U.S. Geological Survey

U.S. Department of the Interior ;: ;

Prepared in cooperation with the

PUERTO RICO ELECTRIC POWER AUTHORITY

100

Sedimentation Survey of Lago Guajataca, 

Puerto Rico, January 1999

Water-Resources Investigations 

Report 00-4044

science 

USGS

for a changing world

U.S. Department of the Interior 

U.S. Geological Survey

Sedimentation Survey of Lago Guajataca, Puerto Rico, 

January 1999

By Luis R. Soler-Lopez, Richard M.T. Webb, and Ramon A. 

Carrasquillo-Nieves

Water-Resources Investigations Report 00-4044

In cooperation with the

PUERTO RICO ELECTRIC POWER AUTHORITY

San Juan, Puerto Rico: 2000
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, 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:

District Chief

U.S. Geological Survey

GSA Center, Suite 400-15

651 Federal Drive

Guaynabo, Puerto Rico 00965-5703

Copies of this report can be purchased from:

U.S. Geological Survey 

Branch of Information Services 

Box 25286 

Denver, CO 80225
CONTENTS

Abstract..............................................................................................................................................................'.. 1

Introduction................................................;.......................................................................................................... 1

Dam and reservoir characteristics.................................................................................................................. 2

Method of survey......................................................................................................:,..:...................;....,.>..... 4

Field techniques ................:..........1...................................................................:..... ...............~.......^... 4

Data processing....................................................................................................................................... 7

Actual capacity and sediment accumulation .................................................;..........:....: ... ............... ......;....... 7

Trapping efficiency and sediment yield ............................ ......................................'...............;........................... 20

References..............................................^ 20

PLyVTE

[Plate is in pocket] ' 

1. Lago Guajataca, Puerto Rico, Bathymetry, January 1999

Contents III
FIGURES

1.-4. Map showing

1. Location of Lago Guajataca in northwestern Puerto Rico....................................................................... 3

2. Planned cross-section locations for the January 1999 bathymetric survey of

Lago Guajataca, Puerto Rico................................................................................................................ 5

3. Actual track lines of the January 1999 bathymetric survey of Lago Guajataca,

Puerto Rico........................................................................................................................................... 6

4. Selected cross-section locations of the January 1999 bathymetric survey of <

Lago Guajataca, Puerto Rico................................................................................................................ 9

5. Reference distances for longitudinal profiles of Lago Guajataca, Puerto Rico, for

the January 1999 bathymetric survey......................................................................................................... 10

6. Selected cross sections generated from the TIN surface model of Lago Guajataca,

Puerto Rico, for January 1999.................................................................................................................... 11

7. Longitudinal profiles generated from the TIN surface model of Lago Guajataca,

Puerto Rico, for the January 1999 bathymetric survey.............................................................................. 17

8. Capacity curve for Lago Guajataca, Puerto Rico, for January 1999.............................................................. 18

TABLES

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

2. Summary data for prior and current sedimentation surveys of Lago Guajataca,

Puerto Rico................................................................................................................................................. 8

3. Storage capacity for Lago Guajataca, Puerto Rico, for January 1999............................................................ 19

IV Contents
CONVERSION FACTORS, DATUMS, ACRONYMS, and TRANSLATIONS

Multiply

centimeter

meter

millimeter

kilometer

square meter

square kilometer

square kilometer

cubic meter

million cubic meters

cubic meter

cubic meter per second

cubic meter per second

gram per cubic centimeter

megagram per square kilometer

By

Length

0.03281

3.281

0.03937

0.6214

Area

10.76

0.3861

247.1

Volume

35.31

810.7

0.0008107

Volume per unit time (includes flow)

35.31

15,850

Mass per area (includes sediment yield)

62.43

2.855

To obtain

foot

foot

inch

mile

square foot

square mile

acre

cubic foot

acre-foot

acre-foot

cubic feet per second

gallon per minute

pound per cubic foot

ton per square mile

Datums

Horizontal Datum - Puerto Rico Datum, 1940 Adjustment

Vertical Datum - National Geodetic Vertical Datum 1929 (NGVD 1929)

Acronyms used in this report

DGPS Differential Global Positioning System

GIS Geographic Information System

PREPA Puerto Rico Electric Power Authority

TIN Triangulated Irregular Network

USGS U.S. Geological Survey

Translations

Spanish

Contents

Sedimentation Survey of Lago Guajataca, Puerto Rico, 

January 1999

By Lws R. Soler-Lopez, Richard M.T. Webb, am/Ramon A. Carrasquillo-Nieves

Abstract

Compared to other reservoirs in Puerto 

Rico, sediment accumulation in Lago Guajataca 

has been moderate since its construction in 1928. 

