PGDM Inland Flood Hazard Map: Barbuda

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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 water depth 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.

1. The flood prone zones were defined as ones with very small slopes (about 0.5% to 0.2%). Areas of the 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 eight years of continuous daily rainfall records from one raingauge sited in Codrington. 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 records become available, the 100 year return period should be determined 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).
4. 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
2. Service) United States.
3. a fully saturated watershed with water already on the ground from previous rainfall 24 hours earlier.
5. The mean water depths on the flood plains were based on the volume of the runoff hydrographs and an estimate of the drainage efficiency of the flood zones. Drainage rates were estimated from field surveys and information contained in topographical maps. Drainage was considered restricted if the drains and streams at their outlets were (or there was a high probability that they could have been) blocked or limited by vegetation (mangroves) high sea water levels, or/and debris from within human settlements.

*** 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 (8 year) rainfall database; (ii) coarse topographical contour intervals; and (iii) field interviews on the observed water levels during a single high flow event, Hurricane Lenny (November, 1999).

1. Owing to these limitations, the map is to be regarded as preliminary and the delineated flood zones represent only the approximate extent of flooding at the indicated hazard level.
2. The map is useful for broad predictions about the areas likely to be inundated.
3. The map can also be used for broad assessment of the flood risk associated with development of infrastructure, including roads and housing near and within Codrington.
4. It can be used as a guide for determining areas for further detailed flood studies.

Mapping Information

Projection Transverse Mercator
Spheroid Clarke 1866
Unit of Measurement Metre
Meridian of Origin 63 °00' West of Greenwich
Latitude of Origin Equator
Scale factor at Origin 0.9996
False Coordinates of Origin 500,000 m Easting