Waquoit Bay National Estuarine Research Reserve

A State - Federal Partnership

CoastWatchers

Since October 2000, the approximately 3 miles of Waquoit Bay NERR’s Vineyard Sound-facing (southward-facing) coastline have been surveyed every 2 months from October through April to monitor shoreline change. Most of this coastal area can be defined geomorphologically as barrier beach and barrier spit, as they are backed on their landward (northward) sides by coastal fresh, brackish, and tidal ponds and embayments, and the beaches have been formed by sandy deposits derived from the erosion of adjacent headlands along Cape Cod’s southern shore. The surveyed beaches are exposed to wave action from the south, with easterly and westerly components of wave and wind reversing the direction of alongshore sediment transport. Prevailing southwest winds result in a net or long-term eastward movement of sediment, though extra-tropical storms with a strong southeasterly component can often reverse this movement episodically. The most critical agents of coastal change for this locale are the occasional tropical storm or hurricane, which can produce large storm surges and heavy southerly wave action, which can overtop the barrier beaches and create new tidal inlets.

For survey convenience, the Waquoit Bay NERR coastline is sub-divided into 3 sections. Running east to west these sections are referred to as State Beach (from Flat Pond to the westernmost South Cape Beach boardwalk~0.7 mile), South Cape Beach (from the boardwalk to near the Waquoit Bay inlet~1 mile) and Washburn Island (from Waquoit Bay inlet to Eel Pond inlet~1.2 miles). Within in each section, a series of sequentially numbered primary benchmarks are located landward (northward) of the primary dunes and positioned every 200 feet along an approximate east – west axis. In all there are now a total of 74 shoreline monitoring stations.

During each beach survey two basic types of measurements are carried out: 1) at all stations horizontal distances are acquired from the primary benchmark to four key morphological features located directly seaward of the benchmarks, and 2) at a subset of stations vertical elevations are acquired for the key morphological features relative to the primary benchmark elevations, which have been established relative to NGVD (also referred to as 1929 Sea Level Datum). The key morphological features recognized by our survey are 1) the seaward most or primary dune crest, 2) the seaward most vegetation limit (usually the limit of beach grass), 3) the seaward most or active berm crest that separates the backshore –generally the flatter, tidally dry beach where one can safely relax on a beach blanket without fear of wave swash – from the beachface – the sharper sloping, tidally active beach, and 4) the toe of the foreshore at the seaward foot of the beachface often identified by a noticeable “step” or “drop-off” which approximates the position of mean low water along this low tidal range (about 1.5 feet) shoreline.

Horizontal Distances: Seaward distances from a primary benchmark to the key morphological points are measured at all Coastwatcher stations (every 200 feet) during each survey. This is accomplished using a simple reel tape measure with measurements made to the nearest foot. Most of the morphological features are easily identified with minimal training, with perhaps the most difficulty encountered locating the active berm crest, which can sometimes be a subtle feature. All survey operations are carried out within 2 hours of the low tide, for convenience and consistency of conditions during measurement.

Vertical Elevations: Vertical elevations of the key morphological features relative to the primary benchmark are measured for a subset of stations (about every fourth or fifth station) during each survey. A simplified “Emery” method is used to determine elevations. The essential concept of the Emery method is the use of the seaward horizon as an approximation for the local level line. In its most simplified form, the “sighter” stands astride the primary benchmark facing seaward and notes the intersection of the seaward horizon with a specific “height” on a vertical elevation rod positioned atop the ground surface or morphological feature of interest. These heights are measured to the nearest 0.1 foot. The relative elevation difference between the benchmark and the remote surface or feature then can be calculated by subtracting the specific “height” of the elevation rod from the eye level height of the “sighter” (as previously determined). Knowing the absolute elevation of the primary benchmarks then allows for the calculation of the absolute elevation of the morphological features.