Prepared in cooperation with thePUERTO RICO AQUEDUCT AND SEWER AUTHORITY

U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY

SCIENTIFIC INVESTIGATIONS MAP 3218 Storage Capacity and Sedimentation Trends

of Lago Garzas, Puerto Rico, 1996-2007By Luis R. Soler-López, 2012

Storage Capacity and Sedimentation Trends of Lago Garzas, Puerto Rico, 1996-2007By Soler-López, L.R.

2012

Figure 3. Bathymetric map of Lago Garzas, Puerto Rico, July 2007.

66°45'00" 66°44'30"

18°08'00"

18°08'30"

1.0

1.0

4.0

8.0

21.0

14.0

12.0

18.0

30.0

17.0

9.0

5.0

29.0

26.0

22.0

13.0 4.0

3.0

1.0

9.0

Shoreline modified from the U. S. Geological Survey quadrangle Adjuntas, Puerto Rico, 1960, photo revised 1977. Lambert Conic Conformal Projection. Puerto Rico State Plane coordinate system (Puerto Rico Datum, 1940 adjustment)

RESERVOIR BOTTOM CONTOUR- Depth of the reservoir bottom in 2007, in meters below spillway elevation of 736.12 meters above mean sea level. Contour interval 1-meter. Shoreline shown as red line equal to zero depth orspillway elevation of 736.12 meters above mean sea level.

Spillway structure

0 1,000 2,000 FEET

0 250 500 METERS

Río

Vaca

s bra

nch Southeastern branch

Northweste

rn branch

Northern branch

EXPLANATION

Basin areaStudy

area

PUERTO RICO

EXPLANATION

Lago Garzas basin area

Lago Garzas near AdjuntasStation 50020100

Drainage basin boundary

Caño Tiburones flow directionTunnel flow direction

Town or city

ATLANTIC OCEAN

0

0 1

1

2

2

3

3

4

4 5 MILES

5 KILOMETERS

66°47'30" 66°32'30"

18°27'30"

18°15'00"

Figure 1. Location of Lago Garzas in the upper Río Grande de Arecibo basin, Puerto Rico.

Base from U.S. Geological Survey digital data.

HATILLO

BARCELONETA

JAYUYA

ARECIBO

ADJUNTAS

UTUADO

FLORIDA

Río

Río

Río

Río

Río

Río Ca

onill

as

Caonillas

Río

Río

Jauc

a

Río

Río

Río

Cidra

Río

Río

Gra

nde

Grande

RíoRío Río

Caricaboa

Saliente

Grande

de

de

de Jayuya

Arec

ibo

Arec

ibo

Río

Río

LagoCaonillas

LagoDos Bocas

LagoPellejas

LagoAdjuntas

Lago Viví

LagoGarzas

LagoGarzas

LagoJordan

Caño Tiburones

Tanamá

Limón

Lim

ón

LaVenta Yunes

Viví

Tunnel

Tunnel

Tunnel

Tunn

el

Tunnel

Viví

Vaca

s

Pellejas

Introduction

Lago Garzas is located in west-central Puerto Rico, about 3.5kilometers southwest of the town of Adjuntas, in the confluence of the Río Vacas and three other unnamed tributaries (fig. 1). The dam is owned and operated by the Puerto Rico Electric Power Authority (PREPA), and was constructed in 1943 for hydroelectric power generation and municipalwater use along the southern coast. The dam is a semi-hydraulic earthfill embankment lined withboulders, and has a height of 61.57 meters, a top width of 9.14 meters, a base width of 365.76 meters, and a crest length of 227.37 meters; State Road PR-518 crosses the top of the dam. A morning-glory-type spillway is located near the west abutment of the dam at an elevation of 736.12 meters above mean sea level (Puerto Rico Water Resources Authority, 1969).Figure 2 shows an aerial photograph of the Lago Garzas earthfill dam and the morning-glory spillway section. Additional information andoperational procedures are provided in Soler-López and others (1999). During July 17–18, 2007, the U.S. Geological Survey (USGS)Caribbean Water Science Center, in cooperation with the Puerto RicoAqueduct and Sewer Authority, conducted a bathymetric survey of Lago

Garzas to update the reservoir storage capacity and update the reservoir sedimentation rate by comparing the 2007 data with the previous 1996 bathymetric survey results. The purpose of this report is to describe and document the USGS sedimentation survey conducted at Lago Garzas during July 2007, including the methods used to update the reservoir storage capacity, sedimentation rates, and areas of substantial sediment accumulation since 1996.

