U.S. Geological Survey 

U.S. Department of the Interior 

Prepared in cooperation with the 

PUERTO RICO ELECTRIC POWER AUTHORITY 

Sedimentation Survey of Lago Loco, 

Puerto Rico, March 2000 

Water-Resources Invest gat ons i i

Report 01-4187 
U.S. Department of the Interior 

U.S. Geological Survey 

Sedimentation Survey of Lago Loco, 

Puerto Rico, March 2000 

By Luis R. Soler-López 

Water-Resources Investigations Report 01-4187 

In cooperation with the 

PUERTO RICO ELECTRIC POWER AUTHORITY 

San Juan, Puerto Rico: 2002 
U.S. DEPARTMENT OF THE INTERIOR 

GALE A. NORTON, Secretary 

U.S. GEOLOGICAL SURVEY 

Charles G. Groat, Director 

Use of trade names in this report is for identification purposes only and does not imply endorsement by the 

U.S. Government. 

For additional information write to: Copies of this report can be purchased from: 

District Chief 

U.S. Geological Survey U.S. Geological Survey 

GSA Center, Suite 400-15 Branch of Information Services 

651 Federal Drive Box 25286 

Guaynabo, Puerto Rico 00965 Denver, CO 80225 
CONTENTS 

Abstract ................................................................................................................................................................ 1
 

Sumario ................................................................................................................................................................ 1
 

Introduction .......................................................................................................................................................... 2
 

Dam, Reservoir, and Drainage Basin Characteristics .......................................................................................... 2
 

Method of Survey................................................................................................................................................. 4
 

Field Techniques ............................................................................................................................................ 6
 

Data Processing ............................................................................................................................................. 6
 

Actual Capacity and Sediment Accumulation ..................................................................................................... 14
 

Trapping Efficiency.............................................................................................................................................. 16
 

Sediment Yield ..................................................................................................................................................... 18
 

References ............................................................................................................................................................ 19
 

PLATES 

[Plates are in pocket] 

1. Lago Loco, Puerto Rico, Bathymetry, March 2000 

2. Lago Loco, Puerto Rico, Bathymetry, August 1986 

Contents III 
FIGURES 

1.-5. Map showing 

1. Location of Lago Loco in the Río Guayanilla to Río Loco basins, Puerto Rico ....................................... 3
 

2.	 Planned cross-section locations for the March 2000 bathymetric survey of Lago Loco,
 

Puerto Rico ............................................................................................................................................. 5
 

3. Actual track lines of the March 2000 bathymetric survey of Lago Loco, Puerto Rico ............................. 7
 

4. Distance from dam along the thalweg of Lago Loco, Puerto Rico ............................................................ 8
 

5. Selected cross-section locations for the March 2000 bathymetric survey of Lago Loco, 
 

Puerto Rico ............................................................................................................................................. 9
 

6.-9. Graphs showing 

6.	 Selected cross sections generated from TIN surface model of Lago Loco, Puerto Rico, 
 

for August 1986 and March 2000 bathymetric surveys.......................................................................... 10
 

7.	 Longitudinal profiles along the thalweg of the Río Loco branch of Lago Loco, Puerto Rico,
 

based on August 1986 and March 2000 bathymetric surveys ................................................................ 13
 

8.	 Lake elevation-storage capacity curves for Lago Loco, Puerto Rico, derived from August 1986 
 

and March 2000 bathymetric surveys..................................................................................................... 15
 

9. Reservoir storage capacity to inflow relation established by Brune (1953)............................................... 17
 

TABLES 

1. Principal characteristics of Lago Loco and Loco dam, Puerto Rico .............................................................. 2
 

2. 	Comparison of the 1951, August 1986, and March 2000 sedimentation surveys of Lago Loco, 	 
 

Puerto Rico .................................................................................................................................. 14
 

