GARY B. GRIGGS & MATTHEW J. SLAGEL
A Regional Approach to Coastal Protection
Gary B. Griggs
Director-Institute of Marine Sciences
University of California, Santa Cruz
Matthew J. Slagel
Department of Ocean Sciences
University of California, Santa Cruz
Historically the most common response to coastal erosion along developed shorelines has been the emplacement of some type of armoring, typically riprap or a seawall. Concerns have arisen over the past 20 years, however, over a range of potential impacts of these structures on the shoreline, particularly on public beaches. Much of the existing coastal armor in California was built by individual property owners without any environmental review and as stand alone projects without any regional considerations. Soil nail walls represent a relatively new approach to coastal bluff stabilization that can significantly reduce or eliminate most of the potential impacts of coastal armor. They can be constructed on a regional basis, and can improve the visual, lateral and vertical access and beach area concerns in areas of existing and poorly planned armor.
Additional Keywords: seawalls, revetments, riprap, coastal erosion, coastal protection, armor.
With sea level rising globally and coastlines in most places eroding, public agencies as well as ocean front property owners are becoming increasingly concerned about responding to the ongoing land loss and retreat. Sea level rise isn’t a recent phenomena; it’s been happening for as long as Earth has had an ocean, primarily in response to alternating episodes of global warming and cooling. Sea level has risen over 350 feet since the last Ice Age ended 18,000 years ago. About 10,000,000 cubic miles of water have been added to the oceans in the subsequent years due to the gradual melting of the ice caps and continental glaciers. It is now apparent that the production of greenhouse gases by human activities has added to the natural global warming process, although the magnitude of the human impact on sea level rise is difficult to determine precisely. What is not at all clear is how high sea level may ultimately rise in the decades ahead during this warming cycle and when the maximum might occur. If the global climate suddenly began to cool and sea level rise leveled off, all of the millions of people living on the coastline, or very close to sea level, could begin to ease their minds. Unfortunately, however, the preponderance of evidence points to a slow but continued warming with a progressive rise in sea level through at least this century, and most likely for considerably longer.
Coastal erosion or shoreline retreat has become a more relevant and pressing issue over the past several decades in the United States for several reasons. People have continued to migrate to the coastal states, and as a consequence, coastal property and homes have become increasingly more valuable. Property losses along the Atlantic and Gulf coasts due to hurricanes have been very high in recent years (Figure 1). On the Pacific Coast, the twenty years from ~1978 to 1998, which were characterized by an El NiÃ±o-dominated climate, produced extensive coastal storm damage including accelerated coastal erosion and widespread property losses (Figure 2). As the distance from the cliff edge to a home, business, or public roadway or other infrastructure continues to decrease, concerns for a solution mount.
While cities, counties, and coastal home and business owners are looking at all approaches for dealing with the ongoing erosion, the number of available options is limited. There are essentially three possible responses: 1) retreat, and either move or relocate the structure landward or demolish the building, 2) nourish or add sand to the fronting beach, or 3) armor the coastline with some type of hard structure, either a revetment or some type of seawall or bluff stabilization.
There are many examples where retreat has been carried out (Figure 3a, b), and many more places where, due to site-specific geologic conditions, this is probably the only practical approach left (Griggs, 1986; Griggs, 1995). Although this is not a popular solution for many oceanfront homeowners or for local governments, it may be inevitable and more common with continuing sea level rise.
Beach nourishment has been practiced in one form or another for many years in California, most often as a by-product of either large coastal construction projects or as a result of the initial excavation or maintenance dredging of coastal marinas or harbors (Flick, 1993; Wiegel, 1994). However, the cases where nourishment has been carried out solely for the purpose of beach widening or replenishment are very few (Patsch and Griggs, 2006). There are a number of significant issues that need to be carefully evaluated before any large-scale beach nourishment program is given serious consideration (Griggs, 2005; Patsch and Griggs, 2006). These include: 1) the long-term availability of appropriately sized sand, 2) the life span of a nourished beach, 3) impacts of the nourishment process, and 4) the initial and ongoing costs and who pays for the project. In California, where littoral drift rates are usually quite high, the lifespan of most nourishment projects can be expected to be relatively short unless some sand retention structure is planned as part of the nourishment process.