The storage capacity has been reduced from 48.46 

million cubic meters in 1928 to 42.28 million 

cubic meters in 1999. This represents a storage 

loss of 6.18 million cubic meters or about 13 

percent of the original capacity. The average longterm sedimentation rate was calculated to be 

about 87,000 cubic meters per year or 0.2 

percent of capacity loss per year. Using the 

capacity/inflow relation established by Brune in 

1953, the trapping efficiency of the reservoir was 

estimated to be 97 and 96 percent for 1928 and 

1999, respectively. Based on the average longterm trapping efficiency of the reservoir, the mean 

annual runoff, and the sediment contributing 

drainage area of the reservoir, the sediment yield 

of the Lago Guajataca basin was estimated to be 

about 1,188 megagrams per square kilometer per 

year. At the average long-term sedimentation rate, 

the remaining useful life of the reservoir, in terms 

of flood control and recreation, is about 486 

years, which means the reservoir is expected to 

completely fill with sediments by the year 2485.

INTRODUCTION

The U.S. Geological Survey (USGS) in 

cooperation with the Puerto Rico Electric Power 

Authority (PREPA) has conducted sedimentation 

surveys in several reservoirs around the island of 

Puerto Rico. PREPA owns these reservoirs and uses 

them for hydroelectric power generation, irrigation of 

croplands, and as water supply for domestic and 

industrial use. Lago Guajataca is the principal water 

supply for the northwestern part of the island, and 

during the summer of 1997, it experienced one of the 

largest drawdowns recorded in the reservoir's history. 

This drawdown resulted in rationing water from the 

water-supply system in the northwestern part of the 

island. During January 20-22, 1999, the USGS 

conducted a bathymetric survey of Lago Guajataca to 

determine the current storage capacity, the average 

long-term sedimentation rate, the amount of sediment 

accumulated, the current trapping efficiency, and the 

sediment yield of the basin. This information will help 

PREPA in managing effectively the available 

resources in the reservoir. Data on geographic position 

and water depths were simultaneously collected using 

a differential global positioning system (DGPS) and a 

depth sounder. These data were stored in a personal 

computer in digital form. A hard copy of the data also 

was generated for later editing. The digital data were 

loaded into a geographic information system (GIS) for 

processing and analysis. GIS was used to construct a 

contour map of the reservoir bottom and to calculate 

the January 1999 storage capacity of the reservoir.

Introduction
Dam and Reservoir Characteristics

Lago Guajataca is located in the northwestern 

part of Puerto Rico about 8 kilometers south of the 

town of Quebradillas, and about 10 kilometers 

northeast of the town of San Sebastian (fig. 1). The 

reservoir is the principal water supply for the 

northwestern part of Puerto Rico. The reservoir was 

built in 1928, and originally had the capacity to store 

48.46 million cubic meters of water for domestic, 

agricultural, and industrial use. This original capacity 

was determined by using a mathematical back 

calculation, which is explained on page 7 of this 

report. The Guajataca dam is a hydraulic-fill structure. 

The crest of the dam is 9.50 meters wide, 316 meters 

long, and has an elevation of 202.5 meters above mean 

sea level. The spillway is located on the left abutment, 

and consists of an approach channel; an uncontrolled 

semicircular ogee crest at an elevation of 196.9 meters 

above mean sea level, and a tapering trapezoidal

concrete chute that terminates at the old river channel. 

