Objective: Explain bluff erosional processes, the factors that cause and exacerbate it, and the risks associated with building too close to the bluff. Identify tools and resources that communities can use to communicate and establish sustainable setback standards, thereby preventing purchase and development of at-risk private properties.
Authors: Bridget Faust and Jeffrey D. Stone, Association of State Floodplain Managers
Publication Date: October 10, 2014
Revision Date: May, 2016
Update Note: Since this study was written in 2014, the bluffs along the Lake Michigan shore have remained stable. The recently rising Lake Michigan water level has not yet had a noticeable effect, although Ozaukee County officials anticipate that continued high water levels will have an effect on the bluffs over time. In some places within the county the higher lake level has eliminated the beach, with water now reaching the toe of the bluff. Also, a paragraph explaining the existing 2006 Ozaukee County Shoreland and Floodplain Zoning Ordinace which specifies setback requirements on the bluffs has been added to the “Regulation” part of the final section in this case study,“Strategies to Reduce Risk.”
Ozaukee County, just 25 miles north of Milwaukee, is located along Wisconsin's southeastern shore. As is typical of the coastal counties of most Great Lakes states, the soils in Ozaukee County are composed of sand, gravel, clay, and claylike material called till. These soil layers were deposited when the lakes were carved by the glaciers, leaving behind extensive sloping bluffs which reach from the sandy beach to as high as 140 feet above lake level (A Multi-Jurisdictional Comprehensive Plan for Ozaukee County, 2005). It is atop these great bluffs that individuals frequently seek to construct homes and cabins, both for the aesthetic advantages they provide and the relative ease of building there (The Nature Conservancy, 2012). Despite their perceived soundness, bluffs can erode erratically, progressing slowly for years at a time before sustaining a failure with severe consequences and costly solutions.
One critical example of this erratic bluff erosion can be observed in the Village of Grafton. Located within Ozaukee County, this rural coastal town has had many homeowners face catastrophic bluff erosion overnight. In 2006, a home owner watched a significant portion of the bluff began to slump downward. Over the course of just a month feet of bluff were gone taking with it trees, a functioning septic systems, and a fence. At the cost of the homeowner, a major restoration project was completed in which the bluff was re-graded and vegetated. Less than a year after the bluff restoration was completed a second slumping episode occurred, taking with it approximately 80% of what had been restored. Events like this are not unique to one property owner, one bluff, or one town. Unsustainable development along the bluff tops is a pervasive problem throughout the Great Lakes coastal region.
Grafton Home, Before and After the Bluff Slump
Courtesy of: Barry Sullivan, Ozaukee County
Grafton Home's Septic System Pipes Exposed
Courtesy of: Barry Sullivan, Ozaukee County
Living along these bluffs requires that planners, decision-makers, and residents understand the causes and effects of coastal erosion because they play a critical role in ensuring that homes and buildings are not constructed in dangerous locations. Unnecessary damages can be prevented by establishing a science-based approach for determining safe building setbacks, then passing regulations to ensure their enforcement. The purpose of this case study is to illustrate and describe the causes of coastal bluff erosion, identify science-based tools that can be used to calculate sustainable setback distances, equip coastal resources managers with the tools needed for communicating the risk of coastal bluff erosion, and highlight policy and management strategies to encourage sustainable development on the bluff top.
Causes and Factors Affecting Bluff Erosion
Bluff erosion is a naturally occurring process that is cyclical in nature. The rate at which this erosion occurs is dependent on a combination of factors, some of which are direct drivers of erosion, while others exacerbate those drivers. Because of this, determining where and when coastal bluffs will erode can be a significant challenge. Understanding bluff erosion requires knowledge of the cycles that it typically follows and the processes that influence it. When equipped with this knowledge, planners and decision-makers can, at the very least, recognize the conditions that make bluffs more vulnerable to failure and take measures to ensure that residents are aware of the risks.
