Are mangroves as tough as a seawall? Flow-vegetation interaction in a living shoreline restoration

Stable shorelines are integral to the resilience of coastal communities. Hardened structures, such as seawalls or revetments, may be used to stabilize eroding shorelines. Alternatively, degraded shorelines may be restored using native species and making use of coupled physical and biological processes. However, more complete understanding of the impact of ‘living shoreline’ restoration to physical processes could improve restoration designs. In this study, we assess the impact of an established living shoreline restoration to near-shore hydrodynamics, sediment texture, and organic matter content. We collected data from three estuarine shorelines: one restored; one eroding and stabilized by a seawall; and one in reference condition stabilized by mature mangrove vegetation. The living shoreline site was restored six years prior with a breakwater of oyster reef and plantings of emergent marsh grasses (Spartina alterniflora), and mangroves (Rhizophora mangle and Avicennia germinans). We sampled water depth and incoming instantaneous velocity profiles at 2 Hz using a 2 MHz Acoustic Doppler Current Profiler (ADCP, Nortek), stationed approximately 10 m offshore. A 2 – 3 cm velocity profile above the bed was sampled on the shoreline at 100 Hz, using a Nortek Vectrino profiler. The onshore probe was placed within vegetation in restored and reference sites (respectively N = 250 and 200 stems/m² and solid volume fractions φ=0.009 and 0.027).. Sediment cores were collected to 20 cm depth onshore and offshore of each site, and were processed for loss on ignition before being pooled by site for analysis of grain size distribution. While incoming velocity profiles were similar between sites, hydraulic conditions onshore within the vegetated sites deviated from the seawall site, which was devoid of vegetation. Hydrodynamic gradients from offshore to onshore reflect attenuation by vegetation; onshore velocities in restored and reference sites were orders of magnitude lower than offshore. Onshore velocities at the seawall site were similar in magnitude to offshore. Near-bed shear stress gradients were likewise flat in the seawall site and increased as water flowed through vegetation in the restored site. However, shear stresses did not increase through the mangrove prop roots at the reference site. The variable shear stress patterns observed at restored and reference sites likely attribute to differences in dominant vegetation-water interactions, mediated by stem density and plant morphologies. Sediment grain sizes in the reference site were finer and contained more organic matter.

Dr. Kelly Kibler is an Assistant Professor in the Department of Civil, Environmental, and Construction Engineering at University of Central Florida. Dr. Kibler’s research targets coupled biological and physical processes in river and estuarine systems. She is particularly interested in flow-biota-sediment interactions and their influence to aquatic organisms at multiple scales. The Kibler Ecohydraulics Lab studies natural hydrologic phenomena, as well as waterways that are modified for human benefit, for instance by, dams, dredging, levies and hardened structures. Applications for Dr. Kibler’s research include development pathways and infrastructure that minimize ecosystem disruption and promote preservation or restoration of aquatic ecosystem services.