Flooding Associated with Typhoon Chata’an, July 5, 2002, Guam

By Richard A. Fontaine 

INTRODUCTION

On July 5, 2002, starting at about 8 a.m., 

the southern half of the eye of Typhoon 

Chataʼan passed directly over the northern 

part of the island of Guam. Data collected on 

Guam indicate that the typhoon had sustained 

winds of 85 to 90 miles per hour (mi/hr) with 

gusts of up to 115 mi/hr (Charles Guard, 

National Weather Service, written commun., 

2003). Storm rainfall totals exceeded 21 

inches (in.) over the mountainous areas in 

south-central Guam. During the peak of the 

storm, rain fell at rates of up to 6.48 inches 

per hour (in/hr). Because of the damage 

caused by Typhoon Chataʼan, the President 

signed a major disaster declaration on July 6, 

2002. 

Damages associated with Typhoon 

Chataʼan, while considered moderate relative 

to other storms that have affected Guam, 

amounted to several tens of millions of dollars. In excess of 1,000 single-family and 

multi-family homes were either extensively 

damaged or destroyed. Electrical power was 

out for several days over most of the island 

and no potable water was available through 

public distribution systems (Federal Emergency Management Agency, 2002). The 

extreme rainfall led to flooding in southern 

Guam and caused numerous landslides and 

severe erosion along water courses. The most 

significant evidence of these effects could be 

found in the Fena Valley Reservoir, where 

elevated sediment concentrations made the 

water unsuitable for use as a domestic water 

supply for several days. During normal operation, Fena Valley Reservoir supplies most of 

the drinking water for the military and some 

of the general public in southern Guam. All 

of the stream-gaging stations operated by the 

U.S. Geological Survey (USGS) on Guam 

were damaged to some extent during the 

flood and three of the stations were totally 

destroyed. 

Peak flows in many rivers in southern 

Guam reached record levels during Typhoon 

Chataʼan. New record peak stages and/or 

flows of record occurred at 14 of 15 sites 

where the USGS has collected data. In some 

areas, the magnitude of flood peaks exceeded 

previous records significantly. Peak flows 

had recurrence intervals of 80 years or more 

at 9 of the 13 sites where sufficient data were 

available to make the computations. Four of 

the 9 sites had recurrence intervals that were 

determined to be greater than 100 years. 

In this fact sheet, storm rainfall totals and 

maximum rainfall totals for durations of 1-, 

3-, 6-, and 12-hours are summarized for 12 

rain gages on Guam. Peak stages and/or flows 

were computed at 15 USGS streamflow-gaging stations and recurrence intervals for the 

peaks determined. Rainfall and streamflowgaging stations operated by the USGS on 

Guam are supported by funding provided by 

numerous agencies including the U.S. Navy, 

the U.S. Army Corps of Engineers (USACE), 

and the University of Guam through the 

Water and Environmental Research Institute 

(WERI). The USGS Office of Surface Water, 

as part of a national program to document the 

effects of extreme floods in the United States, 

provided funding to support the preparation of 

this fact sheet. 

RAINFALL

Total rainfall for the period of the storm 

(July 4-5, 2002) and maximum rainfall totals 

for durations of 1-, 3-, 6-, and 12-hours at 12 

rain gages are shown in table 1. Included in 

table 1 are data for 7 USGS rain gages, provisional data for 3 National Weather Service 

(NWS) rain gages (Charles Guard, National 

Weather Service, written commun., 2003), 

and provisional data for 2 rain gages operated 

as part of the Tropical Rainfall Measuring 

Mission (TRMM) (Mark Lander, University 

of Guam Water and Environmental Research 

Institute, written commun., 2003). The locations of the 12 gages and total rainfall for July 

4-5, 2002 are shown in figure 1. 

Storm rainfall totals for Typhoon Chataʼan 

exhibited a distinctive pattern. The northern 

part of Guam received less than 10 in. of 

rain, and rainfall totals gradually increased to 

values of 10 to 13 in. in the central part of the 

island. Further south there is a sharp gradient 

in rainfall totals between the central part of 

the island and the upland areas of south-central Guam, especially in the vicinity of the 

Fena Valley Reservoir, where rainfall totals 

of 19 to 21 in. were recorded. Rainfall totals 

drop off sharply in the extreme southern part 

of the island. Rainfall totals at Mount Jumullong Manglo, Umatac, and Merizo rain gages 

were 19.20, 14.16, and 8.3 in., respectively. 