Historically, the typical response to coastal erosion or shoreline retreat has been the construction of a seawall, revetment, or other hard structure, intended to reduce or slow the impact of storm waves and rising sea level on the coastline. In California, as of 2004, 110 miles or 10% of the state’s 1100 miles of coastline had been armored (Griggs, 2005). Perhaps not surprisingly, 30% of the shoreline of the four southern and most urbanized California counties (San Diego, Orange, Los Angeles and Ventura) has been protected with some sort of hard structure. While existing coastal protection structures range widely in their size, configuration and effectiveness (Fulton-Bennett and Griggs, 1985), costs for construction today for typical engineered seawalls or revetments vary from about $3000 to over $8000/lineal foot.
In contrast to the oceanfront homeowner, property owner, or local government’s concern about whether or not the structure will provide the necessary protection for their property, home, business, or infrastructure, considerable public and permitting agency concern has arisen in recent years about the perceived direct or indirect impacts of these structures on the coastline (for a more complete discussion of these impacts see Griggs, 2005). Many of these concerns revolve around the issue of to what degree should private property owners be allowed to impact public beaches as they attempt to protect their own property, or in the case of government funded projects, how much taxpayer’s money should be spent on efforts to stabilize the position of an otherwise eroding coastline? There is an important difference here between the private and public interests; seawalls are not designed to protect beaches but to reduce wave impact and halt or slow erosion of the bluff, cliff or dunes landward of the beaches.
As a result, in California, virtually every new proposed coastal protection project faces extensive environmental assessment, permitting agency review, as well as public scrutiny, and typically, opposition. That opposition is usually based on one or more of the following perceived impacts of coastal armoring, which are described in detail in Griggs (2005):
“¢ visual or aesthetic impacts
“¢ sand impoundment or placement losses
“¢ reduction of beach access-lateral or vertical
“¢ loss of sand supply from eroding cliffs or bluffs
“¢ passive erosion
“¢ active erosion
The visual impact of coastal armoring has often been the issue that has concerned the public the most, particularly in places like southern California where there are tens of millions of people who use the beaches and where 30% of the shoreline is now armored. A seawall or riprap is something that anyone can observe directly, and that doesn’t require a scientific explanation or debate among experts. Whether a seawall, revetment or some other form of stabilization or protection, there is a visual impact, which is usually much greater to the beach user or general public than to the owner of the property being protected, who may not even see the structure from their home or property.
Many early armoring projects were completed without any environmental review. Some of these very early projects consisted of dumping broken slabs of concrete off the cliff edge, or a variety of other non-engineered and unsightly solutions (Figure 4). We are still living with many of these early armor efforts today, as well as those that were built without permits, or built under emergency conditions when there was little time for planning or careful review. Many of the older seawalls were also constructed by individual property owners as stand-alone projects, rather than as part of any regional or integrated effort. As a result, there are many areas where an unsightly assemblage of un-engineered or poorly planned structures exist side by side (Figure 5), which not only are seen as undesirable by the public and permitting agencies, but they also do not provide a uniformly resistant alongshore protection effort.
During the devastating 1983 El NiÃ±o winter storms, beach level homes along the shoreline of northern Monterey Bay that were protected by a variety of seawalls and bulkheads, suffered serious damage when the weakest structure failed and then the entire set of structures suffered gradual collapse when the fill which provided the resistance behind the seawalls was scoured out by waves (Figure 6).
IMPROVING ON THE PAST
There has been an increased awareness of the potential impacts of seawalls or revetments in recent years, and to a large degree, this has happened because of the poorly planned or unplanned protection projects of the past that are still covering the shoreline in many places, and that serve as constant reminders of what is undesirable. Because of these visual impacts and a general concern with covering over natural cliffs and bluffs or the beach with riprap or seawalls, far more attention than ever before is being focused on reducing visual as well as other impacts.
One relatively new approach in California has been the use of gunnite or shotcrete, which is colored and textured to match the native rock in the cliffs as closely as possible. While gunnite has been used as a covering over eroding cliffs and bluffs for years, it has not been particularly successful due to lack of steel reinforcement, poor connection to the bluff and thin application, and it has produced significant visual impacts (Figure 7). Colored and textured gunnite or soil-nail walls have been used to stabilize highway road cuts, but only recently has this approach been applied to coastal protection projects as a way to mitigate the visual impacts and also greatly reduce placement losses or the amount of beach covered by a seawall or revetment.
