Supporting aquatic habitat remediation in the Detroit River through numerical simulation

Successful habitat remediation begins with an understanding of the geomorphic processes (e.g., flow regime and sediment transport) that maintain desired habitats. Although field surveys can provide information on site conditions, field measurements are limited to spatial-temporal extents and resolutions that do not always meet the level of detail required to determine the best remediation design for the underling geomorphic processes at a given site. However, hydraulic models are capable of predicting flow characteristics at fine resolutions over large spatial extents and can be developed to supplement field surveys. Hydraulic models were used in conjunction with field surveys to guide the placement of three fish spawning habitats (reefs) in the Detroit River. Model predictions were used to identify reef placement minimizing flow disruptions that could encourage sediment deposition over the reefs. Following reef construction, we revisited the hydraulic models to evaluate their ability to predict the influence of reef design and placement on flow patterns and further examine the influence of the reefs on localized flow heterogeneity at a resolution relevant to fish (<1 m). Three-dimensional computational fluid dynamics (CFD) models were used to predict the flow field over the remediation sites with and without the virtual addition of the three fish spawning reefs. The pre-reef model was calibrated using water velocity measurements made with an acoustic Doppler current profiler (ADCP) prior to reef construction. Following calibration, the three reefs were virtually added by manipulating the bathymetry to reflect post-construction conditions and the CFD model was repeated with the post-construction bathymetry. Comparison between measured and predicted post-construction water velocities revealed a mean difference of 18%, with the largest differences in shallow areas where detailed bathymetry was lacking. Near the reefs the CFD model was able to more accurately capture flow patterns. Post-construction predictions showed shear stress over the reefs was capable of maintaining transport of sand. Lake sturgeon spawning habitat suitability, vorticity and flow heterogeneity also increased locally over the reefs, suggesting the remediation projects created flow conditions favorable to spawning fishes. Hydraulic models allowed examination of detailed flow patterns over a large spatial extent and provided a valuable tool for guiding reef remediation projects.