THE FLOODS OF MAY 17-18, 1985

AND

OCTOBER 6-7, 1985 IN PUERTO RICO

By

Ferdinand Quinones and Karl G. Johnson

U.S. GEOLOGICAL SURVEY

OPEN-FILE REPORT 87-123

Prepared in cooperation with the

PUERTO RICO DEPARTMENT OF NATURAL RESOURCES, PUERTO RICO ENVIRONMENTAL QUALITY BOARD,

AND PUERTO RICO HIGHWAY AUTHORITY

San Juan, Puerto Rico

1987

DEPARTMENT OF THE INTERIOR

DONALD PAUL HODEL, Secretary

U.S. GEOLOGICAL SURVEY

Dallas L. Peck, Director

For additional Information write to:

Chief, Caribbean District, WRDU.S. Geological SurveyGPO Box 4424

San Juan, Puerto Rico OO936

Copies of this report can be purchased from:

U.S. Geological Survey Books and Open-File Reports Federal Center, Bldg. 41 Box 25425 Denver, Colorado 8O225

Ill

CONTENTSPAGE

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

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

The floods of May 17-18, 1985................................ 3

Rainfall................................................ 3

Flooding and damages.................................... 3

Peak discharges......................................... 4

The floods of October 6-7, 1985.............................. 10

Rainfall................................................ 10

Flooding and damages.................................... 10

Peak discharges......................................... 14

Landslides.............................................. 14

Comparison of the 1985 floods with historical floods......... 14

Summary...................................................... 19

References cited............................................. 20

LIST OF ILLUSTRATIONS

Figure 1. Map of Puerto Rico showing locations mentionedin this report:................................ 2

2. Map showing cumulative precipitation over PuertoRico during May 15-19, 1985................... 4

3. Map showing areas in Puerto Rico where floodingwas most severe during May 1985............... 6

4. Map showing network of streamflow gaging stations operated by the U.S. Geological Survey in Puerto Rico................................... 7

5. Graphs showing discharge hydrographs at keystreamflow gaging stations during the May 15-19, 1985 floods at:a) Rio Grande de Manati at Highway 2,

near Manati,b) Rio Fajardo near Fajardo,c) Rio Inabon at Real Abajo, andd) Rio Grande de Anasco near San Sebastian.... 9

IV

LIST OF ILLUSTRATIONS (Continued)PAGE

Figure 6. Map showing cumulative precipitation over PuertoRico during October 6-7, 1985................. 10

7. Graph showing hourly and cumulative rainfall during October 6-7, 1985 at Tallaboa gage near Penuelas, southern Puerto Rico................ 11

8. Map showing areas flooded in southern Puerto Ricoduring the October 6-7, 1985 floods........... 13

9. Graphs showing discharge hydrographs at key streamflow gaging stations during the October 6-7, 1985 floods at:a) Rio Grande de Manati at Highway 2 near

Manati,b) Rio Grande de Loiza at Caguas,c) Rio Inabon at Real Abajo, andd) Rio Cerrillos near Ponce................... 16

10. Map showing location of Barrio Mameyes near thecity of Ponce in southern Puerto Rico......... 17

11. Graph showing comparison of unit discharges duringthe 1960 and 1985 floods in Puerto Rico....... 18

LIST OF TABLES

Table 1. Daily precipitation, in inches, for May 15-19, 1985 at selected National Weather Service- National Oceanic and Atmospheric Administration (NWS-NOAA) stations throughout Puerto Rico....

2. Summary of gage heights and discharges during floods of May 17-18, 1985 at selected U.S. Geological Survey streamflow gaging stations throughout Puerto Rico........................

