Date of Award
Master of Science in Geology
Jon C. Boothroyd
The Narragansett barrier/headland system, located along the southeastern shoreline of Rhode Island, is comprised of a moderately developed barrier spit between two bedrock and till bluff headlands. An estuary is situated to the north and west, with its entrance at the northern end of the barrier spit. Major depositional environments within the study area include barrier spit and headland/bluff beaches, flood-tidal and ebb-tidal deltas, tidal flats, wetlands, and Pleistocene glacial deposits.
Storm climate, barrier/headland morphology, past and present human development, and the distribution and supply of sediment within the Narragansett barrier/headland system greatly influence the present geologic development and evolution of the system and subsequent stability. The study of beach shape and volume changes along the Narragansett barrier revealed a distinct high/low/high volume cycle over an 18 month survey period, in which the total sediment volume of all profiles along the beach decreased from 646 m3·m-1 in September, 1991 to 310 m3·m-1 in March, 1992. The volume then increased to 690 m3·m-1 in October, 1992 before declining again in December, 1992. The sediment volumes along the profiles fluctuated by as much as 40 - 60 m3·m-1 on a weekly basis, and during the December, 1992 blizzard, 118.5 m3·m-1 of sediment was eroded from the NAR-TB profile. These volume changes indicate significant longshore transport as well as offshore and onshore transport of sediment.
The fine grain size and dissipative profile shape along much of the barrier beach prevents the seaward transport of sediment beyond the depth in which it can be transported back onshore. Therefore, little sediment is lost from the barrier-shoreface exchange system. Conversely, the lack of sediment input to the system from fluvial sources and longshore transport, along with a slow beach recovery rate, decreases barrier stability. As sea level rises in future years, the stability of the system will decrease as sediment supply becomes limited due to human interference and the lack of new sources of sediment. A program of beach replenishment and controlled raising and migration of the foredune zone and back-barrier flat by human and natural processes may serve as a possible solution to maintain the barrier/headland system. Possible sources of sediment for such a project include upland sources, shoreface deposits, and tidal delta deposits.
If sea-level rise accelerates over the next 100 years, the depth and extent of flooding along the Narragansett shoreline will dramatically increase during a 100-year frequency storm event. The greatest flooding will occur along Ocean Road on the headland south of the barrier, within the Narragansett Pier area, and along the entire barrier. If sea level rises 125 cm by 2100, flooding will extend as much as 285 m further inland than in 1990, and many structures along the barrier and headland to the south will be inundated and destroyed because they are not set back sufficient distances from the shoreline and not raised to sufficient heights. The future configuration of the shoreline along the barrier/headland system will depend on whether the barrier is maintained in its present position or allowed to migrate as sea level rises.
With an increase in sea level, FEMA flood insurance rate maps will become out-dated, and present guidelines on minimum building elevations and set-backs insufficient given current predictions of future sea-level rise. Current estimates of sea-level rise should be incorporated into the determination of new set-back and building elevation requirements.
Harwood, Ronald A., "COASTAL PROCESSES, SEDIMENTATION PATTERNS, AND SEA-LEVEL RISE ALONG A BARRIER ISLAND AND UPLAND SHORELINE, NARRAGANSETT, RHODE ISLAND" (1993). Open Access Master's Theses. Paper 2398.