This newer approach to bluff or cliff stabilization involves first anchoring long “soil nails” or steel tiebacks deep into the bluff materials. A steel reinforcing rod frame that mimics the shape of the existing bluff is then constructed and attached to the tie backs (Figure 8). This steel mesh is then covered with approximately 12 inches of gunnite, followed by a second 6 to 12-inch thick layer, which is textured and colored to match the adjacent cliff or bluff materials (Figure 9). Weep or drain holes need to be built into the structure in order to avoid the buildup of water in the bluff materials behind the wall. Further improving on this approach are recent examples of taking molds of the adjacent existing rock, which can be used to create a surface texture that is indistinguishable from the original or native cliff materials (Figure 10). Additional benefits include being able to build somewhat freeform steps into the structure that can actually improve beach access (Figure 11). Storm drain outlets can also be incorporated into the design. Concrete, if well mixed and prepared, and if the rebar is epoxy coated and protected from exposure to seawater, can be very resistant to wave impact and erosion. While the color and texture of cliffs and bluffs can be duplicated, this is more problematic when beachfront or dune development is being armored or protected.
OPAL CLIFFS-A REGIONAL APPROACH
The challenge for any government agency, board or commission that is asked to make decisions on new protection projects for coastal property, is how to balance the desire or “right”, in some cases, of a property owner or a group of property owners to try to save their homes and property, with the desire of the public to maximize the beach area available for use and minimize the visual intrusion that most existing armor has created. These have often been difficult decisions in the past, both because of the acknowledged impacts of the proposed seawalls or revetments, and also because so many requests were from individual property owners, many often responding to the construction of a seawall on the adjacent property. In few cases was there any coordination or attempt to design similar or compatible structures or develop a uniform regional approach, but rather each engineer would typically design what they felt was the most appropriate from their own experience. Cost was also no doubt a factor.
The Opal Cliffs area of northern Monterey Bay along the central California coast (Figure 12) offers a good opportunity for evaluating the historic problems associated with cliff erosion and a series of individually planned coastal protection efforts. This area also presents the opportunity to develop a uniform regional solution that has the potential to significantly improve the existing situation by: 1) cleaning up existing unsightly structures, 2) removing concrete and rock from the beach to improve access and increase usable beach area, 3) improving protection for cliff top homes, and 4) providing a model for other areas to follow.
The Opal Cliffs residential area extends along the coastline of northern Monterey Bay for about 3,800 feet and includes 46 individual parcels of cliff top property of which 43 have been developed with single-family dwellings. The cliffs increase in height from about 35 feet on the west at 41st Avenue to about 65 feet near the city of Capitola (Figure 13) and have been eroded into an uplifted marine terrace.
The lower portion of the cliffs consists of 20 to 45 feet of thickly bedded layers of siltstone and sandstone of the Pliocene age (~3-7 million years old) Purisima Formation bedrock, which is capped by 15 to 20 feet of unconsolidated, mostly sandy, marine terrace deposits (Figure 12). The sedimentary bedrock is only moderately consolidated and provides relatively little resistance to wave erosion. Additionally, these rocks are extensively jointed with the northeast-southwest orientation of the coastline following the dominant joint set. These joints typically dip steeply seaward (75 to 85 degrees; Figure 14), such that wave undercutting of the base of the cliff is eventually followed by failure of the now unsupported overlying slabs of bedrock (Figure 15).
This regional geologic structure or weakness, as well as the lack of a wide protective beach, has led to a relatively high rate of cliff retreat, ranging from about 4 to nearly 12 inches/year, on average, based on photogrammetric analysis of stereo aerial photographs spanning the 50-yr period from 1953-1994 (Griggs and Johnson, 1979; Moore, Benumof and Griggs, 1998; Moore and Griggs, 2002). Due to the orientation of this stretch of coast and the predominant angle of wave approach, littoral drift rates are high (300,000 yds3/yr; Griggs and Johnson, 1976; Griggs, 1986) and beaches are narrow or non-existent. As a result, waves attack the base of the cliffs at most high tides.
Aerial photographs taken in 1928 show no development in this area. Construction of cliff top homes began in the 1930s and with the exception of 3 parcels, the Opal Cliffs coastline area is now completely developed. As cliff erosion proceeded and individual homeowners began to feel threatened, a number of property owners constructed individual coastal armoring projects. The first armor for which records exist was placed in the early 1960s, and construction of individual projects has continued almost to the present day.
