Establishing Standards for Building Setbacks: Incorporation of Erosion Rate Variability

Abstract

Building setbacks in coastal areas are often established using average annual erosion rates as a predictor of future shoreline trends. Review of actual shoreline movements in many areas reveals what are often wide variations in erosion rates through time. Setbacks based solely on average annual erosion rates fail to take this variability into account, and may understate the true erosion risk to a structure, depending upon its location relative to the setback. This paper presents a method to review erosion rate variability at a site and to better gauge the likelihood that a structure at a given location will be undermined by erosion. INTRODUCTION Many states and communities have established coastal building setback lines in order to push new development landward, away from the active beach, dune or bluff. These setback lines are traditionally based on three factors: 1) the average annual shoreline erosion rate (AAER), typically expressed in ft/yr or m/yr, 2) an erosion rate multiplier, usually ranging from 30 to 60, and 3) an erosion reference feature -a physical feature that can be located in the field or on a map (e.g., mean high waterline, vegetation line, dune crest, or bluff edge). The common presumption is that structures built at or behind these lines will be safe from erosion for a period of time equal to the erosion rate multiplier expressed in years. This presumption is reasonable as long as two conditions are satisfied: 1) the AAER, or mean erosion rate, used to establish the setback accurately represents past shoreline behavior at the site, and 2) the mean erosion rate will be a good predictor of future shoreline behavior. The first condition holds when there has been little variation in the historical erosion rate over time, i.e., where historical shoreline positions have closely followed a linear trend. The second condition holds where there have been and will be no significant changes in physical conditions and coastal processes (underlying geology, wave and water level conditions, and sediment supply) at the site. 1 Principal, Christopher Jones & Assoc., 5525 Jomali Drive, Durham, NC 27705 USA, cjones@cstone.net 2 Coaslal Engineering Specialist, North Carolina Sea Grant, 5001 Masonboro Loop Road, Wilmington, NC 28409 USA, rogerssp@uncwil.edu