Phase 3: Optimization – Boardwalk Design with Backwoods Bridges

Frederique Beroset MBA & M.S. in Biology

Boardwalks are Intended to Lessen the Impact of Foot Erosion.

As of July 2020, certain areas of the Gulf Coast have gone nearly two decades without experiencing hurricane-related erosion. These coastal properties have accumulated substantial dunes and protective berms that have progressed seaward, often burying their boardwalks. We have observed this phenomenon and noted that many properties, when the time came to replace their aging boardwalk, opted to keep identical placement and design instead of altering their walkover to fit their dunes’ accumulation of new sand. A boardwalk is one of the core components of a successful hurricane resiliency plan. The upfront investment of obtaining the right permits and installing a dune walkover designed to protect your living coastal barriers can save thousands of dollars in the long run. The tradeoff of cutting costs and installing an inadequate boardwalk creates a safety hazard for pedestrians and accelerates dune erosion, thus resulting in a higher risk of hurricane damage. Boardwalks were introduced into the coastal environment to help lessen the erosion and damage caused by foot traffic to the dune ecosystem and provide safe passage to beachgoers. Boardwalks are not designed to last forever; but, within the guidelines provided by regulating authorities, building a boardwalk designed for an ever-evolving dune allows for practical, economical, and sustainable results.

What Happens when a Dune Progresses Past the Boardwalk

When a dune accumulates enough sand to evolve past or bury a boardwalk, the following 5 major issues arise. If sand is spilling onto the boardwalk, a concerning liability is created as loose sand is slippery. Shoveling loose sand off of a boardwalk to avoid accidents is labor-intensive, thus expensive, and nearly impossible to maintain. If the boardwalk is too close to the dune’s surface, native vegetation below the structure will die. Without healthy plants and their complex roots systems to hold the sand under the boardwalk in place, that area of the dune becomes more susceptible to erosion. If the boardwalk ends prematurely, beachgoers are forced to walk through developing dunes to reach the recreational beach area. This ongoing foot traffic kills the native vegetation and destabilizes these newly-created dunes. The section of dune below a boardwalk is often the weakest point because of a phenomenon we call the washing machine effect where surge churns against the poles of the boardwalk eroding sand at an accelerated rate.

Note: If your dune has already progressed past your boardwalk, there are other protective measures you can implement. For further information, read our article “4 Proactive Steps to Protect the Development of your New Berms.”

Interview with Andrew Kuehl from Backwoods Bridges

These observations prompted a conversation with the boardwalk experts at Backwoods Bridges, a custom timber construction company. We asked Andrew Kuehl their chief estimator to weigh in on our findings and discuss what kind of design-thinking may help maximize a boardwalk’s use while protecting the dune and protective berm evolving below and around it.

1. Dune Doctors: “Do you have any important design recommendations you would like property owners to know about?”

Andrew Kuehl: “We typically like to use 6x6s instead of 4x4s for posts. The use of 316 Stainless Steel hardware is best for corrosion resistance, but it does come at a significant cost.”

2. Dune Doctors:In Florida, aside from following the boardwalk specifications from the Florida Department of Environmental Protection (FLDEP), are there other protective measures you take to minimize damage to the dune?”

Andrew Kuehl: “We try to stay within 24”-36” of the boardwalk footprint during construction to minimize damage to the dune system and vegetation.”

3. Dune Doctors:Any boardwalk installation comes with a certain degree of disruption to the environment. When the disruption is significant is there a follow-up plan to help restore the damaged dune? Does the permit include mandatory restoration?”

Andrew Kuehl: “We don’t have a follow-up plan. We recommend they give you guys a call! The last DEP permit we pulled had a stipulation to restore the dune and vegetation to existing conditions. I recommend contacting the DEP for up-to-date information.”

4. Dune Doctors: If a storm erodes a dune and the boardwalk needs to be replaced, do you design a replacement boardwalk that fits the pre-storm dune or the eroded dune?

