2019-Inlet-Hazard-Area-Boundary-Update-NCDEQ.pdf
The first North Carolina Inlet Hazard Areas (IHA) were developed in 1978 to recognize that shorelines adjacent to inlets are more dynamic than those along the oceanfront. At the time, the novel shoreline analysis methodology used the historic migration of inlet shorelines along the coast to define IHAs. Since that time, research has shown that in addition to inlet migration, the oscillations of ocean shoreline adjacent to the inlet are also a significant threat to development. Forty years later, some of the inlets have significantly changed. Several inlets (Mad Inlet, Old Topsail Inlet, and New/Corncake Inlet) have closed completely with little chance of reopening. Others (New Topsail and Shallotte Inlets) have moved outside the limits of the original IHA boundaries. In 2004, the Science Panel on Coastal Hazards began working on revising the IHA methodology, which led to initial recommendations in 2010. Most recently in 2016, the Panel was retasked by the North Carolina Coastal Resources Commission to develop an inlet shoreline change rate calculation methodology and update the IHAs.
Inlet shorelines behave differently than oceanfront shorelines not influenced by inlets. Although dynamic and locally unique, most inlets can be classified as either migrating in the net longshore sand transport direction, oscillating around a general location, or both. The shorelines inside the inlet, between the two islands, can migrate much faster than most other landforms. New Topsail Inlet has been moving south approximately 90 feet per year since the 1930s. Mason Inlet was moving at 365 feet per year before it was relocated and stabilized.
Inlet oscillations occur both directly on the inlet shoreline, between the two islands, and on the ocean shorelines near the inlet. The locations of the inlet shorelines and the width of the inlet are constantly modified by changes in wave height/direction, storms and other factors. In 2013-2014, Tubbs Inlet between Sunset Beach and Ocean Isle Beach widened from around 560 feet to more than 1700 feet, widening by a factor of 3 in less than 2 years. The inlet width has since been narrowing and is likely to return to its previous width.
Oceanfront shorelines near inlets have long-term erosion rates approximately 5 times greater than other oceanfront shorelines. Much larger oscillations in the oceanfront shoreline near inlets can also occur over several years or decades. These fluctuations are most often caused by movements in the primary ebb channel through the offshore bar. As the channel moves closer to one island, sections of that shoreline accrete while the other island erodes near the inlet. When the channel shifts by natural processes or dredging, the oceanfront process reverses. The island previously losing then gains, while the other side of the inlet loses what it previously gained and sometimes more. The oscillations may not contribute to the long-term erosion rate but can be a short-term threat to coastal development.
In 2010, the Panel developed draft IHAs for each of the developed inlets. Public comments criticized the effort in part because then-present IHA rules were not appropriate for the much larger redefined areas. Also, no proposed rule changes were presented to accompany draft boundary updates. The 2010 drafts were also criticized because of the increased size of the draft IHAs, and the fact that inlet risk within the areas varied considerably. In comparison, when defined as a simple box along the shoreline, the Ocean Erodible Area (OEA) component of the Ocean Hazard Area (OHA) is like the IHA. However, the published erosion rates within the OHA identify the relatively higher risk closer to the shoreline.
In response to the public comments on the 2010 IHA drafts, the panel developed the Inlet Hazard Area Method (IHAM) to define the IHA and to identify two risk lines that are calculated similarly to the CRC’s OEA mapping. Away from inlets, the existing vegetation line can be a useful indicator of the long-term erosion trend, offering several advantages in defining the Ocean Hazard Area. However, the migrations and oscillations near the inlets make the vegetation line too volatile to be an effective management tool. A primary finding of this report is that the vegetation line is not a reliable reference feature for certain management purposes near inlets. The dynamic oscillations near inlets were found to be better represented by a fixed, Hybrid-Vegetation Line based on the most landward limits of all vegetation lines over the study period. The Science Panel recommends fixed IHA development boundaries, like the Static Vegetation and Development lines used for large-scale (>300,000 cubic yards) beach nourishment projects.
The IHAM defines the landward limit of the IHA by multiplying 90 years times the annual inlet-shoreline erosion rate, measured landward from the Hybrid-Vegetation Line. This calculation is like that already applied in defining the landward limit of the Ocean Erodible Area and Ocean Hazard Area outside the IHA. A second line, the 30-Year Risk Line, has been mapped similarly to the minimum oceanfront setback distance of 30 times the erosion rate for identifying higher-risk areas. Because inlet shorelines behave differently than non-inlet areas, there are several important differences in how the erosion rates are measured and how they are applied in mapping compared to the non-inlet shorelines:
The maps in this report present the Panel’s recommended IHA for each of the developed inlet shorelines where the inlet risk is equal to or more important than the long-term erosion and storm impacts. Because inlet oscillations make the existing vegetation line a poor indicator of future conditions, the proposed boundaries are fixed relative to the Hybrid-Vegetation Line. The Science Panel on Coastal Hazards recommends that the CRC consider updating subsequent IHA boundaries every five years, to coincide with updates to oceanfront erosion rates and Ocean Erodible Area boundaries. This 2019 report is submitted as a replacement for the 2010 report on the panel’s recommendations.