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Revetment at the National
Gypsum A TOSC Infomation Brief |
TOSC Program |
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Revetment at the National
Gypsum A TOSC Infomation Brief |
TOSC Program |
This TOSC Information Brief addresses National Gypsum's proposed revetment at the Cement Kiln Dust (CKD) site, in Alpena, Michigan. In building the revetment, National Gypsum is seeking to reduce the movement of CKD material into Thunder Bay. The revetment does not address the 80 acres of onshore CKD contamination, nor the approximately 60 acres of CKD material that have entered Lake Huron since the CKD pile was created in the 1950s. Those problems remain under investigation by National Gypsum and the Michigan Department of Environmental Quality (MDEQ).
The Information Brief will provide an overview of the use of revetments to stabilize shorelines, and examine the use of a revetment at the Alpena CKD site. Key questions include:
A revetment is a type of structure that is built along an embankment, shoreline or steep facing slope in order to protect it against erosion generated by wave or current action. A revetment is like a common sea wall in that it is designed to maintain the location and shape of a shoreline. Instead of standing vertically like a sea wall, however, a revetment "coats" the shoreline with protective material. A typical rock revetment, like that in Alpena, is shown below.
Revetments are typically constructed using materials that are able to endure years of wave action. Materials may include quarry stone, concrete, interlocking concrete-block, or fabric. The Alpena revetment is a 'riprap' revetment made of quarry stone. Riprap is a type of design that utilizes layers of stone that are anchored in place. There are four main features found in all revetments. Moving from the shoreline into the body of water, those features are:
Starting from the shoreline and extending on to shore, a revetment will usually have a filter cloth. A filter cloth allows groundwater to drain through the structure without creating erosion channels that could weaken the revetment. On top of the filter cloth different grades of stone are layered. The base layer of stone, called "core stone", is smaller to allow for settling, while the outermost layer of stone, called "armor stone" is larger and heavier. The filter cloth and core stone provide a stable base to build upon, and the armor stone protects the revetment (and thus the shoreline) from wave and ice action.
The layers of stone extend above and below the shoreline surface, with the top extending above the "high water level", the highest anticipated water level based on historical data, preventing waves from going over the revetment. Likewise, the bottom of the revetment extends below water level. This "toe reinforcement" prevents waves from removing sand at the revetment base and thus undermining the structure.
Revetments have been built along the shorelines of many lakes, rivers and oceans to protect a water-shoreline interface from excessive erosion (such as between the CKD pile and Lake Huron). On the Great Lakes, a 13,000-foot rock revetment was built near the Indiana Dunes National Lakeshore, in Beverly Shores, Ind., to control dune erosion into Lake Michigan. Some of the revetments used in the Great Lakes region include:
Location |
Body of Water> | Length of Structure |
Beverly Shores, Ind. |
Lake Michigan |
13,000 ft. (total) |
Michiana, MI |
Lake Michigan |
400 ft. |
Whitefish Township, MI |
Lake Superior |
300 ft. |
Manistique, MI |
Lake Michigan |
204 ft. |
Table 1: Selected Great Lakes Revetments
Revetments have also been used along such ocean coastal regions as Jupiter Island, Fla., where an interlocking concrete-block revetment protects houses from storm surges. In addition, projects to stabilize riverbanks along Michigan’s Pine and Rouge Rivers have used revetments.
While revetments have varying degrees of success, all revetments have the feature of slow failure. Strong storms and the waves they produce will wear away at the structure. One big storm will probably not wipe out the entire structure, but big storms may cause pieces to fail each time. Revetments are best seen as durable but impermanent structures. The most common cause of failure is erosion of the base through wave action. Revetments also fail through erosion at the ends. Proper design and maintenance can prolong the life span.
In the case of the 13,000-foot Beverly Shores revetment described above, portions have failed repeatedly. The Corps of Engineers has placed new stone to reinforce where the structure has failed. The failures in the revetment have occurred at the same locations several times, and yet other portions of the revetment are virtually unchanged since its construction in 1974. No studies have been carried out to examine the basis for the repeated erosion. Generally, it can be said that, in part due to repairs and in part due to the design, the Beverly Shores revetment continues to keep the road along the shore (the main feature the revetment is protecting) in safe, driveable condition.
Few, if any, studies have been done of the long-term success of revetments as shoreline protection measures.