Andrew Kuehl: “Whether in Florida or Alabama, we would build to the eroded dune profile unless the client plans to replace the sand and replant the dune to pre-storm condition. We would work in tandem with you guys to coordinate where it would end… I’m sure FLDEP would also have a big say in the matter.”**

5. Dune Doctors:Many coastal areas are susceptible to high levels of erosion. How do you strike a balance in the design of a boardwalk between planning for the dune’s seaward progression and the property’s regular loss of recreational beach area?”

Andrew Kuehl: “The dunes change so much that we build them to the dune profile at the time of construction. It is hard to predict what the dune is going to do. We’ve had some boardwalks that had to be elevated significantly because they were ‘buried’ in the dune. We’ve also had the opposite where we had to lower the boardwalk because the dune had moved and the structure was too high.”

6. Dune Doctors:If you design a boardwalk for a dune that has a wide recreational beach area, do you consider dune evolution in the design?”

Andrew Kuehl: “It is hard to anticipate how a dune is going to move, so we don’t typically take that into account. The FLDEP typically requires you to end right around the toe of the dune, so that is usually where we will plan to end it. Another point I’d like to bring up is that the lifespan of these structures is typically not very long. The hot/cold, wet/dry cycles along with the salt and sun really take a toll on them, so 10 years is probably about the average lifecycle for a boardwalk. When the dune does its thing over 10 years then it is probably time to rebuild the beach access anyways.”

**NOTE: We confirmed with the FLDEP that dune restoration is mandated if damage occurs during the process of replacing the boardwalk. Marina Borries an Environmental Specialist with the FLDEP says “after installing a boardwalk, any vegetation damaged or destroyed must be restored or replaced. Since heavy equipment is not authorized for walkover construction, there is no reason any additional dune damage should occur, but if for some reason it does, additional restoration would have to be completed.”

Walton County Beach Access: A Joint Dune Rehabilitation Effort Between Backwoods Bridges and Dune Doctors

When dune rehabilitation and seaward progression are part of a property’s long-term vision of hurricane resiliency, the boardwalk design is a significant factor that can determine the property’s level of coastal protection. Several years ago, we worked alongside Backwoods Bridges on a project for Walton County. The area under the county’s public beach access on Dothan Avenue off of 30A had experienced erosion caused by stormwater run-off. To rehabilitate the dune, Backwoods Bridges brought in about 200 tons of beach sand and rebuilt their dune to the estimated pre-erosion form. To help curb the original cause of erosion, they also added a stormwater drain and pipe under the first part of the boardwalk. Dune Doctors installed native vegetation to stabilize the fresh sand. In the event of a storm, Andrew says they would most likely rebuild the dune and beach access in a similar way.

Wind Drift Condominiums, Orange Beach, AL: Achieving Sustainable Coastal Protection through Optimized Boardwalk Design

Dune Doctors also collaborated with Matt Gilley from MG Marine Construction in the replacement of Wind Drift Condominiums’ two boardwalks. Dune Doctors worked alongside boardwalk builders in a joint effort to protect and preserve Wind Drift’s dune ecosystem and initiate a new protective berm. Through proper design, the boardwalks were extended and elevated to provide breathing space for the dune to evolve. To strengthen the existing dune, Dune Doctors revitalized the vegetation and extracted dead organic material and debris. Dune Doctors also installed sand fencing to serve two purposes: initiate a new protective berm and stop foot traffic from damaging the dunes. The integration of both the boardwalk construction and Wind Drift’s enactment of the Dune Master Plan™ allowed Wind Drift to achieve the highest level of sustainable coastal protection. To read more about this project, click here to view our case study.

Boardwalk Design is a Key Factor in Dune Restoration

While the contour of the beach changes often, property owners that invest in sand fencing and coastal vegetation to initiate new protective barriers and accelerate their existing dune’s seaward progression benefit from designing a boardwalk that enhances these efforts. Certain properties do not have enough recreational beach area to extend their boardwalk 10ft past the seaward line of their dune’s vegetation (as regulated by the Florida Department of Environmental Protection), but if the environment is conducive – the boardwalk’s design should be centered on protecting the dune’s evolution. To mitigate storm-related damage, the integration of healthy vegetation, sand fencing, and strategic boardwalk design is key to the protection and growth protect of natural barriers to storm surge.

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