Is the bluff too steep for this system?
Your photos and cross-section drawings show an incline on the bluff to be protected. The angle of the slope shown in your diagram appears to be appreciably less than much of the existing slope on the Siasconset North Bluff, where some of the surface slant is nearly vertical.

When the project at Hinckley Lane on Nantucket was being installed, a "cut" was bulldozed down roughly 15'-20' into the face of the bluff for almost the width of the property, in violation of the Conservation Commission's Order of Conditions.

Are you planning to lower the elevation of the Bluff in similar fashion as the work done at Hinckley Lane, which you cite as a reference? Will the baskets or mattresses filled with stones work if applied to a slope like what exists over more than half of the initial proposed project.

You are correct that the angle of the slope shown in the diagram you received initially is appreciably less than the slope of the North Bluff from the top of the bank down to the toe. However, at the lower elevations of the proposed marine mattresses (~+10 to +27 MLW), the bank slope shown in that generic illustrative diagram is actually steeper than our natural bank slope, which is roughly 25-27 degrees. There have been several iterations of site-specific diagrams since then.

The current project drawings depict sections along the proposed installation taken from a LIDAR topographic survey done last August, based on results processed and received in October 2010. Along the project area, the survey shows the top 10 ft± of the bank slope is more or less vertical, but it has a 38- to 40-degree slope below that to the toe.

Gabions and mattresses highly similar to those included in our design have effectively stopped erosion at a number of other locations such as those described in our case studies document.

In particular, the installation constructed by the Army Corps at Cape May, NJ, with a wave climate comparable to that at 'Sconset, was in service from 1996-2004 before being incorporated into a larger beach nourishment project.

The design of the proposed 'Sconset system has been provided by a reputable coastal engineering consulting firm that's keenly aware of the local site conditions. The design requires that where necessary (i.e. where jute terraces have been installed), the lower portion of the coastal bank (i.e. below EL. +26 ft MLW) will be graded to a maximum slope of 1V:1.5H (33.7 degrees) for a stable placement of the mattresses. As I mentioned above, the natural bank slope at those elevations is, for the most part, significantly shallower than that.

The purpose of the slope preparation is both to ensure that the maximum slope does not exceed 33 degrees and to create an even slope. In areas where the prepared slope is significantly shallower than 33 degrees, the installation will use longer mattresses, which makes for an even more robust design. To further secure the stability of the system, the mattresses will be fixed to the surface of the coastal bank using ground anchors driven to a depth of 12 feet. Each anchor has a pullout capacity of approximately one ton, with a minimum of five anchors required for each 5 ft wide x 18 ft long mattress unit.

The large excavation you mentioned at Hinckley Lane was for installation of a Vegetated Reinforced Slope System (VRSS) to stabilize the upper portion of the coastal bank, which is separate from the marine mattress and gabion installation constructed on the lower slope. No VRSS upper slope stabilization is proposed for the SBPF project. The proposed SBPF design involves no bulldozing or earth movement whatsoever at the top of the bank. That portion of the bank is, as you note, nearly vertical and will be left as it is.

The section of the bank just below that vertical section, down to the upper edge of the sacrificial sand berm covering the mattresses, will be re-vegetated with native plant materials similar to those found on the naturally vegetated bank at the southern end of the project area.

Although the re-vegetated bank is substantially steeper than the naturally vegetated more southerly bank, there are numerous examples on the Cape and Islands of successful vegetation of similarly steep banks composed of glacial deposits. Indeed, steeper sections of the North Bluff supported thick and sturdy vegetation throughout the 20th century, until toe erosion caused it to slide.


How stable is the system?
How will the anchors work? Won't they become new ways for water to get deep into the bluff, accelerating the whole mess to pull away during a big storm?

The mattresses will be fixed to the coastal bank using "platypus" mechanical anchors. As described in this illustration, these anchors have a thin shovel like head and are driven into the bank with a percussion tool. When they are loaded, the "shovel" pulls back and becomes locked into position.

Our design will have the anchor heads driven to a depth of 12 feet into the bank. The anchor is then attached to the mattress via a stainless steel wire cable that's fixed to the mattress via a high-density, polyethylene plate (HDPE) and hardware connection. These anchors create minimal disturbance of the soil during installation and can be stressed to an exact holding capacity, which is fully operational immediately (i.e. no grouting required).

Platypus anchors perform exceptionally well in granular soils, display short load-lock and extension characteristics, keep a broad cone of soil immediately in front of the anchor and carry extremely high loads. Because the soils are granular on the lower portion of the bank, drainage under existing conditions is good and there is no concern about the tendons acting as a pathway for water.

Why don't the first two levels of gabions turn inward on the ends? Isn't that an obvious weak point that will be more likely torn apart in a major storm?

The toe gabions are included to protect against scour at the base of the installation. As the returns curve back into the bank, the beach elevation in front of the structure also rises, so that based on the predicted depth of scour, the two outer rows of gabions are unnecessary. If scour along the returns becomes an issue over the life of the project, then toe gabions can be easily added as additional protection.

After becoming exposed, the rocks inside the mattresses might shift and become mobilized within the mattresses in a big storm. Won't the force of the rocks rip the HDPE geo-grid apart? Won't pieces of the plastic grid then float out into the ocean?

The mattresses are constructed with internal compartments (baffles) that prevent the rock from shifting and settling unevenly in the mattress. Furthermore, pre-filling the mats off-site at a construction yard, where the manufacturer is better able to provide quality control during fabrication, avoids excessive spaces between stones in the internal compartments.

This minimizes the degree to which the stones inside the mats can be "mobilized," or moved around, when struck by waves, and it creates a robust system that will not be torn apart from the inside. In fact, the voids between stones create a porous surface that acts like a sponge to absorb and dissipate wave energy breaking against it.

Similar mattress systems have been in place for 20+ years with no reports of major damage, or widespread failure or losses of significantly sized pieces of geogrid into the environment. These marine mats are capable of sustaining various types of minor damage without compromising the overall integrity or performance of the system.

Although the geo-grid is positively buoyant, it is a very rigid co-polymer material that unlike geo-textile fabric or jute will not wrap around boat propellers in the highly unlikely event that it is released into the marine environment.

Typical damage and repairs are illustrated in the attached "Repair Guidelines for Triton Mats."

Why can't we put a row of very large boulders in front of the whole system? That would blunt the force of the biggest waves and reduce concern about the geo-grid needing to stand up to the full force of the waves.

The Cape May, NJ installation, described in our case studies of marine mattresses, added a row of large boulders to the toe of that system as scour protection after its first few years. Although the large boulders did help break the force of waves against the mats, their size also tended to create additional scour around the boulders, speeding the erosion process beyond what would have been the case if they were not there.

This tendency to cause additional scour would also pose an obstacle to obtaining environmental approval for this type of installation. The 'Sconset design adds three rows of 4' tall x 5' wide x 10' long gabions to the toe of the mats, below the natural level of the beach, to provide additional protection against breaking waves and prevent scour from undermining the installation.

Why not just do one or two experimental areas so we can see how it works and remove if it doesn't work.

The current plan is small. We are proposing two 350-foot-long sections for the initial installation. These installations will be monitored closely to measure their success. SBPF has agreed to establish an escrow at the outset of the project to completely pay for removal of the system if that is ever deemed necessary.