Flash Flood Forecasting on a Flash Flood Forecasting on a Tropical Small Island towards Tropical Small Island towards

Disaster Preparedness – TrinidadDisaster Preparedness – Trinidad

Glendell De SouzaScience & Technology Officer

Caribbean Meteorological Organization

International Workshop on Flash Flood Forecast Workshop, Costa Rica13th – 17th March 2006: Session 8: Disaster Mitigation, Preparedness & Response

Presentation OutlinePresentation Outline

• Introduction• Topography• Land Use Practices• Flood Prone Areas

• Urban Flood Forecasting

• Flood Plain Flood Forecasting

• Community Based Flood Forecasting Early Warning System• Flood Forecast Issued

IntroductionIntroduction• Trinidad perennially experiences flooding, which can have disastrous

consequences on the socio-economic fabric of the twin island state. Flooding in Trinidad and Tobago as defined by its National Meteorological Service is categorized as follows:

• Street Flooding: - Flooding with a low degree of destructive potential, which usually causes roadways to be submerged, traffic to be snarled and impede pedestrian traffic.

• Flash Flooding: - Flooding with a high degree of destructive potential; usually associated with the destruction of property, through the swift action of water. Rainfall is heavy or prolonged or a combination of both factors.

• River Basin Flooding: - Flooding with a high destructive potential; usually associated with prolonged rainfall activity over a period of day(s).

Introduction (Cont’d)Introduction (Cont’d)

• The major causes of flooding in Trinidad include but are not limited to:• Topography• Soil types• Land use practices• Rainfall

TopographyTopography

Land use PracticesLand use Practices

Urban Flood ForecastingUrban Flood Forecasting• From the 1200UTC sounding determine the surface temperature for

free convection to develop. Monitor the air temperature for the attainment of this temperature.

• A weak surface pressure gradient: - light winds at the surface and in the planetary boundary layer.

• Strong insolation during the morning with low atmospheric turbidity and small cloud amounts.

• Evidence of moisture advection from either the sounding or through the precipitable water content from satellite imagery.

Urban Flood ForecastingUrban Flood Forecasting

• Associated with the atmospheric indicators are the following environmental factors:

• State of the ground; whether it is saturated from recent rains.• Precipitation along the foothills from slow moving clouds.• Cell alignment and movement over the same area receiving

precipitation.• Depth of clouds

• Based on the atmospheric and environment conditions street or flash flooding is likely in the afternoons along western areas of Trinidad between the hours of noon and 4:00 pm.

Flood Plain Flood ForecastingFlood Plain Flood Forecasting • Analysis of widespread flood events in Trinidad from 1980 to 2004

indicates flooding occurred twenty-three times during this period. Correlating the flood events with rainfall in the Caroni Basin as measured at the Meteorological Service in Piarco for the following:

• Day of the flood• Three days rainfall accumulation prior to the flood• Seven days rainfall accumulation prior to the flood• Fourteen days rainfall accumulation prior to the flood

• The data indicates that widespread flooding is associated with prolonged rainfall over the two week period and not necessarily with rainfall during the day of the flood event

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Date

8-14 days prior

4 - 7 days prior

3 Days prior

Day of Flood

Comparison of Percentage Totals for the day of the flood, three-days prior; difference between the seven and three day period and difference between the fourteen and seven day period

Flood Forecast Algorithm

Seven day cumulative rainfall >= 50.0mm

Check model output for precipitation forecast over the next three days for accumulations >= 75.0mm

Cumulative rainfall over the past three days >= 75.0mmYES

Widespread heavy rainfall forecast over the next three days

NO

YES

YES

Forecast widespread flooding in the floodplains

No additional rainfall forecast: No Flooding Forecast

Flooding not Forecast

NO

NO

YES

Community Based Flood Community Based Flood Forecasting Early Warning SystemForecasting Early Warning System

• The National Emergency Management Agency brought the stakeholders together to design a Community Based Flood Forecasting System for Disaster Mitigation.

• Stakeholders:• Meteorological Service• Water Resource Agency• Ministry of Community Development• Ministry of Local Government• Ministry of Works: - Drainage Division

Community Based Flood Community Based Flood Forecasting Early Warning SystemForecasting Early Warning System

Community Based Flood Community Based Flood Forecasting Early Warning SystemForecasting Early Warning System

• Dialogue with the communities to identify the hazards

• Using historical rainfall and flood data, to map flooding within communities.

• Create flood hazard mapping for floods with a return period of, 5, 10, 15, 25, 50 & 100 years.

• Build capacity within the communities to monitor, warn and mitigate against identified disaster based on the flood hazard maps.

• Install equipment to monitor stream flow. • Communication equipment provided to the focal point within the communities

to receive the warning and the methods to disseminate the message within the communities.

Flood Forecast IssuedFlood Forecast Issued• Flood Forecast issued to National Disaster Agency (NEMA/ODPM).

• Rainfall forecast issued to Hydrological Authority (WRA).

• Water Resources Agency issues river forecast to NEMA/ODPM and Communities

• NEMA/ODPM issued flood warning to the communities.

• Communities monitor the river levels and warning themselves based on flood maps.

• Communities communicate with the Ministry of Community Development and NEMA/ODPM of their needs after the flood event.

After Action ReviewsAfter Action Reviews

• All stakeholders meet after every event to identify the successes and failures.

• Damage and needs assessments (DANA) are reviewed.

• Bottled necks are identified and processes initiated to remove the bottle necks.