The 4th
largest basin in the Philippines and covers an approximate aggregate area of
10,540 sq. km. (includes the allied basin of Guagua River). The basin extends
over the southern slopes of the Caraballo Mountains, the western slopes of the
Sierra Madre range and the major portions of the Central Plain of Luzon. It
encompasses the provinces of Nueva Ecija; part of Bulacan, Tarlac and Quezon;
and almost whole of Pampanga. The total length of the main river, the Pampanga
River, is about 260 kilometers.
The
basin is drained through the Pampanga River and via the Labangan Channel into
the Manila Bay. The main river is supported by several tributaries, the
principal ones of which are the Penaranda and the Coronel-Santor Rivers on the
eastern side of the basin and the Rio Chico River from the northwest side. The
Angat River joins the Pampanga River at Calumpit in Bulacan via the Bagbag
River. The Labangan channel, on the other hand, acts as a cut-off channel for
the Angat River into Manila Bay. Somewhere between the middle and lower portion
of the basin stands Mount Arayat, about 1,026 meters in elevation. Adjacent to
Mount Arayat, across Pampanga River, just on the eastern side is the Candaba
swamp, covering an area of some 250 sq. km. absorbing most of the flood flows
coming from the eastern sections of the basin (western slopes of a portion of
the Sierra Madre mountain range) and the overflowing of the Pampanga River via
the Cabiao Floodway. This area is submerged during the rainy season but is
relatively dry during summer. At the lower sections of the basin, where the
Pampanga delta lies, the Pampanga River system divide into relatively small
branches, crisscrossed with fishponds to form a network of sluggish, tidal flats
and canals, which eventually find their way to Manila Bay. The main river has a
relatively low-gradient channel particularly at the middle and lower sections.
With the anticipated completion of the Pampanga Delta Project (DPWH), it is
expected that floodflows at the lower section of the Pampanga River will recede
at a much faster rate than before.
The basin experiences, on an average, at least one flooding in a year. The dry season generally occurs from December to May, and wet the rest of the year. The wettest months are from July to September. The frequency of tropical cyclone passage over the basin is about 5 in 3 years.
The Agno River, with a drainage
area of 5,952 square kilometers is the third largest river in Luzon, fifth in
the Philippines, next to the
Cagayan and Pampanga Rivers. The
length of the river system from its source in the Cordillera Mountains to its
mouth in Lingayen Gulf is about 270 kilometers, 90 kilometers of which runs
through mountainous terrain and canyons.
The
Agno River after passing through mountains at an average elevation of some 2,000
above MSL, forms a vast alluvial fan and delta called the Pangasinan Plain and
then flows into the Lingayen Gulf. The
Pangasinan plain has long been economically developed and together with the
Pampanga Plain constitute the rice granary of Central Luzon.
Principal tributaries of the Agno River include the Pila, Camiling,
Tarlac and Ambayoan Rivers. The
main branch of the Agno River is Tarlac River which has its source at Mt.
Pinatubo (elevation 1,745m) in Tarlac and joins the Agno River at Poponto Swamp
near Bayambang. The swamp has an
area of about 25 square kilometers and temporarily retains flood waters from the
Tarlac River. Since the Agno River
basin is characterized by mountainous topography, the flood runoff estimated
annually at about 6,654 million cubic meters reaches the plain in several hours
in the river mouth in about a day. Average
annual rainfall varies from 2,000mm in the neighborhood of Tarlac to 4,000mm in
the upper reaches of the Agno River. Such
great variation in the area distribution of rainfall is attributed to orographic
effect coupled with tropical cyclone and south-west monsoon rainfall.
The
Pangasinan Plain suffers from recurrent and destructive floods.
The catastrophic floods of July-August 1972 and May 1976 inundated the
entire Pangasinan Plain including the flood plains of the Tarlac River.
The target areas for this flood forecasting and warning subsystem are:
a) The entire
Pangasinan Plain including the major city/municipalities of Dagupan, Lingayen,
Bugallon, Sta. Barbara, Bayambang and Rosales
b) The central
part of Tarlac province including the municipalities of Gerona, Tarlac, Paniqui
and Moncada.
The
Bicol River is a medium-size river in Southern Luzon and drains an area of 3,771 square kilometers. Situated
in the Bicol Peninsula, it embraces the central portion of Camarines Sur, the
northern portion of Albay, and a portion of Camarines Norte.
