The Evolution of Coastal Erosion Control Technology – Part 3

Coir Fiber Logs and Mats
Coir fiber products are very commonly used in coastal erosion control projects. Coir, or coconut fiber, is a very tough and slow-degrading natural fiber. Coir fiber logs and mats provide building materials that are at once dense, flexible, and long lasting. This technology has seen extensive use in states and regions that discourage or have even outlawed the use of hard-armor structures.

Eight years ago, a contractor in Massachusetts developed coir fiber mats in the form of very large envelopes. Dave Lager, president of Netco Construction Project Managers Inc., says his company has gone through many iterations of the product, based on field experience, installing more than 50,000 linear feet of coir envelopes in more than 100 coastal locations.

The envelopes have been used successfully throughout southern New England. In one case, they were deployed to protect an eroding beachfront on Plum Island, a beach community near Newburyport, MA, where a Thanksgiving 2008 storm at the site had caused the complete loss of a beachfront home. Once installed, the mattresses were covered with sand and planted with native beach grasses.

Coir logs and mats have gained strong traction as coastal erosion control tools, particularly in the northeastern United States. In particular, this approach is very popular in the Cape Cod Region of Massachusetts. Coir logs share many of the flexible structural characteristics of the practices the Dutch have employed for centuries in the dike construction described earlier.

Anchoring Systems
A growing number of anchoring devices have also been developed. Large screw- or auger-type metal products, known as helical anchors, provide deep penetration belowground. Likewise, another set of products, known asduckbill anchors, work similarly to drywall toggle bolts. These anchors, which have a swivel head attached to a stainless steel cable, are driven deeply into the ground and then pulled upward, setting the anchoring toggle deep below the surface.

Drift Fence
Drift fence has been used for years to capture windborne sand, and this practice continues to be employed quite commonly. Many who employ this practice recommend deploying the fencing in a zigzag configuration for maximum performance. The wavelike zigzag pattern creates more overall length and greater capacity for sand capture. Depending on the site requirements, drift fence can be installed in multiple tiers along the beachfront.

Some users manufacture their own drift fence; others use prefabricated material. Still others have used drift fence to gradually repair breaches in dune structures or even to build new dune structures in successive lifts.

Beach Nourishment
Beach nourishment has been practiced in the United States for approximately 100 years. The beach nourishment process, simply stated, is the replacement of eroded beach sand. By adding sand to beaches, the buffer zone between tidal forces and coastal structures is extended.

Often used in conjunction with other coastal erosion control measures, beach nourishment is almost always a repetitive process. By nature it lessens the impact of coastal erosion, but cannot mitigate its root causes.

Beach nourishment projects are at once complex and costly. Many factors can influence the effectiveness of these projects. First of all, the characteristics of the fill sand must match up as closely as possible with those of the existing sand. Fine sand, for example, when placed over course sand, will erode at an accelerated rate. Additionally, extensive geological research is essential before embarking on such projects. It is critical to understand the site-specific ocean currents and normal sand transport patterns. Normally, beaches hold much more sand underwater than above. The volume of sand, its location, and its relationship to water movement patterns can help coastal geologists predict the likely effectiveness of beach nourishment programs.

Popham Beach is located at the mouth of the Kennebec River in Phippsburg, ME. It is one of Maine’s most popular beaches, and it is also extremely fragile and unpredictable.

Emptying in the Gulf of Maine at this point, the Kennebec is approximately 170 miles long and drains an area of nearly 6,000 square miles. Erosive forces at Popham Beach are very complex. The river mouth is populated by a series of peninsulas and islands, and the smaller Morse River enters the beach area just west of the main beach.

The sands at the beach shift continuously, but up until a few years ago, the beach was left to nature’s forces. That changed after Hurricane Irene in 2011. Unprecedented erosion and a suddenly menacing change of direction by the main current of the Morse River threatened a nearly new bathhouse/restroom structure at the beach. The response was rapid, and by early December a very aggressive beach nourishment program blocked this new eastward surge by the river. As of late 2013, this solution had held up.

Geosynthetic Tubes
Geosynthetic tubes are produced when high-strength geotextile fabric is sewn and fabricated into tubelike structures. For beach applications, the tubes are normally filled with beach sand and deployed as barriers or seawalls.

The tubes can be used as a standalone revetment structure, but can also be effectively used as a submerged foundation for sand dune creation. From an aesthetic point of view, they are certainly not attractive. Worldwide, however, geosynthetic tubes have gained some traction as a structural best management practice.

Sea Walls, Jetties, Groins, and Breakwaters
Hard-armor structures have been used for centuries to slow down or mitigate coastal erosion. More recently, other materials have been used to construct hard coastal structures, with metal sheet piling walls serving as one example.

Sea walls and other coastal structures have been constructed along coastlines everywhere in the world. While effective in many cases, these practices have been called more and more into question.

We now understand that solving a problem in one location can simply transfer the problem elsewhere. A pair of jetties at a river mouth, for example, can cause disruption of normal seaborne sand flows. The jetty facing the normal flow will predictably intercept a large volume of sand. Whole beaches have been built that way. The negative side, or course, is that sand will become depleted on the down-flow side of such structures.

Many states have outlawed new hard structures of any kind. In almost all cases, existing structures have been grandfathered. Others allow such structures only under the most extreme of conditions.

The city of Saco, ME, has been battling severe erosion at Camp Ellis Beach for years. A proposal has been made to extend an existing sea wall and to embark on a beach nourishment program. The project comes with a very expensive price tag. The extended sea wall alone will cost tens of millions of dollars. Of even greater concern is that not everyone agrees this solution will produce positive long-term results. In many ways, the situation at this site underscores the heart of the problem: While erosion prevention itself is a most complex topic, coastal erosion protection takes that complexity to a much higher level.

Drainage and Soil Stabilization
Coastal erosion can be worsened by erosion control practices adjacent to the shoreline. In one example, Maine State Geologist Stephen Dickson described how a septic system installed on a coastal bluff actually “lubricated the bluff” and undoubtedly accelerated its eventual failure.

In general terms, drainage and soil stabilization are two very important issues that must be addressed before any erosion control system is installed. If problems caused poorly drained or unstable soils are not addressed correctly, failure is very predictable. In coastal applications, such considerations are that much more critical.