Bluff Erosion Cycles
Bluff erosion is a process that occurs continuously. Though not always visible to the untrained eye, bluff slopes are constantly changing. Years and even decades can pass without any noticeable loss, but despite their perceived stability, large bluff failure events can occur suddenly and without warning. Slumping and failure can occur at any time when the bluff is unstable. In general, the steeper the bluff slope, the less stable it will become. The cycle of bluff erosion is described in six general phases. Read More...
Figure 1. Source: Keillor and White, 2003.
Major Factors in Bluff Erosion:
1. Wave Action
Waves are the primary long-term cause of bluff erosion. Their constant contact at the base or toe of the bluff over time can lead to instability. The more intense or powerful the wave action along the shore (a function of wind speed and the distance a wave traverses), the more rapidly the bluff toe erodes. Eventually this wave action leads to failure at the base of the bluff itself, or as it is more commonly known, toe erosion (see Figure 1). (Keillor and White, 2003)
2. Lake Levels
Changes in lake levels also have significant impacts on bluff erosion rates. High lake levels allow waves to more readily erode away the base of the bluff; consequently, when lake levels are high the bluffs recede more quickly (The Nature Conservancy, 2012). This correlation is not dependent on timescale, meaning even short-term changes in lake level can have significant impacts on bluff erosion rates. For example, increases in water levels during coastal storms can cause severe erosion and bluff weakening due to the strength of the waves (i.e. storm surge, seiche, and edge waves) associated with them, even as singular incidences. Read more...
Changes in Great Lakes levels have been recorded since 1860. For communities that wish to learn more about the water levels that have been observed historically and what can be expected in the future, the NOAA Great Lakes Water Level Dashboard is a resource that was designed specifically for this need.
3. Surface Runoff
Surface runoff, typically a product of rainfall or snowmelt, loosens and visibly removes exposed soil on the bluff as it moves downhill, resulting in the formation of gullies and rills (see Figure 1). Both the volume and velocity of surface runoff influence the rate at which erosion occurs. While vegetation helps to slow the flow of water over land, hard or paved impermeable surfaces have the opposite effect, and are important to consider with regard to erosion rates in developed coastal bluff areas (Keillor and White, 2003).
As water percolates down into the ground, it can saturate soils by filling small porous spaces between soil particles. Groundwater of this kind can build up over time and flow between different soil layers (i.e. sand, glacial till, and clay), each of which has a different capacity to hold moisture. As soils become saturated they become heavier, and the frictional forces that hold soil particles together decrease. This can lead to fractures in the soils that underlie a bluff, causing catastrophic slump erosion (Keillor and White, 2003).
Figure 2. Source: Keillor and White, 2003.
Other Factors Affecting Bluff Erosion:
Wind can exacerbate the rate at which a bluff erodes because its speed and direction impact the height and frequency of waves on the Great Lakes. Areas that typically receive a lot of wind will generally be susceptible to increased erosion. Wind also affects sediment transport processes, which can either nourish or deplete the beaches that act as a barrier between breaking waves and bluff toes.
Seasonal freezing and thawing of groundwater can also impact erosion rates. When water in saturated or semi-saturated bluff soils freezes, the soil particles break apart and become less stable. With spring thawing, rain and snowmelt can rapidly saturate the already weakened ground, causing the bluff to slump or form shallow slides. Read more...
Bluff Height, Soil Type and Slope Angle
In addition to the previously mentioned natural shoreline processes, bluff geology also has specific implications for erosion rates. In general, erosion rates increase with bluff height, finer (clay) soil types, and steeper slope angles. Read more...
"Lake Michigan Coastal Erosion"
Courtesy of: Tom Gill
Tree Giving Way on Steep and Severly Eroded Bluff,
Courtesy of: Mike Swenson
"Landslide- Scarborough Bluffs"
Courtesy of: Chris Kalbfleisch
Coastal Bluffs Along Lake Michigan
Courtesy of: Mike Swenson
- My bluff is safe, it has not moved in ten years.