The outlet works intake, located through the right 

abutment, consists of a diversion inlet, an intake tower, 

an intake shaft, a horseshoe tunnel, and a gate chamber 

with two side-by-side 1.22 by i.22-meter-high 

pressure gates. Extending from one of the gates is the 

canal outlet works, which consists of a 54-inch 

reinforced concrete cylinder pipe that tapers to a 

42-inch reinforced concrete cylinder pipe, a flow 

meter, a 1.37-meter-diameter sleeve valve for 

regulation, and an outlet canal. Extending from the 

other gate is the river outlet works, which consists of 

a horseshoe section tunnel, a transition structure, a 

0.91-meter-diameter reinforced pipe, and a terminal 

structure. The river structure works discharges into the 

Rio Guajataca, while the canal outlet structure 

supplies water for municipal needs (Puerto Rico 

Electric Power Authority, 1984). The principal 

characteristics of Lago Guajataca and structures are 

listed in table 1.

Table 1. Principal characteristics of Lago Guajataca and Guajataca dam, Puerto Rico (Modified 

from the Puerto Rico Electric Power Authority, 1984)

[All elevations in meters above mean sea level]

Length of dam............................................

Elevation of crest of dam...........................

Maximum flood elevation.........................

Spillway crest elevation.............................

Drainage area at dam site...........................

Elevation of the tower intake structure......

Surcharge storage for flood control...........

Lago Guajataca surface area in 19991 .......

Maximum depth during the 1999 survey2.

..........................316 meters

.......................202.5 meters

......................200.4 meters

.......................196.9 meters

.....79.77 square kilometers

.......................180.5 meters

.11.62 million cubic meters 

.......3.42 square kilometers

.........................27.0 meters

1 At spillway elevation of 196.9 meters. 

* Below spillway elevation of 196.9 meters.

2 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
67°10'38" 66°53'38"

18°30'

ATLANTIC OCEAN

v-v.- -.:u

.EXPLANATION

0112 A SURFACE-WATER STATION 

AND NUMBER

0108 -A- LAKE-ELEVATION STATION 

AND NUMBER

 TOWN OR CITY 

~*~- CHANNEL FLOW DIRECTION 

--- DRAINAGE BASIN BOUNDARY

012345 KILOMETERS

i i t i . i j

012345 MILES PUERTO RICO

Figure 1. Location of Lago Guajataca in northwestern Puerto Rico.

Introduction 3
Method of Survey

The sedimentation survey of Lago Guajataca 

involved planning, data collection, data processing, 

and analysis of the results. A geographic information 

system, Arc/Info (by Environmental System Research 

Institute, Inc.), was used to plan the survey lines and to 

analyze the bathymetric data. Planned cross-section 

locations were spaced 50 meters apart, starting at the 

dam and continuing upstream along the different 

branches of the reservoir (fig. 2). Geographic position 

and water depths were acquired simultaneously by 

using a DGPS interfaced to a depth sounder. The 

soundings were subsequently adjusted to represent 

water depths below the spillway elevation. The pool 

elevation of the reservoir was continuously monitored 

at the USGS lake-level station 50010800 (shown as 

station 0108 on fig. 1). The water level changed 

slightly during the data-collection process. To account 

for this change, a time-elevation correction factor was 

applied to adjust the measured depths to represent 

depths below the spillway elevation. A bathymetric 

map representing the reservoir bottom in January 1999 

was then generated (plate 1).

Field Techniques

The data-collection process took place during 

January 20-22,1999. Data were collected by using the 

bathymetric/land survey system developed by 

Specialty Devices Inc. The system uses two Motorola 

SixGun DGPS receivers. The DGPS units were first 

used in a static mode to establish reference marks at 

several sites overlooking the reservoir. These sites 

were later used as master stations for geographic 

positioning control. Satellite information was 

simultaneously recorded for one hour at a control 

station QUEBRAS 2 (lat 18028'01.989"Nl, long 

66°55'43.316"W.), which was established by the local 

government agency "Centro de Recaudacion de 

Ingresos Municipales", and at the following four 

reference stations overlooking the reservoir: GJ1 

(lat 18°23'51.535"N., long 66°55'31.913"W.) near the 

left abutment of the dam; GJ2 (lat 18°24'00.288"N:, 

long 66°55'26.056"W.) at the middle of the earthfill 

dam; GJ3 (lat 18°24'02.762"N., long 

66°55'25.180"W.) at the right abutment of the dam; 

and GJ4 (lat 18°22'56.636"N., long 66°54'36.789"W.)