3. Available storage capacity, as function of pool elevation, for Lago Loco, Puerto Rico, March 2000 ........... 14
 

IV Contents 
CONVERSION FACTORS, DATUMS, ACRONYMS. and TRANSLATIONS 

Multiply By To obtain 

 Length 

centimeter 0.03281 foot 

millimeter 0.03937 inch 

meter 3.281 foot 

kilometer 0.6214 mile 

 Area 

square meter 10.76 square foot 

square kilometer 0.3861 square mile 

square kilometer 247.1 acre 

 Volume 

cubic meter 35.31 cubic foot 

cubic meter 0.0008107 acre-foot 

million cubic meters 810.7 acre-foot 

 Volume per unit time (includes flow) 

cubic meter per second 35.31 cubic feet per second 

cubic meter per second 15,850 gallon per minute 

cubic meter per second 22.83 million gallons per day 

 Mass per area (includes sediment yield) 

megagram per square kilometer 2.855 ton per square mile 

Datums 

Horizontal Datum - Puerto Rico Datum, 1940 Adjustment 

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)- a 

geodetic datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called “Seal Level Datum of 1929”. 

Acronyms used in this report 

BLASS Bathymetric/Land Survey System 

DGPS Differential Global Positioning System 

GIS Geographic Information System 

GPS Global Positioning System 

PREPA Puerto Rico Electric Power Authority 

TIN Triangulated Irregular Network 

USGS U.S. Geological Survey 

Translations 

Lago Lake (in Puerto Rico, also reservoir) 

Río River 

Contents V 
Sedimentation Survey of Lago Loco, Puerto Rico,
 

March 2000 

By Luis R. Soler-López 

Abstract 

Lago Loco, a small reservoir property of 

the Puerto Rico Electric Power Authority and part 

of the Southwestern Puerto Rico Project, has lost 

64 percent of its original storage capacity. In 

1951, the original storage capacity was about 2.40 

million cubic meters, decreasing to 1.43 million 

cubic meters in 1986 and to 0.87 million cubic 

meters in March 2000. The storage loss or longterm sedimentation rate increased from 27,714 

cubic meters per year from the period of 1951 to 

1986 to 31,224 cubic meters per year for the 

period of 1951 to 2000. This represents a capacity 

loss of about 1.1 percent per year for the period of 

1951 to 1986 and 1.3 percent per year for 1951 to 

2000. The trapping efficiency of the reservoir was 

about 92 percent in 1951, decreasing to about 

87 percent in 1986, and to about 80 percent in 

March 2000. The sediment yield of the net sediment-contributing drainage area increased from 

1,504 megagrams per square kilometer per year 

between 1951 and 1986 to 1,774 megagrams per 

square kilometer per year between 1951 and 

2000, or about 18 percent. At the current sedimentation rate of the reservoir, the life expectancy 

of Lago Loco is about 28 more years or until the 

year 2028. 

Sumario 

Lago Loco, un pequeño embalse de la Autoridad 

de Energía Eléctrica de Puerto Rico y del Proyecto del 

Suroeste de Puerto Rico, ha perdido 64 por ciento de 

su capacidad original de almacenamiento. En 1951, la 

capacidad original de almacenamiento era de unos 

2.40 millones de metros cúbicos, que fue 

disminuyendo a 1.43 millones de metros cúbicos en 

1986, y a 0.87 millón de metros cúbicos en marzo de 

2000. La tasa de pérdida de almacenamiento o 

sedimentación a largo plazo aumentó de 27,714 

metros cúbicos anuales en el período de 1951 a 1986 a 

31,224 metros cúbicos anuales en el período de 1986 a 

2000. Esto representa una pérdida de capacidad de un 

1.1 por ciento anual para el período de 1951 a 1986, y 

un 1.3 por ciento anual para el período de 1951 a 2000. 

La eficiencia de este lago para atrapar sedimento era 

de un 92 por ciento en 1951, que fue disminuyendo a 

un 87 por ciento en 1986 y a un 80 por ciento en marzo 

de 2000. El rendimiento de sedimento del área neta de 

aportación de sedimento aumentó de 1,504 

megagramos por kilómetro cuadrado anuales entre 

1951 y 1986 a 1,774 megagramos por kilómetro 

cuadrado anuales entre 1986 y 2000, o un 18 por 

ciento. De mantenerse la tasa actual de sedimentación 

del embalse, la expectativa de vida de Lago Loco es de 

unos 28 años más o hasta el año 2028. 

Abstract 1 
INTRODUCTION 

The Puerto Rico Electric Power Authority 

(PREPA) operates the Southwestern Puerto Rico 

Project, which consists of a series of reservoirs 

connected by underground tunnels; the reservoir are 

used for hydroelectric power generation and irrigation 

of croplands. Among these reservoirs is Lago Loco, 

which originally provided about 2.40 million cubic 

meters of water storage capacity. Since construction in 

1951, the reservoir has been losing storage capacity 

because of sediment accumulation, reducing water 

resources available for irrigation. 