Bluffs erode unpredictably. Sometimes they recede in imperceptibly small increments feigning safety and stability for years at a time. Under the right conditions, however, feet of bluff can be lost overnight. One of the most common misconceptions related to bluff erosion is that a bluff that is not visibly receding is safe. This is because many of the factors that lead up to catastrophic bluff slumping occur below the earth’s surface. In order to prevent unsustainable development on or near the bluff top, it must be emphasized that these seemingly stable states are short-lived. One way to examine these trends in erosion rates is through the Ozaukee County Bluff Viewer, this interactive resource allows users to view bluff recession overtime and measure the extent to which they erode. This tool can be used to start a dialogue about bluff erosion processes and help to educate individuals’ perceptions of bluff stability.
- My house is safe, my house was built in compliance with all ordinances.
Another common misconception related to bluff erosion and development on the bluff top is that complying with building codes, permitting requirements, and setback ordinances ensures that a new or existing home is built in a safe location. In the state of Wisconsin, many coastal communities require that all new construction be setback 75 feet from the Ordinary High Water Mark. Although this universal standard is easily administered and understood by the general populous, it is not always sufficient to ensure that new construction be safe from erosion in the long-term (Ohm, 2008). Many other Wisconsin communities base their coastal setback requirements on bluff recession rates. The rate in question is calculated based on a variety of factors including the underlying soil type, the slope of the bluff, and the estimated useful life of the structure being constructed. In many cases, the useful life applied is 50-years. Frequently, this standard reflects neither the homeowner’s perceived nor the actual useful life of a home built along the coast. Overtime, larger and sturdier homes have been built along the coast many of which were designed to last upwards of 100 years. This difference between the actual and regulatory useful life of a structure is critical to the creation of sustainable setback because the longer the useful life of the structure, the further back the construction project must be completed. As a result, it cannot be assumed that simply complying with the standards set in zoning ordinances ensures that the newly constructed project is safe from bluff erosion. Read more…
- My shoreline protection structure will not do any harm.
When their property is in danger, many homeowners look to protect or stabilize their bluff by constructing shoreline protections structures. Riprap, revetments, and jetties are all examples of structures that a homeowner frequently construct with the help of a consulting engineer. Some of these structures are designed to protect the bluff toe from wave action, others are designed to help catch sediments and rebuild some of what has been lost due to erosion. There is significant contention surrounding the use of hard shoreline protection structures. In general, shoreline protection structures that are perpendicular or parallel to the shore have been shown to potentially interrupt or alter littoral drift. These change the overall sediment budget, leading to the collection of sediments in some areas and sand starvation in others depending on current direction (Sea Grant, 2013).
Until recently many believed that, with careful planning, hard shoreline protection structures could be constructed without significant adverse impact to neighboring properties. However, recent research along the shores of Lake Michigan has demonstrated that this is not the case. Hard shoreline protection structures have been shown to more than double recession rates of bluff top and toe erosion in certain areas. Read more…
Where are the Hazards?
When faced with the unpredictable nature of coastal bluff erosion, preventative planning is the most effective method for ensuring that homes, businesses, and public infrastructure are not built in at-risk areas.
Steps of bluff erosion hazard identification:
1. Where are the existing failures and vulnerable areas?
2. How much will bluffs erode and at what point will they be unstable?
3. How will future conditions affect bluff erosion rates?
1. Where is Bluff Failure Occurring Already? What Areas Appear to be Vulnerable?
To begin a community- or county-level assessment of bluff erosion risk, communities need access to recent data on the stability of the bluffs within their jurisdictions. Depending on the size and extent of the survey area in question, community officials should consider completing a real-time inventory and evaluation of their bluffs. Recognizing that this is a long, intensive process, described below are three useful resources that coastal municipalities can use to complete a basic shoreline analysis, instead of a full field evaluation.
Wisconsin Shoreline Inventory and Oblique Photoviewer
Courtesy of: Wisconsin Coastal Management Program
A. The Wisconsin Coastal Management Program has partnered with the Association of State Floodplain Managers and the Wisconsin Sea Grant Institute to create the Wisconsin Shoreline Inventory and Oblique Photo Viewer. This mapped, geotagged image database covers the entire Great Lakes shoreline in Wisconsin. In addition, oblique aerial photos taken from a 1976 University of Wisconsin project have been aggregated and mapped alongside the current imagery. When considered together, these images can be used to understand and evaluate how bluffs along the coast have changed over a nearly 40 year interval. This baseline understanding of past trends could in turn be used to inform predictions of how these bluffs might change in the next 40 years.