This rainfall pattern is evident from the data 

in figure 1 and in table 1, which lists the gages 

from north to south. 

The pattern of storm total rainfall is 

a direct response to the path followed by 

Typhoon Chataʼan across Guam. During the 

passage of a typhoon, the areas within the eye 

of the storm receive lesser amounts of rainfall 

and areas just outside the eye in the eye wall 

of the storm receive the maximum amounts of 

rainfall. Rainfall totals diminish with increasing distance away from the outer edge of the 

eye wall (Mark Lander, University of Guam 

Water and Environmental Research Institute, 

oral commun., 2003). The southern half of the 

eye of Typhoon Chataʼan passed directly over 

the northern part of Guam. The distribution of 

rainfall totals indicates that the southern edge 

of the eye passed near the central part of the 

island in the area where dramatic increases in 

rainfall were recorded. 

Maximum rainfall intensities during the 

storm show the same pattern as total rainfall. 

Rain gages in north-central Guam recorded 

maximum 1-hour rainfall totals that ranged 

from 1.17 in. at Mount Santa Rosa to 2.64 in. 

at Andersen Air Force Base. The Almagosa 

rain gage, near the Fena Valley Reservoir, 

recorded a maximum 1-hour total of 6.48 in. 

Further to the south intensities diminished 

with a maximum 1-hour rainfall total of 

2.24 in. recorded at Merizo. The distribution of hourly rainfall during the storm at the 

University of Guam, Almagosa, and Umatac 

rain gages is shown in figure 1. Rainfall at 

the University of Guam gage, which is in 

an area that was likely within the eye of the 

storm, lacks the significant spikes of high 

rainfall intensities recorded at the Almagosa 

and Umatac gages, which were both in an area 

to the south of the eye in the eye wall of the 

typhoon. 

Maximum rainfall intensities were 

recorded between 8 and 10 a.m. (fig. 1). This 

period corresponds with the time when the 

eye of the typhoon reached and transversed 

the northern part of the island. In the areas of 

the typhoon eye wall, the period of maximum 

U.S. Department of the Interior Fact Sheet 061-03

U.S. Geological Survey
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Table 1. Maximum rainfall totals, in inches, for selected durations, Typhoon Chata’an, July 4–5, 2002, Guam 

[USGS, U.S. Geological Survey; TRMM, Tropical Rainfall Measuring Mission; NWS, National Weather Service; --, not available] 

Rainfall gage 1 hour 3 hours 6 hours 12 hours July 4–5 

Andersen Air Force Base 2.64 4.14 4.82 6.40 7.12b 

TRMM gagea 

Mount Santa Rosa 1.17 1.91 2.36 3.39 5.50 

USGS gage 133209144545301 

Weather Forecast Office, Guam 2.48 5.27 8.64 10.44 

NWS gagec 

Sinajana 10.90 

NWS gagec 

University of Guam 2.33 3.84 6.97 9.17 10.88 

TRMM gagea 

Mount Chachao 12.72d 

USGS gage 132617144423366 

Windward Hills 3.24 6.48 8.64 10.44 13.32 

USGS gage 132234144441966 

Fena Reservoir Pump Station 5.64 10.56 13.92 16.08 19.08 

USGS gage 132132144422366 

Almagosa 6.48 12.24 15.60 17.88 21.12 

USGS gage 132105144405166 

Mount Jumullong Manglo 5.88 10.80 13.68 15.96 19.20 

USGS gage 131921144401301 

Umatac 4.20 7.44 10.08 11.76 14.16 

USGS gage 131729144393766 

Merizo 2.24 5.89 8.3e 

NWS gagec 

a Provisional rainfall data from Mark Lander, University of Guam Water and Environmental Research Institute
 b Rainfall total through 12:00 a.m. on July 5
 c Provisional rainfall data from Charles Guard, National Weather Service, Guam
 d Estimated storm total based on proration of accumulated total for period July 2–15, 2002
 e Estimated storm total based on ratio of maximum 6-hour to storm-total rainfall at the Umatac rain gage


