module 2_hydromet hazards 2

7/22/2019 Module 2_Hydromet Hazards 2

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SUSAN R. ESPINUEVA

HYDRO-METEOROLOGY DIVISION,PAGASA, DOST

Department of Science and Technolog y

Phi l ippine Atmosph er ic, Geophysical and A stronomical Services Administ rat ion

Hazard 102:Hydro-meteorological Hazards (Part 2)

Types & Causes of flooding & mitigation measures

Training on Trainers on DRR/CCA for Local Partners8 January 2013, Tagaytay City

Project on Enabling Cities in Cagayan de Oro and Iligan to Cope

with Climate Change (Project Climate Twin Phoenix)
http://www.google.com/imgres?imgurl=http://www.nidm.net/images/flood6.jpg&imgrefurl=http://www.nidm.net/flood5_ii.asp&h=317&w=498&sz=16&tbnid=xQsMwHR6MnYJ::&tbnh=83&tbnw=130&prev=/images?q=flood+disaster+pics&usg=__OxfdyzuXcEc7in4yM2hADUqjZi8=&sa=X&oi=image_result&resnum=2&ct=image&cd=1http://h/wiki/New_Orleans


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Definition Flood, Flooding/Inundation

Flood is defined as a

rise, usually brief, in

the water level in astream to a peak from

which the water level

recedes at a slower

rate (UNESCO-WMO,1974).

Flooding/Inundation

a great flow of water that rises and spreads over the land.

may result when a volume of water from lakes/streams/

rivers exceeds its carrying capacity and escapes from its

usual boundaries.
http://www.google.com/imgres?imgurl=http://www.nidm.net/images/flood6.jpg&imgrefurl=http://www.nidm.net/flood5_ii.asp&h=317&w=498&sz=16&tbnid=xQsMwHR6MnYJ::&tbnh=83&tbnw=130&prev=/images?q=flood+disaster+pics&usg=__OxfdyzuXcEc7in4yM2hADUqjZi8=&sa=X&oi=image_result&resnum=2&ct=image&cd=1


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Types of flooding

A. Based on location or place of occurrence:

a.1 River flooding occurs when a large amount of rain falls in riversystems with tributaries that drain large areas containing manyindependent river basins. They may last a few hours or many days

depending on the intensity, amount and the distribution of the rainfall.

Marikina River, Tropical Storm

Ondoy, Sept. 2009

Agno River, Pangasinan, Typhoon

Pepeng, October 2009


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Types of flooding

Communities with a steeper

continental will not see as much

surge inundation.

A shallow slope off the coast will

allow a greater surge to inundate

coastal communities.

a.2 Coastal flooding may occur due to storm surges, hightide and tsunamis (waves produced by earthquakes at sea).


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Types of flooding

a.3 Urban flooding occurs in an area where roads areusually paved. During rainy episodes, water cannot

infiltrate the ground and is normally retained in the surface.This type of flooding is often associated with the limitedcapacity of the sewerage system to drain the heavy rainsthat are falling.

Marikina, September 2009 Pasig City, September 2009


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Types of flooding

B. Based on duration of occurrence:

b.1 Flash Flooding is the result of heavy rainfall orthunderstorm over a relatively small drainage area. Flash floods

carry highly destructive flood waves and are most common in

mountainous areas or in steep places that have streams flowing

though narrow canyons. It happens quickly and move with little

warning.

Flashfloods are

common in river basin

with circular shape.

Pinut-an, Panaon Island, Southern Leyte, December 2003

(Photo courtesy of Mines and Geosciences Bureau)


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Types of flooding

b.2 Sheet flooding is caused by comparatively shallowwater flowing over a wide area and is very common in the

flood plain area which is normally flat. Sheet flooding mayalso result when water from a river channel with insufficientcarrying capacity overtop its bank, inundating the adjacentareas.

Flooding in Poblacion Mainit,

Surigao Del Norte, January

2011

Rosales, Pangasinan, Typhoon Pepeng, October

2009


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Causes of flooding

1. Heavy, continuous

rains that persist for

days or ceases only

briefly.

2. Meandering river


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Causes of flooding

Dating lawaksiltation

Pampanga River after the Mt. Pinatubo

eruption in 1991

(Photo courtesy of Pampanga River Basin

Flood Forecasting and Warning Center)

3. Heavy siltation of

the river system which

decreases the carryingcapacity of the river.

4. Dam spill

Maris Dam (located downstream of

Magat Dam) spilling operation during

the passage of Typhoon Yoyong

(December 2004)


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Causes of flooding

Brgy. Carael (Botolan, Zambales) during the passage of Typhoon

Kiko in August 2009

Example of widespreadflooding due to dike or

levee failure

5. Over-topping of dikes and levees


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Aggravating factors

1. Altering the ecological system in a river basin, whichcould have an impact on the hydrology of the watershed

- denudation of forest and watershed areas

Deforestation


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Aggravating factors

2. Increasing level of urbanization

- as the soil is paved, there will be less infiltration,

the natural land cannot absorb the run-off

Due to the effect of

urban development

which is being

undertaken at a veryfast and alarming rate,

the runoff coefficient

was significantly

increased from 50%

as originally designedto 95% (Gatan, 2009,ADB Semin ar).


