U.S. Department of the Interior 

U.S. Geological Survey 

Sedimentation Survey of Lago El Guineo, 

Puerto Rico, October 2001 

By Luis R. Soler-López 

Water-Resources Investigations Report 03-4093 

In cooperation with the 

PUERTO RICO ELECTRIC POWER AUTHORITY 

San Juan, Puerto Rico: 2003 
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


Introduction .......................................................................................................................................................... 1


Dam, Reservoir, and Basin Characteristics .......................................................................................................... 1


Method of Survey................................................................................................................................................. 3


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


Data Processing ............................................................................................................................................. 9


Storage Capacity and Sediment Accumulation.................................................................................................... 9


Trapping Efficiency.............................................................................................................................................. 18


Sediment Yield ..................................................................................................................................................... 20


References ............................................................................................................................................................ 21


PLATES 

[Plates are in pocket] 

1. Lago El Guineo, Puerto Rico, Bathymetry, October 2001 

2. Lago El Guineo, Puerto Rico, Bathymetry, July 1986 

Contents III 
FIGURES 

1.-7. Map showing 

1. Location of Lago El Guineo in the Río Grande de Manatí Basin, Puerto Rico ........................................ 2


2.	 Planned cross-section locations for the October 2001 sedimentation survey of Lago El Guineo, 


Puerto Rico ............................................................................................................................................. 4


3. Longitudinal distance from the dam, along the thalweg of Lago El Guineo, Puerto Rico ....................... 5


4.	 Comparison between the 1960 USGS Jayuya quadrangle shoreline, the 1:5,000 USGS 


digital orthophoto quadrangle data and the photo-rectified shoreline of Lago El Guineo, 


Puerto Rico ............................................................................................................................................. 7


5.	 Actual track lines where bathymetric data were collected for the October 2001 sedimentation 


survey of Lago El Guineo, Puerto Rico.................................................................................................. 8


6. Lago El Guineo color-coded depth values and contour lines generated using Arcplot ............................ 10


7. Lago El Guineo triangulated irregular network (TIN) surface model generated using Arc/Info.............. 11


8.-11. Graph showing


8.	 Selected cross sections generated from the triangulated irregular network (TIN) surface model 


of Lago El Guineo, Puerto Rico, for July 1986 and October 2001 ........................................................ 12


9.	 Longitudinal profiles along the thalweg of Lago El Guineo, Puerto Rico, for July 1986 

and October 2001.................................................................................................................................... 15 

10.	 Relation between water-storage capacity and pool elevation of Lago El Guineo, Puerto Rico, 


for July 1986 and October 2001 ............................................................................................................. 17


11.	 Reservoir trapping efficiency as a function of the ratio between storage capacity and annual 


water inflow volume ............................................................................................................................... 19


TABLES 

1. Principal characteristics of Lago El Guineo and Guineo dam, Puerto Rico.................................................... 3


2. Comparison of the 1931, July 1986, and October 2001 sedimentation surveys of Lago El Guineo, 


Puerto Rico .................................................................................................................................. 16


3. Storage capacity of Lago El Guineo, Puerto Rico, October 2001 ................................................................... 16


IV Contents 
CONVERSION FACTORS, DATUMS, ACRONYMS, and TRANSLATIONS 

Multiply By To obtain

centimeter 

millimeter 

meter 

kilometer 

square meter 

square kilometer 

square kilometer 

cubic meter 

cubic meter 

million cubic meters 

cubic meter per second 

cubic meter per second 

cubic meter per second 

megagram per square kilometer 

 Length 

0.03281 

0.03937 

3.281 

0.6214 

 Area 

10.76 

0.3861 

247.1 

 Volume 

35.31 

0.0008107 

810.7 

 Volume per unit time (includes flow) 

35.31 

15,850 

22.83 

 Mass per area (includes sediment yield) 