B. Similarly, the Superior Watershed Partnership has partnered with NOAA and the Michigan Department of Natural Resources and Environment to create the Great Lakes Shoreviewer. This database contains professional quality photos of Michigan's entire Upper Peninsula. The images contained in the database were captured in 2011 and can be searched and downloaded using a web-based map.
C. For those located outside the State of Wisconsin, the U.S. Army Corps of Engineers has captured and aggregated images in a Great Lakes Oblique Photo Viewer as part of the Great Lakes Coastal Flood Study. The images featured were captured in 2012 and cover the entire United States Great Lakes shoreline. Although this database does not offer historic images to use as a comparison, the modern images can be used to complete a basic evaluation of bluff conditions. Similar to the Wisconsin Shoreline Inventory, this tool is especially useful for those who do not wish to complete a field study.
2. What is a Stable Condition? Where is it Safe to Build on the Bluff Top?
To determine whether a bluff is in a stable state and how long a property can legitimately expect to remain a safe distance from its edge, a few measurements must be taken or obtained from existing datasets. By determining the height of the bluff, the underlying soil type, and the current slope angle, individuals can then leverage multiple tools to determine estimates of safe setback distances. It is worth noting that the first two tools listed below are designed to be applied to the shores of Lake Superior and Michigan only. Further, these resources can provide only a rough estimate of the rate and extent of bluff erosion. Communities are strongly encouraged to seek out a consulting engineer or staff member with expertise in shoreline analysis to complete the calculations required to determine a "safe" setback requirement.
Stable Slope Angle Calculator
Courtesy of: WI Coastal Management Program
A. Resources Available for a Baseline Estimate:
The Stable Slope Angle Calculator, produced by the University of Wisconsin, Wisconsin Coastal Management Program, Wisconsin Sea Grant Institute, and others, automatically determines the distance a bluff top is expected to recede were it to reach a stable state. To use this tool, current data on the bluff height, slope angle, and soil type are required.
The Building Setbacks Calculator, located on the same website, calculates a scientifically sound setback distance based on slope angle, annual bluff recession rate, and projected use life (in years) of the structure in question. The output of this calculator is a rough estimate of the distance from the bluff's top edge a person must build to ensure their property will not be lost or need to be relocated as a result of erosion.
If additional information on underlying soil types is needed, the Natural Resources Conservation Service provides easy access to soil data for 95% of the counties in the United States. The Web Soil Survey tool allows users to easily search a location by address, state, or county, select an area of interest, map the soil types present in that area, and discover the characteristics of each of the soil types displayed. Measurements of slope angle and bluff height may be more difficult to determine, requiring field work or the help of a consulting engineer.
Flood & Erosion Prediction System
Courtesy of: Baird & Associates
B. Resources Available for a Detailed Estimate:
For those who wish to complete a more precise assessment, Baird & Associates has produced a Flood & Erosion Prediction System. This software program uses a deterministic model to produce predictions, maps, and visualizations of bluff erosion over time.
Another technical resource that can provide detailed guidance on estimating safe setbacks for new development is the Coastal Processes Manual, published by Wisconsin Sea Grant. This manual, targeted towards coastal engineers, includes detailed instructions for identifying different types of bluff erosion, evaluating erosion rates, and estimating a safe construction setbacks.
Additional methodologies and tools are currently being developed to add to the resources available for communities who are seeking to evaluate coastal bluff stability. For example, the Pennsylvania Coastal Program has recently been using LiDAR data to predict bluff recession rates. Although the findings of this study have not yet been released, its outcomes should be examined closely as they will likely have practical applications elsewhere.
