Sedimentation Surveys in Puerto Rico

Science Center Objects

Reservoirs provide most of the water supply for Puerto Rico. Most of them were constructed in the early and middle 20th century and sediment accumulation has significantly reduced their original storage capacity.  The U.S. Geological Survey (USGS) has been conducting a series of bathymetric surveys using a global positioning system (GPS) and a geographic information system (GIS) technology on the principal reservoirs in Puerto Rico, and comparing the results with previous surveys to quantify storage capacity loss. 

Hydrologist conducting a sedimentation survey at Lago La Plata, Puerto Rico

Hydrologist conducting a sedimentation survey at Lago La Plata, Puerto Rico.

The humid tropical environment and mountainous terrane of Puerto Rico are conducive to high rates of sedimentation.  Washed from the hillslopes and construction sites, sediments settle out in the calm waters of the reservoirs, reducing the storage capacity and eventually ending their useful life.  Major floods associated with hurricanes and tropical disturbances cause extensive land erosion and sediment transport that rapidly deplete the storage capacity of reservoirs.

The USGS, in cooperation with the Government of Puerto Rico, has conducted sedimentation surveys since 1994 in 14 reservoirs used for domestic and industrial water supply, power generation, and irrigation.  A GPS, a survey-grade echo sounder, and GIS, were used to plan, collect, and analyze hydrographic data to determine the existing storage capacity, the sedimentation rates, and the sediment yield of the reservoir drainage areas.

Bathymetric surveys were conducted by collecting data of geographic location and water depths simultaneously, and storing the data in a portable personal computer.  The standard methodology consisted of navigating the reservoirs at pre-established cross sections spaced at 50 meters (or less as needed), starting at the reservoir dam and continuing upstream along the different branches of the reservoirs.  The collected data were then edited to eliminate spurious positions and depths, and adjusted to represent water depths at spillway or normal pool elevations.  A triangulated irregular network (TIN) of reservoir bottoms, contour maps, cross sections representing the reservoir bottom depths from shore to shore, and longitudinal sections along the thalweg of the different branches of the reservoirs were generated using GIS, to graphically show the location and amount of sediment deposition.

Sediment accumulation has substantially reduced the storage capacity of the principal water-supply reservoirs in Puerto Rico over time.  Their storage losses range from 12 to 81 percent of the reservoirs’ original storage capacities.  Some of the smaller reservoirs studied are near the end of their useful life.  The individual sedimentation trends of the various reservoirs vary with land use, topography, development, and principally, rainfall magnitudes and frequencies within the basins.

The capacity loss rates per area and basin sediment yields, increase with rainfall magnitudes and frequencies, and with human impacts.  Also, the drainage area extension and morphology influences sediment erosion processes and sediment transport rates within the basins.