In this second piece of our mini-series about coastal resiliency, we take a closer look at what adaptations can help us optimize both human and natural elements along coastlines. How do we successfully integrate artificial features like docks, marinas and bulkheads with natural habitats, and what kinds of initiatives can help restore the equilibrium needed for communities and their ecosystems to thrive?
To explore these questions, Jesse Davis, a Senior Coastal Engineer in GHD’s Miami, FL office, shares the insights he learned while conducting a study for the Marine Corps Air Station (MCAS) Cherry Point in Havelock, North Carolina about the potential for implementing living shorelines to help stabilize shoreline erosion around existing infrastructure.
Q: What were the original site conditions like, and what did you discover were the main issues of concern?
A: MCAS Cherry Point is an 8,000-acre military base that was built in 1941-42, on swampland that was cleared and drained. It is on the south side of the lower Neuse River, which at 250 miles long, is one of the longest free-flowing rivers in the Southeast. The MCAS Cherry Point base is close to the mouth, before the Neuse enters Pamlico Sound, on its way out to the Atlantic Ocean. The project area we were looking at consisted of both hardened and natural, unprotected shorelines. The hardened shoreline consisted of eight vertical wall bulkhead structures ranging from approximately 300 feet to 1,500 feet in length, sporadically placed rock and/or broken concrete revetments, and shore parallel timber structures.
We discovered that the natural, unprotected shorelines contained undercut banks, escarpments up to 8 feet tall, and were severely eroded with recession rates of up to 5 feet per year. We also observed advanced signs of deterioration along the majority of the bulkheads.
So the design question was, “How we do we sustainably repair/replace the existing bulkheads, which are necessary for the base to continue operations, while also increasing the resiliency of the natural shorelines and ecosystems?”
Q: How did you determine what measures needed to be taken?
A: We performed both a desktop evaluation and site visit to determine the range of living shoreline options available. Living shorelines use green infrastructure techniques and materials such as native plantings, sand and rocks as an alternative to hard shoreline stabilization methods like bulkheads. While many environmental conditions were considered, the driving factor for this site was wind generated wave energy. This was due to the shoreline’s sheltered location from ocean waves, 20,000ft waterbody width, and infrequent passage of commercial vessels.
Fetch is the unobstructed length of water that waves travel to reach the shoreline and we can correlate this to a certain level of wave intensity. This, along with wind direction, makes it one of the most important factors to consider when designing a living shoreline in a protected water body. For Cherry Point, the 20,000ft fetch length corresponded to a moderate wave environment, but after considering the compounding effects of seasonally high water levels and the high frequency of wind occurring along this same fetch, we determined that winter wind waves likely played a critical role in the loss of shoreline along MCAS Cherry Point.
To confirm this, we conducted a shoreline change analysis using historic aerial imagery. The results indicated that the entire 15,500 feet of shoreline has either receded or remained unchanged since 1994, with a maximum horizontal recession of 110 feet. This works out to an average loss of nearly 5 feet a year, and demonstrates a critical need for stabilization measures to prevent further loss of valuable Base land and habitat.
Q: What were your ultimate recommendations?
A: We determined that living shoreline components would be both cost-effective and feasible along the entire length of natural shoreline segments and the majority – 5 out of 8 – of the bulkheads. While they are not structurally necessary in front of the bulkheads (if these are replaced) they will provide ecological enhancements and maintain a riparian buffer that will dampen wave energy hitting the bulkheads during storm events, particularly the northeasters.
The design needed to be functional, resilient, and sustainable. To be functional and reduce erosion, we proposed building rock sills offshore of the existing shoreline to dampen the winter wave forces, and create a more protected space for native vegetation and intertidal marsh grasses to grow and establish themselves. To be resilient, we provided the client with a living shoreline option to protect adjacent, high topography backshore locations from Hurricanes, which can submerge the normal shorelines by up to 7 feet. These planted revetment areas would provide a dual benefit: protection from extreme storm events now, and increasing the shoreline’s resiliency to future sea level rise. To be sustainable, we recommended repurposing the debris located along the shoreline to divert waste from landfills and limit the amount of virgin material required. We also recommended exploring the incorporation of carbon capture technology into any new concrete used during the replacement of the seawalls.
Q: Are there any other recommendations that will ensure compatibility between all of the newly planned installations and the existing natural ecosystem?
A: Yes, absolutely. Overall, we want the design to be both resilient and adaptable. For example, the design could address both short and long-term projections of sea level rise. We can use heavier stones to resist an increase in wave energy associated with short term rise projections, and build a wider rock sill that would allow us to adapt for an increase in crest elevation to meet longer term projections.
Other design considerations include the geometry and layout of the rock used for the sills. Our rock choice should allow us to create large interstitial spaces, providing pathways for marine life and good conditions for fish habitat. We can also repurpose existing debris within the lee of the rock sill to create habitat diversity, or use offshore of the deeper water bulkheads to create submerged artificial reefs. There may also be the opportunity to reuse dredged sediment, if the existing Navy Boat Dock basin requires dredging in the future – we’d need to evaluate the quality, but it could be used as a planting substrate for the living shoreline installations.
We also want to prioritize using native vegetation that will minimize any need for fertilizer or pesticides, and in our long term monitoring plan we’ll want to consider including controls for invasive species.
For more on all of these topics, watch for the next piece in this mini-series about coastal resiliency – we’ll be following up with other team leads for their insights on living shorelines, co-existing with our natural habitats, and how coastal cities can become more resilient. You can read the first in the series about the challenges of increasing our coastal resiliency here.
Meet Jesse
Jesse Davis, PE is based in Miami, Florida, and has 13 years of experience in coastal engineering and has provided design, permitting, environmental field assessments, and construction phase services for projects located within Florida, Alabama, Mississippi, New York, New Jersey, Washington, Alaska, Puerto Rico, St. Croix, and Brazil. Career highlights include playing a key role through the entire project life cycle for the permitting, design, and construction of a nature-based island storm protection system in Fort Pierce, Florida that was the recipient of the 2016 ASCE-COPRI Project Excellence Award for large projects, the construction of an 11.6 acre artificial reef off the Port of Miami; and the design/construction engineering services for a 1,500 foot shoreline stabilization project on Whidbey Island in Puget Sound, WA that was recognized in NAVFAC's Environmental Restoration News as a success story.
His project experience includes living shorelines, marinas, boat ramps, dredging, shoreline stabilization, seawalls, propeller wash modeling, vessel berthing & mooring calculations, disaster response, design of fall protection for water control structures, inspection of breakwaters and water control structures, water quality sampling, hazwaste & drycleaner site sampling and contaminated soils removal oversight. For more information, please contact Jesse.Davis@ghd.com.