floods

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Caribbean Disaster Mitigation ProjectImplemented by the Organization of American StatesUnit for Sustainable Development and Environment

for the USAID Office of Foreign Disaster Assistance and the Caribbean Regional Program

NATURAL HAZARD MAPPING ANDVULNERABILITY ASSESSMENT

Floods

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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT

Riverine Floods

• They occur when the carrying capacity of a stream or riverchannel is exceeded, leading to bank overflow.

• They become a hazard where development occurs in floodprone areas.

• They are controllable??

Rainfall Scenarios

• Short duration, high intensity rainfall flash flooding.• Long duration, low to moderate intensity rainfall slow flood.

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Natural Causes Rapid excessive runoff from slopes to streams.

Triggered by heavy rains.

Promoted by steep slopes, low permeability soils, high orographicrainfall.

Failure of dams and levees along rivers.

Backing up of flood waters by sand barriers at river mouth.

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Human Contributors Undersized drainage network. Poor maintenance of drains and culverts. Indiscriminate garbage disposal in rivers. Poor or lack of maintenance of river channel. Low flow capacity of bridges. Inappropriate use of protective structures, e.g. levees. Changes in land use leading to increased runoff.

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1. Flood Plains of Main Rivers

A. Narrow flood plains of steep, relatively small catchments (smallrivers) in volcanic islands and areas of youthful terrain.

High-intensity short-duration rainfall in upper watershed lead tohigh surface runoff upstream and flash floods downstream.

Many towns, villages located at mouth of river.

Rivers have social/economic importance to riverbankcommunity.

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B. Extensive flood plain of larger streams, e.g. Caroni Plains,Trinidad.

Flooding due to due to sustained rain over catchment and highsurface runoff from catchment hill slopes.

2. Alluvial fans, e.g. Liguanea Fan, Yallahs Valley, Jamaica.

All fan may be at risk due to high water velocity, numerouschannels across fan surface.

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3. Downstream of Potentially Unstable Engineered Dams, e.g.Roseau (Compton) Dam, St. Lucia.

Flooding due to dam breach / overtopping.

Floods may be high velocity, rapid, deep - little warning.

Downstream communities at risk.

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4. Upstream of Potential Sites of Landslide Dams.

Area upstream of dam likely to be flooded as water levels indammed river rise.

Deep water, lives may be lost.

5. Downstream of Potentially Unstable Landslide Dams.

Dam break or overtopping will cause flooding downstream;high velocity flow, little warning. e.g. Layou landslide dams,Dominica.

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6. Areas Behind Unsafe or Inadequate Levees.

Where river lies above of floodplain, flooding is prolonged.

Why are these areas at greatest risk?

Flood plains favorable site for development. Towns (including capital cities) are located at mouths of rivers. Represent only flat land on island. High fertility of flood plain. Where flood plains are narrow (confined between steep valley

walls), impact is greater.

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Flood Hazard Maps

Purpose To define extent, frequency and magnitude of hazard.

Flood Hazard Mapping

Purpose To identify areas prone to flooding and/or at risk. To determine magnitude of the hazard – height of water levels. To determine frequency of occurrence.

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Simple Approach Map past and/or existing flood zones as indication of area at risk.

Method

Field Mapping Document evidence of flood levels, historic flood levels (interview

villagers). Used to “ground truth” remotely-sensed data. In some cases only source of information, e.g. under emergency

conditions.

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Data Required Aerial photographs. Remotely-sensed imagery.

Utility Map shows boundaries of zone affected by flooding. Remote sensed data useful for temporal studies, i.e. examine

variation in flood zone over time with change in basincharacteristics, development of flood plain.

Can be used to identify areas at risk.

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Detailed Approach Use computer models to identify the actual location of water

levels in the event of a flood.

Methodologye.g. Jamaica Flood Mapping Program (Molina, 1987) Apply hydraulic analysis to peak flow data for several river

cross-sections. Cross sections determined from topographic map. Hydraulic analysis will determine the level of water for a given

flood event.

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Information Required

1. Topography Topographic base required to generate cross sections of river. Scale and contour interval may vary depending on relief and

population at risk. Topographic base at 1:2500 scale maximum best. Contour

interval of 0.3 m best.

Sources of Information Lands and Surveys Department. Field mapping.

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Constraints Non-availability of maps at appropriate scale or maps not

available for entire study area. May use 1:5000 maps where 1:2500 not available = less

accurate. Up to 1:10000 scale maps have been used problems in

determining river geometry generalised result.

2. Hydrology Peak flow magnitude and associated return periods. Used to generate flood-flow frequency relationship. Return periods - 100-yr, 50-yr, 25-yr , 10-yr or 5-yr.

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Sources of Information Technical reports. Statistical analyses. Need historical record of peak flow. Rainfall-runoff models. Calculates excess runoff peak flow magnitude. Rainfall intensity, duration, depths and frequency –

Meteorological Office. Peak flow records – Water Resources Department.

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3. Hydraulic Analysis Calculates expected water levels for given peak discharges and

given channel and overbank conditions. Need information on channel characteristics, e.g. roughness

coefficients.

Constraints Difficult to determine some parameters. Well defined river channel difficult to determine for some flood

prone areas, e.g. alluvial fans. Roughness coefficient of channel.

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Concerns Lack or incomplete rainfall database. Obsolete data, e.g. channel modified by flood. Interpretation of data, e.g. flood maps of Trinidad.

Utility of Flood Hazard Maps Basis for flood risk mapping. Plan appropriate land use in areas at risk.

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Organisations to Contact

Ministry of Works (Drainage, Hydrology Divisions) Water Resources Agency National Disaster Offices

floods

Documents