Construction Setbacks
How to Choose Appropriate Setbacks and Limitations

The distance from the edge of a coastal bluff or bank (or other reference point) to a building or other structure is called a setback distance. Selection of a setback distance is one of the most important decisions that a homeowner or builder can make when building a coastal home. The setback distance may determine the life of the building!

Evaluating the safety of an existing setback distance is equally important when deciding whether or not to purchase a coastal home. Many coastal homes being built at present have such large footprints that they are prohibitively expensive to relocate as erosion threatens their existence. Large setbacks are a preferred alternative to demolition for such buildings.

Shore protection and bluff or bank stabilization are costly measures to prevent the retreat of coastal land. The cost is comparable to the cost of coastal land. On the Great Lakes, lakebed erosion in some locations and the freeze-thaw fracturing of armor stone accelerate the deterioration of most types of shore protection structures.

There are different reasons for choosing the distance that separates the edge of a coastal bank or bluff from a building or other structure that is planned for construction. This is also true in considering purchase of an existing coastal structure. The reasons selected depend on the situation and the perceived risk to investment in the structure.

Setback Approaches:

Minimum Setback Distance 

Purchasers of coastal property often approach a decision to build a dream home by asking; "How close will we be allowed to put our house to the lake?"  This can be a fateful approach that may doom a beautiful coastal home to an early death.

Required minimum setback distances established by state or local governments are not a guarantee that a coastal structure will avoid damage or loss by erosion during the structure’s useful life.  Minimum setback distances are established for reasons other than protection from erosion, reasons such as protection of shoreline habitat and water quality, or maintenance of an environmental corridor along the coast. 

Most counties and/or state governments have a minimum required setback either from the edge of a bluff or bank, or from the ordinary high water mark (OHWM). 

Two Types of Minimum Setbacks
 

In Wisconsin, the statewide setback for all unincorporated lands along the state's  Great Lakes shorelines is 75 feet from the OHWM, which is the lakeward boundary of terrestrial vegetation. The setback is measured or estimated as the horizontal distance from the OHWM. There are minimum required setbacks for homes and other buildings as well as other constructed structures including septic systems.

Rationale
Coastal construction projects affect the public welfare, health and safety. When facilities are built too close to the edge of a bluff or bank, or too close to the water’s edge, the natural vegetated boundary, nearshore habitat, and water quality are degraded. Minimum setbacks provide an environmental corridor along the margins of a water body.

Advantages
Minimum setbacks are easily justified, simple to administer and clearly understandable. They can be politically palatable. Usually they do not create undue hardship for property owners. 

Disadvantages
Minimum required setbacks may not provide adequate protection of constructed facilities from damage and loss through erosion. Minimum setbacks may not adequately protect the natural vegetated boundary, nearshore habitat and water quality.

Example
Consider a Great Lakes coastal lot in Wisconsin. The statewide minimum setback is 75 feet from the OHWM. The existing coastal bluff has a stable slope of 2.5:1 (horizontal distance : vertical distance). For bluffs and banks of different heights, what are the setbacks from the bluff/bank edge that would satisfy the statewide minimum setback from the OHWM? The setback distance is measured as the horizontal distance from the OHWM to the setback line.

Table of setbacks from a bluff/bank edge for a 75 feet setback from a OHWM Slope of 2.5:1 (horizontal:vertical) Distances in feet.

Bluff/bank height

Setback over slope

Setback from bluff edge

5

13

62

10

25

50

15

38

37

20

50

25

25

63

12

30

75

0

This minimum setback allows little time if erosion were to begin at one foot per year or more. The  setback allows little space to bring in moving equipment for relocation of an erosion- threatened structure.  The 75 feet setback from the OHWM provides no protection at all from structure loss to erosion for a bluff 30 feet in height because it would allow construction at the bluff edge!

Reference: Roger Springman and Stephen M. Born. 1979. Wisconsin's Shore Erosion Plan:An Appraisal of Options and Strategies. University of Wisconsin-Extension. Report prepared for the Wisconsin Coastal Management Program.