3. Daily precipitation, in inches, for October5-8, 1985 at selected National Weather Service- National Oceanic and Atmospheric Administration (NWS-NOAA) stations throughout Puerto Rico.... 12

A. Summary of gage heights and discharges during floods of October 6-7, 1985 at selected U.S. Geological Survey streamflow gaging stations throughout Puerto Rico............... 15

CONVERSION FACTORS

For the convenience of readers who may want to use the International Systems of Units (Si), the data may be converted by using the following factors:

Multiply inch-pound units

inch (in.)

foot (ft)

mile (mi)

square mile (mi )

acre-foot (acre-ft)

cubic yard (yd )

3 cubic foot per second (ft /s)

cubic foot per second

by To obtain SI units

25.4 millimeter (mm) 2.54 centimeter (cm)

0.3048 meter (m)

1.609 kilometer (km)

2 2.590 square kilometer (km )

3 1,233 cubic meter (m )

3 0.7646 cubic meter (m )

0.02832 cubic~meter per second (m /sec)

.2-per square mile [(ft /s)/mi ) 0.01093

Specific Combinations

cubic meter per second per~square9kilometer [(in /s)/km ]

1 acre-ft = 226.3 gal/min, during one day

1 ft3/s = 448.8 gal/min

1 ft /s =0.65 Mgal/d

THE FLOODS OF MAY 17-18, 1985

AND OCTOBER 6-7, 1985 IN PUERTO RICO

By Ferdinand Quifiones and Karl G. Johnson

ABSTRACT

Severe floods occurred in Puerto Rico twice in 1985. During May 15-19, 1985, as much as 25 inches of rainfall produced significant floods along north and north-central basins in the island. A nearly stationary tropical depression affected Puerto Rico during October 5-8, 1985, resulting in 24-hour precipitation totals of as much as 23 inches and severe floods along the south-central coastal areas.

During the May 17-18, 1985 event, the areas most seriously affected by flooding were along the north coast. These included the lower reaches of the Rio Grande de Manati and the Rio Grande de Arecibo. Significant flooding also occurred at Utuado and Jayuya. The recurrence interval of most of the flood peaks was generally less than 25 years.

The floods of October 6-7, 1985, affected mostly rural areas in southern Puerto Rico, but caused significant loss of life and widespread property damages. Landslides near Ponce, the collapse of a bridge at Rio Coamo, and the destruction of homes near Ponce resulted in about 170 fatalities and more than 125 million dollars in damages. Flooding was also severe at Barceloneta on the north coast. Recurrence intervals equal to or greater than 100 years were estimated for peak discharges at several index stations.

INTRODUCTION

During 1985, severe floods occurred twice throughout Puerto Rico (fig. 1) resulting in significant losses in life and property. The first event occurred during May 15-19, when a low-pressure system resulted in precipitation totals exceeding 14 inches throughout most of south-central and eastern Puerto Rico. A second event was produced by a tropical depression that affected south-central Puerto Rico during October 5-8. Landslides and collapses of several key bridges during the October floods resulted in the death of as many as 170 people. Property losses from both floods were estimated at about 162 million dollars.

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INTRODUCTION (Continued)

The U.S. Geological Survey, Water Resources Division, collected and analyzed data on the magnitude, frequency and extent of the 1985 floods. The investigations were conducted as part of the ongoing cooperative water resources investigation program in cooperation with the Puerto Rico Department of Natural Resources, the Puerto Rico Environmental Quality Board, and the Puerto Rico Highway Authority. Information related to rainfall quantities and intensities throughout the island was provided by Robert Calvesbert, of the National Weather Service, National Oceanic and Atmospheric Administration (NWS-NOAA), U.S. Department of Commerce.

This report summarizes the rainfall and streamflow data collected during the two 1985 flood events. This information should be of value and interest to engineers, developers, planners and government officials. Additionally, it will provide a historical record of data from the floods to supplement several flood atlases now in preparation.

THE FLOODS OF MAY 17-18, 1985

Rainfall

Severe flooding affected most of the north and north-central parts of Puerto Rico during May 17-18 as a result of intense precipitation that began on May 15 and continued through May 19. The rains were produced by a nearly stationary low-pressure center that moved from the northwest on May 15. During the next five days, almost continuous precipitation of varying intensity was recorded throughout most of the south-central and eastern part of Puerto Rico.