Revetment design varies based on the wave climate (or type of waves), shoreline geology and materials available for construction. The primary goal is to build a structure that is strong enough to absorb or deflect the energy of waves at a given site. There are three main considerations in the design phase:
A design consideration is the height of the revetment, which is chosen to prevent normal waves from running over the structure. If this "overtopping" occurs regularly, erosion of the shoreline behind the revetment may reduce the strength of the structure and lead to its failure. As described above, a revetment is built above the "high water level", that is, the highest anticipated water level based on historical data, to resist overtopping.
Another design consideration is preventing "bottom scour", caused by the rolling action of waves removing sediments from the base of the revetment. Bottom scour can be reduced through adequate toe reinforcement, particularly extending the toe sufficiently far into the lake. If scour is allowed to occur, the revetment may eventually lose support at its base and collapse into the water. The ends of the revetment must also be properly reinforced. Without reinforcement at the ends, waves will slowly erode the sides of the revetment and allow waves to get behind the structure. When waves come into contact with a revetment that is not sufficiently "tied in" on the sides, the wave energy will be redirected behind the structure and cause flank erosion. Flank erosion is shown occurring on the left side of the revetment in Figure 2 below. Flank erosion can easily be prevented by designing the revetment to run back into shore, as shown on the right edge of the revetment in the figure.
Figure 2: Revetment figure showing flank erosion on left and control of flank erosion on right
Some type of relief from ground water pressure from behind the structure (on shore) must also be provided. This is typically accomplished by incorporating a filter cloth or "geo-membrane" into the design. Small holes at the base of the revetment can also provide relief from ground water pressure but are not as common.
Revetments are usually designed to be long-lasting structures. In addition to choosing a proper design and suitable materials, a long-term maintenance plan needs to be implemented. Routine maintenance along with occasional repairs helps ensure a durable revetment. Routine maintenance includes checking for structural deficiencies within the revetment. Flaws in the structure may be due to uneven settling, inconsistent size and strength of the armor stone, erosion around and within the structure or other causes. Fixing a problem while it is small is much easier, safer and more cost effective than waiting until a large structural failure occurs.
Typically, revetments are designed to withstand the most severe storms that may occur every 50 to 100 years. The term "100-year storm" represents a storm, size and duration that is statistically probable, on average only once every 100 years. The Alpena revetment was designed to withstand a 50-year storm event.
The National Gypsum revetment will be a large rock riprap type of revetment. Its length will be 1,850 linear feet and contain an access road along the base of the CKD pile necessary for the long-term maintenance of the structure. The structure will stand approximately 10 feet above the Lake Huron water level. The revetment is intended to be a durable shoreline structure. Quantities of material necessary for construction of National Gypsum revetment:
The National Gypsum revetment will be constructed of three main layers of stone: armor stone, filter stone and core stone. Lafarge Corp. is providing the funding for the stone, which will come from the Glawe, Inc., quarry, located two miles west of the CKD site. Between the core stone and filter stone, a geotextile membrane will be placed to allow water to migrate through the structure. A generalized diagram of the embankment shows how each of these layers will be placed to protect the future shoreline.
Figure 4. Cross Section of National Gypsum Revetment
The construction of the revetment will require a portion of the lake bottom to be excavated and a portion of the of the existing CKD pile to be excavated. An estimated 4,000 cubic yards of lake bottom material will be excavated to construct the toe reinforcement. More than four times that amount of onshore CKD will be moved back from the shoreline. The onshore excavation will allow the pile to be regraded and an access road to be installed. That material will placed into a depression near the western end of the CKD pile. The CKD pile near the revetment will be graded to slope down towards the structure; however, the slope will be much less than before the revetment’s construction. The area of the pile near the revetment will also be strengthened with topsoil and vegetation.
National Gypsum was issued permits by the Michigan DEQ and the Corps of Engineers. Both agencies have stated that the revetment is only a portion of the overall shoreline protection plan. To support that conclusion, the revetment permit includes a figure labeled "The Shore Protection Sequence Plan", which shows future projects of an additional 1,000-foot revetment to the west of the current construction and an additional 2000-foot revetment to the north.
1. The revetment should not be seen as a permanent solution to the problem of bulk movement of CKD material into Lake Huron, given that the revetment has a limited life span. A permanent solution needs to be found during the life span of the revetment.
2.Given its location along a Great Lakes shoreline, the National Gypsum revetment is especially vulnerable to erosion at the base and at the ends of the structure.
3.Water running off the CKD pile, behind the revetment, may cause erosion and consequential dislocation of the revetment. The surface water movement should be controlled to prevent this from occurring.
4. Due to the unique characteristics of CKD, there are many questions that need to be addressed. For example, the ability of the CKD material to resist compaction and sinking under the weight of the revetment is not known. Revetments are not commonly built on CKD material; thus its durability over time is not known.