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A PreliminaryEngineering Analysis of the
Marikina River Basin Flood ofSeptember 26, 2009
Learning Lessons, Carrying Onin the Wake of Storm Ondoy (Ketsana)
UP National Hydraulic Research Center
October 2, 2009
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Contents
A. Description of the Marikina River Basin
B. Hydrologic model of the river basinC. Interpretation of the peak rainfall, peak flood
discharge, and peak water level
D. Hydraulic model of the channels and floodplains
E. Recommendations1. Monitoring, forecasting, and early warning
2. Information dissemination
3. Decision-support system
4. Immediate response and preparation
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Part A:
Description of theMarikina River Basin
The NHRC, with the Institute of Civil Engineering at the College of
Engineering of UP Diliman, had used the SWATCH software and the UNET
software for previous studies of the Marikina River Basin and other Metro Manila
river basins for DENR, DPWH, and LLDA until 2005.
The existing SWATCH hydrologic model of the Marikina River Basinencompassed the river system from the mountains down to the Sto. Nio station.
The existing UNET hydraulic model covered each channel & its two overbanks.
Steep slopes characterize the upper catchment of the Marikina River Basin.
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Marikina River Basin
For one major tributary basin,
the Marikina River Basin(535 sq. km. drainage area)in the northeast, the only outletis through Pasig River,to Manila Bay in the west;
and partly through the MangahanFloodway diversion intoone extensive lake region,the Laguna de Bay Basin (withtotal basin area of 3,229 sq.km.)in the southeast.
MarikinaRiver
Pasig River
Laguna
de Bay
ManilaBay
Source of composite: UP NHRC3
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Metro-Manila Rivers and River-Basin Areas
Name of River Basins (RB) Drainage Area(sq.km.)
Marikina River Basin 535Mangahan Floodway-Taytay RB 63Taguig-Napindan RB 45
Meycauayan RB 169Obando-Malabon-Navotas Estuary 35
Novaliches Reservoir-Tullahan RB 72San Juan RB 94Pasig River Basin (north and south) 91
Paraaque-Las-Pias RBs 73Zapote-Bacoor-Imus RBs 168 Source: UP NHRC
4
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Top: Marikina River @Wawa Dam at Montalban gorge
(as seen in the early 1990s).
Left:Along Marikina River,
the towns / cities ofRodriguez (Montalban),San Mateo,Marikina &Pasig.
Source of composite:
UP NHRC5
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(Badilla 2008)
Elevations at Upper Marikina River Basin
Highest
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Elevations along Marikina River
Source: UP NHRC
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Part B:
Hydrologic model of theMarikina River Basin
Hourly rainfall data at stations within the river basin would be needed. The reported rainfall of 448 mm in 12 hours, converted into hourly rainfall, was
used in prelim. SWATCH model with an applied area reduction factor of 0.6. The
peak rainfall occurred around 11AM on Sept. 26, 2009.
The prelim. SWATCH computation indicated peak flood discharge at Sto. Niostation of abt. 5,770 cu.m./s around 12 noon on Sept. 26, 2009.
30-45 minute extreme warning, or even longer, could be enabled by proper
monitoring and simulation such as demonstrated in this preliminary modeling.
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Tabak
Bosoboso
Pasig
Science Garden
101
102
103
104
105
106107
108
109110
111
112
113
114
115
116
117
118
119 120
121122
123124
125126
127
128
129
130
131
132133
134135
136
137138
139
140
141
142
143
144
145
146
147
148
149
150
151152
153
154
155
156
157
158
159
160
161
162
163164
165166
167
168169
170
171
172
173
174
175
176177
178
179
180181182
183
184
Rainfall Stations
Rivers/Tributaries
Sub-Basin Divides
Marikina River Basin Boundary
LEGEND
Marikina River Basin
Sto. Nino Gaging Station
Sto. NinoGaging Station
121 00'o
14 50'o
121 00'o
14 35'o121 20'o
14 35'o
121 20'o
14 50'o
Rosario Weir
Rosario Weir
NORTH
Marikina River Basin Delineation
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The detailed river network of Marikina River Basin above Sto. Nio(DA=535 sq.km.): the basis of the UP NHRC SWATCH physics-based
distributed (rainfall-runoff) hydrologic model. Source: UP NHRC10
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Model Components: rainfall interception and retention storage
infiltration excess rainfall soil moisture storage in the root zone evapotranspiration interflow storage and discharge
groundwater recharge flow stream-aquifer return flow overland flow channel flow
Method of Solution: analytic and semi-analytic
methods with the use of discretekernels
Watershed discretization in SWATCH model showing overland flowplanes and river reaches.
