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Disaster Management for 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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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 MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
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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FLOOD HAZARD MAPPING AND VULNERABILITY ASSESSMENT
Organisations to Contact
Ministry of Works (Drainage, Hydrology Divisions) Water Resources Agency National Disaster Offices