Journal of Natural Disaster Science, Volume 26, Number 2, 2004, pp95-100

Rainfall-Runoff Analysis of Flooding Caused by Typhoon RUSA in 2002
in the Gangneung Namdae River Basin, Korea

Hidetaka CHIKAMORI
Faculty of Environmental Science and Technology, Okayama University
3-1-1 Tsushima-naka, Okayama, 700-8530, Japan(Received for 16. Aug., 2004 and in revised from 6 Jan., 2005)

ABSTRACT
Gangneung City on the east coast of the Korean Peninsula was devastated by flooding from August 31 to
September 1, 2002 caused by Typhoon Rusa. The purpose of this study was to clarify the magnitude of the 2002
flooding caused by that typhoon in the Gangneung Namdae River Basin. The peak discharge observed at
Gangneung was compared to the maximum peak discharge predicted empirically based on parameters for the climatically similar Tohoku district of Japan. The observed peak discharge from the Gangneung Namdae River
exceeded the maximum peak discharge predicted for the Tohoku district. In addition, a rainfall-runoff analysis of
the storm showed that the total areal rainfall in the basin, estimated from the discharge recorded at Gangneung, forthe storm may have exceeded 1,000 mm, considerably more than the 898 mm reported for the observed rainfall at
Gangneung during that period.

1. INTRODUCTION
From August 31 to September 1, 2002, the entire Korean
Peninsula suffered from extensive flooding and landslides on being
hit by Typhoon Rusa. Gangneung, a city on the east coast of thepeninsula, was one of the most devastated areas, 870 mm/d being
recorded on August 31, 2002 - the highest historical record - and a
total rainfall of 898 mm recorded from August 31 to September 1.
The heavy rainfall caused severe flooding in the region, inundating
the entire urban area of Gangneung.
The purpose of this study was to clarify by two methods themagnitude of the flooding in 2002 caused by Typhoon Rusa in the
Gangneung Namdae River Basin. One was to compare the magnitude of flooding in Gangneung with flooding that occurred inJapan. The other was to estimate the actual areal rainfall in the
Gangneung Namadae River Basin by rainfall-runoff analysis.
The magnitude of the flooding in Gangneung was analyzed bymeans of equations of enveloping curves used to assess regional
flood peaks in the Tohoku district of Japan. The equations used inJapan were applied to the Gangneung Namdae River Basinbecause the east coast area of the Korean Peninsula is considered
to have climatic conditions similar to those of the Tohoku.
Areal rainfall in the basin was reevaluated in order to examine
the possibility of there have been much more rainfall than wasrecorded at observatories. This could be assessed by analysis of
the water balance during the storm, as well as from analysis ofradar rainfall data. In addition, rainfall-runoff was analyzed bymeans of a kinematic runoff model, and the calculated discharge
estimated from observed rainfall data compared to the observed

discharge. In another approach, areal rainfall throughout theGangneung Namdae River Basin was estimated by means of apower-type Tank Model from discharge data recorded in Gangneung.
2. RIVER BASIN AND DATA
2.1 Gangneung Namdae River Basin
The Gangneung Namdae River rises in the Taebaek Mountain
Range, runs through the city of Gangneung after joining theWangsan River, and then flows into the Japan Sea (Fig. 1). The
basin area is 258.7 km2, and length of the main channel is 32.9 km(Ministry of Construction and Transportation, Korea, MOCT). In
this study, the area of focus was the region upstream of the urban
area of Gangneung City, an area of 171.1 km2.
This area is separated into two parts; upstream and downstream of the Gangneung Reservoir. The upstream area consists of
two sub-basins; the Namdae River and the Wangsan River basins.
The terrain of the study area is very steep with a elevation
range of as much as 1,400 m, and the main channel is only 32.9 km
long. Landuse in the area is mainly for agriculture and forestry.
2.2

Hydrological Data
Rainfall and runoff data for flooding in the basin are available
for August 30 to September 4, 2002. The Korea Meteorological
Administration (KMA) has installed two rain gauges at Gangneungand Daegwallyeong, and the MOCT has observed water levels at
Gangneung. Water level data was converted to observed discharge
by means of two rating curves determined by the MOCT (seeSection 3.2).

KEY WORDS: Typhoon RUSA, Gangneung, Korea, flood magnitude, rainfall estimation

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H. CHIKAMORI

Ja

pa

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Fig. 2 Hyetographs at Gangneung and Daegwallyeong.