3. What Weather and Climate Conditions are Predicted for the Coming Season?
Recognizing that seasonal conditions (i.e. wet spring, hard freeze, etc.) will impact erosion rates, communities can benefit from having access to a basic synopsis of these conditions. One tool available to meet this need is the NOAA Great Lakes Quarterly Climate Impacts and Outlook. Produced in partnership with Environment Canada, the Midwestern Regional Climate Center (MRCC), and other keystone partners within the region, this two-page document can be used as a simple predictor for regional temperature, precipitation, and water level trends, as well as their impacts on industry and water quality. This basic reference tool can be used by communities to roughly predict the severity of the erosion that can be expected in any given season. Baseline predictions of this nature, however, must be taken with a grain of salt. There are many variables (soil saturation, subsurface conditions, etc.) that determine bluff erosion rates, and without a complete understanding of these conditions it must be assumed that a bluff is vulnerable to failures, both large and small.
Strategies to Reduce Risk
Allowing development too close to the bluffs edge can lead to a range of costly adverse impacts including property loss or relocation. It is predicted that in the next 60 years nearly 87,000 homes or structures located on low-lying land and bluffs are likely to erode into the oceans or the Great Lakes ("Evaluation of Coastal Erosion Hazards: Results from a National Study and a Massachusetts Perspective"). Although those properties will likely be lost, many strategies exist for discouraging the purchase and development of potentially threatened properties on the bluff's edge as well as protecting homes that are threatened by recession.
Planners and decision-makers should consider using existing websites to inform citizens and developers about what bluff erosion looks like and the natural cycle it undergoes. By linking homeowners and developers with the tools needed to establish and calculate a safe setback distance, they can be empowered to protect their property. These tools could also promote broader acceptance of city- or county-wide regulations because they take an objective, transparent, and science-based approach to determining setback distances. These qualities tend to increase residents' trust in and understanding of the need for regulatory requirements and encourage the acceptance of protection programs, ultimately deterring opposition to their implementation (Ozawa, 1993).
Counties and municipalities are generally placed in a position of unique authority when it comes to planning for future development. Though the rights conferred to these localities vary from state to state, most are granted the power to create comprehensive plans, to administer zoning, and to pass ordinances that shape when and how development occurs. For example, comprehensive planning requires that the general public and city officials come together to envision what they want their community to look like in the future, and to then set goals and objectives to realize that vision. Comprehensive plans thus give these localities the administrative authority to incorporate new zoning designations, building codes, and other regulations into their respective codes of ordinances. Many of these regulatory tools (overlay zoning, impact fees, exactions, setback requirements, conservation subdivision, etc.) can be used to improve coastal resilience, while remaining flexible enough to meet the needs of the community. Actively involving the public in discussion of community goals and priorities and the various mechanisms that could be used to achieve them gives citizens a sense of ownership of future regulations and policies implemented. This ultimately helps to foster community acceptance of new regulations (Pace, 2012).
This is the strategy that has been used thus far in Ozaukee County. The county's comprehensive plan has specifically expressed throughout that in order to protect public safety and the health of Lake Michigan, it will make efforts to ensure that buildings are placed at a safe distance back from the bluff top. The county has also prioritized the coding of identified sensitive bluff areas into its zoning ordinances, thereby making development on top of them no longer permissible ("A Multi-Jurisdictional Comprehensive Plan for Ozaukee County, 2005"). These priorities were established with the support of the general public, who were included in the comprehensive planning process as required by law. In taking these preliminary steps, Ozaukee County officials have laid a strong foundation for future regulatory activities.
Additionally, in April 2006 Ozaukee County adopted a Shoreland and Floodplain Zoning Ordinance. Section 7.0308, Erosion Hazard Setback from Bluffs, specifies a required new or expanded structure setback requirement based on one of two determination methods. The first method uses a horizontal top-of-bluff set back distance based on several bluff slope angle measurements as determined by a professional engineer or land surveyor. The second method simply requires a minimum set back distance of 75 feet. The required set back using one of these two criteria is whichever distance is greatest. Both measurements must be documented in county building permit applications and apply both to new construction and to additions to existing structures.