Setback for a Stabilized Slope

Rationale
Coastal buildings can be reasonably free from damage or loss due to erosion if: 1) the base of a bluff or bank is stabilized, 2) the building is set back landward of an ultimately stable slope, and 3) the stability is maintained.

Advantages
The erosion cycle is broken. Toe erosion is stopped. Further slope erosion only increases the stability of the bluff or bank. The amount of future erosion is accounted for, and the new building will probably be safe from future unexpected erosion damage or loss. The view of the water from the structure improves as the bluff/bank slope moves toward a more stable and smaller angle from the horizontal.  Another benefit of the approach is provision of a natural buffer between the building and the water body.

Disadvantages
The stability of the bluff or bank depends upon maintaining the stability of the slope. Stability of the toe of the slope may require a shore protection structure. Stability of the slope and shore protection structure can be threatened by:

  1. lakebed erosion,
  2. unexpected combinations of high water levels and severe storms,
  3. freeze/thaw fracturing of armor stone in the structure,
  4. unexpectedly-large precipitation events causing groundwater saturation and bluff,
  5. instability, or
  6. runoff causing surface erosion on the bluff face.

The stable slope angle appropriate to the bluff/bank top loads, bluff soil properties, soil conditions, and future climatic conditions may not be known, or it may be underestimated. 

Calculation of a setback distance may be difficult because:

  1. an estimate of the existing slope angle is needed and
  2. the existing bluff or bank face may have several different slopes.

Aggrieved neighbors may litigate, charging that the shore protection structure retards natural sediment transport and beach building, causing damage to their shoreline. Shore protection structures are costly. The combination of initial cost and maintenance cost during the period of ownership is likely to exceed the cost of the initial investment in land and utilities.

Setback for an Eroding Slope

Rationale
Constructed buildings and other facilities on coasts can be made safe from damage or loss by choosing a setback distance that allows for natural erosion of the shore to continue during the useful lives of the facilities.

View of an Erosion Setback

Advantages
This is the least expensive alternative approach for determining a setback. There is no need for expensive construction of shore protection structures and expensive stabilization of bluff for bank slopes. This approach focuses on the net result of all of the complex conditions and properties that affect erosion. This approach does not retard natural sediment transport and beach building, removing one basis for litigation by aggrieved neighbors. Another benefit of the approach is a natural buffer between a structure and the water body.

Disadvantages
There may be little or no recession rate data available. The determination of past recession rates is useful but requires careful analysis of aerial photos, which can be an expensive and complex process requiring trained professionals. There are errors in measuring past recession rates from aerial photos. Recession rates vary in space and time. These errors, variations and unknowns reduce the usefulness of past recession rates for estimating future recession and for determining setback distances. Accurate estimates of future recession rates are desirable but depend on:

  1. unknown future climatic conditions,
  2. unknown combinations of high water levels and storm wave activity,
  3. possibly unknown soil properties and,
  4. soil conditions hidden within bluffs and banks.

Neighbors may charge that failure to halt erosion is threatening structures on their property.

Example
An undeveloped lot is for sale along the Lake Michigan coast in Ozaukee County, Wisconsin. There is no recession rate information for this property. However, recession rates were measured at 179 other locations on this stretch of coast. The average measured recession rate was 0.7 feet/year. There are not enough recession rate measurements over enough different time intervals to determine how the recession rates vary with time, changing lake levels and changing storm and bluff conditions.

Assume that a minimum recession rate is one foot per year due to the accumulation of errors possible in measuring recession rates from aerial photographs. Use this minimum chosen rate to estimate a minimum recession setback distance. Notice that this minimum setback is measured from the bluff edge and is not the same as a minimum setback distance measured from the Ordinary High Water Mark as required by the state.

A maximum recession rate can be calculated from the available recession rate data and statistics by someone acquainted with statistics for each of several different levels of confidence. Such a calculation leads to this kind of statement: 

"We can be X% confident that the average annual recession rate on the property of interest will be less than Y feet per year, assuming that the site has similar soil conditions and properties to those properties where the recession rates were measured".

It will take larger maximum recession rate values to increase confidence in this statement. 