Currently 31 of the 43 homes (72%) along Opal Cliffs have some form of coastal protection in place. These structures include cliff top retaining walls, sea cave plugs, shotcrete patches, vertical or sloping concrete seawalls, poured concrete cylinders, riprap, concrete rubble, and a combination of these and other materials (Figure 16 a,b). Many of these structures are failing or in a degraded condition such that they no longer provide adequate protection from wave attack. In addition, some of the structures at the base of the cliffs do not necessarily prevent the toppling or failure of rock slabs from the upper portion of the cliff. A number of these structures also present significant visual impacts, and where riprap has been placed, it limits access along the narrow beach.
As part of a regional assessment of the hazards to the homes in the Opal Cliffs area from continuing cliff retreat, a parcel-by-parcel assessment was made of the distance from each home to the cliff edge. This distance was then compared to the long-term average annual cliff erosion rate in the specific area in order to arrive at a critical time or year when each structure would theoretically be in imminent danger of collapse. This evaluation assumes that the average cliff erosion rates are uniform over time. All experience in this and other areas indicates, however, that coastal cliff or bluff erosion tends to be episodic (Griggs and Johnson, 1979; Kuhn and Shepard, 1984) with most failures occurring during the simultaneous occurrence of very high tides and extreme storm waves (Griggs and Johnson, 1983), heavy rainfall (Griggs, 1982), or large earthquakes (Griggs and Scholar, 1997). Observations of cliff failures in the Purisima Formation in this general area indicate that individual failure blocks may be five to ten feet in thickness.
Individual homeowners with unprotected properties along Opal Cliffs have been unsuccessful in obtaining approval from the California Coastal Commission to construct shoreline protection in recent years for various reasons. The primary reasons cited include that there are potentially significant impacts associated with any armor that need to be addressed or mitigated and that the homes do not warrant shoreline protection because they are not in imminent danger of collapsing into the ocean. The Coastal Commission has defined imminent danger as the potential to suffer a structural loss within one to two storm cycles (usually one to two years). Due to the unpredictable nature of storms or large earthquakes, the lengthy permit approval process, and the difficulty of constructing cliff protection devices during the winter months, or with the limited remaining land area, some of the more threatened property owners are understandably concerned about losing their homes.
From an engineering perspective, the foundation of any structure will be compromised well before the cliff edge reaches the foundation. In addition, if any protective structure were to be built or emplaced, a certain amount of access is required between a house and the cliff edge in order to actually build a structure due to the very limited access at beach level. For these reasons, calculations were made of critical times or lifetimes when each house would be threatened under several different setback distances, ten, twenty and twenty-five feet from the cliff edge. An additional consideration that was not factored into the analysis was the projections for higher future sea levels and the potential for increased cliff erosion rates. Thus our projections should be viewed as conservative and, in all probability, the erosion rates may likely increase.
Recent large-scale orthophoto maps of the Opal Cliffs area were obtained from the Santa Cruz County Surveyor and were used as the topographic base. Distances were measured from each of the 43 primary structures (homes) to the edge of the cliffs. These distances (as of 2004) varied from 10 to 107 feet, with twelve of the homes within 25 feet of the cliff edge (Figure 17). Knowing the long-term erosion rates for individual segments of the Opal Cliffs area, a “lifetime” for each house was calculated assuming an average annual erosion rate and the current distances to the cliff edge. One map was generated that shows the lifetimes of the houses using the average annual erosion rates and the current distances from the cliff edge. Under these conditions, four of the 43 houses (9%) are at the edge of the cliffs within 20 years, and 15 additional houses (44%) are at the edge of the cliffs within 55 years.
A second map was created to show the lifetimes of the houses using the same average erosion rates and either a single episodic bluff failure event of ten feet, or using a setback of ten feet (Figure 18). A failure of this magnitude could be attributed to sustained heavy rainfall, wave undercutting and subsequent collapse, or a large earthquake. Given the size of past failures in the Opal Cliffs area, which is in large part related to joint spacing in the Purisima bedrock, a failure of this magnitude is possible although it would not occur along the entire stretch of the coast at any one time. This map shows that with the assumed bluff failure of ten feet, or using a ten-foot setback, that six of the 43 houses (14%) are at the edge of the cliff within 20 years, and 21 additional houses (63%) are at the edge of the cliffs within 55 years.