Fig. 1 Physiographical map of the Gangneung Namdae River Basin.

3. MAGNITUDE OF FLOODING CAUSED BYTYPHOON RUSA
Fig. 3 Hydrograph at Gangneung.

3.1

RainfallFig. 2 shows variation in rainfall recorded at Gangneung andDaegwallyeong from August 31 to September 1. At Gangneung,the maximum hourly rainfall was 98 mm/h, and the maximum
daily rainfall 870 mm/d; both recorded maximums.
The waveform of rainfall variation has two peaks. The cumulative rainfall from the onset of precipitation exceeded 200 mm
when the first peak occurred. Therefore, it is likely that the soil ofthe basin would have been almost saturated when the second peakoccurred. Moreover, with saturation of the soil surface adversely
affecting rainwater infiltration, almost all of the precipitation
would have become surface water and caused severe flooding.

3.2

Discharge
Variations in discharge at the Gangneung gauging station during the storm were estimated by converting water level data bymeans of two different rating curves derived by the MOCT at thatstation (equations (A) and (B)). Fig. 3 shows those variations.
Here, equation (A) was mainly used to calculate discharge because
the usable water level range for equation (A) is greater than that ofequation (B). Hereafter, discharge calculated by means of equation (A) is referred to as observed discharge. The upper limitof the range for equation (A) was closer to the highest water level
than was that of (B), but the water levels around the time of peakdischarge during the flooding were beyond the usable ranges of theequation (A) and equation (B). The highest water level on the staff
gauge was 5.34 m, whereas the respective upper limits of theusable ranges of equations (A) and (B) are 3.5 m and 0.83 m.
Peak discharge recorded at the Gangneung gauging station
was compared with the maximum peak discharge computed bymeans of enveloping curve equations that often are used to design
flood control measures in Japan. The enveloping curves weredrawn from regional flood-peak data for each district in Japan.
Because the climate on the east coast area of the Korean Peninsula
is considered to resemble that of the Tohoku district in Japan, calculation of maximum peak discharge at Gangneung is assumed to
be derivable by the use of the enveloping curve equations for the
Tohoku district.
In Japan, the following enveloping curves generally are used:(1) Creager Enveloping Curves
The general form of the Creager equation is:
Q = CA ( A- 0.05) -1)(1)

RAINFALL - RUNOFF ANALYSIS OF FLOODING CAUSED BY TYPHOON RUSA IN 2002 IN GANGNEUNG, KOREA

Table 1.

Table 2.

Comparison of observed and estimated probable maximum peak discharges in the Namdae River basin.

m3/s/km2 by equation (2). The observed discharge also was comparable to the maximum peak discharges predicted for Japan, 24.9
m3/s/km2 by enveloping curve equation (1) and 24.0 m3/s/km2 byequation (2). These results indicate how large the magnitude of
the flood was.

Water balance during the storm period.

where Q is the peak discharge (m3/s/km2), C a coefficient dependent on the location of the objective drainage basin, and A thedrainage area (km2). The value of C is 34 for the Tohoku districtof Japan and 80 for the southern Shikoku, southern Kyushu, andOkinawa islands, the largest peak values for flood discharge inJapan.
(2) Kadoya’s Enveloping Curves
Kadoya and Nagai (1979) presented the enveloping curve
equation:(97

Q = KA - 0.06 exp - 0.04 A 0.45)

(2)where Q is the peak discharge (m3/s/km2), K a coefficient dependent on the location of the objective drainage basin, and A thedrainage area (km2). The value of K is 26 for the Tohoku districtof Japan and 49 for the southern Kii Peninsula of Honshu Island,
southern Shikoku Island and Kyushu Island where the equation
gives the largest values for peak discharge in Japan.
A comparison of peak discharges estimated from the enveloping curves and observed discharges is shown in Table 1. The
observed discharge exceeds the maximum peak discharge predicted for the Tohoku district. The observed peak discharge was calculated as 22.1 m3/s/km2 by means of the rating curve (A), whereas
the peak discharge for a basin in the Tohoku district of Japan withthe same area as that of Gangneung Namdae River Basin was estimated as 10.6 m3/s/km2 by enveloping curve equation (1) and 12.7