2.855 ton per square mile 

foot 

inch 

foot 

mile

square foot 

square mile 

acre

cubic foot 

acre-foot 

acre-foot

cubic feet per second 

gallon per minute 

million gallons per day

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 

DOQ Digital Orthophoto Quadrangle 

GIS Geographic Information System 

PREPA Puerto Rico Energy Power Authority 

TIN Triangulated Irregular Network 

USGS U.S. Geological Survey 

Translations 

Spanish English 

Lago Lake (in Puerto Rico, also reservoir) 

Río River 

Contents V 
Sedimentation Survey of El Lago Guineo, 

Puerto Rico, October 2001 

By Luis R. Soler-López 

Abstract 

Lago El Guineo has lost about 17.5 percent 

of its original storage capacity in 70 years because 

of sediment accumulation. The water volume has 

been reduced from 2.29 million cubic meters in 

1931, to 2.03 million cubic meters in 1986, and to 

1.89 million cubic meters in 2001. The average 

annual storage-capacity loss (equal to the 

sedimentation rate) of Lago El Guineo was 4,727 

cubic meters for the period of 1931 to July 1986 

(or 0.21 percent per year), increasing to 5,714 

cubic meters for the period of 1931 to October 

2001 (or 0.25 percent per year). Discrepancies 

that could lead to substantial errors in volume 

calculations in a small reservoir like Lago El 

Guineo, were found when transferring the fieldcollected data into the geographic information 

system data base 1:20,000 U.S. Geological 

Survey Jayuya, Puerto Rico quadrangle. After 

verification and validation of field data, the Lago 

El Guineo shoreline was rectified using digital 

aerial photographs and differential global 

positioning data. 

INTRODUCTION 

Lago El Guineo, located in the Río Grande de 

Manatí Basin in the municipio of Villalba in central 

Puerto Rico (fig. 1), is owned by the Puerto Rico 

Electric Power Authority (PREPA). The reservoir was 

originally designed to provide 2.29 million cubic 

meters of water for irrigation of croplands in southern 

Puerto Rico and for hydroelectric power generation. 

Since completion in 1931, the reservoir has been 

losing water storage capacity because of sediment 

accumulation. Although agricultural practices within 

the reservoir basin are limited, the high average annual 

rainfall of 2,540 millimeters (Calvesbert, 1970) and 

steep slopes promote erosion and transport of 

sediment into Lago El Guineo. 

The U.S. Geological Survey (USGS), in 

cooperation with PREPA, conducted a bathymetric 

survey of Lago El Guineo in October 2001 to assess 

the extent of sediment accumulation in the reservoir. 

Data on geographic position and water depths were 

acquired using a differential global positioning system 

(DGPS) interfaced to a depth sounder. The fieldcollected data were then loaded into a geographic 

information system (GIS), which was used to 

determine the existing water storage capacity, 

sedimentation rates, and sediment distribution, and to 

predict the useful life of the reservoir. This report 

provides PREPA with the necessary information to 

effectively manage the available water resources and 

to develop strategies to mitigate the storage-capacity 

loss in Lago El Guineo. 

DAM, RESERVOIR, AND BASIN 

CHARACTERISTICS 

The Lago Guineo dam construction started in 

1928 and was completed in 1931. The dam is located 

about 4.8 kilometers north of the town of Villalba in 

central Puerto Rico on the Río Toro Negro. The Lago 

Guineo dam is a rockfill embankment 172.2 meters in 

length with a vertical concrete corewall continuous 

from abutment to abutment. The height of the dam at 

the top of the corewall is 906.78 meters above mean 

sea level. The dam is part of the Toro Negro 

Hydroelectric System, which was developed for 

electric generation and for irrigation of croplands in 

the southern part of Puerto Rico. 

Dam, Reservoir, and Drainage Basin Characteristics 1 
 Figure 1. Location of Lago El Guineo in the Río Grande de Manatí Basin, Puerto Rico. 