watershed discretization showingoverland flow planes and riverreaches
each overland flow plane hasunique hydrologic and hydraulicproperties
!"# $
11 Source: UP NHRC
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(PAGASA)
12
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Wind Directionand Wind Speed
Hourly Rainfal, Average Pressure and Winds
in Manila Observatory (26 Sep 2009)
0
10
20
3040
50
60
70
12 AM 3 AM 6 AM 9 AM 12 PM 3 PM 6 PM 9 PM
Time (pht)
Ra
in(m
m)
985
990
995
1000
1005
Pressure
(hPa
)
Rain
Barometric Pressure
Highest rainfall about 60 mm/hr at Manila Obs. was measured bet. 10 AM and 11 AM. Higher rainfall might have been measured at other stations. Hourly rainfall data at stations within the river basin would be needed.
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Sept. 26, 2009 peak flood discharge of Marikina River @ Sto. Nio.
Hourly rainfall (mm/hr) @ Science Garden, QCStation, converted based on 448 mm in 12 hours(used in prelim. SWATCH hydrologic model withapplied area reduction factor = 0.6.)
Max hourly rainfall abt 90 mm/hr at 11AM.Peak flood discharge computed 5,770 cu.m./s at 12NN
Source of prelim. computation:UP NHRC
Level of 30-yr design-basis flood
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Part C:
Interpretation of the peak rainfall,flood discharge, and water level
The reported rainfall of 448 mm in 12 hours corresponded with extreme
rainfall with a return period of 180 years.
The preliminary computed peak flood discharge of 5,770 cu.m./s had an
estimated return period of more than 100 years.
For 4 hrs, the computed flood exceeded the 30-yr flood that is currently the
design basis for the Pasig-Marikina River Channel Improvement Project.
The computed rise of water level by as much as 8 meters matched with some
observed markers of flood level at Marikina.
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Highlights of the Hydrologic Design Parameters of Pasig-Marikina River Channel Impr. Proj.
Historical Annual Maximum
Discharge of Marikina River
at Sto Nino, Marikina
Year Max. Discharge, m3/s
1986 2650 *
1970 2464
1959 2072
1977 2051
1966 2036
2000 1895
1998 1680
1995 1676
1999 1642
1967 1609
etc.
Probable Annual Max. Discharge of Marikina
River at Sto Nino station (by Log Normal Prob.
Dist. and Flood Runoff Model)
Return Max. Discharge, m3/s
period Existing (1997) Future (2020)
(years) Land Use Land Use
Sto Nino Sto Nino Rosario
2 1350 1470 1480
5 1870 2020 2000
10 2210 2350 2320
20 2550 2740 2720
30 2740 2900 2890
(design flood)
50 2980 3120 3070
100 3310 3430 3440
The 20-year flood of 2,550 m3/s was exceeded in 1986; however the 30-year design-basis flood of 2,740 m3/s was not yet experienced in the42-year period of record, 1958-2000.
The 2009 flood was greater than a 100-year flood.
Source: UP NHRC16
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The 2009 rainfall correspondedto 180-year return period.
17 Source: UP NHRC
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The 2009 flood corresponded toabout 180-year return period.
18 Source: UP NHRC
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Computation of MAXIMUM RIVER WATER LEVEL
Using the published DPWH-JICADischarge Rating Curves (March 2002)
in the case of Marikina River at Sto Nio (DA = 535 sq.km.):Q = 17.01 (H 0.00)^1.85 for H < 5.33 metersQ = 0.20 (H 0.00)^4.49 for H > 5.33 meterswhich relates river gage height, H (meters),
to flow discharge, Q (cu.m./sec), by regression of historical data -
then the Peak Flood Flow = 5,770 cu.m./seccomputed by the SWATCH hydrologic modelcorresponds to a gage height of H = 9.8 meters,
which means that starting from an initially low H = 1 to 2 meters,the gage height (river water level) can rise by about 8 meters.
these computations matched with the observed markers ofmaximum flood water levels on 26 Sept. 2009 relative to the low banks.
Flood discharge Water level
19
Source: UP NHRC
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8 meters high5 meters high
Marikina Riverbank
9/28/2009.
Downstream view of Rosario Weir
along Marikina River.
Water level markers
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Source of composite:UP NHRC
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Part D:
Hydraulic model of the riverchannels and floodplains
The UNET model considered the network of channels and adjoining floodplains,
from Sto Nino, to Napindan at Taguig, to Marikina River, to Pasig River down to
Del Pan Bridge at mouth of Manila Bay, including the vicinity of Provident Village.
