# Presentation: 499854

## Flash Flood Forecasting on a Tropical Small Island towards Disaster Preparedness – Trinidad

- Glendell De Souza
- Science & Technology Officer
- Caribbean Meteorological Organization

- International Workshop on Flash Flood Forecast Workshop, Costa Rica
- 13th – 17th March 2006:  **Session 8: Disaster Mitigation, Preparedness & Response**** **

## Presentation 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

## Introduction

- 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).

- Urban Flooding

## Introduction (Cont’d)

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

## Topography

## Land use Practices

## Slide 7

## Urban 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 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 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

## 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 

- 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 >= 75.0mm

- YES

- 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

## Slide 13

## Community Based Flood Forecasting 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 Forecasting Early Warning System

## Community Based Flood Forecasting 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 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 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.

## Thank You