Repeat Bathymetric Surveys and Model Simulations 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 sonar. These serial bathymetric surveys, conducted in 2015, 2018, 2021, and 2022, identified active sand bedform fields in the vicinity of 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 the bedform field on several of the reef beds. The Middle channel reef was comprised nine reef beds also oriented across the river channel. Sedimentation of the Middle Channel reef was more comprehensive as most of the beds in the 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 through the 36-meter-long gravel reef beds on a time scale of several days. The rate of sediment movement predicted by the model was more rapid than the speed of bedforms slip face translation measured in the field, a maximum of approximately 0.3 meters per day. Further, as the supply of sediment from upstream is continuous, once a reef fills with sediment it generally remains so, although some small variations (+/- 0.2 m) in the elevation of the sand overlying the reef surface were observed. Taken together, this measurement and modeling methodology could be used to monitor and simulate potential sources of bedload sediment that could impair the longevity of reef placements. Efforts directed toward enhancement and/or maintenance of reefs impaired by sedimentation would benefit from continued monitoring through periodic high-resolution bathymetric surveys, detailed inspection by diving, and collection of underwater imagery.