Simulation could include such interior conditions as: bridges, culverts, gate
spillways, weirs, rating curves, levees, pumping stations, and channel constrictions.
Using the prelim. SWATCH flood discharge at Sto. Nio station as input, theUNET model computed water level rises of 5 meters or 8 meters which matched
with observed flood levels at several locations in Marikina.
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Sept. 26, 2009 peak flood discharge of Marikina River @ Sto. Nio.
Hourly rainfall (mm/hr) @ Science Garden, QCStation, converted based on 448 mm in 12 hours(used in prelim. SWATCH hydrologic model withapplied area reduction factor = 0.6.)Max hourly rainfall abt 90 mm/hr at 11AM.
Peak flood discharge computed 5,770 cu.m./s at 12NN
Source of prelim. computation:UP NHRC
Level of 30-yr design-basis flood
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River constrictionRiver constrictionexampleexample
22
Source ofcomposite:UP NHRC
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MANILA
BAY
PASIG RIVER CHANNEL IMPROVEMENT
PASIG RIVER
SANJ
UANR
IVER
MCGS
ROSARIO WEIR
MANGAHAN
FLOODWAY
LAG
UNA
LAKE
NAPINDANRIVER
LAKESHORE DIKE
SPILLWAY
PARA
AQUE
MARIKIN
AR
IVER
NHCS
500 cumecs
2400cumecs
2900
cumecs
UPPER
MARIKINARIVE
R
CHANNE
LIM
PROVE
MEN
T
LOWE
RMARIKIN
ARIVER
CHANNE
LIMPROVEMENT
MARCOS BRIDGE
MARIKINA BRIDGE
MANALO BRIDGE
LAMBINGAN BRIDGE
1200 cumecs
MAINT. BRIDGE
RODRIGUEZ BRIDGE
Schematic of Modeled Area
23Source of composite: UP NHRC
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282000 284000 286000 288000 290000 292000 294000 296000
1608000
1610000
1612000
1614000
1616000
1618000
Pasig-Marikina River System including Floodplains
Provident Village
SM Mall
Rosario Weir
Napindan
Manila
Bay
Santo Nino
24
Source:UP NHRC
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UNET is an unsteady flow, quasi, two-dimensional modelin that the network of channels is represented by
interconnected one-dimensional channel reaches andeach reach is composed of several cross sections withdistinct main channel flow and floodplain area.
Simulate interior boundary conditions such as:
bridges culverts gate spillways weirs
rating curves levees pumping stations
UNET : River and Floodplain HydraulicsUNET : River and Floodplain Hydraulics
(U.S. Army Corps of Engineers)(U.S. Army Corps of Engineers)
25Source: UP NHRC
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0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0 36.0
Distance, (km)
10.012.014.016.018.020.0
22.024.0
Eleva
tion(m
)
Typhoon Ondoy 12:00 Hrs: Water Surface, Discharge and Velocity ProfilesPasig-Marikina River (Manila Bay at km0.0 - Sto Nino at km33.7)
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0 36.0Distance (km)
1000
2000
3000
4000
5000
6000
Disch
arge
(CMS)
0.0 4.0 8.0 12.0 16.0 20.0 24.0 28.0 32.0 36.0Distance (km)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Ve
loc
ity
(m/s)
Flood water elevation
26
Source: UP NHRC
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!
"
#$% $ & '& ( )* ! !
+*
!
), ,!!
(-%
RightRight overbankoverbank
LeftLeft overbankoverbank
Flood water elevation
Ground elevation
27
Source: UP NHRC
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RightRight overbankoverbank
LeftLeft overbankoverbank
28
Source of composite: UP NHRC
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RightRight overbankoverbank
LeftLeft overbankoverbank
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Flood inundated areasaccording to computersimulations by UP NHRC
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Part E:
Recommendations Monitoring, forecasting, and early warning. With advances in field
instrumentations, geomatics, information & communication technologies, and
engineering simulations, prior warnings may be enabled.
Information dissemination. Geographic-information-system (GIS)-based datamay be catalogued, updated, processed, and relayed effectively to ground units
and/or to mass media outlets.
Decision-support system. The same technologies may be used for medium-term
and long-term studies and models for governance, planning, and engineering toreduce exposure and vulnerability to a multitude of natural hazards.
Immediate response and preparation. Infrastructures may have to be
reengineered to withstand greater hazards; communities may have to be
reorganized to operate under extreme catastrophic conditions.
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