During March 2000, the U.S. Geological Survey 

(USGS) in cooperation with PREPA conducted a 

bathymetric survey of Lago Loco to determine the 

existing storage capacity of the reservoir, to assess the 

amount and location of sediment accumulation, and to 

estimate the sediment trapping efficiency of the 

reservoir and the sediment yield of the Lago Loco 

drainage area. The purpose of this report is to provide 

PREPA officials with the necessary information to 

manage effectively the water resources available in the 

reservoir and to develop strategies to mitigate the 

storage-capacity loss of Lago Loco. 

DAM, RESERVOIR, AND BASIN 

CHARACTERISTICS 

The construction of Lago Loco dam was 

completed in 1951. The dam is located on Río Loco in 

southwestern Puerto Rico, about 10 kilometers west of 

the town of Guayanilla, 4 kilometers west of the town 

of Yauco, and about 8 kilometers southeast of the town 

of Sabana Grande (fig. 1). The reservoir is the 

downstream unit of the Southwestern Puerto Rico 

Project and originally held about 2.40 million cubic 

meters of water. The power generation associated with 

the Southwestern Puerto Rico Project takes place at 

two power plants upstream of Lago Loco, and water 

stored in Lago Loco is used only for irrigation of 

croplands in the Lajas Valley farmlands (fig. 1). 

The dam is a concrete gravity structure with a 

total length of 182.90 meters, a height of 21.94 meters 

and a base width of about 18.0 meters (table 1). The 

valley occupied by the dam is characterized by a 

comparatively long, shallow left abutment, a river 

channel about 46 meters wide and a steep right 

abutment. The left abutment section has an elevation 

of 73.76 meters above mean sea level and the right 

section has an elevation of 74.98 meters above mean 

sea level. The dam has two non-overflow sections, the 

left one is 118.70 meters long and the right one is 

18.29 meters long. 

Table 1. Principal characteristics of Lago Loco and Loco dam, Puerto Rico (modified 

from Sheda and Legas, 1968) 

Total length of dam, in meters 182.90 

Maximum height of dam, in meters 21.94 

Maximum base width of dam, in meters 18.0 

Length of left non-overflow section, in meters 118.70 

Length of right non-overflow section, in meters 18.29 

Length of spillway section, in meters 45.72 

Elevation of spillway crest, in meters above mean sea level 70.10 

Crown elevation of irrigation intake structure, in meters above mean sea level 62.03 

Crown elevation of sluiceway structure, in meters above mean sea level 55.78 

Drainage area at damsite, in square kilometers 21.76 

Reservoir surface area in March 2000, in square kilometers 0 .29 

Maximum depth during the March 2000 bathymetric survey, in meters 9.0 

2 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
Figure 1. Location of Lago Loco in the Río Guayanilla to Río Loco basins, Puerto Rico. 

Dam, Reservoir, and Drainage Basin Characteristics 3 
An ungated overflow spillway section, 45.72 

meters long is located near the right abutment and has 

an elevation of 70.10 meters above mean sea level. 

The spillway was designed to pass a maximum flow of 

694 cubic meters per second between the nonoverflow sections. The intake structure for irrigation 

releases is located at the right abutment non-overflow 

section at a crown elevation of 62.03 meters above 

mean sea level (table 1). The sluiceway structure is 

located at the spillway section at a crown elevation of 

55.78 meters above mean sea level (Sheda and Legas, 

1968). 

The Lago Loco drainage area is located in the 

Caguabo-Mucara soil association (Gierbolini, 1975). 

The Caguabo soil series consists of shallow, welldrained soils that are slightly acid and moderately 

permeable. These soils formed in very gravelly 

residuum weathered from basic volcanic rocks. The 

slopes are high, ranging from 20 to 60 percent. The 

Mucara soil series consists of moderately deep, welldrained soils that are moderately permeable. These 

soils formed in residual material weathered from 

volcanic rock. The slopes are moderate to high, 

ranging from 5 to 60 percent and are covered mostly 

with brushy pasture and minor subsistence crops. 