A third map was created to show the lifetimes of the houses using the average erosion rates and a theoretical bluff failure of 20 feet, or using a setback of 20 feet. Using a 20-foot set back, 17 houses (40%) are at the edge of the cliffs within 20 years, and 16 additional houses (78%) are at the edge of the cliffs within 55 years. Negative lifetime values indicate that the cliff edge would have reached and retreated past the house prior to this time period.
A final map was generated to show the lifetimes of the houses using the average erosion rates and a more extreme theoretical bluff failure of 25 feet, or using a setback of 25 feet (Figure 19). This is a realistic scenario; in most cliff top situations or municipalities in California, a house within 20 or 25 feet of a cliff or bluff edge might well be posted as unsafe to occupy, simply because of the uncertainties of the magnitude of the next failure. During the 1989 Loma Prieta 7.0 magnitude earthquake, for example, seismically induced coastal cliff failure occurred up to 50 miles from the epicenter. Failure and tension cracks extended up to 30 feet in from the cliff edge along the northern Monterey Bay coastline and at least 3 homes and 6 apartment units were demolished as a result (Figure 20; Plant and Griggs, 1990; Sydnor et. al, 1990; Griggs and Scholar, 1997). The 25-foot setback map indicates that 24 houses (56%) are at the edge of the cliff within 20 years, and an additional 10 houses (79%) are at the edge of the cliffs within 55 years.
CONSIDERATIONS FOR REGIONAL CLIFF STABILIZATION
The present situation along the Opal Cliffs area is undesirable for a number of reasons: 1) the cliffs along much of the area continue to fail; 2) a number of homes are very close to the cliff edge, are either unprotected or inadequately protected, and are already or may be threatened in the near future; 3) much of the present armor is ineffective and unsightly; and 4) the existing armor, particularly the riprap and rubble, reduces horizontal access along the shoreline and also covers a portion of the narrow beach. In addition, there have long been discussions about regional approaches to shoreline stabilization or protection but few instances where homeowner organizations or adjacent property owners have come together to solve a coastal erosion problem. Is there an approach or solution that could resolve all of these issues, mitigate the significant impacts, and also be acceptable to the permitting agencies?
We believe that the combination of removal of the existing rock, concrete debris and failed structures, and the construction of a 3800 foot long soil nail wall at the base of the Opal Cliffs area could provide the solution and also be a potential model for other eroding cliffed areas.
A good model for this approach is a project recently completed on an emergency basis just a few thousand feet upcoast in the Pleasure Point area (Figure 21). The geological conditions are very similar although the cliff increases in height from about 25 to 35 feet in the Pleasure Point area to 35 to 65 feet in the adjacent Opal Cliffs area. Continuing bluff retreat at about 6 inches/yr in the Pleasure Point area has led to closure of one lane of an oceanfront county street and now threatens the remainder of the street as well as the underlying water and sewer lines. The Santa Cruz County Redevelopment Agency initiated a bluff stabilization proposal in 2001 to protect ~1100 feet of this threatened coastline (Figure 22). The proposal and associated Environmental Impact Report have been reviewed and revised several times since 2001 (TetraTech, 2006) but have not yet received an approval from the California Coastal Commission. Concerns were expressed regarding impacts of the structure on surfing, reduction of sand supplied by the eroding bluffs, and passive erosion with a rising sea level by several environmental organizations, some of the surfing community, as well as Coastal Commission staff. In the interim, however, as erosion continued to threaten the public street, about 300 feet of bluff were stabilized under an emergency authorization. A soil nail wall was constructed to stabilize the terrace deposits along the upper portion of the bluffs (Figures 11 and 21), which was nearly indistinguishable from the native bluff materials. Contractors were also able to build in a “goat trail” within the structure that has provided an improved and safer access for surfers. Overall, these “emergency” stabilization segments provided a good example of the difference between bluff stabilization through a soil nail wall that effectively replicated the natural bluff materials, and a large vertical or curved face concrete seawall. The project so effectively resembled the natural bluff materials that some locals could not recognize the area that had been stabilized (Figure 21). The revised plan and EIR are now working their way back through the permitting process but the EIR provides a useful comparison for the proposed regional approach for stabilizing the Opal Cliffs area. Each of the potential impacts or concerns are discussed in detail in Griggs (2005) but are briefly discussed below as they relate to the Opal Cliffs site.