The recession rate data in this example had a standard deviation of + 0.6 feet per year. 

At a 90 percent confidence level, the maximum recession rate will be less than 1.5 feet/year. At a 95 percent confidence level, the maximum recession rate will be less than 1.9 feet/year. At a 99 percent confidence level, the maximum recession rate will be less
than 2.3 feet/year. These rates are used to estimate setback distances that will be adequate for three different time periods.


Table. Recession-based setback adequate for the life of a structure. Distances in feet.

Guideline

50 years

70 years

100 years

min. chosen setback

50

70

100

max.setback, 90%confidence

75

105

150

max.setback,95%confidence

95

133

190

max.setback,99%confidence

115

161

230

The table contains a reasonable accommodation for local variability in recession rates from lot to lot, but may not adequately account for recession rate variability over time. 

Will future climatic conditions be like those of the past 30 to 50 years in which aerial photos have been taken? As the coast recedes, will soil properties and groundwater conditions be the same as they have been in the past 30 to 50 years? 

Some coastal areas in Milwaukee and Ozaukee counties have had four or five rainfall events within four years that were expected to occur only once in 100 years. Unforeseen land tilling changes, regrading of natural slopes, paving of parking lots and roads, and alterations in surface and subsurface drainage patterns can cause greater subsurface seepage and surface runoff on coastal bluffs and banks, decreasing their stability and increasing their rates of recession.

One way to hedge against greater future recession rates is to pick one of the higher confidence levels and longer time period setback values in the table.


Setback for a Home

Rationale
A coastal residence can be, and should be, sited to be safe from erosion damage or loss throughout its entire useful life of 70 to 100 years. Uses of the residence that are greatly enhanced by close proximity to the shore can be achieved by constructing temporary and easily relocated structures (like gazebos) intended only for those select uses. In this approach, a recession-based setback and a stable slope-based setback are examined to see their effect on the view of the water. A recommended and preferred alternative to the following approach is to hire a geotechnical engineer to determine stability of the bluff or bank and a coastal engineer to determine the feasibility and cost of constructing shore protection to stabilize the base of the bluff or bank.

Advantages
This approach should not require future relocation of the residence. Relocation may be either prohibitively expensive due to the large footprint of the house or impossible due to inadequate lot depth or a future increase in the minimum setback from the local street or road. A fringe benefit of this approach is that it provides a natural buffer between the facility and the water body.

Disadvantages
This approach is subject to some of the disadvantages common to other setback approaches. There may be little or no recession rate data available. The determination of past recession rates is useful but requires careful analysis of aerial photos, which can be an expensive and complex process requiring trained professionals. There are errors in measuring past recession rates from aerial photos. Recession rates vary in space and time. These errors, variations and unknowns reduce the usefulness of past recession rates for estimating future recession and for determining setback distances. Accurate estimates of future recession rates are desirable but future recession rates depend on 1) unknown future climatic conditions, 2) unknown combinations of high water levels and storm wave activity, 3) unknown soil properties and 4) conditions hidden within bluffs and banks.

Stability of the slope and a shore protection structure can be threatened by: 1) lakebed erosion, 2) unexpected combinations of high water levels and severe storms, 3) unexpectedly large precipitation events causing groundwater saturation and bluff instability or runoff  and surface erosion on the bluff face. The stable slope angle appropriate to the bluff/bank top loads, bluff soil properties, soil conditions, and future climatic conditions may not be known, or it may be underestimated. 

One option with this approach has the disadvantages of dependence upon shore stabilization measures. Shore stabilization is costly and may equal the purchase price of the lot with utilities. This approach may render as "unbuildable" some coastal residential lots in developed or partially-developed areas.

Example 1
A new home is to be constructed in Ozaukee County, Wisconsin on a coastal lot located on a bluff that is 40 feet above the lake level. The existing bluff slope shows signs of erosion and has a slope ratio of about 1: 2.0 (vertical: horizontal). What is the safe setback distance from the bluff edge for the residence? Considerations: a stabilized slope with shore protection, leaving the bluff in its natural state, and the limitations on the view of the lake from the house.