METHOD OF SURVEY 

The bathymetric survey of Lago Loco involved 

planning, data collection, data processing, and data 

analyses. Arc/Info Geographic Information System 

(GIS) was used to plan the survey and to analyze the 

collected data. An electronic file depicting the 

reservoir shore line was loaded into a portable 

personal computer from the GIS, and served as the 

borderline for bathymetric data collection. Using the 

GIS cross sections were planned at a spacing of 50 

meters, starting at the dam and continuing upstream 

along the Río Loco branch of Lago Loco (fig. 2). 

Bathymetric data were collected during March 2000 

using a depth sounder coupled to a Differential Global 

Positioning System (DGPS) to control the horizontal 

position of the survey boat. The reservoir-pool 

elevation was monitored continuously at the Lago 

Loco lake-level station at damsite, near Yauco (USGS 

station 50128900, fig. 1). During the survey, the pool 

elevation of Lago Loco was below spillway elevation, 

so the collected depth data were adjusted to represent 

depths at spillway elevation by applying a timeelevation correction factor. 

The bottom-depth profile data were down 

loaded onto a GIS and contour lines of equal depth 

were drawn at 1-meter intervals. An additional 

0.5-meter contour was added to represent the 

shallowest part of the reservoir. These contour lines 

were then converted into a triangulated irregular 

network (TIN) to generate a reservoir-bottom surface 

model. From this model, the storage capacity and the 

amount and location of sediment accumulation was 

calculated using the GIS. In addition, a 1986 

bathymetric map of Lago Loco (F. Quiñones, USGS 

1986, unpublished report) was scanned, georeferenced and loaded into the GIS where a TIN 

reservoir bottom surface model was generated and 

the storage capacity of Lago Loco for 1986 was 

determined. By comparing the two bathymetric data 

layers, the amount and location of sediment 

accumulation and the sedimentation rate from 1986 

to 2000 were determined. 

Lake-elevation storage-capacity curves for 

both surveys were generated using the TIN reservoir 

surface models by calculating the reservoir volumes 

at 1-meter lake-elevation intervals. Selected cross 

sections representing the reservoir bottom from shore 

to shore, as well as longitudinal profiles of the 

reservoir bottom along the thalweg of the different 

branches of the reservoir were constructed from the 

August 1986 and March 2000 TIN bottom surface 

models. 

4 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
Figure 2. Planned cross-section locations for the March 2000 bathymetric survey of Lago Loco,

 Puerto Rico. 

Method of Survey 5 
Field Techniques 

The bathymetric survey of Lago Loco took 

place on March 10, 2000. Data were collected using 

the Bathymetric/Land Survey System (BLASS) 

developed by Specialty Devices, Inc. The system 

consists of two Novatel global positioning system 

(GPS) receivers coupled to a depth sounder (model 

SDI-IDS Intelligent Depth Sounder). The GPS 

receivers determine and help control the horizontal 

position of the survey boat, whereas the sounder 

collects water-depth data. The GPS units were first 

used in static mode to establish benchmarks at two 

sites overlooking the reservoir. Satellite 

information was recorded simultaneously at the 

Centro de Recaudación de Ingresos Municipales 

benchmark “Lajas 2” (lat. 18°04’26.3034”N., 

long. 66°57’34.4487”W.) and at two sites overlooking 

the reservoir, “Loco 1” (lat. 18°02’39.225”N., 

long. 66°53’18.099”W.) and “Loco 2” 

(lat. 18°02’39.114”N., long. 66°53’12.797”W.). 

Their geographic locations were calculated using the 

post-processing software CENTIPOINT. These new 

benchmarks indicated a horizontal error of less than 

10 centimeters. 

Once established, the benchmark with the best 

view of the survey area was occupied as the reference 

station. One GPS unit was installed at the reference 

station, while the other GPS unit was installed in the 

survey boat to be used as the mobile station. The GPS 

on board the survey boat independently calculates a 

position every second while receiving a set of 

correction signals from the reference station, 

converting the system into a DGPS. The DGPS 

information was used to maintain the survey boat 

within 2 meters of the true geographic location. The 

bathymetric-survey software HYPACK was used to 

navigate and to collect data. The software integrates 

the depth and position data, storing the x, y, 

(geographic location) and z (depth) coordinates in a 

portable personal computer. A total of 40 lines were 

planned using the GIS (fig. 2), however, low reservoir 

pool elevation and sediment accumulation in the 

upper, riverine portions of the reservoir limited the 

data collection to 21 lines (fig. 3). 