Method of Survey and Analysis

The field techniques and bathymetric data reduction processes used for the 2007 survey were performed following proceduresestablished by the USGS Caribbean Water Science Center and described in the previous bathymetric survey report of Lago Garzas (Soler-López and others, 1999). The July 2007 bathymetric survey was conducted using a differential global positioning system (DGPS) coupled to a digital depth sounder similar to the setup used for the survey conducted in September 1996 (Soler-López and others, 1999). Survey navigation lines were

66°44'20"44'30"44'40"44'50"66°45'00"

18°08'50"

08'40"

08'30"

08'20"

08'10"

08'00"

07'50"

18°07'40" 0 0.25 0.50.125 KILOMETERS

0 0.25 0.50.125 MILES

Figure 4. Aerial photograph of the Lago Garzas and its immediate surroundings. Photograph from the U.S. Army Corps of Engineers, Puerto Rico.

~6.0 m

~5.0 m

0

5

10

15

20

25

30

35

400

5

10

15

20

25

30

35

400

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Vertical scale greatly exaggerated

0 500 750250 1,000 1,5001,250 2,0001,750 2,5002,250

2007

EXPLANATION

1996

Figure 6. Longitudinal bottom profiles along the thalweg of Lago Garzas, within the northern, northwestern, Río Vacas, and southeastern branches of the reservoir, for 1996 and 2007.

Northern branch

Northwestern branch

Río Vacas branch

Southeastern branchDept

h, in

met

ers

belo

w s

pillw

ay e

leva

tion

of 7

36.1

2 m

eter

s ab

ove

mea

n se

a le

vel

Distance from the dam, in meters

network (TIN) surface model of Lago Garzas was then created from the bathymetric map, and the reservoir volume was calculated using GIS. The TIN surface model was used with the digital data for 1996 and 2007 to generate a stage-storage curve and table, as well as longitudinal profiles along the different tributary branches of Lago Garzas.

Storage Capacity, Sedimentation Rates, and Useful Life

The storage capacity of Lago Garzas decreased from 5.11 million cubic meters in 1996 (Soler-López, 1999), to 4.99 million cubic meters by July 2007 (table 1). This decrease represents a reduction from 1996 to 2007 of 120,000 cubic meters (or 2.3 percent over 11 years), for an annual storage-capacity loss rate of about 10,909 cubic meters. The long-term sedimentation rate of Lago Garzas has decreased slightly from 13,019 cubic meters per year in 1996 to 12,656 cubic meters per year in 2007 (table 1). The nearly unchanged sedimentation rate of Lago Garzas from 1996 to 2007 (table 1) may be due to the largely undisturbed Lago Garzas drainage area (fig. 4). An actualized storage capacity table and curve were generated by calculating the TIN volume at 1-meter elevation intervals (table 2, fig. 5). Sediment accumulation within the majority of Lago Garzas between 1996 and 2007 has been minimal, with the exception of one deposition area along the northwestern branch of the reservoir (fig. 6). The profiles shown in figure 6 indicate that a patch of sediment about 5 meters thick has accumulated along the northwestern branch of the reservoir between about 550 to 650 meters upstream of the dam. The Lago Garzas long-term drainage area sediment yield rate has remained relatively unchanged, decreasing slightly from 565 to 550 cubic meters per square kilometer per year from 1996 to 2007 (table 1; Soler-López and others, 1999). As of 2007, Lago Garzas has lost 14 percent of its original storage capacity of 5.80 million cubic meters (Soler-López and others, 1999). Based on the current long-term storage-capacity loss of 12,656 cubic meters per year estimated for 2007, the Lago Garzas has an estimated useful life of about 394 years, which may end by the year 2401.

Summary and Conclusions During July 2007, the U.S. Geological Survey, in cooperation with the Puerto Rico Aqueduct and Sewer Authority, conducted a bathymetric survey of Lago Garzas to update the reservoir storage capacity and estimate the reservoir sedimentation rate by comparing the 2007 data with the previous 1996 bathymetric survey results.