The basin is oriented in a northwest-southeast direction and bounded on
the east by a chain of volcanoes and on the west by highlands and lowhills.
The flat alluvial land in the Bicol Plain occupies the area between the
Eastern Bicol Cordillera and the Ragay Hills.
The runoff which is estimated annually at 5,100 million cubic meters,
starts from the Mayon Volcano with an elevation of 4,421m, and meanders in a
generally northwestward direction. After
being regulated through lakes Bato, Buhi, and Baao, the streamflow reaches the
mainstream of the Bicol River which owing to its very gentle slope, is affected
by tides as far as upstream of Naga City.
The principal tributary of the Bicol River is the Sipocot River. Unlike the main river, the Sipocot River cuts through mountainous terrain and has a steeper slope. The Bicol River after joining the Sipocot River, widens to more than 1,000 meters across at the estuary before discharging into San Miguel Bay.
The average annual rainfall varies from 2,000mm in the southwestern area to 3,600mm in the southwestern section of the basin. Flood-producing rains in the basin from May to February are mainly due to tropical cyclone passages, northeast monsoon and orographic effect.
Flooding from the sea in the alluvial plain near the mouth of the river is caused by storm surges associated with the passage of slow-moving typhoons near or over San Miguel Bay particularly during high tides.
The target areas for the
Bicol River Flood Forecasting and Warning Subsystem are:
a)
The Central part of the basin, from lake Baao to Lake Bato.
b)
The alluvial plain extending from Naga City to the river
mouth.
The
Cagayan River is the largest river in the Philippines with a drainage area of
27,280
square kilometers at its mouth in Aparri.
The basin encompasses parts of Cagayan, Isabela, Mountain Province, Nueva Vizcaya, Quirino and Quezon Provinces. The basin is roughly elliptical in shape with its major axis oriented in a north-south direction. The Cagayan Valley is relatively flat but mountains with elevations up to 2,000m surround the east, south and west of the drainage basin.
The
Cagayan River, which is the main drainage channel of the basin, flows in a
northerly direction from its head waters in Nueva Vizcaya to its mouth in the
Babuyan Channel near Aparri. Its
principal tributaries include the Siffu-Malling, Chico, Ilagan and Magat Rivers.
The estimated annual discharge is 53,943 million cubic meters.
The Magat River is the largest tributary with an estimated annual discharge of 9,808 million cubic meters. It lies in the southwestern portion of the basin, stretching approximately 150 kilometers from Nueva Vizcaya down to its confluence with Cagayan River about 55 kilometers from the river mouth. Both the Magat and the Chico Rivers have extensive drainage areas which comprise about 1/3 of the whole basin. The Ilagan River originates from the western slopes of the Sierra Madre and drains the eastern central portion of the Cagayan River basin with an estimated annual discharge of 9,455 million cubic meters. It flows westward and joins the Cagayan River at Ilagan, Isabela, 200 kilometers from the mouth. The Siffu-Malling River lies on the slope of the Central Cordillera ranges flowing almost parallel to the Magat River. Marshes and swamps are found in some parts of its lower reaches.
The average annual rainfall in 1,000mm in the northern part and 3,000mm in the southern mountains. Floods caused by this river flow down very slowly because of surface retention over the extensive flood plain, extremely gentle slope, retardation of flood by several gorges and river meander.
The
target areas for the flood forecasting and warning system of the Cagayan River
Basin are:
a)The areas along the lower reaches, from Tuguegarao to Aparri
b) The
alluvial plain along the river course from Ilagan to Tumauini, Isabela.
Before the Flood
1. Find out how often your location is likely to be flooded.
2. Know the flood warning system in your community and be sure your
family knows it.
3. Keep informed of daily weather condition.
4. Designate an evacuation area for the family and livestock.
5. Assign family members instructions and responsibilities according to
an evacuation plan.
6. Keep a stock of food which requires little cooking and
refrigeration; electric power may be interupted.
7. Keep a portable transistorized radio and flashlight with spare
batteries, emergency cooking equipment, candles, matches, and first-aid kit
handy in case of emergency.
8. Store supplies and other household effects above expected flood
water level.
9. Securely
anchor weak dwellings and items.
When Warned of Flood
1. Watch for rapidly rising flood waters.
2. Listen to your radio for emergency instructions.
3. Store drinking water in containers. Water service may be
interrupted.