To date, many Great Lakes states have made efforts to regulate development along the shoreline. For example, in Bayfield County, Wisconsin, local planning and zoning department staff and academics received funding from the Wisconsin Coastal Management Program to devise a formula that can be used to determine a setback that is more realistic and defensible than the fixed 75-foot distance required under existing state statute (Kastrosky et al. 2011; Northwest Regional Planning Commission, 2012).The science-based methodology outlined in their report is already being applied in other coastal communities, and can be readily applied throughout the region. For those who are seeking examples of statutory language for coastal setbacks of this nature, a comprehensive list can be referenced in "ASFPM State/Province Coastal Erosion Setback Summary" and "Protecting Shoreline Investments: Example Regulations from Wisconsin's Coastal Communities".
For communities hoping to avoid the occasionally negative public relations issues that can accompany the passage of regulations, partnering with a local land trust to purchase conservation easements on shoreline and/or bluff top properties can provide an incentive-based approach to shoreline management. Easements are a legally binding agreement between a landowner and a land trust or agency. The terms of these agreements vary from property to property, but always require the property owner to sell some or all of their property rights to the trust or agency in question. Landowners typically receive a property tax break in addition to the sales price for placing an easement on their land. Along the bluff top, individuals and communities are sometimes eligible for a special subset of conservation easements targeted at erosion control. These easements can either prohibit development up to a fixed distance landward from the bluff edge or they can "roll", meaning that they move inland as the bluff erodes ("Erosion Control Easements"). This latter type is what is termed a rolling easement.
Transfer for Development Rights (TDR) programs can also be implemented by a local unit of government to help protect land from development pressures. As opposed to easements, where a set of property rights are sold in perpetuity for a payoff and tax break, TDR programs incentivize preservation by allowing property owners to sell their rights to development in the free market. When designing a TDR program, a local unit of government typically establishes zoning districts for the area intended to be preserved through the program as well as the area designated for denser development. These parcels are called sending districts and receiving districts, respectively. Upon their establishment, landowners in the sending district are given the option of forfeiting their property development rights to the local unit of government. In return landowners receive "credits" which can be sold to developers who, upon their purchase, build structures in the receiving district. In this way, municipalities can protect environmentally sensitive lands within the community while incentivizing growth in areas better suited for development (Pace, 2012; "Mequon, Wisconsin").
In Ozaukee County, Mequon, a neighboring city just 10 miles south of Grafton has developed a Transfer of Development Rights program. Grafton city officials and planners could consult with the city of Mequon staff if they were interested in implementing a similar program to incentivize coastal conservation adjacent to areas that are particularly vulnerable to bluff erosion ("Mequon, Wisconsin").
For homeowners who are looking to take immediate action, re-vegetating the bluff is a great first step towards increasing the stability of the bluff. Vegetation plays an essential role in stabilizing a bluff. Plant roots help to remove water from the bluff and hold soils in place, the canopy created by vegetation helps to shade the ground during dry spells ultimately helping to maintain soil moisture, and finally, the presence of vegetation helps to reduce the impacts of severe storms, wind, and ice that aggravate bluff erosion. Best of all, homeowners can maintain and grow vegetation without the help of city staff, making this a cost-effective strategy for increasing bluff stability. For additional guidance, the Pennsylvania Coastal Management Program has created a best practices manual for bluff re-vegetation (Cross et al. 2007).
When a home and the family that dwells within are living dangerously close to the bluff edge, retreating from the bluff edge is sometimes the best solution. As was the case for the home owner in Grafton, both structural and natural stabilization solutions like shoreline protection structures and re-vegetating exposed bluff soils could not prevent another major bluff failure from occurring. As a result, the property owner elected to move his house back from the bluffs edge. Although this process can be time consuming and costly, it is one of the only ways that a property owner can actually save a home from tumbling over the bluff edge in these types of situations.
Bluff erosion is a naturally occurring process that, when allowed to take its course, has beneficial functions. Human interference with these processes through the construction of shoreline protection structures and other measures to stop bluff erosion cannot stop it from occurring altogether, just shift it to the next exposed area. If homeowners and communities continue trying to control this process, they should consider working with neighboring communities to create an integrated approach to bluff management strategy that recognizes the long-term impacts of altering the region's sediment budget.