Date of Award
Master of Science in Geology
J. C. Boothroyd
Beach profiles and littoral environment measurements from one site on Charlestown Beach, Rhode Island, have been used to determine various beach states and modes of variability over a five-year period. The major profile configurations of this microtidal beach, were classified by the wave climate responsible for their shape. Storm beaches typically exhibit a wide, concave beachface and landward-displaced berm crest. Post-storm recovery beaches are characterized by the rapid onshore migration and welding of swash bars to the concave beachface. Berm development is influenced by tidal conditions, wave climate, and pre-storm configuration, and occurs within 4-7 days. This recovery rate is more rapid than on mesotidal beaches due to the greater interaction time with wave processes during a tidal cycle. Mature beaches at Charlestown often consist of a high, wide berm with a flat to gently landward-dipping berm top and a steeply-dipping beachface.
The storms found to cause the most beach erosion were either tropical or extratropical in origin but tracked closely to the west of Rhode Island. These storm paths produced southeasterly (onshore) winds of extended duration and incident waves with a long fetch. Intense storms (1-2 per year) cause extensive beach erosion (25-30 m3m-1) and may result in foredune retreat and/or backbarrier accretion by overwash. Moderate storms (6-7 per year) produce more frequent but smaller beach changes (10-20 m3m-1) such as storm berms/scarps, berm top runnels, and berm top or foredune accretion.
Short-term storm-fairweather (erosion-accretion) beach cycles were ubiquitous fluctuations were identified only during the first three years. This temporal variation may result from sediment budget changes due to long-term storage, alongshore transport, and large-scale climatic fluctuations.
A principal components analysis of the data set produced three major modes of above mean low water profile change: 1) the 1st eigenfunction indicates an onshore-offshore transport of sediment (e.g. storms and accretionary periods), 2) the 2nd eigenfunction corresponds to a beachface-berm top exchange of sediment (e.g. berm erosive events and periods of greater berm top accretion than, or in conjunction with, beachface erosion, and 3) the 3rd eigenfunction is most applicable during infrequent foredune activity (e.g. erosion, overwash events, and dune fill).
A high correlation exists between profile volume and the 1st temporal coefficients. This indicates that the majority of changes in profile volume occur within the region of 1st eigenfunction profile variability and are interpreted to result from onshore-offshore transport.
The landward limit of 2nd eigenfunction profile variability may be used as an unbiased delimiter of the inland boundary of a coastal feature, from which coastal resource setback distances are measured. On Charlestown Beach, this corresponds to the middle of the foredune ramp, marking maximum dune erosion during the Blizzard of 6-7 Feb 1978. Due to the annual fluctuation of storm frequency and intensity, five years of profile data are more accurate than a single year for the determination of the inland boundary.
Rosenberg, Murray J., "Temporal Variability of Beach Profiles, Charlestown Beach, Rhode Island" (1985). Open Access Master's Theses. Paper 2042.