Table 2. Peak flows and stages and associated recurrence intervals for Typhoon Chata’an on July 5, 2002 and previous record peak stages and 

flows at U.S. Geological Survey streamflow-gaging stations on Guam 

[ft3/s, cubic feet per second; CSG, crest-stage gage; n.a., not applicable; n.d., not determined; <, actual value is less than value shown; >, actual value is greater 

than value shown] 

Previous record July 5, 2002 

Station 

number Station name 

Peak flow 

(ft3

/s) 

Peak 

stage 

(feet) 

Date of 

previous peak 

Period of peak 

records 

Peak flow 

(ft3

/s) 

Peak 

stage 

(feet) 

Recurrence 

interval 

(years) 

16807200 Agana River at Agana (CSG) 600 n.a. 09/06/1985 1985–1989 1,720 n.a. <10a 

16807600 Asan River at Asan (CSG) n.a. 10.10 11/03/1997 1997–2002 3,920 12.27 >100a 

16807650 Aplacho River at Apra Heights 929 23.55 09/07/2000 2000–2002 1,690 25.22 >100b 

16808120 Namo River at Santa Rita (CSG) 1,220 n.a. 09/11/1989 1985–1989 1,810 9.60 <10b 

2002 

16809500 La Sa Fua River near Agat n.a. n.a. n.a. 2002 1,980 13.5 100b 

16809600 La Sa Fua River at 310 feet near 1,440 6.05c 09/27/1978 1954–1960 2,140 17.73 50 

Umatac 1977–1983 

2000–2002 

16816000 Umatac River at Umatac 7,460 7.04 10/19/1960 1954–1976 6,240d 10.75d 15 

2002 

16846000 Tolaeyuus River at mouth n.a. 20.47 12/17/1997 1994–1995 n.d. 28.06 n.d. 

1997–2002 

16847000 Imong River near Agat 6,100 11.30 09/27/1978 1961–2002 5,920d 13.53d 80 

16848100 Almagosa River near Agat 2,650 7.78 09/27/1978 1972–2002 6,330 10.80 90 

16848500 Maulap River near Agat 2,420 9.20 09/27/1978 1972–2002 5,340 11.23 >100 

16849000 Fena Dam Spillway near Agat n.a. 4.50 10/15/1953 1953–2002 n.a. 5.36 >100e 

16854500 Ugum River above Talofofo Falls 5,890 14.2 02/26/1980 1977–2002 14,700 21.50 80 

16858000 Ylig River near Yona 3,410 18.63 12/16/1997 1953–1998 n.d. 23.45 n.d. 

16865000 Pago River near Ordot 10,100 20.15 05/21/1976 1952–1982 16,500 23.55 90 

1999–2002 

a Recurrence interval based on peak discharge data published in the Flood Insurance Study, Territory of Guam (Federal Emergency Management Agency, 1998) b Recurrence interval based on regional, peak discharge, regression equations published in the Flood Insurance Study, Territory of Guam (Federal Emergency 

Management Agency, 1998) c Gage datum was changed by 10.00 feet in 2000; the previous peak stage of record would be 16.05 feet using the current gage datum d During Typhoon Chata’an a new record peak stage was established but not a new record peak flow. This was the result of changes in the stage-discharge ratings 

between the time of the previous record peaks and Typhoon Chata’an e Recurrence interval based on analysis of peak stages 
rainfall intensities likely corresponded to 

times when wind speed was high. It is well 

documented that rain gages under-register 

rainfall in windy conditions and that the 

magnitude of this error increases with wind 

speed; Larson and Peck (1974) reported rainfall catch deficiencies of about 20 percent at 

winds speeds of 20 mi/hr. Wind speeds during 

the passage of the typhoon and the periods of 

maximum recorded rainfall intensities were 

likely to have been significantly greater than 

20 mi/hr. Actual maximum rainfall intensities were therefore likely to have been much 

greater than the intensities recorded. 

PEAK FLOWS

As part of this study, peak flows and/or 

peak stages were determined at 15 current 

or former USGS stream gages in Guam. The 

peak flow and stage data are summarized 

in table 2 and the locations of the gages are 

shown in figure 1. Northern Guam has essentially no stream drainage because the area is 

composed primarily of permeable limestone 

through which rainfall rapidly infiltrates and 

there is little or no surface water runoff (Ward 

and others, 1965). Southern Guam has a dense 

network of streams because the area is composed primarily of volcanic rocks with limited 

infiltration capacities. During the Typhoon 

Chataʼan storm, rainfall totals were in excess 

of 10 in. over most of southern Guam, generating significant volumes of surface-water 

runoff. Peaks flows and/or stages in this area 

exceeded previous maximums at 14 of the 15 

USGS stream gages where flood measurements were made. 

