Coastal study sites south coast Puerto Rico

The south coast is affected by seas approaching from the east, southeast and south. High amplitude, low period seas occurred during the summer and winter months when easterlies were stronger. Swell was not common in the area and long period, high amplitude waves were related to storm occurrence. The insular shelf is less wide to the east, but the shelf width is 5 km at Arroyo. The shelf width is from 16 to 21 km from Las Mareas to Jacaguas, with many natural barriers such as coral reefs, mangroves, sand bars, and beach rock present. The majority of the coral reef areas were located between Las Mareas and Pozuelo and west of Ponce. Few submarine canyons occur on the south coast except for those tied to the few well-defined river systems. The Guayanilla submarine canyon is the largest of these canyon systems.

The southeast coast is dominated by alluvial deposits and rocky shoreline with some sand and lag gravel beaches developed in front of them. Alluvial fan sediments from the mountains form unconsolidated shorelines with the sediments containing large amounts of gravel and dark sands. These are alluvial fans and plains from Patillas to Ponce. Limestone rocky shorelines are at Guanica and the southwest coast.

The 6th field trip will be to sites on the south coast of Puerto Rico, starting at Guanica














South Coast Sites

Playa Santa
El Tuque Beach
Matilda Beach

week 6 trip

Playa Santa, Guanica

Net low erosion rates occurred at all stations at Playa Santa beach from 1963 to 1987, but there was accretion from 1963 to 1971. The beach was not affected directly by storm waves due to the northwest shoreline orientation and the high degree of natural protection. Extensive coral reefs and seagrass beds in the area acted as wave energy buffers.

High erosion rates were recorded at all stations from 1971 to 1978. Higher erosion rates were detected on both corners of the beach located near the limestone cliff. Loss of sand was due to wave refraction patterns caused by the location of these outcrops. A combination between accretion and erosion was found on Playa Santa from 1978 to 1987. Accretion occurred from station 1 to 2, but there was erosion from stations 3 to 4.

Playa Santa had transitional profiles ranging from 5.0 to 8.0 degrees. Reflective stages were observed in some beach segments. Dissipative stages occurred occasionally from station 1 to 2. Loss of sand at these stations could be related to the effect of a rocky outcrop located to the southeast of the stations. No significant changes were found on the beach face except during February, 1993 when the beach face was flatter. Dissipative stages were related to tropical lows and cold fronts that affected the area. Another event of high wave amplitude occurred in July, 1992 when station 3 suffered loss of sediment. Very few changes were observed on the beach due to the presence of coastal vegetation and nearshore seagrass beds. This seagrass acted as an energy buffer reducing the direct wave impact on the beach. Nearshore slopes ranged from 0.3 to 3.3 degrees with the flattest nearshore profile found during the winter. Wave refraction was not available for the area.

A moderately well sorted carbonate sand was found along the beach (sorting ranges from 0.5 to 0.6). A shift from moderately to well sorted sediments occurred in February, 1993. Fine sand from biogenic sources such as calcareous algae occurred in the nearshore. The grain size distribution was unimodal for most of the stations because of a singular sediment source. A bimodal distribution was found in station 3.

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El Tuque Beach, Ponce

Two main shoreline trends were measured at El Tuque from 1962 to 1987. These were net accretion on the northwest beach (from stations 1 to 6) and severe erosion on the southeast beach accretion (from stations 7 to 10). Severe erosion occurred from stations 7 to 10 because the stations were affected by human activities.

As described in the beach profile section, El Tuque beach was modified several times by human activities from 1960. The area was known as Punta Cuchara before it was changed to El Tuque Beach after the Commonwealth of Puerto Rico acquired it as a park in the early 1960's. US Army Corp of Engineers, 1978 The beach has a northwest and southeast orientation and it was composed of alluvial fan sediment sequences, with extensive mangrove areas in some portions of the beach until 1960. As the park was developed, the mangroves were destroyed and alluvial fan deposit sequences were removed in some beach sections during 1960 to 1965.