2 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001
An uncontrolled morning glory-type spillway is 

located at the left abutment of the dam at an elevation 

of 902.21 meters above mean sea level. The spillway 

has a design discharge capacity of 198.24 cubic meters 

per second at a maximum reservoir water-surface 

elevation of 904.13 meters above mean sea level. The 

power outlet is located in a former stream diversion 

tunnel through the right abutment of the dam. The 

principal components of the outlet works are an 

entrance trashrack at the upstream end of the 

horseshoe conduit leading from the upstream toe of 

the dam to a gate tower located at the diversion tunnel 

portal, a horseshoe tunnel from the gate tower to the 

corewall, and a horseshoe tunnel from the corewall to 

the outlet portal. The gate tower consists of a double 

shaft structure. Two slide gates admit flow into the 

downstream shaft of the gate tower, which eventually 

flow out of the dam (Puerto Rico Electric Power 

Authority, 1988). Additional information about the 

dam structures and the reservoir is listed in table 1. 

Lago Guineo Basin is located in the HumatasLos Guineos-Alonso soil series association. This 

association, found mainly on the slopes and ridgetops 

in the mountainous north-central part of Puerto Rico, 

ranges from moderate to well drained clayey soils. The 

Humatas soils make up about 55 percent of the 

association, the Los Guineos soils make up about 25 

percent, and the Alonso soils make about 20 percent 

(Acevido, 1982). In general, these types of clayey soils 

tend to be more plastic, cohesive, and are more 

difficult to erode than non-compacted sandy-type 

soils. 

METHOD OF SURVEY 

The bathymetric survey of Lago El Guineo 

involved planning, data collection, data processing, 

and analysis. An Arc/Info GIS was used to plan the 

survey and to analyze the collected data. Cross 

sections were planned at a spacing of 50 meters, 

starting at the dam and continuing upstream along the 

different branches of Lago El Guineo (fig. 2). The 

longitudinal distance along the thalweg of Lago El 

Guineo is shown in figure 3. 

Table 1. Principal characteristics of Lago El Guineo and Guineo dam, Puerto Rico 

(modified from the Puerto Rico Electric Power Authority, 1988) 

[GIS, Geographic Information System] 

Drainage area at dam, in square kilometers 4.07 

Drainage area at dam from GIS calculations, in square kilometers 4.27 

Reservoir surface area, in square kilometers 0.19 

Discharge capacity at flood elevation of 904.13 meters above mean sea level, in 198.24 

cubic meters per second 

Elevation of top of corewall, in meters above mean sea level 906.78 

Elevation of top of dam, in meters above mean sea level 905.56 

Elevation of spillway crest, in meters above mean sea level 902.21 

Elevation of invert power tunnel intake, in meters above mean sea level 874.17 

Elevation of top of power tunnel intake, in meters above mean sea level 876.30 

Original storage capacity, in million cubic meters 2.29 

Method of Survey 3 
Figure 2. Planned cross-section locations for the October 2001 sedimentation survey of 

Lago El Guineo, Puerto Rico. Shoreline from the 1960 U.S. Geological Survey, Jayuya 

topographic map quadrangle. 

4 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 3. Longitudinal distance from the dam, along the thalweg of Lago El Guineo, 

Puerto Rico. Shoreline from the 1960 U.S. Geological Survey, Jayuya topographic map 

quadrangle. 

Method of Survey 5 
A digital map of the Lago El Guineo shoreline, 

extracted from the 1960 USGS Jayuya quadrangle, 

Puerto Rico, was loaded into a portable computer to 

serve as guideline for the reservoir navigation and 

field data collection. However, some inconsistencies 

were found between the field data and the GISgenerated shoreline map. The DGPS field data did not 

conform adequately with the shoreline extracted from 

the 1:20,000 USGS topographic map of Jayuya, 

Puerto Rico, suggesting that one of the two datasets 

was inaccurate. To verify if the field data were 

accurate, an aerial 1:5,000 digital orthophoto 

quadrangle (DOQ) of the reservoir shoreline was 

overlaid with the GIS topographic map shoreline and 

with the DGPS data. The DOQ and DGPS data 

matched appropriately (fig. 4). However, the 

topographic map did not match with the other two sets, 

indicating that the 1:20,000 map did not have the same 

accuracy as the DGPS and the DOQ data. 