at a hill on the upper reach of the reservoir. Once 

established, one DGPS unit was installed at the station 

with the best view of the area to be surveyed, and the 

other DGPS unit was installed in the survey boat. The 

DGPS on board the survey boat independently 

calculated a position every second while receiving a 

set of pseudo-range correction factors from the 

master station to maintain a positional accuracy 

within 2 meters. Water depths were measured by using 

a RAYTHEON DE-719 depth sounder coupled to an 

ODOM DIGITRACE to record data both in analog 

and digital form. The depth recorder shows depth to 

the nearest 0.1 meter, and was calibrated at water 

depths of 3 and 15 meters. The bathymetric survey 

software HYPACK (Coastal Oceanographies Inc.) was 

used to receive and record the geographic position and 

depth once every second while in survey mode. A total 

of 41,047 data points were collected over the entire 

reservoir. HYPACK, which runs on a portable 

personal computer, can be used to record data and to 

navigate. The helmsman of the survey boat is provided 

with a graphical display showing the lakeshore, the 

location of the planned cross sections, the actual 

position of the survey boat while underway, and 

indicators of speed and the amount of deviation from 

the planned survey lines.

A total of 187 cross sections spaced 

approximately 50 meters apart were planned in the 

office; however, low water depths in the riverine 

sections and vegetation in some areas of the reservoir 

limited data collection to only 155 cross sections (fig. 

3). Additional data were collected in a random pattern 

along the reservoir deltas. At the beginning and end of 

each survey day, the survey boat was secured to an 

assigned point on the shore and positional data were 

recorded for at least 30 seconds to verify the precision 

of the DGPS-calculated positions.

Random errors can be generated in positional 

data because of DGPS calculation errors or 

topographic interference by elevated land surfaces. 

When this occurred, either a new master station was 

occupied or a signal repeater was deployed to regain 

positional corrections. Also, errors in depth data can 

occur because of insufficient signal gain or because of 

debris or bubbles obstructing the transducer face. 

Physical or electronic problems encountered in the 

field were corrected as soon as they were detected.

4 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
66°56'00 66°55'00 66°54'00

18°24'00n

180 23'00n

18°22'00"

Dam

EXPLANATION

Planned cross section 

" * and identification number

Rio Chiquito de Cibao branch

400 METERS

0 1,000 FEET

Figure 2. Planned cross-section locations for the January 1999 bathymetric survey of Lago Guajataca, Puerto Rico.

Introduction
66°55'00" 66°54'00"

18°24'00

18°23'00

18°22W

Dam

0 400 METERS I I 1 I "l I '

0 1,000 FEET

Track lines where bathymetric 

data were collected

Random data collected

Rio Chiquito de Cibao branch

Figure 3. Actual track lines of the January 1999 bathymetric survey of Lago Guajataca, Puerto Rico.

6 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
Data Processing

Initial editing and verification of the position 

and depth data were performed by using the HYPACK 

program. Position data were corrected to eliminate 

anomalous positions that occurred when satellite 

information was obstructed by local topographic 

features by interpolating between the nearest correct 

anterior and posterior positions. The edited data were 

then transferred into the GIS for processing and 

analysis. Contour lines were drawn by using the 

collected data. These data points were color coded 

according to depth so that the drawn contour lines 

matched the corresponding color and depth ranges. 

The bathymetric contour lines were then converted 

into a surface model by creating a triangulated 

irregular network (TIN). The TIN surface model of the 

reservoir bottom consists of thousands of adjoining 

triangles with x, y, and z coordinates assigned to all 

vertices (Environmental Systems Research Institute, 

Inc., 1992).The volume of the reservoir was then 

calculated at incremental pool elevations of 1 meter to 

develop a stage-storage curve.

The original total storage capacity (live plus 

dead storage) of the Lago Guajataca reservoir is not 

reported in any existing literature. Only the useful 

(live storage) volume of 40.21 million cubic meters is 

reported (Puerto Rico Electric Power Authority, 1998). 