Severe erosion was measured on the southeast stations from 1962 to 1971 as a result of those activities. The loss of shoreline was caused by the destruction of extensive mangrove areas for the construction of parking facilities as fine sediments were easily removed after the mangroves were removed. Accretion occurred on the northwest beach during the same period. Sand added to the beach came from sediment losses from the southeast stations.

Low to moderate erosion was measured in most stations from 1971 to 1978, except from stations 8 to 10 were accretion occurred. Loss of shoreline still occurred because of the construction activities in the park. According to the Corp of Engineers, the subaerial beach was composed of fine sand to the northwest and coarse and medium sand to the southeast during 1978. US Army Corp of Engineers, 1978 Finer sediments were related to the presence of mangrove and coastal lagoon sediments transported to the northeast. Coarser sand was related to the presence of a coral reef located on the east side from stations 9 and 10. Gravel and coarse sand was found on the nearshore and very fine sand was shifted to the offshore for the same period.

Accretion was measured from stations 1 to 7 and 10 from 1978 to 1987, with low erosion rates on stations 8 and 9. Shoreline accretion was caused by replenishment activities done in 1979 at the park. According to the Corps of Engineers, the beach fill was to restore the shoreline to its historic seaward limit (1960) with an average width of 10 meters. The beach fill (coarse sand and gravel obtained from the mouth of Rio Bucana) was extended from the northwest park boundary in a southerly direction. The actual subaerial beach and beach face showed an homogenous coarser sand sediment facies which was different to 1978, when the beach sediment facies had two main facies, fine and coarser sands. The sand fill was stabilized by a shore connected breakwater and an offshore breakwater parallel to the northerly section. Also, additional fill material was placed north of the breakwater to protect the older beach fill. US Corp of Engineers, 1978

Loss of shoreline on stations 8 and 9 was caused by trapping longshore sediment transport by the groin located at the southeast beach. Also, stations 8 and 9 were located behind the offshore breakwater causing major loss of shoreline on these stations.

Shoreline change measurements were not available after 1987, but field observation showed that coarse sand coming from replenishment activities was lost after this period. The actual subaerial beach and beach face showed a very fine sand facies. Also, extensive very fine sand and mud facies were observed in the nearshore. Changes in sediment facies could be caused by a change in the local wave refraction pattern due to the presence of breakwaters and from other replenishment activities on the area.

According to wave refraction plots, the southeast stations at El Tuque beach received high wave energy when waves approached from southwest.

El Tuque beach was modified by human activities several times in the last 34 years. Mangrove destruction and sand removal were conducted for the construction of the El Tuque recreational park during 1960 to 1965. Other activities were attempted to restore it as a public beach and retard erosion caused by prior activities, from 1970 to 1977 by the US Corp of Engineers. Activities such as sand replenishment and coastal structure construction were done. This resulted in the presence of both a groin (perpendicular to the shore) and a breakwater (parallel to the shore) in the longshore transport zone after 1977.

A beach survey by the Corps of Engineers during 1977 prior to beach stabilization activities, showed two main sediment facies on the subaerial beach. These were:

The analysis of sediments indicate a composition that averages approximately 75 percent poorly graded coral and shell material along the southern beach. The coarser sands could be coming from reef sources located near these stations. Along the northern beach, the material consist of gray silty sand and silt with some gravel-size coral and shell fragments. Fine sands could be coming from the lagoon and finer sediment components from alluvial fan sources. A shift from fine and coarse sand to coarse sand and gravel was identified on the subaerial beach after 1978 due to sand replenishment activities.

Beach surveys conducted during 1992 to 1993 indicated a shift from gravel and coarser sand to very fine sand in the backbeach and beach face. Very fine sand was found along the swash zone from stations 3 to 5 and mud occurred especially at stations 4 to 5 which were behind the breakwater. Medium and fine sands were identified on stations 1 to 2 for the same period. Mud facies found along all the nearshore were different from the gravely, coarse and medium sands found prior to 1977.

Both reflective and transitional stages were measured at stations 5 and 4 respectively during 1992 to 1993 beach survey. These stations were replenished with river sand dredged from the Rio Bucana mouth and then protected by a breakwater built during 1977. These changes may explain the fact that part of the fill material has remained from 1977 to the present. A groin located southeast of station 5 acted as a dam blocking the littoral sediment transport. No changes in the reflective stage occurred at station 5 during the study due to the stabilization caused by this structure.

