A Photogrammetric Survey of Backbarrier Accretion on the Rhode Island Barrier Beaches

Author

Elizabeth Jean Simpson, University of Rhode Island

Date of Award

1977

Degree Type

Thesis

Degree Name

Master of Science in Geology

Department

Geology

First Advisor

John Fisher

Abstract

Washover fans and tidal deltas are known to be significant sediment storage sites on the barrier islands of the Outer Banks of North Carolina and the Texas Gulf Coast. No previous studies, however, have attempted to quantify the importance of these sediment sinks in the littoral sediment budget.

The relative importance of washover and tidal delta sedimentation on the erosional shoreline of Rhode Island has been determined from a photogrammetric analysis of the backbarrier shoreline changes on the south shore barrier beaches from Napatree Point to Point Judith over the period of 1939 to 1975 (the dates of the earliest and latest aerial photographic coverage). Amounts of real changes were measured directly or were calculated from direct measurements. Determination of volumetric changes required additional information concerning annual rates of vertical washover sedimentation (0.03-0.04 m/yr, from Godfrey, 1976; and 0.05 m/yr, this study) and an estimate of volumetric change per areal change of eroded beach from the U.S. Army, Coastal Engineering Research Center (1973) in which a change of 0.09 m² along the shoreline is equivalent to a volumetric change of 0.76 m³, or a change of 8.44 m³/m².

Backbarrier areas were measured using a square grid point-counting technique. These direct areal measurements were converted to ground areas using the representative fractional scales determined for each individual photograph from ground truth measurements, and the amounts of areal changes of supratidal and subtidal washover and tidal delta deposits and of eroded beach were calculated for the period of 1939 to 1975. Total areal change of supratidal plus subtidal washover deposits was +522,792 + 267,953 m² = +790,745 m²; total areal change of supratidal plus subtidal delta deposits was +188,238 + 562,322 m² = +1,050,560 m². Total area of eroded beach for the whole south shore was -608,558 m². Annual rate of areal changes was washover deposits for the whole south shore was calculated to be +21,965 m²/yr; for annual tidal delta accretion, +29,182 m²/yr; and for annual rate of beach erosion, -16,904 m²/yr.

According to these values of areal changes, subtidal plus supratidal tidal delta sedimentation is 1 1/3 times more effective than subtidal plus supratidal washover sedimentation in the landward transportation, deposition, and storage of sediment. Supratidal washover accretion.

Using the derived annual rate of vertical washover sedimentation of 0.05 m/yr to compute the approximate volumetric values of changes for both washover and tidal delta deposits (in the absence of any indication of vertical tidal delta accretion rates), and using the Coastal Engineering Research Center’s value of 8.44 m³ sediment loss (or gain) per 1 m² areal units of beach erosion (or accretion) to compute the volume of eroded beach, the following results were obtained. Washover accretion was determined to be +1,354,809 for the whole south shore over the entire study period, tidal delta accretion is +1,822,476 m³, and the amount of eroded beach is -5,138,934 m³. According to these values, overwash can account for 26% of the sediment eroded from the beaches and tidal delta sedimentation for 35%. Losses to alongshore and offshore transport of sediment therefore total 39% of the volume of sediment eroded from the beaches of the south shore of Rhode Island.

The greatest factor controlling the occurrence and amount of washover accretion appears to be an erosional beach. At 27% of the transects at which washover accretion was significant (i.e., more than the mean value of +18,000 m²), beach erosion was also significant (i.e., less than the mean value of -6,000 m²). At 66% of the transects at which washover accretion was significant to moderate (i.e., greater than +18,000 m² or greater than 0 and less than +18,000 m²), erosion was also significant to moderate (i.e., less than -6,000 m² or greater than -6,000 m² but still less than 0). Other related controlling factors are the height and continuity of the dunes, the development of transitory inlets, and the width of the barrier beach (which is a function of the development of tidal deltas and washover backbarrier deposits and of the amount of beach erosion).

Recommended Citation

Simpson, Elizabeth Jean, "A Photogrammetric Survey of Backbarrier Accretion on the Rhode Island Barrier Beaches" (1977). Open Access Master's Theses. Paper 2031.
https://digitalcommons.uri.edu/theses/2031