Dead storage (that volume below the elevation of the 

intake structure 180.5 meters above mean sea level) 

was not determined. The original total capacity of the 

reservoir is paramount in adequately estimating the 

sedimentation rate of the reservoir, the trapping 

efficiency of the reservoir, and the sediment yield of 

the drainage area. To account for this important 

information, a dead-storage loss rate was calculated 

and used to carry out a mathematical back calculation 

to estimate the original dead storage.

In 1978, the USGS conducted an unpublished 

bathymetric survey of Lago Guajataca (F. G6mezG6mez, U.S. Geological Survey, written commun., 

1999). The 1978 dead storage was determined to be 

5.82 million cubic meters. The 1999 dead storage was 

calculated to be 4.80 million cubic meters. The 

resulting average annual loss of dead storage was 

calculated to be 48,571 cubic meters per year. This 

value then was used to estimate the total dead storage 

loss for the 50-year period from 1928 to 1978; the 

estimated loss was 2.43 million cubic meters. When 

this 50-year loss is combined with the 1978 deadstorage loss of 5.82 million cubic meters, the 

estimated original dead-storage capacity of the 

reservoir is 8.25 million cubic meters. Combining the 

reported original live storage volume of 40.21 million 

cubic meters with the estimated original dead storage 

volume of 8.25 million cubic meters produces a value 

of 48.46 million cubic meters, which could be 

considered as the original total capacity of the 

reservoir. The estimated total capacity of 48.46 million 

cubic meters was used to calculate the sedimentation 

rate, the trapping efficiency, and the sediment yield of 

the reservoir basin.

The storage capacity in 1999 was computed to 

be 42.28 million cubic meters. This represents a 

reduction of 6.18 million cubic meters in 71 years or 

about 13 percent of the original capacity. The average 

long-term sedimentation rate of the reservoir was 

calculated to be about 87,000 cubic meters per year, or 

0.2 percent of storage loss per year. At this 

sedimentation rate, the remaining useful life 

expectancy of the reservoir is about 486 years. As a 

result, the reservoir is expected to fill completely by 

the year 2485. However, at the average dead-storage 

loss rate of 48,571 cubic meters per year, the reservoir 

bottom is expected to reach the elevation of the intake 

structure in about 99 years or by the year 2098. Data 

for prior and current sedimentation surveys of Lago 

Guajataca are summarized in table 2.

Actual Capacity and Sediment Accumulation 7
Table 2. Summary data for prior and current sedimentation surveys of Lago Guajataca, Puerto Rico

[Elevation datum, National Geodetic Vertical Datum 1929, , not applicable]

Total capacity at spillway elevation of 196.9 meters above mean sea level 

(million cubic meters)

Live storage (million cubic meters)

Dead storage (million cubic meters)

Sediment accumulation (million cubic meters)

Storage loss (percent)

Annual loss of capacity (cubic meters)

Annual loss of capacity (percent)

Approximate year that reservoir is expected to completely fill with sediment

1928

48.461

40.2 1 2

8.253

0

0

0

0

1999

42.28

37.48

4.80

6.18

13

87,000

0.2

24854

1 Original total capacity estimated from mathematical back-calculation procedure.

2 Reported live storage (Puerto Rico Electric Power Authority, 1998).

3 Original dead storage estimated from mathematical back-calculation procedure.

4 Assuming that the reservoir would continue to fill at the average long-term sedimentation rate.

In this report, cross sections representing the 

reservoir bottom are referred to as selected cross 

sections. Selected cross-section locations for the 

January 1999 bathymetric survey of Lago Guajataca 

are shown in figure 4. Longitudinal distances along the 

different branches of the reservoir are presented in 

figure 5. Selected cross sections representing the 

reservoir bottom in January 1999 were generated from 

the TIN surface model of Lago Guajataca, and are

shown in figure 6. Longitudinal profiles of the 

reservoir bottom along the different branches of Lago 

Guajataca are shown in figure 7. A stage-storage 

curve, which is showir in 'figure 8, was generated by 

calculating the resef^^r vblume at 1-meter 

incremental pool elevations. The stage-storage relation 

of figure 8 is shown in tabulated form in table 3.