Precipitation totals for the 5 days of the storm ranged from 8 to 25 inches (fig. 2). The most intense rainfall occurred over an area extending west of a north-south line at the town of Orocovis toward Mayaguez. A second cell of less intense rainfall affected the Rio Grande de Loiza basin, in the eastern part of the island. The recorded precipitation total for the 5-day period May 15-19 was 25.21 inches at Jayuya. Precipitation totals for the 5-day period at individual stations operated by the NWS-NOAA are summarized in table 1.

Flooding and Damages

The most significant floods occurred on May 17 and 18. Flooding was severe in the lower reaches of streams in the Rio Grande de Arecibo, the Rio Tanama, the Rio Grande de Manati, the Rio Grande de Jayuya, the Rio Orocovis, the Rio Turabo, the Rio de La Plata, and the Rio Grande de Afiasco basins. Flooding of urban areas was reported at the towns of Arecibo, Utuado, Jayuya, Orocovis, Vega Baja, Dorado, Toa Baja, Barceloneta, Caguas, and Loiza.

18 30'

67 15' 67 00' _____45'______

^TL^ANTICARECIBO

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18 00*

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OCEAN6600' 6545*

MAYA

CARIBBEAN SEA0 10 20 KILOMETERS

10 20 MILES

Figure 2. Cumulative precipitation (Isohyets In Inches) over Puerto Rico during May 15-19, 1985.

THE FLOODS OF MAY 17-18. 1985 (Continued)

Flooding and Damages (Continued)

The worst flooding occurred at Arecibo and Barceloneta, (fig. 3). The flooding at Arecibo covered slightly less area than during the 1928 record flood (Hickenlooper, 1968). The flood at the town of Barceloneta was lower and inundated a lesser area than the 1970 flood. Undefined conditions appear to have caused backwater effects in several areas in Barceloneta on the Rio Grande de Manati flood plain. The floods at the towns of Jayuya and Utuado were lower than the record floods of 1975 (Johnson, 1982).

Damage estimates to private and public property during the floods reached 37 million dollars according to the Federal Emergency Management Administration (FEMA).

Peak Discharges

The U.S. Geological Survey collected peak stage and flow data from its network of gaging stations throughout Puerto Rico (fig. A). At sites where recording instruments failed or were damaged during the floods, high-water marks were surveyed shortly after the event to determine the peak stage and discharge. Flood stages, discharges, and other information pertinent to the gaging stations are summarized in table 2. Flood hydrographs at key gaging stations for the period from May 15-21, 1985 are shown in figures 5a to 5d.

Table 1. Dally precipitation, In Inches, for May 15-19, 1985 at selected NationalWeather Service-National Oceanic and Atmospheric Administration

(NWS-NOAA) stations throughout Puerto Rico

IJOCATION

NORTH COASTAL

Borinquen AirportCandelaria-Toa BajaRio Piedras AESSan Juan WSFOToa Baja 1 SSW

SOUTH COASTAL

Aguirre CentralCentral San FranciscoLajas AESMagueyes IslandPonce 4 EPonce CitySanta Isabel 2 ENE

NORTHERN SLOPES

CanovanasFajardoIsabela AESI .a Muda - CaguasMauati 3 ETrujillo Alto 2 SSW

SOUTHERN SLOPES

Corral ViejoJuana Diaz CampMaunaboPat il lasPuerto RealRoosevelt RoadsSabana Grande 2 ENEYauco 1 NNW

EASTERN INTERIOR

Caguas 1 WCayey 1 ECidra 1 EGurabo AESJuncosPi co del EsteSan Lorenzo 3 SSan Lorenzo Farm 2 NW