Dissipative stages were found for stations 1 to 3 at El Tuque during the profiling. These stations showed loss of sand due to the groin blocking sediment transport to the northwest. A higher dissipative stage was measured on station 3 because it was directly affected by both structures. Nearshore profiles were flat, ranging from 0.3 to 2 degrees. No significant changes in the nearshore slope was observed during the period of profiling.

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Matilda Beach, Ponce

High to severe net erosion rates were measured in all stations at Matilda Beach from 1964 to 1987, except for station 19 where the beach was accreting. The highest erosion rates at Matilda Beach were from 1964 to 1971. Severe erosion occurred in all stations, except for station 19 which had accretion. Erosion could have been caused by an increase of storm occurrence during this period causing higher amplitude waves approaching the shoreline. A decrease in erosion rates developed from 1971 to 1987. Moderate erosion occurred from stations 1 to 3, and 17 to 21.

A shift from erosion to accretion, and from accretion to erosion was noted at the stations from 1977 to 1987. Low to moderate erosion occurred from stations 9 to 16. The loss of beach width was caused by commercial sand extraction from the beach at these stations from the 1970s until the present. Sand added at stations 17 to 21 were from the Rio Matilda.

According to wave refraction plots, high wave energy reached the west side of the beach, except when waves approaching from southeast had a period of 8 seconds, when low wave energy occurred. Stations 6 to 17 were protected from high amplitude waves the majority of the time due to the submarine canyon that diverges waves from the canyon head to Rio Matilda and from stations 1 to 4. The continuous sand extraction and lack of planification on the beach have made this system the ugliest beach plain on the Island.







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The 7th field trip will be to sites on the south coast of Puerto Rico, starting at Ponce











South Coast Sites

Jacaguas-Manzanillo
Playa Cortada

week 7 trip

Jacaguas-Manzanillo

High to very high net erosion rates were recorded in all stations at Jacaguas beach from 1962 to 1987, with major losses on the stations near Rio Jacaguas. The highest erosion rates were noted from 1962 to 1971 in all stations. The erosion could be caused by an increase of storm occurrence during this period. The alluvial plains were significantly eroded creating losses in the local sediment budget. Reduction in erosion rates occurred from 1971 to 1978, where low erosion rates were measured from stations 1 to 6 and moderate to high on the east side of the beach. Decrease in erosion rates continued from 1978 to 1987. Low rates of erosion could be caused by decrease in the flood and storm occurrence. No data were available to identify other causes of erosion in the area.

High wave energy occurred at the shoreline, when long period waves approached from the southeast. Low wave energy is developed at the shoreline, when waves came from the south, making of Isla Caja de Muertos act a natural barrier providing protection from direct wave impact.

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Playa Cortada

The beach extends from west of Rio Coamo to the east side of Rio Descalabrado. According to Morelock, 1984 there was accretion on the west beach from 1936 to 1951, with erosion on the east beach near the old dock.

Net accretion occurred in all stations at Playa Cortada from 1963 to 1987, except for stations 7 to 9 where very little erosion was observed. Erosion was caused by the emplacement of riprap built on these stations.

Accretion occurred in all stations from 1963 to 1971. Sand added to the beach was coming from river sources such as Ro Coamo which during high floods carried large amounts of terrigenous sediments that were then transported along the beach by local currents. Flood events were possibly generated by storms that occurred during this period.

Moderate to high erosion was measured from 1971 to 1978, except from stations 14 to 17 when no shoreline changes were noted. Stations 1 to 13 had a shift from erosion to accretion from 1978 to 1987. No shoreline changes occurred from stations 14 to 17. According to wave refraction plots, high wave energy occurred when long period waves approached from a south-southeast direction.