Data Processing 

Initial editing of the depth and positional data 

was performed within the HYPACK program. 

Erroneous positions were corrected by eliminating 

anomalous spikes or “positional jumps” that occur 

when the correction signal from the stationary DGPS 

station is lost for a period of time. The correction 

signal can be lost because of physical obstructions 

such as topographic features or because of 

electromagnetic interference. When this occurred, the 

erroneous positions were interpolated between the 

correct anterior and posterior positions. Depth data 

were also corrected to eliminate erroneous depth 

readings collected by the fathometer. Floating debris 

and/or bubbles can interfere with the fathometer’s 

transducer causing erroneous depth readings. Incorrect 

depth readings were interpolated between the correct 

anterior and posterior depth values. If the amount of 

incorrect positional and depth data was substantial, the 

cross section was re-measured. 

Once corrected, the edited data were then 

transferred into the GIS database, where the final data 

processing took place. The Arc/Info software was 

customized to color code the depth-data points 

according to different depth values. Data points with 

the same color were connected to generate a 

bathymetric-contour map of the reservoir bottom 

(plate 1). The 1986 contour map (plate 2) was 

previously delineated and the only adjustment 

performed was to assign real-world coordinates to 

the map. 

The contour maps (plates 1 and 2) were used to 

create the TIN reservoir-bottom surface models for 

August 1986 and March 2000, respectively. Distances 

from the dam upstream along the Río Loco branch of 

the reservoir were calculated using the GIS (fig. 4). 

The selected cross-section locations are shown in 

figure 5. 

Sampling the TINs every 5 meters along the 

selected cross sections generated a graph representing 

the reservoir bottom from shore to shore for August 

1986 and March 2000. The same procedure was 

employed to generate the longitudinal profile along 

the thalweg of the Río Loco branch of Lago Loco for 

August 1986 and March 2000. The selected cross 

sections (fig. 6) were located following a pattern that 

included all flooded areas of reservoir, whereas the 

longitudinal profile (fig. 7) was located at the deepest 

part of the reservoir thalweg. 

6 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
Figure 3. Actual track lines of the March 2000 bathymetric survey of Lago Loco, Puerto Rico. 

Method of Survey 7 
Figure 4. Distance from dam along the thalweg of Lago Loco, Puerto Rico. 

8 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
Figure 5. Selected cross-section locations for the March 2000 bathymetric survey of Lago Loco, Puerto Rico. 

Method of Survey 9
 
Figure 6. Selected cross sections generated from the TIN surface model of Lago Loco, Puerto Rico, for 

August 1986 and March 2000 bathymetric surveys. Refer to figure 5 for cross-section locations. Cross 

sections are oriented with the observer looking in the downstream direction. 

10 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
Figure 6. Selected cross sections generated from the TIN surface model of Lago Loco, Puerto Rico, for 

August 1986 and March 2000 bathymetric surveys. Refer to figure 5 for cross-section locations. Cross 

sections are oriented with the observer looking in the downstream direction—Continued. 

Method of Survey 11 
Figure 6. Selected cross sections generated from the TIN surface model of Lago Loco, Puerto Rico, for 

August 1986 and March 2000 bathymetric surveys. Refer to figure 5 for cross-section locations. Cross 

sections are oriented with the observer looking in the downstream direction—Continued. 

12 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
Method of Survey 13
 

Figure 7. Longitudinal profiles along the thalweg of the Río Loco branch of Lago Loco, Puerto Rico, based on August 1986 and 

March 2000 bathymetric surveys. 
---

ACTUAL CAPACITY AND SEDIMENT Table 3. Available storage capacity, as a function 

ACCUMULATION of pool elevation, for Lago Loco, Puerto Rico, 

March 2000 

The storage capacity of Lago Loco decreased [All elevations, in meters above mean sea level; all capacities, in from 2.40 million cubic meters in 1951 to 1.43 million million cubic meters]

cubic meters in August 1986, and to 0.87 million 

cubic meters in March 2000. This represents a storage 

loss of 0.97 million cubic meters by August 1986 and Pool elevation Storage capacity 