4. Move household belongings to upper levels.
5. Get livestock to higher ground.
6. Turn off electricity at the main switch in the
building before evacuating and also lock your house.
During the Flood
1. Avoid areas subject to sudden flooding.
2. Do not attempt to cross rivers of flowing streams where water is
above the knee.
3. Beware of water-covered roads and bridges.
4. Avoid unnecessary exposure to the elements.
5. Do not go swimming or boating in swollen rivers.
6. Eat only well-cooked food. Protect left-overs against contamination.
7. Drink
clean or preferably boiled water ONLY.
After the Flood
1. Re-enter the dwellings with caution using flashlights, not lanterns or torches. Flammables may be inside.
2. Be alert for fire hazards like broken wire connnections.
3. Do not eat food and drink water until they have been checked for flood water contamination.
4. Report broken utility lines (electricity, water, gas, and telephone) to appropriate agencies/authorities.
5. Do not turn on the main switch or use appliances and other equipment until they have been cleaned, dried or thoroughly checked by a competent electrician.
6. Consult health authorities for immunization requirements.
7. Do not go "sight-seeing" in disaster
areas. Your presence might hamper rescue and other emergency operations.
KINDS OF BASIN FLOOD BULLETINS
Basin Flood Bulletin is a hydrological information prepared and
issued due to possibility, threatening or expectation of flooding in the
near future at a certain locality within a river basin where there are
real-time rainfall and water level reporting stations.
-
Flood Outlook - Issued when there is an existence of a potentially flood-causing weather system but no impending threat yet of flooding.
-
Flood Alert - Issued when there is an impending threat of flooding.
-
Flood Warning - Issued when there is a real threat of flooding.
RAINFALL INTENSITY CLASSIFICATION
|
Category |
6-Hour |
12-Hour |
24-Hour |
|
Light |
<15mm |
<30mm |
<60mm |
|
Moderate |
15 - 45mm |
30 - 90mm |
60 - 180mm |
|
Heavy |
>45mm |
>90mm |
>180mm |
River-flood forecasts are prepared by FFB and
disseminated by FFB Flood Forecasting Centers to the public. During periods of
flooding, the FFB centers issue forecasts for the height of the flood crest, the
date and time when the river is expected to overflow its banks, and the date and
time when the flow in the river is expected to recede to within its banks. These
forecasts are updated as new information is acquired.
To develop flood forecasts, the FFB develops and
calibrates complex mathematical models of how the certain rivers and streams
respond to rainfall and storm surges. These models are developed for
pre-selected forecast service points, which are usually located along major
rivers or on small streams near population centers that have a history of
flooding. In every case, records of river discharge must be available so the FFB
can develop a river model. An important hydraulic input to these models is the
stage/discharge rating. The resulting model is rarely exact, but it provides
estimates of river response to rainfall. Thereafter, when heavy rainfall is
forecast for the river basin, those amounts are entered into the model, and the
model estimates the river stage and discharge that will result. As new river and
rainfall data are collected during a storm, the new data are entered into the
computer, and new river forecasts are produced.
As forecasts are prepared, water that flows
into large rivers from upstream points and tributary streams must be considered;
in fact, gauging important tributary streams is often needed even at locations
where forecast services are not provided. These points are used in the forecast
models as model control points. Because none of the models can predict exactly
what will happen on a river, the use of river stages and the associated rating
curve/table to reassess continuously how much water is in every stream is a
vital part of the forecast process.
Even a well-calibrated model is an ephemeral
commodity. Once a river model is developed, changes in watershed
characteristics, such as increasing urbanization, drainage improvements, and
construction of dams and levees, can make the model obsolete. A continuing cycle
of model calibration, collection of river-discharge and rainfall data, and model
recalibration is required to provide a current, useful, and accurate
flood-forecasting tool.
People living in other prone areas/basins should be on lookout for
Weather Advisories and Severe Weather Bulletins to know the existence of a
severe weather system or the whereabouts of the tropical cyclone: and Public
Weather Forecast which partly describes the expected rainfall nationwide, all
issued by the Weather Branch (WB), PAGASA.
Awareness of the factors affecting the occurrence,
possibility/cause and characteristic of flood; actual observation of continuous
heavy fixed reference points in the river, etc., may help one foretell an
impending natural flood for himself.
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