WESTERN INTERIOR

Ad juntas AESArecibo ObservatoryCerro MaravillaColosoCorozal AESDos BocasJayuyaLares 2 SEMaricao Fish HatcheryMorovis 1 NNegro-CorozalSan Sebastian 2 WNWUtuadoVillalba 1 E

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5-DAY TOTALS

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THE FLOODS OF OCTOBER 6-7, 1985

Rainfall

Intense precipitation induced by a nearly stationary tropical depression occurred over most of southern Puerto Rico during October 6-7, 1985. As much as 23 inches of rainfall occurred north of the towns of Ponce and Coamo during a 24-hour period (fig. 6). The intensity of the precipitation near the town of Penuelas (fig. 7) was as high as 2.75 inches in one hour and 5.5 inches in two hours. The rainfall was equally distributed between the afternoon of October 6 and the early hours of October 7. Less intense precipitation occurred on October 5. Similarly less intense precipitation occurred on October 8 after the very intense storm of October 7. Precipitation totals for the 4-day period (October 5-8) were as high as 30.66 inches (table 3).

67 16* 6700* 45' 30' 16' 6600' 6546'

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A TLANTiCARECJBO

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MACAO

V 2U V CARIBBEAN SEA

0 10 20 KILOMETERS

6 10 20 MILES

Figure 6.~Cumulative precipitation (Isohyets In Inches) over Puerto Rico during October 6-7, 1985.

Flooding and Damages

The most severe flooding occurred along the south coast from the town of Santa Isabel, west towards the city of Ponce (fig. 8). Severe flooding also occurred in the lower reaches of the Rio de La Plata (near the towns of Toa Alta and Dorado) and the Rio Grande de Manati (at the town of Barceloneta). The urban area of Barceloneta was flooded for the second time in 1985. Preliminary highwater marks surveyed in the center of the town showed that the October flood was about one foot higher than the May 1985 flood. Local less severe flooding was reported at the mountain towns of Utuado and Jayuya.

113.0

R °0 10 20 30 HOURS BEGINING AT

0100 OCTOBER 6, 1985

8 12 16 OCTOBER 6

8 12 16 OCTOBER 7

Figure 7. Hourly and cumulative rainfall during October 6-7, 1985 atTallaboa gage near Penuelas, southern Puerto Rico.

(Data courtesy of Puerto Rico Energy Power Authority.)

THE FLOODS OF OCTOBER 6-7, 1985 (Continued)

Flooding and Damages (Continued)

The floods along the south coast in the Santa Isabel to Ponce area exceeded the historical floods of 1970 (Haire, 1971). Flooding in the lower valley of the Rio Jacaguas (south of the town of Juana Diaz) was not as severe as in the Rio Coamo area in spite of about equal precipitation in both basins. The Toa Vaca reservoir attenuated most of the peak, storing a significant amount of the runoff. The reservoir filled-up to its capacity (original capacity of about 55,000 acre-feet) only for the second time since its construction in 1972. Flooding along the Rio Cerrillos and the city of Ponce was also minimal in comparison to previous events of lesser magnitude. The Rio Cerrillos flood channel contained most of the flood runoff within its boundaries.

Damages to private and public property during the floods were estimated by FEMA at about 125 million dollars. About 3,000 homes were damaged, of which 1,300 were a total loss. The deaths of about 40 people (other than fatalities related to landslides) were attributed to the floods. The worst incident occurred at the Rio Coamo bridge on Las Americas Expressway (Highway 52). The approach to the west bound lanes on the bridge collapsed sometime between midnight and 3:00 a.m. on October 7. An unknown number of vehicles plunged into the river, with at least 24 fatalities. Flooding at the Quebrada del Agua (west of the city of Ponce) resulted in 16 fatalities.