1.53 million cubic meters by March 2000, or a storage 70.10

loss of 40 and 64 percent, respectively. The long-term 

69.10 annual capacity loss or sedimentation rate of Lago 

Loco was 27,714 cubic meters in August 1986, 68.10

increasing to 31,224 cubic meters in March 2000, or 67.10 an average annual storage capacity loss of 1.1 and 

1.3 percent, respectively. Table 2 summarizes the 66.10

results of the 1951, August 1986, and March 2000 65.10

sedimentation surveys of Lago Loco. 64.10

The available storage capacity of Lago Loco is 

listed in table 3 at 1-meter elevation intervals and the 63.10

graphical relation between pool elevation and storage 62.10

capacity is shown in figure 8. 61.10

60.10

 871,458 

626,068 

460,198 

347,430 

252,700 

168,850 

 97,245 

 44,527 

 12,275 

441 

0 

Table 2. Comparison of the 1951, August 1986, and March 2000 sedimentation surveys of 

Lago Loco, Puerto Rico 

[---, undetermined] 

Year of survey 1951 1986 2000 

Total capacity, in million cubic meters 2.40 1.43 0.87 

Sediment accumulated, in million cubic meters 0 0.97 1.53 

Years since construction 0 35 49 

Storage loss, in percent 0 40 64 

Annual capacity loss, in percent 0 1.1 1.3 

Long-term sedimentation rate, in cubic meters per year 0 27,714 31,224 

Trapping efficiency, in percent1 92 87 80 

Sediment yield, in megagrams per square kilometer per year 1,504 1,774 

Year that the reservoir could be filled with sediment --­ 20512 20283 

1 Using the capacity-inflow ratio of Brune (1953).
 

2 Assuming that the reservoir will continue to fill at the sedimentation rate of 27,714 cubic meters per year.
 

3 Assuming that the reservoir will continue to fill at the sedimentation rate of 31,224 cubic meters per year.
 

14 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
Figure 8. Lake-elevation storage-capacity curves for Lago Loco, Puerto Rico, derived from 

August 1986 and March 2000 bathymetric surveys. 

Actual Capacity and Sediment Accumulation 15 
The selected cross sections presented in figure 6 

reveal substantial sediment accumulation in most 

areas of the reservoir, indicating sediment 

accumulation on the bottom as well as along the 

reservoir shore. These depositional patterns could 

result from large floods. Lago Loco is a small, shallow 

reservoir and hydraulic and geomorphic processes 

during major flood events, such as those generated by 

hurricanes or other tropical disturbances, cause the 

reservoir to behave like a natural river channel. 

Reservoir areas having high water velocities are sites 

of erosion along the outside of meanders, and areas 

having low water velocities are sites of deposition 

along the inside of meanders. Sediment deposited in 

Lago Loco since 1986 could have scoured in high 

velocity areas and accumulated in the low velocity 

areas, re-arranging the morphology of the reservoir 

bottom. 

Sediment accumulation in the vicinity of the 

sluiceway structure at the dam has reached an 

elevation of about 61.10 meters above mean sea level 

according to the March 2000 data. The crown 

elevation of the sluiceway structure is 55.78 meters 

above mean sea level; as a result, the sluiceway 

structure is buried under a layer of about 5 meters of 

sediments. The irrigation intake structure, at an 

elevation of 62.03 meters above mean sea level, is in 

danger of being disabled because the reservoir bottom 

is less than a meter (0.93 meter) below the upper part 

of the intake. The intake soon will become useless and 

if not operated on a regular basis, and even with 

regular operation, the intake is at risk of being 

overwhelmed with sediment. 

Dead storage is the volume below the crown 

elevation of an intake structure; the intake is some 

distance above the bottom of the reservoir used to 

accommodate infilling sediments without disabling 

the structure. The dead storage capacity of Lago Loco 

has been substantially reduced from 62,091 cubic 

meters in August 1986 to 11,065 cubic meters in 

March 2000. This represents a reduction of about 

82 percent in dead storage between August 1986 and 

March 2000. 