The Lago Garzas storage capacity in September 1996 was 5.11 million cubic meters, and decreased to 4.99 in million cubic meters by July 2007. This difference represents a reservoir storage capacity loss of 120,000 cubic meters between 1996 and 2007. The long-term (1943-2007) sedimentation rate of the reservoir is 12,656 cubic meters per year. As of 2007, the reservoir has experienced a total storage loss of 14 percent compared to its original storage capacity of 5.80 million cubic meters. Most of the storage capacity loss of Lago Garzas is distributed uniformly across the reservoir bottom; however, one area of sediment deposition is evident along the northwestern tributary branch, where an average of about 5 meters of sediment has accumulated between 1996 and 2007. The long-term Lago Garzas drainage area sediment yield has remained relatively unchanged from 1996 to 2007, decreasing slightly from 565 to 550 cubic meters per square kilometer per year. The 2007 life expectancy of about 394 years for Lago Garzas is not a pressing concern, assuming a long-term storage-loss rate of about 12,656 cubic meters per year. Sediment accumulation along the northwestern and southeastern branches of the reservoir, however, will continue forward towards the deepest parts of Lago Garzas, which will eventually start filling the reservoir near the dam. If the long-term sedimentation rate of 12,656 cubic meters estimated for 2007 continues, the useful life of Lago Garzas may end by the year 2401.

References Cited

Puerto Rico Water Resources Authority, 1969, Engineering and Construction Division: Inspection and Maintenance Reports of the Garzas Dam and Related Facilities, 8 p., 4 pls.

Soler-López, L.R., Webb, R.M.T., and Pérez-Blair, Francisco, 1999, Sedimentation survey of Lago Garzas, Puerto Rico, September 1996: U.S. Geological Survey Water-Resources Investigations Report 99-4143, 20 p., 2 pls.

established at 50-meter intervals andoriented parallel to the dam face of the reservoir, continuing upstream to thereservoir tail along the different river branches. Geographic positions and water depths were acquired simultaneouslyusing a DGPS interfaced to a depth sounder. The pool elevation of thereservoir was measured at the USGS lake-level station 50020100, Lago Garzas near Adjuntas, Puerto Rico. The soundings were subsequently adjusted to represent water depths below the spillway elevation. The 2007 data were stored and transferred into the USGS geographic information system (GIS), where final analysis and volume calculations were made following procedures similar to those used to develop the 1996bathymetric map of Lago Garzas reservoir (Soler-López and others, 1999). The field data were used to generate a bathymetric map representing the reservoir bottom in July 2007 (fig. 3). A triangulated irregular

Table 1. Lago Garzas data comparison for 1996, and July 2007.

Data descriptorYear of survey

1996 2007

Years since construction1

Years since previous surveyTotal storage capacity, in million cubic metersSediment accumulated since construction, in million cubic metersSediment accumulation since previous survey, in million cubic meters

Storage loss since construction, in percent

Annual loss of capacity since construction, in cubic metersAnnual loss of capacity since construction, in percent

Estimated sediment trapping efficiency,in percent

Sediment yield, in cubic meters per square kilometer per yearEstimated year the reservoir would fillwith sediments

5318

5.11

.69

.47

12

13,019

.23

95

565

2388

6411

4.99

.81

.12

14

12,656

.22

95

550

2401

1 Using the construction date of 1943.

Table 2. July 2007,

Storage capacity for Lago Garzas, Puerto Rico,

Pool elevation, in above mean sea

meters level

Storage capacity, in million cubic meters

736.12 735.12 734.12 733.12 732.12 731.12 730.12 729.12 728.12 727.12 726.12 725.12 724.12 723.12 722.12 721.12 720.12 719.12 718.12 717.12 716.12 715.12 714.12 713.12 712.12 711.12 710.12 709.12 708.12 707.12 706.12

4.99 4.56 4.18 3.82 3.49 3.20 2.92 2.66 2.41 2.19 1.98 1.79 1.61 1.44 1.28 1.13 1.00

.87 .76 .65 .55 .46 .38 .31 .24 .19 .13 .09 .05 .02 .00

Normal pool elevation = 736.12 meters above mean sea level

740.00

700.00

705.00

710.00

715.00

720.00

725.00

730.00

735.00

0.0 2.001.00 4.003.00

Storage volume, in million cubic meters

6.005.00

Elev

atio

n, in

met

ers

abov

e m

ean

sea

leve

l

Figure 5. Relation between pool elevation and storage capacity of Lago Garzas for 1996 and 2007.

2007

EXPLANATION

1996

Figure 2. Aerial photograph of the Lago Garzas earthfill dam and spillway section, taken during a dam safety inspectioin May 2006. Photograph courtesy of the Puerto Rico Electric Power Authority.

Spillway

Earthfill dam

Spillway

Earthfill dam

n