The location is picked for convenience so that results from examples used with the recession setback approach and the stable slope setback approach can be used here. The same stable slope ratios and recession setbacks will be considered as are shown in those approaches on other pages of this site.. 

Minimum setback distances of 50 to 100 feet are added to the recession and stable slope setback distances as contingency setbacks; one of the logical choices made in this example. The largest contingency setback value is used with the steepest stable slope. The smallest contingency setback is used with the gentlest slope which is assumed to have a larger margin of safety. Similarly, the largest contingency setback is used for the shorter time span in calculating a recession setback. There are other logical ways to look at the issue.

Recession setback distances will be selected for a 95% confidence level in terms of spatial variability. It is assumed that this will also accomodate the uncertainty about future recession rates.

Setback for an Eroding Shore


The view limitations are calculated with simple trigonometry to show the difference in view for the various setback options (see the figure below). The view limit is defined as the horizontal distance (y) from the base of the bluff to the water surface as seen from the house. A simple formula gives this distance:

Y = H[( x/h) – s]                      where:

H = bluff and beach height above water, 
x = setback from bluff edge, 
h = height of sight above bluff surface, 
s = the horizontal part of the slope ratio (1:s).

These relationships are shown in a Sight Lines Diagram.
Figure. Diagram for calculating lake view impairment due to setbacks



Results are shown in Table 1 for h = 7 feet and H = 40 feet. The bluff regraded to stable slope values provides the clearest view with the water surface visible from the house at minimum distances of 64 to 460 feet from the base of the bluff. 

The greatest view limitation occurs when a recession setback is used. The water surface visible from the house is a minimum distance of 1040 to 1360 feet from the base of the bluff; roughly ¼ mile offshore if there is no beach.

With a stabilized toe, as the bluff recedes from a 1:2 slope to a stable slope, the view improves. With a setback picked for a stable slope ratio of 1:4.0, the initial view distance will be 600 feet but drops to 120 feet when the stable slope is reached. If the property has a broad beach, that stable slope may allow a view of the water’s edge.

Table 1. Setbacks and view limit for house on a 40 feet high bluff with a 1:2 slope. Distances are in feet.

Condition

Variable

Primary setback

Minimum setback

Total setback

View distance,y

Regrade bluff to stable slope

S = 2.5

0

100

100

460

S = 3.0

0

85

85

360

S = 3.5

0

70

70

260

S = 4.0

0

50

50

120

Bluff recedes naturally to stable slope

S = 2.5

20

100

120

580*

S = 3.0

40

85

125

600*

S = 3.5

60

70

130

620*

S = 4.0

80

50

130

600*

Natural bluff recession

T = 50yr

95

100

195

1040

T = 70yr

133

75

208

1120

T=100yr

190

50

240

1280

* Initial view distance. 

The view is greatly improved by regrading to the gentler slopes. Another alternative is to build a gazebo closer to the bluff edge to get a better view. The structure can be designed to be easily relocatable and made into a three-season structure if desired.

Example 2
The same conditions and considerations used in Example 1 apply to another property in Ozaukee County, Wisconsin, except that the bluff height is 100 feet. 

A comparison of Tables 1 and 2 shows the effect of bluff height on the total setback and the resulting view distance.

Table 2. Setbacks and view limit for house on a 100 feet high bluff with a 1:2 slope. Distances are in feet.

Condition

Variable

Primary setback

Minimum setback

Total setback

View distance,y

Regrade bluff to a stable slope

S = 2.5

0

100

100

1180

S = 3.0

0

85

85

910

S = 3.5

0

70

70

650

S = 4.0

0

50

50

310

Bluff recedes naturally to stable slope

S = 2.5

50

100

150

1890*

S = 3.0

100

85

185

2340*

S = 3.5

150

70

220

2790*

S = 4.0

200

50

250

3170*

Natural bluff recession

T = 50yr

95

100

195

2590

T = 70yr

133

75

208

2770

T=100yr

190

50

240

3230

* Initial view distance. 