TRAPPING EFFICIENCY 

Heinemann (1981) suggested that the single 

most informative descriptor of a reservoir is its 

trapping efficiency. This value is the proportion of the 

incoming sediment that is deposited or trapped in a 

pond, reservoir, or lake. Trapping efficiency is 

dependent on several factors, including particle size 

distribution and water residence time (for example, the 

average time that the incoming runoff remains in the 

reservoir). The trapping efficiency is controlled by the 

characteristics of the inflow hydrograph, particularly 

with regard to high- and base-flow volumes and 

frequency characteristics. The reservoir shape and 

outlet structures also influence the water retention 

time, water current velocities in the reservoir, and 

hence the trapping efficiency (Verstraeten and Poesen, 

2000). 

There have been many empirical studies of the 

relation between reservoir storage capacity, water 

inflow, and trapping efficiency of which Brune’s 

(1953) is the most widely used and accepted. Brune 

(1953) developed a curve (fig. 9) that basically 

estimates the trapping efficiency of a reservoir based 

on the relation of storage capacity to annual inflow. 

The trapping efficiency of Lago Loco was estimated 

using this relation. 

The Lago Loco drainage area was increased 

artificially when four other reservoirs (Lago Guayo, 

Lago Prieto, Lago Toro, and Lago Yahuecas) were 

connected by underground tunnels to Lago Lucchetti, 

a reservoir that supplies water to Lago Loco. These 

reservoirs are part of the Southwestern Puerto Rico 

Project and divert flow from the mountainous portions 

of the project area into Lago Lucchetti, providing 

additional water for hydroelectric power generation at 

power plant no. 1. Water inflow from these reservoirs 

into Lago Lucchetti is subsequently released via the 

power outlet, into power plant no. 2 for power 

generation, and subsequently into Lago Loco. 

Regardless of water diversion into Lago Loco, the 

amount of water released yearly from Lago Loco into 

the irrigation canal is greater than the yearly water 

influx from Lago Lucchetti. Rarely does water flow 

16 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
over the spillway of Lago Loco and the reservoir-pool 

elevation remains low as a result of the outflow to 

accommodate irrigation demands and for flood 

control. Thus, inflow from Lago Lucchetti is 

considered to be relatively unimportant in estimating 

water and sediment inflow to Lago Loco, and 

estimates here are restricted to runoff from the natural 

drainage basin. 

Lago Loco has no stream gaging station to 

measure the inflow entering from the Río Loco. Thus, 

the neighboring basin of Río Guayanilla was used to 

estimate the average annual runoff of the Lago Loco 

drainage area. The hydrologic characteristics of these 

two basins are somewhat similar in topography and 

land use, and so the runoff per unit area resulting from 

rainfall is considered to be similar per unit surface 

area. 

The runoff/rainfall ratio of the Río Guayanilla is 

0.38 (Giusti and López, 1967). The Lago Loco 

drainage area has an average annual rainfall of 1,651 

millimeters (Calvesbert, 1970). Multiplying the 

average annual rainfall by the runoff/rainfall ratio of 

0.38 gives the estimated runoff of 0.63 meters per year 

for the Lago Loco basin. Multiplying this value by the 

Lago Loco drainage area gives an estimated annual 

inflow value of 13.71 million cubic meters. Using the 

median curve of Brune (fig. 9) the estimated trapping 

efficiency of Lago Loco is about 92, 87, and 80 

percent for 1951, 1986, and 2000, respectively. This 

gives an average long-term trapping efficiency of 90 

percent for August 1986, and 86 percent for March 

2000. 

Figure 9. Reservoir storage capacity to inflow relation established by Brune (1953). 

Trapping Efficiency 17 
SEDIMENT YIELD 

Sediment yield has been defined by the 

American Society of Civil Engineers as the total 

sediment outflow from a catchment or drainage basin, 

measurable at a point of reference and a specified 

period of time per unit of surface area (McManus and 

Duck, 1993). For Lago Loco, the total amount of 

sediment that has entered the reservoir (1.53 million 

cubic meters) was divided by the average long-term 

trapping efficiency (0.86), giving a reservoir sedimentaccumulation volume of 1.78 million cubic meters. To 

determine the rate of sediment influx, 1.78 million 

cubic meters was divided by the age of the reservoir 

(49 years), yielding 36,326 cubic meters of sediment 

per year that enters the reservoir. The sediment yield 

of the Lago Loco drainage basin can be calculated by 

dividing 36,326 cubic meters of sediment per year by 

the net sediment-contributing area of 20.47 square 

kilometers (the total drainage area minus the reservoir 

surface area), which gives 1,774 cubic meters per 

square kilometer per year. Assuming a sediment drybulk density of one gram per cubic centimeter, the 

estimated sediment yield of the Lago Loco drainage 

area is about 1,774 megagrams per square kilometer 

per year for March 2000. Using the same calculations, 

the sediment yield of Lago Loco (using the average 

long-term trapping efficiency of 90 percent for August 

1986) was 1,504 megagrams per square kilometer per 

year, which represents an increase of about 18 percent. 