8 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
66°56'00

18°24'00" 

66°55'00 66°54'00

18023'00"

18°22'00

Dam

 2-

EXPLANATION

Selected cross section 

and identification number

Rio Chiquito de Cibao branch

0 400 METERS

I i 1 i 'i 1 '

0 1,000 FEET

Figure 4. Selected cross-section locations for the January 1999 bathymetric survey of Lago Guajataca, Puerto Rico.

Actual Capacity and Sediment Accumulation 9
66°56'00

18°24'00

18°23'00

18°22W

66°55'QOB

I

66°54'00

Dam

EXPLANATION

Distance from the dam, in meters

000 Rfo Chiquito de Cibao branch

0 400 METERS I i' i "i \ '

0 1,000 FEET

Figure 5. Reference distances for longitudinal profiles of Lago Guajataca, Puerto Rico, for the January 1999 

bathymetric survey.

10 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
HI (jj 

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VERTICAL EXAGGERATION X 9.75

Figure 6. Selected cross sections generated from the TIN surface model of Lago Guajataca, Puerto Rico, for 

January 1999. -

[Cross sections are oriented with the observer looking in the downstream direction (refer to figure 4 for cross-section 

locations).]

Actual Capacity and Sediment Accumulation 11
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Figure 6. .Selected cross sections generated from the TIN surfacei model of Lago Guta;jataca, Puerto Rico, forJanuary 1999 Continued.

[Cross sections are oriented with the observer looking in the downstream direction (refer to figure 4 for cross-section 

locations).]

12 Sedimentation Survey of Lago Guajataca, Puerto Rico. January 1999
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100 200 , 300 400 500 600. 700 

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Figure 6. Selected cross sections generated from the TIN surface model of Lago Guajataca, Puerto Rico,.fqr 

January 1999 Continued.

[Cross sections are oriented with the observer looking in the downstream direction (refer to figure 4 for cross-section 

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Actual Capacity and Sediment Accumulation 13
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100 200 300 400 500 600

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700 800 900 1,000 

VERTICAL EXAGGERATION X 9.75

Figure 6. Selected cross sections generated from the TIN surface mo'del of Lago Guajataca, Puerto Rico, for 

January 1999 Continued.

[Cross sections are oriented with the observer looking in the downstream direction (refer to figure 4 for cross-section 

locations).]

14 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
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Figure 6. Selected cross sections generated from the TIN surface model of Lago Guajataca, Puerto Rico, for 

January 1999 Continued.

[Cross sections are oriented with the observer looking in the downstream direction (refer to figure 4 for cross-section 

locations).]

Actual Capacity and Sediment Accumulation 15
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SECTION 19 

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100 800 900 1,000 

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DISTANCE FROM LEFT BANK, IN METERS

Figure 6. Selected cross sections generated from the TIN surface model of Lago Guajataca, Puerto Rico, for 

January 1999 Continued.

[Cross sections are oriented with the observer looking in the downstream direction (refer to figure 4 for cross-section 

locations).]

16 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
LJJ 

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1,000 2,000 3,000 4,000

DISTANCE FROM THE DAM, IN METERS

5,000 6,000 7,000 8,000 

VERT.CAL SCALE GREATLY EXAGGERATED

Figure 7. Longitudinal profiles generated from the TIN surface model of Lago Guajataca, Puerto Rico, for the 

January 1999 bathymetric survey.

Actual Capacity and Sediment Accumulation 17
SPILLWAY ELEVATION 

196.9 i i i i | i i i r

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LU

VOLUME, IN MILLION CUBIC METERS

Figure 8. Capacity curve for Lago Guajataca, Puerto Rico, for January 1999.

