Changing Coastline

Hard Engineered Solutions

There are several types of hard engineered solutions that are commonly used to reduce the affect of erosion, which are consequently not employed within the National Seashore but are typically used elsewhere, including the bay side and areas of the outer beach that are not within the park. There are two primary categories of hard engineered solutions: shore parallel and shore perpendicular.

Shore Parallel Erosion Control Structures

Shore parallel erosion control structures can be defined as armored structures that run parallel to the shoreline, which include but are not limited to revetments and sea walls. Revetments can be designed in many different ways, serving as some form of slope protection. Gabion revetments, a commonly constructed type, are made up of coated wire baskets filled with trap rock, built into the beach two to three rows deep. Gabion revetments are used to offset the affect of storms, reduce general loss of beach and stabilize the cliff face, which can help vegetation take hold, catalyzing a more long-term erosion solution. The drawbacks of these types of revetments are that they are costly to install, need to be routinely maintained and have only about a five to fifteen year lifespan due to wear and tear on the baskets. Once the baskets fail, they can introduce non-indigenous rocks onto the beach, requiring a costly and labor-intensive clean up.[i]

Another commonly utilized shore parallel structure is a seawall otherwise known as a bulkhead. In the context of the Cape, sea walls are typically engineered walls that are erected to protect a property that has been built directly on the water from strong waves and storms. They can be constructed out of a variety of material ranging from concrete to fiberglass sheeting, steel, boulders or wood. Bill Fitts, who started Turtle Works Construction, installed many seawalls throughout Provincetown using a method he inherited from his predecessor, which had been inspired by the local trap boat fisherman. The greatest challenge in constructing the wall was driving the pilings down and setting them that was made possible by this technique, which Fitts has illustrated:

Driving the hickory poles was a matter of using water pressure. They would have a pump on board with 2 ½ inch firehouses and they would pump salt water through the hoses through a nozzle, which mounted to a long pipe and they would send that down with the piling and when the piling hit the ground and went in the water pressure from the hose would soup up the water and the piling would just sink down. Hickory is extremely, extremely heavy so its own weight helped it sink down. Then they would pull the hose back out and the piling would be set ready to go onto the next one.

One benefit of seawalls is that while they have high upfront costs, they require a relatively low amount of maintenance. Fitts discusses the extra life he recently gave to an older bulkhead by performing routine maintenance:

About eight or ten years ago I just went through my predecessor’s bulkhead… and replaced all its bolts, replaced the rod that was anchoring it behind and I would say that that bulkhead was 40 years old then and I probably gave it another 20 – 40 years more of life. Eventually it will need to be replaced, you can only go so far but they last a good long time.

Vital to the construction of seawalls is that the pilings must be driven down below the zone of influence of churning water because incoming waves can cause the ground around the piling to get soupy. If the sand becomes more fluid it can wash out from underneath, resulting in structural failure, which is why Fitts’ method of using water pressure helped him ensure that the seawall would last for several generations.

Shore Perpendicular Erosion Control Structures

Shore perpendicular erosion control structures run perpendicular to the beach and work to preserve the toe directly by providing a catchment system, versus shore parallel structures that work by stabilizing the bluff to prevent further sand from cascading down onto the toe. A typical shore perpendicular structure is a groin otherwise known as groyne, made up of rock or wood. A groin can be installed as a single groin or as a groin field, with many groins running parallel down the beach.

A groin would be installed to assist the growth of the beach on the updrift side. While effective, the drawback of a groin is that the updrift beach is amply fed by trapped sand, but the downdrift side of the groin is not fed, causing deep cuts to develop on the beach. In general groins “have a significant impact on the landscape and can create barriers to the recreational use of the upper beach. They often cause downdrift erosion unless there is a long term management commitment to beach recycling or nourishment.”[ii] For this reason, if a groin is installed, it must be kept to entrapment capacity to allow the traveling sand to continue on down the beach at all times. This means the beach must be artificially fed on the downdrift side so that the area can naturally erode, continuing the cycle of feeding further downdrift beaches. There is a similar policy to maintain normal sand levels for gabion revetments; if a gabion revetment is constructed it has to be covered with a enough sand to cascade down during a storm to keep feeding adjacent beaches. These are examples of the type of policies that have been implemented to try and offset the interruption to coastal processes by these types of methods.

Invasive erosion control techniques involving hard engineered structures are not benign because they interrupt coastal processes rather than strategically working with the existing elements. In addition to being costly and artificially altering the environment they are ineffective at providing a sustainable solution. The presence of a hard engineered structure may be effective for the section of beach that it spans, but the incoming wave energy will be displaced and will have a greater negative impact for neighboring sections of beach. If ranked, seawalls are the least disruptive over other methods and appropriate in areas where there is a heavy concentration of buildings on the waterfront like in Provincetown and an absence of preexisting sand. Sand however is the most efficient in dissipating wave energy. When waves come in contact with a hard material other than sand, the energy is not dissipated but merely redirected causing intensified wave power for adjacent areas, while also interrupting the long shore sediment transport system.

When determining which methodology should be adopted, several questions should be raised to decide what is the most appropriate solution.[iii]

• Over what time scale will it be effective?
• What will be the impacts on the natural environment and the landscape over different timescales?
• Are the resources (materials, equipment, labor, funding) available for both initial implementation and long term maintenance?
• Are the short and long term costs justified by the benefits?
• What will be the impacts on adjacent areas?
• Are there more appropriate options?

This line of inquiry leads us to the last question of more appropriate options. A full range of options including soft solutions, which are defined in the following section, should be considered when deciding which type of solution will be implemented.

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[i] “A guide to managing coastal erosion in beach/dune systems.” Scottish Natural Heritage. Web. 15 Nov. 2009. <http://www.snh.org.uk/publications/on-line/heritagemanagement/erosion/appendix_1.8.shtml>.

[ii] “A guide to managing coastal erosion in beach/dune systems.” Scottish Natural Heritage. Web. 15 Nov. 2009. <http://www.snh.org.uk/publications/on-line/heritagemanagement/erosion/appendix_1.12.shtml>.

[iii] “A guide to managing coastal erosion in beach/dune systems.” Scottish Natural Heritage. Web. 15 Nov. 2009. <http://www.snh.org.uk/publications/on-line/heritagemanagement/erosion/5.1.shtml>.