12

Table 3. Dally precipitation, In Inches, for October 5-8, 1985 at selected National Weather Service-National Oceanic and Atmospheric Administration (NWS-NOAA) stations throughout Puerto Rico

IXJCATION

NORTH COASTAL

Borinquen AirportCandelaria - Toa BajaRio Piedras AESSan Juan WSFOToa Baja 1 SSW

SOUTH COASTAL

Aguirre CentralCentral San FranciscoLajas AESMagueyes IslandPonce 4 EPonce CitySanta Isabel 2 ENESanta Rita

NORTHERN SLOPES

CanovanasFajardoIsabela AESLa Muda-CaguasManat i 3 ETrujillo Alto 2 SSW

SOUTHERN SLOPES

Corral ViejoJuana Diaz CampMaunaboMayaguez AirportPatillasPuerto RealRoosevelt RoadsSabana Grande 2 ENEYauco 1 NNW

EASTERN INTERIOR

Cayey 1 ECidra 1 EGurabo AESJuncosPico del EsteSan Lorenzo 3 SSan Lorenzo 2 NW

WESTERN INTERIOR

Ad juntas AESArecibo ObservatoryBarranquitas 2 SSWCerro MaravillaCorozal AESDos BocasJayuyaLares 2 SEMorovis I NNegro - CorozalSan Sebastian 2 WNW

OCTOBER5

0.45.03

1.70.29

1.04

.40

.14

.16

.22

.52

.58

.38

.00

1.50.09.02

1.08.80.50

.42ft

.85

.081.65.00

1.04.28.14

.44

.561.00.87.57

1.15.50

.10

.041.00.41.95.30.49.04.62.58.06

6

0.603.021.452.923.50

.93

.21

.08*

.75*.85

15.00

2.452.16.19

1.09.20

1.25

.38A

2.21.00

1.10.00

5.81.00.23

1.511.251.061.206.753.603.60

.20

.186.001.95.42.18.79.15.72.40.02

7

2.507.963.152.627.28

12.3012.008.4510.2518.203.70

21.522.30

3.557.603.558.106.094.15

15.9022.236.332.014.9010.80

.5413.7515.05

11.6011.077.388.385.005.606.20

11.707.8010.0022.907.587.00

16.416.707.502.664.02

8

0.75.32.32T.22

1.102.101.302.642.611.161.93.00

.38

.52

.64

.51

.79

.65

1.013.02.11.50

1.16.86T2.251.30

1.632.001.192.12.45

1.05.80

3.551.901.055.40.36

1.676.661.60.49

1.131 .08

/.-DAYS TOTALS

4.3011.336.625.8312.04

14.7314.459.9913.1122.085.44

24.6817.30

7.8810.374.4010.787.886.55

17.7125.259.502.598.8111.667.3916.2816.72

15.1814.8810.6312.5712.7711.4011.10

15.559.9218.0530.669.319.15

24.358.499.334.775.18

EXPLANATION:

T - Trace.* - No reading from rain gage.

66

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-7,

19

85

flo

od

s.

CO

14

THE FLOODS OF OCTOBER 6-7, 1985 (Continued)

Peak Discharges

The U.S. Geological Survey collected peak stage and discharge information during and after the floods throughout its network of gaging stations in Puerto Rico. Historical peak flows were recorded at several stations in the network. These included gaging stations at the Rio Coamo, the Rio Inabon, the Rio Descalabrado, the Rio Jacaguas, the Rio Cerrillos, the Rio Portugues, and the Rio Grande de Manati (table A). Recurrence intervals equal to or greater than 100 years were estimated for most of the peaks recorded at the gaging stations from Santa Isabel to Ponce. Discharge hydrographs at key gaging stations showed that the peak discharges occurred during the early hours of October 7 (fig. 9).

Landslides

Landslides occurred throughout Puerto Rico as a result of saturation of the soils during the intense rains of October 5-8. The most severe landslide occurred on a hill at Barrio Mameyes, on the northwestern fringe of the city of Ponce (fig. 10). A slab of calcareous sandstone detached from the crest of the hill, slipping toward the bottom of a populated ravine. Field estimates showed that about 260,000 cubic yards of material slid from the hill.