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The 8th field trip will be to sites on the south coast of Puerto Rico, starting at las Mareas











South Coast Sites

Pozuelo, Guayama
Las Mareas, Guayama
Arroyo

week 8 trip

Pozuelo, Guayama

Net low erosion rates were measured from stations 1 to 15 from 1963 to 1987, with very little shoreline changes from stations 16 to 21 on the east side of the beach. From 1963 to 1976, low erosion rates occurred on stations 1, 5 to 14, and 18 to 21. Natural erosion was caused by increase in storm occurrence and human activities related to changes from agriculture to industrial land-use. Accretion occurred on stations 2 to 4 and 15 to 17. Very few changes occurred in most of the stations because the area is well protected by natural barriers.

For the period, from 1976 to 1987, there was an increase in erosion rates. Stations 2 to 4, and 10 to 15 had moderate to high erosion rates. Erosion was increased by the coastal modification of port construction. Accretion occurred from stations 18 to 21.

Waves approached from south-southeast and short period waves caused high energy waves to reach the shoreline, except for long period waves coming from the southeast that caused very low wave energy concentration at the beach.

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Las Mareas, Guayama

Very little net change was measured at Las Mareas beach from 1963 to 1987, except for accretion on station 5 and from 11 to 13 located on the west side of Rio Guamani. Low to moderate erosion rates occurred in all stations from 1963 to 1971. No shoreline data was available from stations 1 to 4 from 1963 to 1977. Erosion can be related to human activities of transition from agriculture to industrial land use in the area. Large sugar cane plantations were cultivated at Las Mareas before 1963. The residential areas were limited to colonies of employees for the sugar cane plantations. Few coastal structures such as a dock and a navigation channel were located in the bay close to Central Aguirre during this time. Seguinot-Barbosa, 1986 After 1963, a drastic change from agriculture land use to industrial occurred causing many changes to the coastal features including partial destruction of mangrove areas. These activities caused erosion. Also, sheet runoff through the cane fields contributed to erosion and slumping caused by the high occurrence of storms and heavy rains during this period. Bush, et al., 1995 Increase of storm occurrence could be a cause of erosion during this period.

All stations from 1971 to 1978 had a shift from erosion to accretion with major accretion in the east stations close to the Rio Guamani. Moderate to low erosion rates were noted on stations 2 and 8 to 12 from 1978 to 1987 with accretion on stations 4 to 7 and 13. Loss of shoreline was related to the building of port facilities after 1972. The construction of these new facilities caused more destruction in the mangrove area. Dredging activities in the navigation channel occurred frequently. This caused waves with higher amplitude to reach the coast due to the changes in bathymetry. Seguinot-Barbosa, 1986

According to wave refraction plots, high wave energy occurred when long period waves approached from south, southeast, southwest and short period waves approaching from east-southeast.

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Arroyo

Arroyo beach was examined from Punta Figuras to Punta Guilarte. Three main areas were defined based on net shoreline changes of the beach from 1972 to 1987. These are: These do not constitute significant shoreline changes because these stations are protected by a coral reef. Also, the partial losses and gains of the sand on the beach shifted between stations at different periods causing temporary sand storage between areas.

Severe erosion occurred at stations 1 and 2 on the west beach from 1972 to 1974. Sand was added to station 3 for the same period, as well as on stations located on the south of the beach (stations 4 and 5). There was a shift from erosion to accretion on stations 1 and 2 from 1974 to 1978, however, station 1 shifted back to erosion from 1978 to 1987 while station 3 eroded in this period.

The south beach experienced accretion in all stations from 1972 to 1974, causing the development of a sand bar at stations 4 and 5. Sand added to the beach was coming from sediment removed by natural erosion from the west stations. About 24 meters of sand were added to the beach during this period. Severe erosion occurred in the bar from 1974 to 1978. Very little sand (2 meters) was added to the bar from 1978 to 1987.

Accretion occurred at stations 6 and 7 on the east side of the beach from 1972 to 1974 (Figure 116); more than 10 meters of shoreline were added. Erosion occurred from stations 8 to 11 with major erosion rates at stations 10 and 11. A shift from erosion to accretion and viceversa occurred in all stations from 1974 to 1978. Shoreline changes were apparently caused by changes in wave direction approaching the beach, causing a different wave energy distribution.

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