LOWER MOVES FASTER: CONTRASTING RETREAT PATTERNS OF GRAZED AND UNGRAZED SEGMENTS OF THE NORTH BAY BARRIER, CEDAR ISLAND, NC

The North Bay barrier (NBB) on Cedar Island comprises two fetch-limited barrier spits in southwestern Pamlico Sound, North Carolina. The two spits trend northwest and southeast away from a central Pleistocene sand ridge. Several beachfront residences occupy the NW spit. No humans reside on the SE spit, but it has been used as grazing pasture for horses and cows for more than 60 years. A ferry terminal bounded by jetties separates the two barrier spits, and a fence near the ferry terminal bisects the subaerial portion of the barrier, restricting the horse and cow grazing to the SE spit. Grazing limits or eliminates dune stabilization by vegetation on the SE spit, resulting in lower and flatter topography (~1 m in height), whereas on the ungrazed NW spit, dunes >3 m high are observed.

We mapped changes in the NBB spits over time using georeferenced aerial imagery in Google Earth. The ‘seaward’ edge of the barrier (facing Pamlico Sound) was demarcated by the water line. The SW barrier edge was mapped as the landward-most boundary between higher-elevation sand and lower-elevation backbarrier environments. In most places, dune and washover sand migrates landward over backbarrier Juncus marsh but in isolated zones, barrier sand encroaches onto Spartina alterniflora marsh or into open water. Since 2004, a backpack-mounted differential GPS system has been used to groundtruth features traced from air photos by repeatedly mapping the same boundaries on foot.

The grazed and ungrazed spits differ both in retreat rates and the morphodynamics of landward sand transport. The average landward retreat of the grazed SE spit is faster (~4 m/year) than the ungrazed NW spit. The subaerial barrier sand is transported by storm overwash (always landward) and by eolian sand transport (both seaward and landward). On the ungrazed NW barrier spit, vertical build-up of the vegetated dunes slows their landward movement and restricts storm overwash to relatively narrow channels between the dunes. The greater topographic relief of the NW spit is also associated with a more stochastic and spatially heterogeneous barrier retreat pattern, whereas the more uniform and rapid retreat of the grazed SE spit results from broader sheet-like overwash that more frequently inundates large areas of lower and flatter topography.