After eliminating the uncertainty of the field 

data, the Lago El Guineo shoreline was rectified using 

the DOQ and the DGPS data, which have a horizontal 

positional accuracy of less than 2 meters. The use of 

an inaccurate reservoir shoreline for contouring and 

volume calculations can yield substantial errors, 

particularly in a small reservoir like Lago El Guineo. 

Bathymetric data were collected using a depth 

sounder coupled to a DGPS, to control the horizontal 

position of the survey boat. The reservoir pool 

elevation was continuously monitored at the USGS 

lake-level station 50032290, Lago El Guineo at 

Damsite near Villalba (fig. 1). The pool elevation of 

Lago El Guineo was lower than the crest of the 

spillway during the survey; therefore, the collected 

depth data were adjusted to the spillway elevation by 

applying a time-elevation correction factor. 

The adjusted depths along the cross sections 

were plotted in the rectified shoreline, and contour 

lines of equal depths were drawn manually at 1-meter 

intervals from the shoreline to the deepest parts of the 

reservoir. The procedure used to contour the reservoir 

bottom is explained later in this report. These contour 

lines were then converted into a triangulated irregular 

network (TIN) to create a model of the reservoir 

bottom. In addition, a bathymetric map from a July 

1986 sedimentation survey was analyzed and 

compared with the current survey (October 2001) to 

establish historical sedimentation trends. 

Field Techniques 

The bathymetric survey of Lago El Guineo was 

conducted on October 16, 2001. Data were collected 

using the bathymetric/land survey system (BLASS) 

developed by Specialty Devices, Inc. The system 

consists of two Novatel global positioning systems 

(GPS) interfaced to a digital depth sounder model 

SDI-IDS intelligent. The GPS receivers monitor the 

horizontal position of the survey boat while the depth 

sounder collects water-depth data. The GPS units were 

used first in static mode to establish a benchmark at a 

site overlooking the reservoir. Satellite data were 

recorded simultaneously at the known benchmark, the 

USGS District office roof (latitude 18°25’N., 

longitude 66°06’W), and at the new benchmark, 

“Guineo dam” (latitude 18°09’N., longitude 

66°31’W). The new benchmark coordinates were 

calculated using the GPS post-processing software 

CENTIPOINT. This new benchmark indicated a 

horizontal error of less than 10 centimeters. One GPS 

unit was deployed at the “Guineo dam” benchmark to 

serve as the reference station, while the other unit was 

mounted on the survey boat to serve as the mobile 

station. The GPS unit in the survey boat was used to 

independently calculate a position every second while 

it received a set of radio signal corrections from the 

reference unit, which converted the system into a 

DGPS maintaining the horizontal precision of the boat 

within 2 meters. The bathymetric survey software 

HYPACK was used to navigate and collect data. The 

software integrates the depth and position data, storing 

the x, y (geographic locations), and z (depth) 

coordinates in a portable personal computer. 

A total of 35 cross sections were planned using 

the GIS. However, vegetation growth inside the 

reservoir at a shallow area in the northern reach 

impeded navigation and limited data collection to 33 

transects, for a total of 3,221 data points over the 

entire reservoir (fig. 5). 

6 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 4. Comparison between the 1960 U.S. Geological Survey (USGS) Jayuya 

quadrangle shoreline, the 1:5,000 USGS digital orthophoto quadrangle data, and the 

photo-rectified shoreline of Lago El Guineo, Puerto Rico. 

Method of Survey 7 
Figure 5. Actual track lines where bathymetric data were collected for the October 2001 

sedimentation survey of Lago El Guineo, Puerto Rico. Shoreline from the 1:5,000 U.S. 

Geological Survey digital orthophoto quadrangle. 

8 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Data Processing 

Initial editing to verify the position and depth 

data was performed using the HYPACK software. 