The increase in sediment yield between 1986 

and 2000 was likely caused by land-use practices 

within the basin, and further accelerated sediment 

erosion and influx to the reservoir caused by 

Hurricanes Hortense (1996) and Georges (1998), 

which have been shown to have caused greatly 

accelerated sediment influx to other reservoirs in 

Puerto Rico (Soler-López, 2000 and 2001). 

Floods associated with these hurricanes not only 

transported eroded material deposited in past flood 

events, but also temporarily stored sediment in the 

river channel. Eventual transport of the temporarily 

stored material will reduce the storage capacity of the 

reservoir further. 

The August 1986 and March 2000 surveys show 

that the life expectancy of Lago Loco is a critical 

concern (table 2). Storage capacity is being reduced by 

extreme flood events. The March 2000 survey 

indicates that large storm events such as Hurricanes 

Hortense and Georges can substantially reduce the life 

expectancy of the reservoir. These storm events induce 

erosion and transport large volumes of material that is 

naturally weathered or made available by human 

activities such as farming and construction. 

Although these flood events do not regularly 

occur, such events will likely effect Puerto Rico in the 

future. If the sedimentation rate recorded in March 

2000 continues, Lago Loco has a useful life of about 

28 more years. However, the actual life expectancy of 

Lago Loco probably ranges between the August 1986 

(51 years) and the March 2000 (28 years) estimates, 

depending on the frequency of intense rainfall events. 

18 Sedimentation Survey of Lago Loco, Puerto Rico, March 2000 
REFERENCES 

Brune, G.M., 1953, Trap efficiency of reservoirs: 

Transactions of the American Geophysical Union, 

v. 34, no. 3, p. 407-418. 

Calvesbert, R.J., 1970, Climate of Puerto Rico and the 

U.S. Virgin Islands: U.S. Department of 

Commerce, Environmental Science Services 

Administration, 29 p. 

Gierbolini, R.E., 1975, Soil survey of Mayagüez area 

of western Puerto Rico, U.S. Department of 

Agriculture, Soil Conservation Service, p. 38-59, 

1 pl. 

Giusti, E.V., and López, M.A., 1967, Climate and 

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Heinemann, H.G., 1981, New sediment trap efficiency 

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McManus, J., and Duck, R.W., eds., 1993, 

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reservoirs; Chapter 6 of Reservoir sedimentation 

rates in the Southern Pennine Region, UK: 

London John Wiley & Sons, p. 73-92. 

Sheda, H.E., and Legas, James, 1968, Condition of 

Loco Dam, Puerto Rico, in Condition of concrete 

dams in Puerto Rico: U.S. Department of the 

Interior, Bureau of Reclamation, section 10, 15 p., 

6 pls. 

Soler-López, L.R., 2000, Sedimentation survey of 

Lago Dos Bocas, Puerto Rico, October 1999: 

U. S. Geological Survey Water-Resources 

Investigations Report 00-4234 19 p. 1 pl. 

Soler-López, L.R., 2001, Sedimentation survey of 

Lago Caonillas, Puerto Rico, February 2000: 

U. S. Geological Survey Water-Resources 

Investigations Report 01-4043, 25 p. 1 pl. 

Verstraeten, G., and Poesen, J., 2000, Estimated trap 

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yield: Progress in Physical Geography, v. 24, 

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References 19 
District Chief 

Caribbean District 

U.S. Geological Survey 

Water Resources Division 

GSA Center, Suite 400-15 

651 Federal Drive 

Guaynabo, Puerto Rico 00965-5703 

Soler-López—SEDIMENTATION SURVEY OF LAGO LOCO, PUERTO RICO, MARCH 2000—WRIR 01–4187