Conclusions about the Examples
The view from a house on a 100 feet high bluff has a more limited view of the water than a house on a 40 feet high bluff setback an equal distance. The width of the nearshore waters invisible from the house is greater. The view improves more dramatically as the higher bluff recedes to a stable slope angle with a stabilized toe. With a setback picked for a stable slope ratio of 1:4.0, the initial view distance will be 3170 feet (6/10 ths of a mile) but drops more than 90 percent to 300 feet when the stable slope is reached. 

A gazebo, or similar enclosed structure built closer to the bluff edge will help compensate for the loss of the view of nearshore waters because of the setback. Such a structure will be more important on the 100 feet high bluff site than on the 40 feet bluff site.

Future unforeseen changes on the site and on neighboring properties can decrease bluff/bank stability and increase recession rates. Such changes include: 1) ground tillage that increases absorption of precipitation and subsurface seepage and drainage from the bluff face; weakening bluff soils, 2) nearby development that includes regrading of natural slopes and creation of paved surfaces that alters surface drainage patterns and increases drainage over the bluff face, existence and use of septic systems that increase subsurface seepage and drainage; weakening bluff soils. 


Setback for a Major Facility

Major facilities include college buildings, office buildings, apartment buildings, industrial plants, sewage treatment plants, power generation stations, water pumping and conditioning plants, and coastal highways. Proper siting of such facilities along the shores of Great Lakes and ocean coasts requires the services of competent, experienced coastal and geotechnical engineers.

One of the primary considerations is elevation of the facility above highest possible water levels. One source of this information for Canadian and U.S. shores of the Great Lakes is Wisconsin Sea Grant's Coastal Processes Manual. See the Publications page for details.  Additional sources are needed to help anticipate future extreme levels due to climate change. See Internet Links for some of these sources.

Rationale
Major facilities can be sited along the coast with construction setbacks that are adequate to account for the effect of structure loading on future bluff and bank stability under expected future climatic conditions without an over-dependence upon the long-term adequacy and integrity of shore stabilization measures including shore protection structures.

Advantages

This approach accounts for the dependence of many people on the continued safe operation of the facility over its expected life. The approach safeguards the facility from unexpected catastrophic damage or loss due to sudden failure in one weak link of a bluff/bank or shore stabilization system. One of the options considered allows for natural erosion to continue and saves the cost of constructing and maintaining shore protection structures. Uncertainties about the erosion hazard are likely to be reduced by obtaining the services of a trained professional. A side benefit of the approach allows for maintenance of a natural buffer between the facility and the water body.

Disadvantages
The setback determination is subject to uncertainties inherent in picking future recession rates, safe ultimate stable slope angles, and adequate shore protection structures. Safety factors used to determine stable slopes for residential construction may not be appropriate for construction of major facilities because of the weight of these facilities on the bluff and because of a lower tolerable level of risk. Site loads, altered surface drainage patterns, foundation requirements and facility operating conditions (vibrations, movement of heavy vehicles, etc.) complicate the site stability issue. Because this approach involves rather large setback distances, it will reduce the number of coastal sites which are suitable for the facility.

Example
A new industrial plant is to be built on one of two sites- either a terrace 10 feet above highest lake levels, or a bluff 40 feet high. Both sites are on the Ozaukee County, Wisconsin shoreline of Lake Michigan. The present terrace/bluff face slope ratios at both sites are 1:2.0 (vertical:horizontal). Compare setbacks for both natural recession and stabilized slope options. 

1. Allowing Recession to Continue
Recession data from other sites in this part of the county indicate that the annual recession rate has been between one and 2.3 feet per year (at a 99% confidence level). This recession rate information seems more reliable than it really is. Unfortunately, we cannot know what the future recession rate will be at these two sites. One might assume that the variability of recession rate over time will be no more than the measured variability of past recession rates over the spatial distance along the Ozaukee County coast. That assumption is a matter of judgment.

