR 20 was built in 1961 utilizing the 1905 railroad grade. Childs Creek has been confined between berms (sugar dikes) for 100+ years in an alignment that enhances localized deposition near the highway and reduces the area for sediment to deposit across its historical alluvial fan. Aggradation upstream and downstream of the undersized 1962 culvert results in erosion of the SR 20 road prism, erosion and overtopping of the berm along the highway, and regular inundation of the roadway during high-frequency, low-magnitude storm events. WSDOT is proposing to acquire about 7 acres of land directly north of SR 20 and west of the historical alluvial fan to re-establish a floodplain that can serve as an alluvial fan for long-term sediment storage while accommodating multiple channel configurations. The intent is to decrease the sediment load at the crossing downstream by storing it across the floodplain, thereby decreasing the need for maintenance through a new, wider crossing. Large wood integrated into floodplain design emulates the functions of a mature riparian forest by creating areas of long-term sediment storage, substrate for conifers to grow, enhanced revegetation ability, storage potential for wood originating upstream, and serving as hard-points to encourage multiple potential flow pathways – all of which provide benefits to aquatic habitat over the current confined system. The proposed crossing is based on geomorphic parameters (about 4.4x of the bankfull width) instead of stream simulation to accommodate multiple channel configurations during low flow conditions and flood events.