Connecting flood-related fluvial erosion and deposition with vulnerable downstream road-stream crossings

Fluvial erosion is increasingly responsible for infrastructure and building damages associated
with floods as the intensity of extreme rainfalls hit rural and urban rivers in a variety of climate
settings across the United States. Extreme floods in 2016 and 2018 caused widespread culvert
blockages and road failures, including extensive damage along steep tributaries and ravines in
the Marengo River, Wisconsin, watershed during 2016 and 2018. A study conducted by the U.S.
Geological Survey (USGS), Wisconsin Wetlands Association (WWA), Ashland County, and the
Northwest Wisconsin Regional Planning Commission (NWRPC) investigated the special
concern of fluvial erosion hazards (FEHs) associated with gullying, streamside landslides, and
the loss of wetland storage in headwaters. In 2019, a pilot study was begun to map and classify
ephemeral and perennial streams and wetlands in terms of their sensitivity to FEHs. This study
combined data from field-based rapid geomorphic assessments (RGAs) coupled with a stream
network-wide geographic information system (GIS) approach for mapping stream segments,
referred to as fluvial process zones (FPZ), sensitive to erosion, deposition, and channel change.
The GIS approach used nationally available 10-meter (m) resolution topology and an extended
stream network to map FPZs based on Strahler stream order, stream power, channel slope,
presence of adjacent steep valley sides and headwater flats, and adjacent landform setting.
Bankfull channel widths derived from RGA-based hydraulic geometry curves combined with
drainage areas, an estimate of bankfull flow, and channel slope were used to calculate specific
stream power for the FPZs. Lastly, the FPZs were characterized by their location within three
major landform settings that affect erosion potential. The resulting vulnerability maps provided
a screening framework to identify FPZs that are sensitive to incision, gullying and mass wasting
along steep headwater ephemeral channels, as well as downstream perennial channels that have
the potential for valley-side landslides, coarse sediment deposition, and channel change. Lastly,
each FPZ was characterized in terms of hydrologic alteration associated with ditching. The
vulnerability mapping products and rankings of sensitivity of FPZs will ultimately be used by
Ashland County and their collaborators to prioritize natural flood management projects that
mitigate FEHs, restore hydrology, and reconnect channels with adjacent wetlands and
floodplains.