PGDM Inland Flood Hazard Map: Antigua

Antigua Flood Hazard Map

A higher resolution map image is also available (JPG 875k)


Map Text

EXPLANATORY NOTES

Flood Hazard Qualification

For the maximum 24 hour cumulative rainfall from Hurricane Lenny (equivalent to the 100 year storm***), the mean waterdepth within hazard zones will be as follows:

This depth is a measure of the volume of water (from rainfall) running onto the zone from surrounding lands, after abstracting rainfall amounts due to infiltration and from natural land drainage. Since this is a mean value, there may be places within the zone that are not inundated, and other places (e.g., close to the drains and in depressions) where the water depth exceeds the mean value.

  1. Potential flood prone zones were defined as ones with small slopes (about 0.2% to 0.5%. These would most likely be drained by streams and rivers with similar slopes. The drainage with such limited slopes are likely to be inadequate for draining floodplains. Areas of the potential flood zones and the catchments draining into them were determined using ArcView Version 3.1 and 3D Analyst, Version 1.0.
  2. Rainfall frequency was determined from a limited database, consisting of six years of continuous daily rainfall records from one raingauge sited at the Lester Bird Airport. Water depths from the 100 year return period storms are not expected to be substantially different from the Hurricane Lenny records and so these were used for determining the flood hazard zones. When additional records become available, the 100 year return period should be determined, compared with the Hurricane Lenny value, and the map accordingly updated.
  3. Runoff hydrographs for Hurricane Lenny (assumed equivalent to the 100 year storm) were developed using the HEC-1 procedure developed by the U.S. Army Corps of Engineers (1990). The hydrographs were based on the following:
    1. the Type III temporal distribution curve of the 24 hour rainfall produced by the National Resource Conservation Service, (formerly the Soils Conservation Service) United States.
    2. a fully saturated watershed with water already on the ground from previous rainfall 24 hours earlier.

*** A 100 year return period storm has a 1/100 chance of occurring in any year. Storms with high return periods, though seldom occurring, have high rainfall amounts; small return period storms, which occur frequently, have small rainfall amounts.

USE OF THE MAP

The map was produced using (i) a small (6 year) rainfall database; (ii) coarse topographical contour intervals; and (iii) limited information on observed water levels during a single high flow event, Hurricane Lenny (November,1999). Detailed information such as flood levels and water level variation during storm events cannot be confidently inferred unless these constraints are removed.

  1. The map is therefore to be regarded as preliminary; and the delineated flood areas represent only the approximate extent of flooding at the indicated hazard level.
  2. The map provides information on areas of focus during large rainfall events, particularly within the Cook Creek Watershed where a substantial portion of the population lives. The map also shows roadway sections likely to be under water during these events; this is not exhaustive information, as other localized flooding may occur on other roadway sections.
  3. The map is useful for broad predictions about the areas likely to be inundated on forecasting of particular extreme rainfall events following several days of inclement weather during the wet season.
  4. The map can also be used for broad assessment of the flood hazard associated with development of infrastructure, including roads (and their elevations) and housing, within the Island of Antigua.
  5. It can be used as a guide for determining areas for further detailed flood studies.
  6. The map DOES NOT provide any assessment of the hazard due to FLASH FLOODING.

Survey and Mapping Information

Grid: British West Indies
Projection: Transverse Mercator
Latitude of Origin: Equator
Longitude of Origin: 62 deg 00 min West of Greenwich
Scale factor on central meridian: 0.9995
False co-ordinates of Origin: 400 000 metres East, nil North
Unit of measurement: metre

Detailed surveys within Cooks Creek floodplain along Bendal's Government Quarry Road to the east and the Creekside Bridge to the west were conducted by Anwar Deonanan and Associates, 3rd Street, Barataria, Trinidad West Indies, using Total Station and data collectors.

Controls points obtained from the Land and Surveys Department, Antigua Government, were used for placing the survey data on the National Grid.


USAID/OAS Post-Georges Disaster Mitigation: http://www.oas.org/pgdm

Page last updated on 14 May 2001