Variation of mean flow and turbulence characteristics within canopies of restored intertidal oyster reefs as a function of restoration age

The ecological benefits of healthy oyster populations have led to mounting interest in restoration of degraded reef habitats and design of nature-based reef features. However, hydrodynamic studies of restored oyster reef remain sparse, and little is known about changes in mean flow and turbulence as a function of time since restoration. In this study, we investigate hydrodynamic differences between restored (restoration age: <1 y, 2 y, 4 y), degraded, and intact intertidal oyster reefs in a shallow, microtidal estuary. Field experiments conducted at each reef were designed to characterize variability in flow and turbulence associated with differences in reef morphology and restoration age, addressing research questions of whether and when hydrodynamic function is reestablished following restoration. Minor differences in normalized turbulence parameters within the canopies of restored and reference reefs (2 cm above bottom) were associated with variable channel-to-reef velocity attenuation, which was linked to heterogeneity in oyster canopy structure as characterized by high-resolution laser scans. Within-canopy turbulence characteristics and normalized Reynolds stresses were significantly elevated on restored and reference reefs compared to the degraded reef, emphasizing the role of reef restoration in changing near-bed hydrodynamics. Above-canopy (9 cmab) turbulence was hydrodynamically similar across live reference and restored reefs of all restoration ages, with estimated roughness heights that scaled with the canopy height. Comparisons between intact and restored reefs indicate that properly restored reefs can reach hydrodynamic similarity with historically intact reefs within 1 year of restoration, a conclusion that supports the use of reef restoration as a tool to reestablish hydrodynamic functions on degraded reefs.