3.3 Water Balance during the Storm
Table 2 shows the total runoff depth at Gangneung and total
amount of rainfall at Gangneung and Daegwallyeong during
Typhoon Rusa. The two values for total runoff depth shown in thistable were derived from water level data by means of rating curves(A) and (B).
The estimate for total observed runoff depth (calculated byrating curve (A)) appears inordinately large given the total rainfalls
at Gangneung and Daegwallyeong. Moreover, the ratio ofobserved runoff depth to observed rainfall at Gangneung was ashigh as 88 % even though the basin probably was dry before thestorm. The amount of rainfall for the ten days preceding the storm(August 21 - 30) was as little as 4 mm at Gangneung and 18.9 mm
at Daegwallyeong. The high ratio is due to a water imbalance
between the observed rainfall and runoff. The reason is that theobserved discharge calculated by a rating curve was greater than
the actual discharge and/or the observed rainfall was in fact lessthan the actual amount that fell in the basin.
4. RAINFALL-RUNOFF ANALYSIS USINGKINEMATIC RUNOFF MODEL
To clarify the reasons for the water imbalance describedabove, discharge at Gangneung was simulated in a kinematic
runoff model. The simulated discharge at Gangneung, derived
from the observed rainfall, was compared to the observed discharge calculated by rating curve (A).
4.1 Model of Gangneung Namdae River Basin
Three sub-basins make up the Gangneung Namdae River
Basin, two upstream of the Gangneung Reservoir and one downstream. Each sub-basin could be expressed as a combination of
two rectangular planes and one channel in the kinematic runoff
model. The Gangneung Reservoir was expressed by a Power-type
Tank Model which considers the retardation effect of the reservoir.
The upstream and downstream parts of the basin are connected by

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H. CHIKAMORI

Table 3. Parameters used in the kinematic runoff model for
the Gangneung Namdae River Basin.

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The continuity equation isFig. 4

Schematic illustration of the kinematic wave runoff
model for the Namdae River basin.

∂h ∂q
+
= re
∂t ∂x
where t is the time (s), x the distance (m), and re the effective rainfall (m/s).
(2) Channel flow parameters
The momentum equation for channel flow is
W = KQ P

Fig. 5Retention curve for mountainous area. The solid line
shows the relationship between cumulated rainfall
and rainwater that did not contribute to direct runoff.
The dashed line shows the case in which no rainfall
contributes to direct runoff.

the reservoir. A schematic illustration of the rainfall-runoff modelfor the Gangneung Namdae River Basin is shown in Fig. 4. The
area of the planes and the channel length of each sub-basin are
given in Table 3.
Rainfall distribution was assumed to be as shown in Fig. 4,
which considers the distance between each sub-basin and each rain
gauge station. Effective rainfall was calculated by means of the
cumulated rainfall-retention relationship (Fig. 5) which is based on
rainfall and runoff data observed in Japanese forests.
4.2 Basic Equations and Parameter Calibration(1) Surface flow parameters
The momentum equation for surface flow is
h = kqp
where h is the flow depth (m), q the flow discharge per unit widthof a slope (m2/s), and k and p are parameters. Parameter p was set
at 0.6 based on Manning’s law. The initial value for k was set at
1.0 because k was found to range from 1.0 to 2.0 in Japanese
forests (Nagai et al., 1988). Parameter k subsequently was calibrated and the hydrograph calculated fitted to the observed one.
Parameter k, used to calculate runoff in each sub-basin, is shown in
Table 3.

where W is the cross section of channel flow (m2), Q the discharge
in a channel (m3/s), and K and P are parameters. Initial values for
K and P respectively were set at 1.0 and 0.7 based on findings of aJapanese study (Kadoya and Fukushima, 1976). These parameters
were calibrated, then the hydrograph calculated was fitted to theobserved hydrograph.
The continuity equation is
∂W ∂Q
+
=q
∂t
∂x
where q is the total discharge per unit channel length from theslopes adjoining both sides of the channel (m2/s).
(3) Retardation effect of the Gangneung Reservoir
Discharge from the upstream areas of the basin flows into theGangneung Reservoir, before being released downstream. To
account for the retardation effect of the reservoir, discharge from it
was expressed by means of a one-layer Power-Type Tank Model.
The discharge estimated with this model is expressed as
q = a (h - z)