The Mameyes landslide destroyed about 90 homes causing the death of about 130 people. Most of the houses were buried under mud and debris. The time of occurrence of the incident (around 3:00 a.m. on October 7) and the intensity of the rain at that time, affected the rescue efforts (only about 50 bodies were recovered). Although many other smaller landslides occurred throughout the island, the incident at Mameyes was the only one that resulted in loss of life. The Puerto Rico Department of Natural Resources is currently conducting an inventory of the landslides recorded during the floods.

COMPARISON OF THE 1985 FLOODS WITH HISTORICAL FLOODS

The floods of May and October 1985 were of severe magnitude and extreme frequency. Record peak discharges occurred at several basins including some of the largest in Puerto Rico. The recurrence interval of many of the instantaneous peak discharges equaled or exceeded the 100-year flood.

The "Myers Rating" method can be utilized to compare the intensity of floods from different basins and/or different events (Haire, 1972). In the Myers's technique, the unit discharge (flow per unit area) of a particular flood is compared with the maximum known floods throughout the world or in the study area. In Puerto Rico, the most recent extreme floods occurred in 1960, 1970, and 1975. Data from the 1960 floods and the 1985 floods were compared using the Myers's technique (fig. 11).

15

Table 4. Summary of gage heights and discharges during floods of October 6-7, 1985 at selected U.S. Geological Survey streamflow gaging stations throughout Puerto Rico

MAXIMUM PREVIOUSLY RECORDED

STAT'ON NAME

MAXIMUM DURING 6-7, 1985 FLOOD/V

50010600

50011200

50011400

5001480050015700

500277505002800050028400

5003120050035000

50038100

50038320500395005004300050046000

50050900

5005115050051180

5005131050053050

5005500050055650500564005005690050057000

5006180050063440500635005006380050065500

500670005007100050075000

50092000

50106500

50108000

50111500501125005011400050115000

5011590050124200501299005013600050138000

50144000

50147800

Rio Guajataca above Lago deGuajataca

Rio Guajataca below Lago deGuajataca

Rio Guajataca above mouth nearQuebradillas

Rio Camuy near BayaneyRio Camuy near Hatillo

Rio Grande de Arecibo above AreciboRio Tanama near UtuadoRio Tanama at Charco Hondo

Rio Grande de Manati near MorovisRio Grande de Manati at Ciales

Rio Grande de Manati at Hwy 2near Manati

Rio Cibuco below CorozalRio Cibuco at Vega BajaRio de la Plata at Proyecto La PlataRio de la Plata at Hwy 2

near Toa Alta

Rio Grande de Loizaat Quebrada Arenas

Quebrada Blanca at el JaqualQuebrada Salvatierra near

San LorenzoRio Cayaguas at Cerro GordoRio Turabo at Borinquen

Rio Grande de Loiza at CaguasQuebrada Caimito near JuncosRio Valenciano near JuncosQuebrada Mamey near GuraboRio Gurabo at Gurabo

Rio Canovanas near Campo RicoQuebrada Sonadora near El VerdeQuebrada Toronja at El VerdeRio Espiritu Santo near Rio GrandeRio Mameyes near Sabana

Rio Sabana at SabanaRio Fa jar do near Fa jar doRio Icacos near Naguabo

Rio Grande de Patillas nearPatillas

Rio Coamo near Coamo

Rio Descalabrado near Los Llanos

Rio Jacaguas at Juana DiazRio Inabon at Real Aba joRio Cerrillos near PonceRio Portugues near Ponce

Rio Portugues at Hwy 14 at PonceRio Guayanilla near GuayanillaLaguna Cartagena near BoqueronRio Rosario at RosarioRio Guanajibo near Hormigueros