During the peak of the storm, it was neither safe nor practical to make direct measurements of peak flows and stages. As a result, a 

variety of indirect methods based on hydraulic 

models and hydrologic techniques were 

used to compute peak stages and flows. The 

indirect methods used are site specific and had 

to be tailored to the conditions at each of the 

USGS stream gages where computations were 

made (Rantz and others, 1982, chapter 9). 

The peak stage recorded at the Ylig River 

stream gage was 23.45 feet (ft), 4.82 ft higher 

than the previous maximum level recorded 

since 1953. The peak stage recorded in the 

Fena Valley Reservoir was 5.36 ft above the 

spillway crest and 0.86 ft higher than the 

previous maximum recorded since 1953. 

The pattern of maximum peak flows closely 

corresponds to the distribution of rainfall 

over Guam. For example, the Umatac River 

was one of the two sites where a new record 

peak flow was not established during the 

typhoon. This is also an area where rainfall 

totals declined sharply from the areas just 

to the north. USGS stream gages located 

in the vicinity of the Fena Valley Reservoir 

had particularly high peak flow rates. USGS 

stream gages have been operated on the 

Almagosa and Maulap Rivers since 1972 and 

on the Ugum River since 1977. The previous peak flows of record for these gages took 

place in 1978 and 1980 when the Almagosa 

River peaked at 2,650 cubic feet per second 

(ft3

/s), the Maulap River peaked at 2,420 ft3/s, 

and the Ugum River peaked at 5,890 ft3

/s. 

Peak flow rates during Typhoon Chataʼan on 

the Almagosa, Maulap, and Ugum Rivers of 

6,330 ft3

/s, 5,340 ft3/s, and 14,700 ft3/s are 

239, 221, and 250 percent greater than the 

previous records. 

The distribution of streamflow during 

Typhoon Chataʼan is shown in the plots of 

hourly discharge included in figure 1 for the 

Pago River, Maulap River, and La Sa Fua 

River stream gages. Flow at these selected 

stream gages responds quickly to periods of 

high rainfall intensity. At each of the stream 

gages the sharp increase in flows coincides 

closely with the onset of the periods of maximum rainfall intensities. The peak flow rates 

at the three stream gages took place within 

an hour of the end of the maximum rainfall 

intensities. Streamflow dropped off sharply 

from the peaks in conjunction with the rapid 

reductions in rainfall. 

FLOOD FREQUENCY

The recurrence intervals associated with 

the flood peaks were computed following the 

guidelines outlined in Bulletin 17B (Interagency Advisory Committee on Water Data, 

1982). Because a generalized map skew has 

not been computed for Guam, the analyses 

were based on the station skew at each of the 

stream gages. At five USGS stream gages, 

sufficient data to compute recurrence intervals 

were not available. For those gages the recurrence intervals were estimated using computations summarized in a published flood insurance study for Guam (Federal Emergency 

Management Agency, 1998). Recurrence 

intervals were not computed for the Tolaeyuus 

and Ylig Rivers because peak flow computations for Typhoon Chataʼan could not be made 

at those sites. 

The recurrence interval is an estimate of 

the average number of years between floods 

of a magnitude that equal or exceed the computed value. For example, a peak flow with 

a recurrence interval of 100 years is one that 

would be equaled or exceeded, over the long 

term, an average of once every 100 years. An 

alternative way to view the recurrence interval 

is to take its reciprocal (for example 100 

years becomes 1/100 or 0.01), which yields 

the annual exceedance probability. Therefore, 

a 100-year flood has an annual exceedance 

probability of 0.01, or a 1 percent chance of 

being equaled or exceeded in any given year. 

The computed recurrence intervals in years 

are summarized in table 2 and are shown in 

figure 1. With the exception of four stream 

gages, the computed recurrence intervals 

were greater than or equal to 80 years. The 

recurrence intervals at the lower La Sa Fua, 

Umatac, and Agana River stream gages were 

50, 15, and less than 10 years, respectively. 