Positions were corrected to eliminate anomalies that 

occur when the correction signal from the reference 

station is lost because of local topographic features or 

electromagnetic interference. Position errors were 

corrected by interpolating the correct preceding and 

following positions. The depth readings also were 

corrected to eliminate incorrect depth readings. 

Incorrect depth readings can result from insufficient 

signal gain or because floating debris interfere with 

the transducer face. The incorrect depth readings were 

corrected by interpolating the correct preceding and 

following depth readings. Once corrected, the edited 

data were then transferred into the GIS database. The 

Arc/Info software was customized by generating a 

look-up table, a set of directives that color-coded the 

depth values according to different depth ranges 

(fig. 6). It should be noted that figure 6 is provided for 

illustrative purposes only, and so the color values are 

not defined in a legend. Data points of the same color 

were then connected by a line between them, and a 

contour map of the reservoir bottom was generated 

(plate 1). Because contours from the July 1986 

bathymetric survey were already delineated (plate 2), 

the only adjustments made were to digitize and georeference the map (assign real-world coordinates). 

The bathymetric contour maps (plates 1 and 2) 

were used to create the TIN surface models of the 

reservoir bottom for July 1986 and October 2001, 

respectively. The TIN represents the reservoir bottom 

as thousands of adjoining triangles with x, y, and z 

coordinates assigned to each vertices (Environmental 

System Research Institute, 1992). An example of the 

October 2001 TIN surface model of Lago El Guineo is 

shown in figure 7. 

Selected cross sections representing flooded 

areas of the reservoir bottom for each survey also were 

generated from the TINs. The TINs were sampled 

every 5 meters along these cross sections to generate 

profiles representing the reservoir bottom from shore 

to shore for July 1986 and October 2001 (fig. 8). 

Longitudinal profiles along the thalweg of Lago El 

Guineo for July 1986 and October 2001 are shown in 

figure 9. These longitudinal profiles represent the 

deepest part of the reservoir bottom, along the central 

portion for each survey. 

STORAGE CAPACITY AND SEDIMENT 

ACCUMULATION 

The storage capacity of Lago El Guineo has 

decreased from 2.29 million cubic meters in 1931 to 

2.03 million cubic meters in July 1986, and to 1.89 

million cubic meters in October 2001 (table 2). This 

represents a total storage loss of 260,000 cubic meters 

by July 1986 and 400,000 cubic meters by October 

2001, or a loss of 11 and 17.5 percent, respectively. 

The average annual storage-capacity loss (equal to the 

sedimentation rate) of Lago El Guineo was 4,727 

cubic meters for the period of 1931 to July 1986 (or 

0.21 percent per year), increasing to 5,714 cubic 

meters for the period of 1931 to October 2001 (or 0.25 

percent per year). 

The storage capacity of Lago El Guineo as a 

function of pool elevation is listed at 1-meter intervals 

in table 3. The graphical relation between pool 

elevation and storage capacity is shown in figure 10. A 

comparison between the July 1986 and October 2001 

selected cross sections to analyze sediment 

accumulation and distribution within the reservoir 

bottom is not adequate for most of the cross sections 

presented in figure 8, because of the difference 

between the two sets of shorelines and contour 

locations. For example, a comparison of cross section 

number 10 (fig. 8) shows what appears to be a 

different orientation in the plotting origin of the cross 

sections. However, if the 1986 cross section is mirrorimaged, the resulting cross section aligns fairly well 

with the 2001 DGPS-collected data and cross section. 

This misalignment may have happened when 

manually plotting the depth data in the 1986 

bathymetric map (plate 2), since the difference is also 

noted when comparing plates 1 and 2. Although the 

1986 shoreline and contours could be adjusted to fit 

the rectified October 2001 shoreline, it would create 

inconsistencies in the contour data, which would result 

in discrepancies. To properly adjust the 1986 dataset, 

the raw field data would be required, which are not 

available. 

Method of Survey 9 
Figure 6. Lago El Guineo color-coded depth values and contour lines generated using 

Arcplot. The figure is for illustrative purposes only. 