Rio Grande de Anasco nearSan Sebastian

Rio Culebrinas at Hwy 404 near Moca

ND

ND

ND

NDND

140.0018.4057.60

55.20128.00

197.0015.1099.1054.80

200.00

6.003.25

3.7410.207.89

89.800.8216.402.30

60.20

9.841.010.068.626.88

3.9614.901.26

18.3046.00

12.90

49.809.7017.808.82

18.6018.90

ND16.40

120.00

134.0071.20

1984-85

1984-85

1969-701984-85

19851985

1982-851960-851969-711983-851965-851960-85

1963-851970-851959-851960-85

1960-85

1978-851985

19851978-85

1985

1960-851985

1972-851985

1960-85

1967-851983-851983-851966-851969-731983-85

1979-851961-851945-531962-661979-85

1966-851967-68

19701984-851966-691984-85

19851964-851964-851965-85

1963-851981-851984-851975-851973-85

1963-851969-85

05-18-85

09-18-84

09-19-84

05-18-8505-18-85

05-18-8505-18-8505-18-85

05-18-8510-09-70

05-18-8511-07-7912-13-8108-27-61

09-06-60

07-18-7905-17-85

05-17-8508-31-7905-17-85

09-06-6005-18-8505-18-8505-18-8509-06-60

12-13-8212-02-8307-05-8312-02-8309-04-73

04-21-8310-24-7404-21-83

09-16-7510-09-70

05-21-69

05-18-8510-09-7009-16-7509-16-75

09-16-7509-12-8206-11-8409-16-7509-16-75

09-16-7509-16-75

12.03

9.94

8.10

14.4220.67

18.2217.4517.95

17.8924.00

33.5419.8018.8432.20

36.35

13.4014.58

17.109.4417.06

31.1712.0721.308.43

27.70

13.108.601.71

12.0713.02

19.3513.628.96

12.4521.40

11.50

18.7820.6011.2010.10

17.3820.4011.0419.6028.50

33.9036.60

2600

590

3090

35106250

458001219015000

48000125000

136000136003030059600

95500

89507400

93201320011600

71500700

257001190

74600

150001410

181240019800

9010196002860

1480022000

7000

127005720

2240013100

1450014700

9433800128000

14000069000

--

--

--

NDND

330660260

870977

6909013061090

478

14902280

249012901470

79685015705201240

1520140028114402880

228013202270

809478

543

26054012601490

780778--

20601070

1480969

ND

ND

ND

NDND

ND50ND

5030

25103015

25

20ND

ND10ND

20ND25ND15

5NDND1010

51075

515

10

ND7

10025

30NDND10020

10025

6

6

6

77

77

77

7776

7

66

666

66676

66776

666

67

7

7777

77777

77

10.10

10.40

7.97°

17.7024.81

15.6917.11°

13.17°19.75

1550

730

2970

649010500

784011500

2850075400

33.80 150000813.0817.9822.00°

24.58

10.039.24

10.3915.2613.27

19.8013.3711.337.66

19.40

9.807.561.607.598.57

13.1910.485.18

11.04--

24.37C

29.42°--

20.8320.20°

43202020028900

75000"

38701500"

198039505120

2280010007470840

19400

6960330>1034905850

22006860620

585054000

18800"

37000°15000"24000"15500"

17.70 [>1600019.13°14.3810.0228.00°

21.94°26.94

119007400"664030000"

53600"23000

ND

ND

ND

NDND

ND430200

520590

761290200530

375

650460

530390650

2501220460370320

710330

>170400850

560460490

3201170

1450

740155013501760

860630ND400250

400320

ND

ND

ND

NDND

5ND

1215

302

155

20

2ND

ND2

ND

3ND2

ND3

3NDND22

222

2100

100

ND100

>10070

35NDND25

305

EXPLANATION EXPLANATION

a - Estimate based on rating extension b - Estimate based on poor highwater mark > - Greater than given value

ND - Not determinedc - High-water mark elevatione - Estimated, subject to revision.

