Dredged Material Disposal cont'd

Dredged Material Disposal

Figure 57. Historic shoreline change from last inlet breach cycle (1851-1926)

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the north end of South Beach and possibly threaten the mainland shoreline in the

vicinity of and to the south of the lighthouse. If the 140-year cycle continues, this

South Beach will eventually retreat west and south and weld to the Morris Island

and North Monomoy Island shore. This would leave the Town of Chatham main-

land shore open to wave and tidal forces until Chatham Inlet and Nauset Spit

migrate south and reform the barrier spit. The use of some alternate disposal sites

may require non-Federal coast sharing. The continued placement of sand on the

beach at Andrew Hardings Lane and the beach in front of the Chatham Light-

house is also recommended to protect the upland property in this area as the main-

land beach evolves in response to the inlet migration. Additional disposal of

material on the seaward edge of the ebb shoal (in around 10 ft (3.1 m) of water

depth) is also possible. This nearshore disposal site will keep material in the

littoral zone and allow sand bypassing to continue to the South Beach area.

It is difficult to predict accurately the evolution of the shoreline and inlet over

the next 50 years, but by examining the patterns of shoreline adjustment from the

past cycle, a general idea of the change can be achieved. A review of historic

shoreline evolution from the 1850s to the 1920s (Weishar, Stauble, and Gingerich

1989) indicated that the last breach occurred in 1846 approximately 2 miles

(3.2 km) to the north of the present inlet (just off Allen Point at the southern end

of Pleasant Bay). For some 20 to 30 years the ebb and flood shoals of this new

inlet developed. The South Beach barrier island was deprived of this normally

uninterrupted southerly sand transport and experienced accelerated erosion, and

decreased island width in the vicinity of the Town of Chatham (Figure 57). In

November 1871, a new second breach cut through this low barrier just opposite

the then twin Chatham Lights (almost in the same position as the 1987 breach).

The 1873 shoreline has distinct similarities to the present inlet configuration of the

early 1990s. By the early 1880s, this sand-starved island had breached in numer-

ous places. The town was unprotected from the ocean waves and currents and

suffered severe erosion to the mainland shorefront, including the loss of the two

lighthouses. Street ends were lost and several houses had to be moved inland.

Between 1886 and 1893 a smaller South Beach barrier island had formed closer to

the mainland. By 1920 (a little less than 50 years after the 1871 breach) the barrier

island finally welded to the mainland in the vicinity of Morris Island. Monomoy

Island and Morris Island were now part of a continuous spit attached to the south

end of Chatham's mainland. The mainland beach was wide at this time, with no

offshore barrier spit or island. Nauset Spit reformed and migrated south so that by

1950 (99 years after the breach), the new spit had again formed a protective

barrier to the mainland.

With the former cycle, the inlet dynamics were slightly different in that the

1846 larger northern inlet and 1871 southern inlet coexisted and were for the most

part connected through north Chatham Harbor for some 10 years. Chatham

Harbor was also open to the south until the 1880s. The present configuration with

one inlet carrying the prism for the estuary system presents a different evolution-

ary twist. It is hypothesized that the sand starved South Beach will still repeat the

cycle of erosion through overwash, island breaching, and landward migration.

Since Tern Island is privately owned and has reached its capacity, this site may

not be available for future dredge material placement. Future placement may be

Chapter 6 Dredging Plan Recommendations

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