10 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 7. Lago El Guineo triangulated irregular network (TIN) surface model generated 

using Arc/Info. 

Method of Survey 11 
Figure 8. Selected cross sections generated from the triangulated irregular network (TIN) 

surface model of Lago El Guineo, Puerto Rico, for July 1986 and October 2001. Refer to 

figure 2 for cross-section locations. 

12 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 8. Selected cross sections generated from the triangulated irregular network (TIN) surface 

model of Lago El Guineo, Puerto Rico, for July 1986 and October 2001. Refer to figure 2 for crosssection locations—Continued. 

Method of Survey 13 
Figure 8. Selected cross sections generated from the triangulated irregular network (TIN) 

surface model of Lago El Guineo, Puerto Rico, for July 1986 and October 2001. Refer to 

figure 2 for cross-section locations—Continued. 

14 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 8. Selected cross sections generated from the triangulated irregular network (TIN) 

surface model of Lago El Guineo, Puerto Rico, for July 1986 and October 2001. Refer to 

figure 2 for cross-section locations—Continued. 

14 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 9. Longitudinal profiles along the thalweg of the Lago El Guineo, Puerto Rico, for July 1986 and October 2001. 

Method of Survey 15

--

--

Table 2. Comparison of the 1931, July 1986, and October 2001 sedimentation surveys of Lago 

El Guineo, Puerto Rico 

[--, undetermined] 

Year of survey 1931 1986 2001 

Available storage capacity, in million cubic meters 2.29 2.03 1.89 

Sediment accumulated, in million cubic meters 0 0.26 0.40 

Years since construction 0 55 70 

Storage loss, in percent 0 11 17.5 

Annual storage capacity loss, in percent 0 0.21 0.25 

Long-term sedimentation rate, in cubic meters per year 0 4,727 5,714 

Trapping efficiency, in percent1 97 96 95 

Drainage basin sediment yield, in megagrams per square kilometer per year2 1,159 1,471 

Year that the reservoir would fill with sediment 2415 23323 

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


2 Assuming a sediment dry bulk density of 1 gram per cubic centimeter.


3

 Assuming that the reservoir would continue to fill at the 2001 calculated long-term sedimentation rate. 


Table 3. Storage capacity of Lago El Guineo, Puerto Rico, 

October 2001 

[All elevations in meters above mean sea level, all capacities in million cubic meters] 

Pool elevation Storage capacity 

902.21 1.89 

901.21 1.70 

900.21 1.53 

899.21 1.36 

898.21 1.20 

897.21 1.06 

896.21 0.91 

895.21 0.78 

894.21 0.66 

893.21 0.56 

892.21 0.46 

891.21 0.38 

890.21 0.31 

889.21 0.25 

888.21 0.19 

887.21 0.14 

886.21 0.10 

885.21 0.07 

884.21 0.04 

883.21 0.02 

882.21 0.10 

881.21 0.003 

880.21 0.00 

16 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 10. Relation between water-storage capacity and pool elevation 

of Lago El Guineo, Puerto Rico, for July 1986 and October 2001. 

Method of Survey 17

Although these discrepancies impeded the 

comparison of these two cross-section datasets, the 

overall calculations of volume for the July 1986 

survey are considered to be adequate because no 

anomalies are evident when comparing the historical 

sedimentation trends of Lago El Guineo (table 2). The 

increase in sedimentation rate and annual storage loss 

from 1986 to 2001 is probably the result of large 

sediment loads associated with Hurricanes Hortense 

and Georges in 1996 and 1998, respectively, as these 

storms severely impacted other upland reservoirs in 

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

The longitudinal profiles of Lago El Guineo for 

July 1986 and October 2001 are more appropriate than 

cross-section data for the analysis of the extent of 

sediment accumulation in the reservoir bottom 

because the profiles are more easily overlaid, and the 

discrepancies between the two sets of shorelines are 

negligible using the side-view perspective. From the 

profiles presented in figure 9, the rise on the reservoir 

bottom caused by sediment accumulation is evident. 