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Effect of urbanization

Hydrologic

and man-made factors

Small streams in urban areas can also rise quickly after heavy raindue to higher run-off generated and the smaller time of concentration.

f


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Aggravating factors

3. Improper waste disposal/encroachment along the riverchannel which constricts the pathway of floodwaters

Flood prone (left) and informalsettlers along Mangahan Floodway(Source: Bongco, 2009)

A ti f t


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Aggravating factors

4. Insufficient capacity of drainage system

Carrying capacities of drainage system in Metro Manila

A ti f t


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Aggravating factors

Other factors: Population growth

Philippines stats:

2000 Population: 76.0 M10-year Avg. Annual Pop.Growth: 2.2%GDP: 2000$310.0 billionGDP per capita: $4,079GDP growth: 3.6%

GNI per capita2000: $1,040World Bank ClassificationLower Mid. Inc PopulationBelow Poverty Line: 25.5 M

Population data of the National Capital Region

Ave. Populationdensity:

1903 517

1960 3,872

2000 16,495

Distribution among cities/

municipalities as of 2000:e.g. Navotas 88,617

Pateros 5,520Growth rate: 2000 to 2007 = 2.11%

A ti f t


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Aggravating factors

Other factors: Cultural & religious attitudes

Certain cultu ral and rel ig io us att i tudes todisasters m ay also need to b e overcome.

According to Oxfam, an assessment ofparticipatory capacities and vulnerabilities foundthat disasters were perceived as Gods

punishment or a fact of life.Since a disaster was cons idered to b e a

natural phenomenon, many people

expressed doubts that they can actual ly do

someth ing about i t .

This fatalist ic att i tude is also reinforced by

stron g relig ious beliefs. Bahala na ang

Diyos(God will take care of everything) is

the usual prayer in the face of an impending

disaster in the communi ty. Oxfam , 2001:66

Miti ti th i t f h d t h d


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Mitigating the impacts of hydromet hazards

1. Understanding the hazard

characteristics,occurrence, frequency, areas to be affected2. Gauging the hazard installing monitoring

facilities i.e. automatic weather stations

(AWS), radar, upper-air stations, etc.3. Analyzing the observed information

numerical weather prediction (NWP)

products, synoptic charts, satellite

imageries4. Issuing the forecast/warnings

I t f fl di


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Impacts of flooding

There is no one description for floods. Rather there aredifferent situations such as flash floods, monsoon flooding and

wind storms such as tropical cyclones. The scale of floods inthe Philippines is increasing, as is the number of floods.

T d & i t f fl di i M t M il


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Trends & impacts of flooding in Metro Manila

TC events: 55 (51.4%)Non-TC events: 52 (48.6%)

Sour ce: HMD, PAGASA

Max 1-hr for TC events: 75.1 mmMax 1-hr for Non-TC events: 97 mm

Metro Manilas regional GDP in 2008

was 468 billion PHP (National

Statistical Coordination Board).

Damage costs range from 3% of GDP

(SQ-EX-10) to 24% (A1FI-EX-100).

Climate change costs represent 1%

(1-in-10 flood), 6% (1-in-30 flood) &

10% (1-in-100 flood) of GDP.

Source: Climate Risks and Adaptation in A sian CoastalMegacit ies: A Synth esis Report, 2010, ADB , JICA, WB

WithoutCC

With CC

Flood Risk/ Damage Mitigation Meas res


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Flood Risk/ Damage Mitigation Measures

1. Structuralto keep flood

away from the people

- Construction of dikes andlevees

- Construction of

dams/reservoirs

-Modification of catchmentcondition & on-site storage

-Improvement of channel

flow conditions

- Discharging water by

pumping

2. Non-structural - to keep people

away from the flood

- Land use management- Property acquisition and

floodway clearance

- Early Warning System (EWS)

- Hazard mapping- Public information and

education

- Flood proofing of buildings

- Evacuation from endangered

areas

- Flood fighting

- Flood insurance

- Flood adaptation

Adaptation measure: flood proofing


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Adaptation measure: flood proofing

Extreme flood events in the PH


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Flashfloods in Quezon in December 2004



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Causes and impacts of flashfloods in Albay in Nov 2006



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Causes of flooding in Metro Manila due to passage ofTS Ondoy (Sep 26, 2009)



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Impacts of flooding due to passage of TS Ondoy (Sep 26, 2009)



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Causes and impacts due to passage of TS Ondoy (21 June 2008)



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Causes and impacts due to passage of TY Pepeng (Parma Oct 2009)



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Impacts of TS Sendong (Washi) in Cagayan de Oro City &Iligan City 16 Dec 2011

Flash Flood due

to TS Sendong(Washi) 16 Dec 2011 inCagayan deOro City &Iligan City

module 2_hydromet hazards 2

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