These stream gages are located in areas that 

received less rain than the upland areas of 

south-central Guam. A recurrence interval of 

less than 10 years was computed for the Namo 

River. No rainfall data were available in the 

immediate area to either support or contradict 

this apparent outlier. Recurrence intervals 

were determined to be greater than 100 years 

for the Asan, Aplacho, and Maulap River and 

Fena Valley Reservoir gages. 

REFERENCES CITED

Federal Emergency Management Agency, 

1998, Flood Insurance Study, Territory 

of Guam: June 8, 1998 revision, variously paged. 

Federal Emergency Management Agency, 

2002, Mitigation Operational Strategy, 

Typhoon Chataʼan, FEMA-1426-DR, 

GU, 9 p. 

Interagency Advisory Committee on 

Water Data, 1982, Guidelines for 

determining flood flow frequency: 

Water Resources Council Bulletin 17B, 

28 p., 14 app., 1 pl. 

Larson, L.W., and Peck, E.L., 1974, 

Accuracy of precipitation measurements for hydrologic modeling: Water 

Resources Research, v. 10, no. 4, p. 

857-863. 

Rantz, S.E., and others, 1982, Measurement and computation of streamflow, 

volumes 1 and 2: U.S. Geological Survey Water-Supply Paper 2175, 631 p. 

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ISLAND OF GUAM


17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

0 

2000 

4000 

6000 

8000 

10000 

12000 

14000 

16000 

18000 

20000 

DISCHARGE, IN CUBIC FEET PER SECOND 

Pago Station 16865000 

� Provisional Discharge Data 

University of Guam Rain Gage 

� July 4-5 total rainfall 10.88 inches 

July 4 July 5 

DATE AND TIME, IN HOURS 

DATE AND TIME, IN HOURS 

0 

1 

2 

3 

4 

5 

6 

16 17 18 19 20 21 22 23 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

RAINFALL, IN INCHES 

0 

1000 

2000 

3000 

4000 

5000 

6000 

DISCHARGE, IN CUBIC FEET PER SECOND

July 4 July 5 

Maulap Station 16848500 

� Provisional Discharge Data 

Almagosa Rain Gage 

� July 4-5 total rainfall 21.12 inches 

3 

4 

5 

6 

1500 

2000 

2500 

BIC FEET PER SECOND 

La Sa Fua Station 16809600 

� Provisional Discharge Data 

Umatac Rain Gage 

� July 4-5 total rainfall 14.16 inches 

Pago 

River 

Ylig River Namo River 

Agana R

Asan

R 

Almagosa R 

Aplac h o River 

Talofofo River 

Imong R 

La S a Fua

R 

Umatac R 

Ugum R 

Maulap R 

Umatac 

14.16 

Mt. Jumullong 

Manglo 

19.20 

Mt. Santa 

Rosa 

5.50 

Mt. Santa

Rosa

5.50

University 

of Guam 

10.88 

Sinajana 

Mt. 10.90

Chachao 

12.72 

Weather Forecast Office 

Guam 

10.44 

Andersen 

Air Force Base 

7.12 

Merizo 

8.3 

Almagosa 

21.12 

Fena Pump 

Station 

19.08 

Windward 

Hills 

13.32 

80720 

<10 

84850 

>100 

84810 

90 

80950 

100 80960 

50 

81600 

15 

85450 

80 

84700 

80 

84600 

n.d. 

84900 

>100 

85800 

n.d. 

86500 

90 

80812 

<10 

80765 

>100 

80760 

>100 

Mangilao 

Umatac 

Talofofo 

Dededo 

Yigo 

Agana 

Umatac

Agat 

Inarajan Merizo 

Dededo

Yigo

a Harbor 

Talofofo 

Bay 

Ylig Bay 

Fena Valley 

Reservoir 

Tumon Bay 

Agana Bay 

Agat Bay 

Pago Bay 

PHILIPPINE 

SEA 

PACIFIC 

OCEAN 

144 ̊40' 

144 ̊50' 

13 ̊30' 

 

0 

0 

2 MILES 

SCALE 

2 KILOMETERS 

Base from U.S. Geological Survey 

Island of Guam, 1:62,500, 1953