A geotechnical engineer hired to analyze the stability of the two sites recommends a conservative minimum facility setback distance of 200 feet, a conservative stable slope ratio of 1:4.0 (vertical:horizontal) and an expected facility effective life of 100 years. The uncertainties about future recession rates and future bluff/bank stability during unknown future climatic conditions suggest that the conservatively large minimum facility setback distance is a prudent contingency to add. The following recession-based setback calculation is made for both Site A and Site B (lacking information to suggest differences in recession at the two sites):

Recession setback = 100 years life x 2.3 feet/year = 230 feet
Plus a minimum facility setback…………………. + 200 feet
Total setback allowing natural recession to occur:…..430 feet

Diagram of a Recession-Based Setback

2. Stabilizing the Slope - Halting Recession
Site A: bank 10 feet high. A stable slope-based setback.

Stable slope setback = (4.0 – 2.0) x 10 feet =....................20 feet
Plus the minimum facility setback………………………..200 feet
Total setback with adequate maintained shore protection:  220 feet

Diagram of a Stable Slope-Based Construction Setback

Site B: bluff 40 feet high. A stable slope-based setback.

Stable slope setback = 40 feet: (4.0 – 2.0) 40 feet =..........80 feet

Plus the minimum facility setback…………………….….200 feet

Total setback with adequate maintained shore protection:  280 feet

Comments
On small residential-sized parcels, coastal land is priced more by the shoreline foot than by the square foot, hectare or acre of land. On large properties, area has a large effect on price. If there are no serious area constraints on either parcel, the better choice seems to be: site the facility 430 feet back from the shore without shore protection structures. The site with the bank that is 10 feet high requires a setback just 210 feet further from the bank edge. The site with the bluff that is 40 feet high requires a setback just 150 feet further from the bank edge.

For an industrial plant, the initial cost of stabilizing the shore may be higher than the land cost. When the maintenance costs are considered over the life of the facility, the total cost of stabilizing the shore will be probably be much more than double the land cost. 

If the recession setback option is chosen, shore protection structures can be added later if future recession rates prove to be much higher than assumed. 

Future unforeseen changes on the site and on neighboring properties can decrease bluff/bank stability and increase recession rates. Such changes include: 1) nearby ground tillage that increases absorption of precipitation and subsurface seepage and drainage from the bluff face, weakening bluff soils 2) regrading of natural slopes and creation of paved surfaces on site or on nearby properties that alters surface drainage patterns, and increases drainage over the bluff face, 3) changes in precipitation frequency and intensity and changes in water level, storm frequency and intensity due to climate change.  Web sites of sources with information on anticipated effects from climate change are given in Internet Links.



Some Limitations
Following these demonstrated approaches will help reduce risks of structural loss or damage from erosion but will not guarantee freedom from such losses or damage.

The examples used here presume construction in undeveloped areas. The setback distances calculated may appear much larger than those used in practice. One reason for this is that the common compromises of balancing risk with other goals were ignored for the sake of simplicity and brevity.


In many situations, a coastal lot being developed, or redeveloped, sits between adjoining developed lots. It is common practice in coastal counties to use the average of the existing structural setbacks as the new construction setback . Such a practice is similar in rationale to the minimum setback approach given in the first example: there is no consideration of a) recession rates, b) useful structure life on the site, and c) slope stability, in permitting the new structure. Setback averaging for a new home between existing homes can result in reducing the time that the new home can be occupied on the site.

Determination of a safe construction setback distance requires good information, good judgment and a bit of luck. Some factors are impossible to predict. Future climatic conditions, future bluff conditions and unseen bluff/bank properties are examples of these unknowns. Changes in land use practice on neighboring property can cause unforeseen adverse changes in the stability and erosion of a coastal site.


Cultivation of soil on nearby inland parcels can increase absorption of precipitation into the soil and groundwater, increasing the seepage of groundwater from the face of a coastal bluff or bank, and increasing the rate of erosion. The installation of on-site or nearby septic systems may have a similar effect.


Construction of subdivisions landward of coastal lots may alter surface drainage patterns and increase the amount of water running off of coastal banks and bluffs. Neighbors' installation of shore protection structures may starve a site's beach and diminish a natural measure of protection from the erosive power of storm waves.






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