18 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
Table 3. Storage capacity for Lago Guajataca, Puerto Rico, for January 1999

[amsl, above mean sea level]

Elevation, in meters amsl

196.9

195.9

194.9

193.9

192.9

191.9

190.0

189.9

188.9

187.9

186.9

185.9

184.9

183.9

182.9

181.9

180.9

179.9

178.9

177.9

176.9

175.9

174.9

173.9

172.9

171.9

170.9

169.9

Capacity, in million cubic meters

42.28

38.91

35.64

32.48

29.40

26.48

23.72

21.15

18.74

16.52

14.45

12.61

10.88

9.30

7.82

6.48

5.24

4.16

3.19

2.40

1,69

1.13

0.63

0.36

0.16

0.07

0.02

0

Actual Capacity and Sediment Accumulation 19
TRAPPING EFFICIENCY AND SEDIMENT YIELD

The rapid rates of sediment accumulation in 

many upland reservoirs make it important to estimate 

the trapping efficiency if a true erosion rate is to be 

calculated. Heinemann (1981) considered trap 

efficiency as the most informative single descriptor of 

a reservoir. A number of empirical studies have been 

conducted on trapping efficiency; the best known and 

most commonly used is that of Brune (1953). The 

Lago Guajataca trapping efficiency was estimated by 

using the capacity/inflow ratio described by Brune 

(1953). The capacity/inflow ratio is the reservoir 

capacity divided by the mean annual flow. In the Lago 

Guajataca drainage area, the average long-term 

rainfall is 2.18 meters per year and the rainfall/runoff 

ratio is 0.6 (Giusti and Lopez, 1967). Thus, the mean 

annual runoff reaching Lago Guajataca is calculated to 

be 1.31 meters per square kilometer. Based on the 

79.77-square-kilometer drainage area, the inflow to 

the reservoir is about 104.5 million cubic meters per 

year. The capacity/inflow ratio was calculated to be 

0.5 for 1928 and 0.4 for 1999. These values then were 

plotted on the curve generated by Brune (1953) with 

the estimated trapping efficiency interpolated from the 

curve. The trapping efficiency of Lago Guajataca was 

estimated to be 97 percent in 1928 and 96 percent in 

1999. The average long-term trapping efficiency of 

Lago Guajataca is about 96 percent.

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 over a specified 

period of time (McManus and Duck, 1993).

Based on this definition, the sediment yield for 

the Lago Guajataca basin was calculated by first 

dividing the amount of deposited material in Lago 

Guajataca (6.18 million cubic meters) by the average 

long-term trapping efficiency (0.96), which yielded 

the total amount of sediment eroded from the Lago 

Guajataca catchment area (6.44 million cubic meters). 

This number then was divided by the sedimentcontributing area of the reservoir, which was 

calculated to be 76.35 square kilometers (for example, 

the total drainage area, 79.77 square kilometers minus 

the reservoir flooded area, 3.42 square kilometers), 

and the number of years since construction (71 years).

Based on an estimated dry-bulk density of 1 gram per 

cubic centimeter, the sediment yield for the Lago 

Guajataca basin was calculated to be 1,188 

megagrams per square kilometer per year.

REFERENCES

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

Transactions of the American Geophysical Union, 

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

Environmental Systems Research Institute, Inc., 1992, 

Surface modeling with TIN, Surface analysis and 

display: Environmental Systems Research 

Institute, Inc., Redlands, California, p. 4-1, 6-1.

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

streamflow of Puerto Rico: Caribbean Journal of 

Science, v. 7 no. 3-4, p. 87-93.

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

curve for small reservoirs: Water Resources 

Bulletin 7, p. 825-830.

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

sedimentation rates in the Pennine Region, UK, 

Geomorphology and sedimentology of lakes and 

reservoirs: in Chapter 6: Chichester, John Wiley 

& Sons, p. 73-92.

Puerto Rico Electric Power Authority, 1984, Guajataca 

Dam, Isabela, Puerto Rico: Designer's Operating 

Criteria, U.S. Department of the Interior, Bureau 

of Reclamation, Chapter 1, General Description, 

Sections 1-2, p. 1-3.

Puerto Rico Electric Power Authority, 1998, Guajataca 

Dam, Isabela, Puerto Rico: Phase I Inspection 

Report: Unit of Inspection and Regulation for the 

Safety of Dams and Reservoirs of Puerto Rico, 

May 1998, Section I Executive Summary, p. 1.

20 Sedimentation Survey of Lago Guajataca, Puerto Rico, January 1999
District Chief

Caribbean District

U.S. Geological Survey

Water Resources Division

GSA Center, Suite 400-15

651 Federal,Drive

Guaynabo, Puerto Rico 00965-5703

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