There are three distinct areas where sediment 

accumulation is substantial; approximately 750 

meters, 1,050 meters, and 1,250 meters upstream from 

the dam. Although sediment accumulation is evident 

in practically all areas of the reservoir, the greater 

sediment accumulation has occurred in the upper 

reach, approximately 1,250 meters upstream from the 

dam, where the RíoToro Negro enters the reservoir 

(fig. 9). An average of about 5.5 meters in thickness of 

sediment has accumulated in this area, giving a 

sediment deposition rate of about 0.4 meter per year 

for the period of July 1986 to October 2001. 

Approximately 750 meters upstream from the dam, a 

thickness of 4 meters of sediment has accumulated, for 

a deposition rate of about 0.3 meter per year during the 

same period. From the dam to about 500 meters 

upstream, the material accumulation is less and more 

uniform; a layer about 1.5 meters thick has 

accumulated in the same period, resulting in a 

deposition rate of about 0.1 meter per year. 

The power-generating intake structure is located 

in the deepest part of the reservoir, approximately 120 

meters upstream from the dam and has a crown 

elevation of 876.30 meters above mean sea level. 

According to the 2001 survey data, the reservoir 

bottom has reached an elevation of 880.21 meters 

above mean sea level and although the intake tunnel is 

surrounded by a layer of sediment of about 4 meters 

thick, the structure is still operational. It is likely that 

the daily water releases for operation of the irrigation 

canal and power-generating process, along with the 

trashrack, maintain the intake clear of material 

deposition. This accumulation of material, however, 

has depleted the reservoir of the dead-storage volume 

used to accommodate sediment without disabling the 

intake structure. In addition, if water releases are 

discontinued for a prolonged period, the intake 

structure could be rendered inoperative. This situation 

has been noted in many of the reservoirs in Puerto 

Rico (Soler-López, 2001b). 

TRAPPING EFFICIENCY 

Heinemann (1981) considered trapping 

efficiency to be the most informative descriptor of a 

reservoir. 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 parameters including sediment size, 

distribution, the time and rate of water inflow to the 

reservoir, the reservoir size and shape, the location of 

the outlet structure, and water discharge schedules 

(Verstraeten and Poesen, 2000). 

Many empirical studies showing the relation 

between reservoir storage capacity, water inflow, and 

trapping efficiency have been conducted, of which 

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

Brune developed a curve that is used to estimate the 

trapping efficiency of a reservoir based on the ratio 

of storage capacity to annual water inflow volume 

(fig. 11). The trapping efficiency of Lago El Guineo 

was estimated using the relation established by Brune. 

18 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
Figure 11. Reservoir trapping efficiency as a function of the ratio between storage capacity 

and annual water inflow volume. 

 The Lago El Guineo drainage area has no 

stream gaging station to measure annual inflow to the 

reservoir. To estimate how much rainfall becomes 

runoff in the Lago El Guineo drainage area, the 

neighboring basin of Río Saliente was used to estimate 

the rainfall/runoff ratio of the Lago El Guineo 

drainage area. The Río Saliente drainage area has 

similar topography, land use, elevation, and hillslopes, 

and therefore the rainfall/runoff ratio can be used for 

the Lago El Guineo Basin. 

The USGS surface-water station Río Saliente at 

Coabey near Jayuya, Puerto Rico (number 50025155), 

has an average annual runoff of 1.12 meters for the 

period from 1989 to 1999 (Díaz and others, 2000). The 

average annual rainfall for the Río Saliente Basin is 

2.29 meters (Calvesbert, 1970). Thus, the rainfall/ 

runoff ratio equals 0.49. The Lago El Guineo drainage 

area has the same average annual rainfall of 2.29 

meters as the Río Saliente drainage area (Calvesbert, 

1970), therefore, the Lago El Guineo estimated runoff 

is assumed to be approximately 1.12 meters per unit of 

area per year. Multiplying this value by the Lago El 

Guineo drainage area (4.27 square kilometers) gives 

an estimated annual inflow value of 4.78 million cubic 

meters. Using the median curve of Brune (fig. 11), the 

estimated trapping efficiency of Lago El Guineo was 

about 97, 96, and 95 percent during 1931, 1986, and 

2001, respectively, resulting in an average long-term 

trapping efficiency of 96 percent for Lago El Guineo. 

