COASTAL EROSION AND BEACH MORPHOLOGY CHANGES IN RINCÓN, PUERTO RICO Krystina R. Scott
1*,
Miguel Canals
2,
and Francisco Velez
3
Geology Department, University of Puerto Rico at Mayaguez (UPRM), 2 Ocean Engineering Laboratory, University of Puerto Rico at Mayaguez (UPRM), 3 Civil Engineering, University of Puerto Rico at Mayaguez (UPRM) 1*
Introduction
Beach Morphology
During the last several decades beaches in Rincón have suffered severe erosion, putting at risk private and public property. Thieler and Rodriguez (2007) analyzed the long-term coastal erosion from Punta Higüero to Punta Cadena and found an average erosion rate of 0.4 ± 0.2 meters of shoreline retreat per year in Rincón. In the present study, we examine beach erosion at shorter timescales during nine months from early September 2010 to late May 2011 at Villa Cofresí Beach, using weekly beach profiles to better understand the beach response to storm events and swells. We also obtained sediment samples at the beaches and at an offshore sand reservoir and compared their characteristics.
The beach at Villa Cofresí is protected by a large seawall. The figures below depict the weekly beach profiles at each station. The black line represents the average beach profile for each station. Vertical changes in beach profiles as large as 1.5 to 2 meters can be seen as well as a large temporal variability in response to storm events. The lower limit of the beach profile is limited by a rock fill placed several years ago in an attempt to protect the beach and the nearby hotel. It is also evident that the beach recovers most of its sand several weeks after a storm. STATION 2
STATION 1
Study Area Elevation (m)
Elevation (m)
Summer
Seawall
After Tropical Storm Otto
TPMR
Cross-shore distance (m)
Cross-shore distance (m)
Cross-shore distance (m)
Station 1 Station 2
Hurricane Igor
Ikono Image 2004
Location of beach profile stations
Tropical Depression Otto Tropical Storm Thomas
Sand Texture and Distribution
N
Villa Cofresí Beach Morphology Changes
2mm
0.125mm
SAND TEXTURE
SAND NOMENCLATURE
GRAVEL
QUARTZ
0.25mm
TERRIGENOUS
CaCO3
SILT
SAND
1mm
Sand Photos by: Hector Ruiz and Oliver Bencosme / SGPPR
The Bajo Blanco sand bar is part of a natural reservoir of sand close to Punta Higüero, Rincón. Sand samples collected in different beach areas in Rincón help us determine the sediment distribution and characteristics of the area. To characterize the sand texture and composition at the Bajo Blanco sand bar, we collected sediment cores and deployed sediment traps. Beach sand collected in the Tres Palmas Marine Reserve (TPMR) and Villa Cofresí Beach was compared with the sand of the Bajo Blanco.
December 23 Winter Swell
Sand Sample Location Villa Cofresí Berm Zone Villa Cofresí Swash Zone Villa Cofresí Offshore TPMR Winter Season TPMR Summer Season Bajo Blanco Station 1 Bajo Blanco Station 2 Bajo Blanco Station 3
Divers extracting sediment cores
2005 Provided by: Leon Ritcher
During Hurricane Earl, 2010
Post-Tropical Storm Otto, 2010
Summer, 2011
Provided by: SGPPR
Conclusions and Recommendations • From September to December 2010, physical forcing from hurricanes, tropical storms and winter swells caused erosion of Rincón beaches and exposed the structural foundations of the Villa Cofresí Beach Hotel. • Tropical Depression Otto impacted the Island with heavy rainfall and waves for several days, causing flooding and severe erosion in Villa Cofresí Beach. • At short timescales, beaches were observed to recover quite quickly and within several weeks of a storm event. • This indicates that these beaches are dominated by cross-shore sediment transport. • At longer timescales, beach face morphology in Villa Cofresí (2010-2011) is divided by eroded beach during September to December and build up beach during January to May. • The sand of the Bajo Blanco sand bar is a viable source for beach fill material for Rincón beaches.
Seabed ripples at Bajo Blanco
Core samples collected in the Bajo Blanco sand bar were analyzed in the laboratory to compare with the sand in local beaches and determine if this sand is suitable to serve as beach fill material for Rincón beaches.
References Thieler, R. E., and Rodriguez, R. W., 2007; Historical Shoreline Changes at Rincón, Puerto Rico 1936-2006. USGS Open-File Report 2007-1017