As discussed earlier, the visual appearance of many existing coastal protection structures is probably the single greatest issue or concern held by the general public who use the coastline or the beach. The newest generation of soil nail walls or bluff stabilization projects that has been completed, however, has nearly eliminated this concern. As in any design or with any workmanship, there can be a range in the appearance of the final product. However, we now have enough good examples and there are enough firms involved in this type of work that quality and appearance of the bluff treatment can be demanded (Figure 10). In the Opal Cliffs area, the residents and the California Coastal Commission have an opportunity to greatly improve the visual appearance of the coastline by removing all of the unsightly remnants of earlier armor projects and by providing a consistent alongshore treatment that can reproduce the appearance of the existing cliff materials.
Sand Impoundment or Placement Losses
By removing the existing rock, rubble and concrete structures that now encroach onto the narrow beach, and replacing this with an 18 to 24 inch thick reinforced and tied- back concrete wall, placement loss can be essentially eliminated as a concern and a significant amount of beach can be reclaimed (Figure 23). Where riprap exists, it may extend up to 30 feet seaward of the base of the cliff, thereby eliminating any use of this beach area. Removing the rock and rubble as well as any other concrete structures that encroach a significant distance onto the beach and then constructing a thin soil nail wall that followed the cliff edge would allow these shoreline areas to be reclaimed for public use.
Reduction of Beach Access-Lateral and Vertical
In addition to covering significant beach area, the existing riprap and rubble and some of the protruding concrete structures also restrict lateral access along the shoreline (Figure 23). Although Opal Cliffs has only narrow discontinuous and seasonal beaches, the existing lateral access at low tide is restricted by the rock and debris on the beach. Removing these materials and structures would improve public access in these impacted areas and the soil nail wall proposed would not impact lateral access.
Vertical access is not an issue in this area. With the exception of one limited access public stairway to the beach approximately midway along this 3800 foot reach of coastline, the cliffs are near vertical and the property is all developed with single family homes, so there is no vertical access that would be affected by the construction of a soil nail wall.
Loss of Sand Supply from Eroding Cliffs
The contribution of sand from eroding coastal cliffs to the littoral drift budget can be relatively straightforward to determine (Runyan and Griggs, 2003; Patsch and Griggs, 2006)). We need to know the alongshore length of cliff, the height of the cliff, the percentage of sand or littoral-size material in the bluff materials and also the average annual erosion rate. Multiplying all of these factors together will produce a volume of sand or beach-compatible material that would be provided on an average annual basis. We can then compare this volume with average littoral drift rate and assess whether or not eliminating this volume of sand through stabilizing the cliff is significant. This calculation was carried out for the adjacent Pleasure Point area as part of the Environmental Impact Report for that project (TetraTech, 2006). It is about 0.22 yds3 of sand per foot of coastline/yr. Scaling this value up for the somewhat higher bedrock cliffs in the Opal Cliffs area produces an average annual input of about 0.34 yds3 of littoral sand per foot of coastline, or about 1292 yds3 of littoral sand annually for the 3800 feet of cliffs. Comparing this volume to the ~300,000 yds3 of annual littoral drift indicates that natural erosion of this portion of seacliffs contributes about 0.4% of the total, an insignificant amount. There are large variations in annual littoral drift in any one location due to yearly variations in wave climate and sediment input from streams such that this small volume would not be detectable.
Where a hard structure is built along a shoreline undergoing net long-term erosion as a result of sea level rise, the shoreline will eventually migrate landward behind the structure (Figure 3a). The effect will be gradual loss of the beach in front of the seawall or revetment as the water deepens and the shoreface profile migrates landward (Griggs, 2005). This process is designated as passive erosion and is the process that has been well documented along many of the armored barrier islands of the Atlantic coast, as well as on Oahu (Fletcher, et. al., 1997), and along sandy shorelines of California and Washington. Passive erosion is not an issue along cliffed coastlines without beaches, however. These cliffed areas owe their existence to the general lack of a beach and also the underlying geology (Figure 23). Where sand does not accumulate and the rock exposed in the cliffs is very resistant to erosion, the retreat and breakdown of the cliffs has not kept pace with sea level rise. In these loc