Method of Survey 19
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 for a specified 

period of time per unit of surface area (McManus and 

Duck, 1993). For Lago El Guineo, the total amount of 

sediment that has entered the reservoir (0.42 million 

cubic meters) was estimated by dividing the 

accumulated sediment (0.40 million cubic meters) by 

the long-term trapping efficiency (0.96). To determine 

the average annual rate of sediment influx, 0.42 

million cubic meters was divided by the age of the 

reservoir (70 years), resulting in 6,000 cubic meters 

per year. The sediment yield of the Lago El Guineo 

drainage basin was calculated by dividing 6,000 cubic 

meters per year by 4.08 square kilometers (the 

drainage area minus the reservoir surface area), 

resulting in a yield of 1,471 cubic meters per square 

kilometers per year. Assuming a sediment dry bulk 

density of 1 gram per cubic centimeter (Webb and 

Soler, 1997), the estimated sediment yield of the Lago 

El Guineo drainage area is about 1,471 megagrams per 

square kilometer per year for the period from 1931 to 

2001. 

Using the same calculations, the average annual 

sediment yield of Lago El Guineo was 1,159 

megagrams per square kilometer per year for the 

period from 1931 to 1986. This represents a 

21-percent increase in erosion rates within the 

Lago El Guineo Basin since 1986. However, the true 

sediment yield of the basin is likely higher because 

these calculations do not account for eroded material 

(that was not trapped by the reservoir) and material 

resulting from Hurricanes Hortense and Georges, in 

addition to other recent rainfall events, which is 

temporarily stored sediment in river channels 

upstream from the reservoir. These high runoff flows 

flush previously eroded material downstream and also 

deposit additional material in the river beds upstream 

of the reservoir. This temporarily stored material has 

the potential to further reduce the storage capacity of 

the reservoir when transported and deposited into the 

reservoir by future high-flow events. 

Based on the October 2001 storage capacity and 

the long-term sedimentation rate, Lago El Guineo has 

a useful life of about 331 more years. However, the 

actual life expectancy of Lago El Guineo could be 

somewhat shorter or longer depending on future 

rainfall frequencies and magnitudes, as well as landuse practices within the basin. Although the life 

expectancy of Lago El Guineo does not seem to be a 

pressing concern, sediment accumulation in the 

vicinity of the water intakes could disable the structure 

if water releases are discontinued for a prolonged 

period of time. This could affect water utilization even 

if the reservoir can store a substantial volume of water. 

20 Sedimentation Survey of Lago El Guineo, Puerto Rico, October 2001 
REFERENCES 

Acevido, G., 1982, Survey of Arecibo area, northern 

Puerto Rico: U.S. Department of Agriculture, 

Soil Conservation Service, Washington D.C., p. 5, 

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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. 

Díaz, P.L., Aquino, Zaida, Figueroa-Alamo, Carlos, 

Vachier, R.J., Sánchez, A.V., 2000, Water 

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

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rates in the Southern Pennine Region, UK: John 

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

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Geological Survey Water-Resources 

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

results of the principal reservoirs of Puerto Rico, 

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unpaginated CD. 

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. 

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

efficiency of small reservoirs and ponds: methods 

and implications for the assessment of sediment 

yield: Progress in Physical Geography, v. 24, no. 

2, June 2000, p. 219-251. 

Webb, R.M.T., and Soler-López, L.R., 1997, 

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Rico, 1953-94: U.S. Geological Survey WaterResources Investigations Report 97-4108, 18 p., 9 

pls. 

Method of Survey 21 
__________________________________________________________________________ 

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 EL GUINEO, PUERTO RICO, OCTOBER 2001—WRIR 03–4093