Author: Mackenzie Taggart and Brandon Puckett
How did Hurricane Florence and Tropical Storm Michael Impact Living Shorelines?
Part II: Monitoring the Performance and Resilience of Marsh Sill Living Shorelines
Regular monitoring of eight constructed rock and oyster sills by scientists with the North Carolina Coastal Reserve and National Estuarine Research Reserve (NCNERR) and the Division of Coastal Management (DCM) is part of an ongoing project to assess the performance and resilience of marsh sills over time in coastal North Carolina. The monitoring addresses 1) structural integrity of the sills, 2) erosion reduction and sediment accumulation because of the sills, and 3) repair and maintenance the sills may need to undergo.
Monitoring results will be used to advise waterfront property owners, contractors and regulatory partners on the shoreline protection benefits, durability, and potential cost-effectiveness of marsh sills. The sills being monitored are located along the coastline from the Chowan River in Edenton to St. James Plantation in Southport. The marsh sills range in size from 79 feet (Springer’s Point) to 505 feet (St. James Plantation) and age from 3 to 14 years (Figure 1).
Post Hurricane Florence
All the monitored sills were visited 1-3 months pre- and post-Hurricane Florence. Data collected during monitoring showed minimal signs of damage to both the rock and oyster sills due to Hurricane Florence. Across all sills, scientists concluded an average loss of 3.5 inches of sediment due to scour on the waterward side of the sills, and an average loss of 1.2 inches on the landward side of the sills (Figure 2). On average, sill width increased by 9.8 inches, whereas sill height decreased by 1.5 inches, which is likely due to slight movement of sill material (rocks or oyster bags) during the high energy storm event (Figure 2).
Behind the sills, the marsh shoreline eroded an average of 11.8 inches with a subsequent loss of 14 percent of marsh vegetation, suggesting most of the marsh was protected and retained after the storm. Only the St. James Plantation Oyster Sill in Southport suffered structural damages in the form of a few displaced oyster bags from the main sill (Figure 2). Overall, the varied sill types proved successful in offering durable shoreline stabilization—by minimizing erosion and protecting marsh vegetation—even after the strength of a hurricane.
The saying goes that pictures are worth a thousand words, so below we’ve include pre- and post-Hurricane Florence pictures of the sills we monitored. The pictures speak for themselves and illustrate what the monitoring data suggests—marsh sill living shorelines perform well during large storm events, providing a resilient shoreline stabilization option along North Carolina’s estuarine shorelines.
Morris Landing Rock Sill | Holly Ridge, NC
This 492-foot rock sill was installed in 2005 at the Morris Landing Clean Water Preserve. The photos were taken looking southwest, along the sill. Notice the similar location of the marsh vegetation line from before to after the hurricane. Changes in vegetation color are largely due to changing seasons (summer to fall). Measuring tapes were used during monitoring.
Springers Point Rock Sill | Ocracoke Island, NC
This 79-foot granite rock sill lies on a very exposed shoreline on the Pamlico Sound side of Ocracoke. The photos are taken looking north along the sill. Changes in vegetation color are due to normal seasonal changes. In the photo on left, the NCNERR staff person is using a highly accurate GPS to measure elevations.
Teaches Hole Rock Sill | Ocracoke Island, NC
Silver Lake Harbor Oyster Sill | Ocracoke Island, NC
This marl rock sill was constructed in 2012 at a length of 105 feet and is located within Ocracoke’s Silver Lake Harbor. The photos were taken along the sill looking northeast. The water level is relatively high in the bottom, right hand picture, so the sill is not as visible.
Chowan River Boat Ramp Rock Sill | Edenton, NC
Measuring 367 feet, this is the northern most sill in the monitoring program and was the least impacted by Hurricane Florence. The photos in the top row were taken looking northwest, along the sill. The photos in the bottom row were taken looking southeast, along the sill. In bottom left photo, the DCM staff person is using a measuring tape to measure the distance from the rock sill to edge of marsh vegetation.
St. James Plantation Oyster Sill| Southport, NC
Measuring 505 feet, this is the longest sill in the monitoring program. First installed in 2007, this oyster bag sill has had several additions to increase its length. The photos in the top row were taken looking west. The photos in bottom row were taken looking east. A few of the oyster bags (oyster shell in plastic mesh bag) at the top of the sill were knocked off of the sill during the storm (not pictured).
Kingsley Street Park Sill| Southport, NC
This 148 foot granite rock sill was installed in 2004 on behalf of the city of Southport. The photos were taken looking northwest, perpendicular to the sill. In the photo on left, the NCNERR staff person is using a quadrat (the large square) to assess marsh vegetation cover.