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Figure 10. Location of Barrio Mameyes near the city of Ponce in southern Puerto Rico.

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19

COMPARISON OF THE 1985 FLOODS WITH HISTORICAL FLOODS (Continued)

The upper line in figure 11 represents the maximum known floods in the world expressed as a correlation between the unit discharge and the drainage area by the equation Q = 10,000/VA. This is commonly referred to as the "100 percent Myers rating." The lower line represents the "50 percent Myers rating" expressed by the equation Q = 5,000/</A.

The data from the 1985 floods show that the unit discharges were not as high as the 1960 events. At least five (5) events during the 1960 floods approached the 100 percent Myers rating. In general, unit discharges for the 1985 floods were in the vicinity of the 50 percent Myers curve. The only events of the 1985 floods that approached the record 1960 unit discharges occurred at the Rio Grande de Anasco near San Sebastian during the May event (table 2) and, the Rio Coamo near Coamo during the October event (table A).

SUMMARY

Puerto Rico was affected by severe floods twice during 1985. During May 15-19, 1985, a low-pressure system that moved from the north resulted in as much as 25 inches of precipitation over the south-central and up to 1A inches over the eastern basins of the island. Flooding was widespread along the mainstem and tributaries of the Rio Grande de Arecibo, the Rio Grande de Manati, and the Rio de La Plata on the north coast. Urban areas in the towns of Arecibo, Barceloneta, Jayuya, and Utuado were affected by the floods. Less severe flooding occurred at Caguas, Loiza, Dorado, Vega Baja, and Toa Baja. Property losses were estimated at about 37 million dollars.

The second flood event occurred during October 6-7, 1985. A nearly stationary tropical depression, preceded by several days of rainfall, resulted in record precipitation and floods along the south-central and eastern part of Puerto Rico. Precipitation totals were as high as 23 inches during 2A hours. Intensities of 5.5 inches in two hours were recorded near the city of Ponce. Flooding was severe and widespread from Santa Isabel to Ponce. The worst flooding occurred in the Rio Coamo basin, but was also significant along the north coast in the towns of Barceloneta, Arecibo, and Toa Baja. Recurrence intervals equal to or greater than 100 years were recorded at several index stations. Property damages were about 125 million dollars.

The rain and floods of October 6-7, 1985 resulted in landslides throughout the affected areas. The worst landslide occurred at Barrio Mameyes, near the city of Ponce. A slab of sandstone with a volume of about 260,000 cubic yards detached from a hill, destroying about 90 homes in the Mameyes community. The death total from the landslide was about 130 people.

The collapse of the approach section of the bridge over the Rio Coamo on Las Americas Expressway resulted in the death of 2A people. Flooding at the Quebrada del Agua, west of Ponce, resulted in an additional 16 fatalities.

20

REFERENCES CITED

Hickenlooper, I.J., 1968, Floods at Arecibo, Puerto Rico: U.S.Geological Survey Hydrologic Investigation Atlas HA-271, 1 plate.

Haire, W.J., 1971, Floods in the Santa Isabel Area, Puerto Rico: U.S. Geological Survey Hydrologic Investigations Atlas HA-448, 1 plate.

Haire, W.J., 1972, Floods of October 5-10, 1970 in Puerto Rico: U.S. Geological Survey Water Resources Bulletin 12, 38 p.

Johnson, K.G., and Carrasquillo, Ramdn, 1982, Floods of October 9, 1970 and September 16, 1975 at Jayuya, Puerto Rico: U.S. Geological Survey Water Resources Investigations 81-346, 1 plate.

Rib Grande de Arecibo at Sector Santa Barbara, near Arecibo, May 19, 1985.

Rio Grande de Arecibo at De Diego Expressway bridge In Arecibo, May 19, 1985.

Coamo Dam, downstream side, October 8, 1985.

Lago Coamo at Coamo dam, October 2O, 1985.