5/ 3

where q is the runoff depth of the tank (mm/h), h the water depth(mm), z the height of the runoff (side) hole, and a the runoff coefficient. Parameters of the tank model, a and z, were calibrated bytrial and error, and respectively set as 0.05 and 700.
A simulated hydrograph is shown in Fig. 6. This observedhydrograph represents the discharge calculated by rating curve
equation (A). This figure shows that the observed discharge tends
to be greater than the calculated one, especially about the time ofthe second peak. The total observed discharge volume is considerably larger than the total calculated discharge. If the discharge
value calculated by rating equation (A) is assumed to be true thenthe actual amount of rainfall exceeded the observed rainfall
amount because the observed discharge attributed to actual rainfall
was greater than the calculated discharge of the observed rainfall at
Gangneung or Daegwallyeong.

RAINFALL - RUNOFF ANALYSIS OF FLOODING CAUSED BY TYPHOON RUSA IN 2002 IN GANGNEUNG, KOREA

99

Fig. 7 Power-type tank model.

Fig. 6 Calculated hydrograph with the kinematic runoff model

5. RAINFALL-RUNOFF ANALYSIS WITH THE
POWER-TYPE TANK MODEL AND INVERSE
ESTIMATION OF AREAL RAINFALL FROMDISCHARGE DATA
An imbalance between the observed rainfall and discharge has
been shown, and the probability suggested that the actual rainfall
in the Gangneung Namdae River Basin was greater than therecorded amount.
To determine how rainfall was balanced with the observed
discharge, rainfall-runoff was simulated with a Power-Type Tank
Model. The tank model for the Gangneung Namdae River Basinwas calibrated, and an areal rainfall hyetograph estimated from theobserved hydrograph by means of this model.
5.1 The Power-Type Tank Model
The model was composed of two tanks in series (Fig. 7).
Runoff was defined as the sum of the discharges from the side
holes of tanks q1 and q2. Infiltration, f1, and percolation, f2, were
defined as discharges from the bottom holes of the tanks. As
shown, runoff from the upper tank was a non-linear function(power form function) of the water depth in the upper tank, h1,
whereas that from the lower tank was a linear function of the water
depth in that tank, h2. Infiltration and percolation were both linear
functions of water depth in a tank. The lag time of runoff, T, wasused to account for runoff propagation in the basin in the calculation of runoff.
5.2 Calibration of the Model
The eight parameters of the Power-type Tank Model: a1, a2, b1,
b2, z1, z2, lag time of runoff, T, and the initial value of h2, were calibrated by the Rosenbrock optimization method. The initial value
for h1 was assumed to be zero. In the calibration, areal rainfall was
the weighted sum of the observed rainfall. Discharge data simulated with the kinematic runoff model were used instead of the dis-charge data based on the observed water level, given the requirement for calibrating a rainfall-runoff model using rainfall andrunoff data that are balanced in terms of the water budget. As earlier shown, the discharge calculated from the observed water levelby means of rating curve equations does not balance with theobserved rainfall. The values of the calibrated parameters werea1=0.00973, b1=0.028, z1=82.7, a2=0.0481, b2=0.0580, z2=28.6,
h1=0., h2=0.000730, T1=0.497.
5.3 Rainfall Estimation Based on Discharge
In the calibrated Power-type Tank Model, the estimated areal
rainfall in the Gangneung Namdae River basin was derived fromthe observed discharge by means of rating curve equation (A).
Rainfall then was estimated by correcting the water depth in theupper tank.
The estimation procedure was as follows:
1) Calculation of discharge at a discrete time step k.
For the upper tank:h1¢ ¨ h1 ( k - 1) + robs ( k )Dt
q1 ( k ) ¨ a ÊË h1¢ - z 1 ˆ¯

5/ 3

f1 ( k ) ¨ b1h1¢

h1 ( k ) ¨ h1¢ - (q1 ( k ) + f1 ( k ))Dt
For the lower tank:h 2¢ ¨ h 2 ( k - 1) + f1 ( k )Dt
q2 ( k ) ¨ a 2 Ê h 2¢ - z 2 ˆË¯
f2 ( k ) ¨ b2 h 2¢()

h 2 ( k ) ¨ h 2¢ - q2 ( k ) + f2 ( k ) Dt
qcal ( k ) ¨ q 1 ( k ) + q2 ( k )where robs(k) is the areal rainfall calculated from the observed rainfall at time step k; h1(k-1) and h2(k-1) are the respective water
depths of the lower and upper tanks;Δt is the time increment; z1
and z2 the side hole heights; q1(k) and q2(k) the respective runoff

