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.
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 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.
