Repeat bathymetric surveys and model simulation of sedimentation processes near fish spawning placements, Detroit and St. Clair Rivers, Michigan

Nine rock-rubble fish spawning placements, or artificial reef complexes, constructed in the
Detroit and St. Clair Rivers between 2004 to 2018 were surveyed periodically with multibeam
sonar. These serial bathymetric surveys, conducted in 2015, 2018, 2021, and 2022, identified
active sand bedform fields impinging two reef complexes: Fighting Island in the Detroit River
and Middle Channel in the St. Clair River delta. The spatial extent over which the bedforms
interacted with these reef complexes differed. The Fighting Island reef complex, which was
comprised of twelve reef beds oriented across the river channel, experienced partial
sedimentation that can be attributed to the streamwise translation and lateral encroachment of
a bedform field on several of the eastern reef beds. The Middle Channel reef complex was
comprised of nine reef beds also oriented across the river channel. Sedimentation of the Middle
Channel reef complex was more comprehensive compared to the Fighting Island reef complex as
most of the beds in the Middle Channel reef complex were within a translating bedform field.
We simulated the temporal evolution of reef sedimentation at the Middle Channel reef complex
using the Wilcock-Kenworthy (WK) two-fraction sediment transport model. In the WK
simulation, sand available upstream of the reef migrated into the 36-meter-long gravel reef beds
over 10 days of model simulation. The rate of sediment infill predicted by the model was more
rapid than the speed of bedform slip face translation measured in the field, approximately 0.3
meters per day. Further, as the supply of sediment from upstream is continuous, once a reef bed
fills with sediment it generally remains in place, although some small variations (+/- 0.2 m) in
the elevation of the sand overlying the reef beds were observed. Taken together, bathymetric
surveys and modeling could be used to identify, monitor, and simulate potential sources of
bedload sediment that could impair the longevity of future spawning reef placements. Efforts
directed toward enhancement and/or maintenance of reefs impaired by sedimentation could
benefit from continued monitoring through periodic high-resolution bathymetric surveys,
detailed inspection by diving, and collection of underwater imagery.