PGDM Inland Flood Hazard Map: Antigua
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:
- Very high = exceeds 600 mm (about 2 feet).
- High = between 300 mm and 600 mm (1 ft to 2 ft).
- Moderate = between 150 mm and 300 mm (0.5 ft to 1ft).
- Low = between 75 mm and 150 mm (3 inches to 6 inches).
- Very low = Less than 75 mm (3 inches).
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.
- 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.
- 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.
- 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:
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- It can be used as a guide for determining areas for further detailed flood
studies.
- 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.