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6. CONCLUSION

Fig. 8Inversely estimated rainfall from discharge observed
at Gangneung

depths from the upper and lower tanks; f1(k) and f2(k) the respective
infiltration rate from the upper to the lower tank and the percolation rate from the lower tank; qcal (k) is the calculated runoff depth.
Areal rainfall, robs(k), was calculated as the weighted sum of theobserved rainfall. Weight computation was based on the percentage of the area for each sub-basin slope to the total basin area.
Correspondence between the observed rainfall and each sub-basin
slope is shown in Fig. 4.
2) Let qobs(k) be the observed discharge at time step k, thenareal rainfall over the basin is{() } +zh1,mod ( k ) ¨ qobs ( k ) - q2 ( k ) / a 1

3/ 5

1

rest ( k ) ¨ h1,mod ( k ) - h1 ( k - 1)
h1 ( k ) ¨ h1,mod ( k ).

where rest (k) is the rainfall estimated from the observed discharge.
Fig. 8 shows an areal rainfall hyetograph estimated from discharge. By 0:00 PM on August 31, there was no significant difference between the observed and estimated rainfall. The first peakof the estimated rainfall at 9:00 AM is below that for the observed
rainfall, but after 2:00 PM the estimated rainfall exceeded theobserved rainfall. The second peak of the estimated hourly rainfall
is 121.9 mm/h, far higher than the observed hourly rainfall of 98
mm/h at Gangneung. The total estimated rainfall, 1256.4 mm also
is far higher than the 898 mm recorded at Gangneung.
These findings indicate that much more rain fell in the basin
during the storm than was previously thought. The rating curve
equation used to calculate discharge needs to be reexamined for
accuracy, in particular for water levels outside of the usable range.
Furthermore, other approaches, such as radar rainfall data analysis,
are needed to clarify the range of the rainfall distribution.

The magnitude of the flooding in the Gangneung Namdae
River Basin from August 31 to September 1, 2002 was examinedfrom several hydrological standpoints. The results of the analyses
can be summarized:
1) The total observed discharge was more than expected given
the amount of rainfall recorded at Gangneung. The reasons for thiswere that the observed discharge, calculated by means of a rating
curve, was greater than the actual discharge and/or the observed
rainfall was, in fact, less than the actual amount that fell in thebasin.
2) The peak discharge observed at Gangneung was comparable to maximum peak discharges estimated by the commonly usedenveloping curve equations in Japan, evidence of how large themagnitude of the flooding was.
3) The maximum hourly rainfall estimated by rainfall-runoff
analysis based on the observed discharge was 121.9 mm/h. The
estimated total rainfall during the storm, based on the observed discharge, was 1251.4mm.
ACKNOWLEDGEMENTS
I thank Prof. Jung Kwansue of Chungnam National
University, Mr. Lee HwanKi, Mr. Lee HanGoo, Mr. Kang TaeHo
and Mr. Lee JunYeol of Korea Water Resources Corporation(KOWACO) for providing data and assistance in the field surveys.
This work was supported by a 2002 Ministry of Education,
Culture, Sports, Science and Technology of Japan, Grant-in-Aid of
Scientific Research (Grants-in-Aid for Special Purposes(14800007) to Principal Investigator: Prof. Takara Kaoru at
Disaster Prevention Research Institute, Kyoto University).
REFERENCES
Kadoya, M and A. Fukushima, 1976. Concentration time of flood in small
or medium river basins, Annuals of the Disaster Prevention ResearchInstitute, Kyoto University, Vol. 19 B - 2, 143 - 152 (in Japanese).
Kadoya, M and A. Nagai, 1979. Enveloping curves for regional flood
peaks in Japan, Annuals of the Disaster Prevention Research Institute,
Kyoto University, Vol. 22 B - 2, 195 - 208 (in Japanese).
Ministry of Construction & Transportation (MOCT), Korea, 2002.
Statistics: An Annual Report, http://www.moct.go.kr/EngHome/DataCenter/Statistic/Statistic01.htm
Nagai, A, A. Yomota and M. Okumura, 1988. Interrelation of parametersof flood runoff models: Runoff in grazing area, Transactions of theJapan Society of Irrigation, Drainage and Reclamation Engineering,
129, 69 - 76 (in Japanese).