A higher resolution map image is also available (JPG 910k)
For the maximum 24 hour cumulative rainfall amount from the 100 year return period storm***, the mean water depth within the hazard zone will be as follows:
Very High = exceeds 600 mm (about 2 feet)
High = between 300 mm and 600 mm (1 foot to 2 feet)
Moderate = between 150 mm and 300 mm (0.5 feet to 1 foot)
Low = between 75 mm and 150 mm (3 inches to 6 inches)
Very low = less than 75 mm. (3 inches)
Equivalent water depths were estimated from runoff hydrographs that drained from surrounding lands into the flood plains. These runoff hydrographs were developed using the HEC-1 procedure by the U.S. Army Corps of Engineers (1990). The hydrographs were based on daily rainfall collected on St. Kitts; for distributing the daily rainfall over short time intervals as required (15 minutes in this case to match the times of concentration of the watersheds), the Type III temporal distribution curve from the National Resources Conservation Service (formerly the Soils Conservation Service), United States, was used.
Some flood prone areas were considered temporary (shown cross-hatched) as they were not due to natural drainage limiting characteristics. Most are due to poor construction, inadequate or dilapidated hydraulic structures, or inappropriate landuse zoning. The existing drainage hazards can readily be removed once the drainage is corrected.
Flood prone areas were identified mainly from field interviews, but they were also derived from digital slope data analyzed by ArcView 3.1 with the 3D Analyst 1.0 software. Flood prone areas from the latter method were ones with limited slopes (<0.5%). Watershed areas draining into the flood areas were determined using the same software.
The estimated flood levels within the zones are only approximations and should not be used for any detailed analysis.
The estimated flood levels represent the mean water elevations; elevations within the zone may be higher or lower as relief varies within the zones. Close to river banks and at depressions, water levels are expected to be highest; areas removed from the rivers (ghauts) would have lower elevations.
The mean water elevations can be improved when daily rainfall data collected on Nevis become available. About ten (10) years of data are required to supplement the 2 years of records available from Newcastle Airport. Rainfall data on the south of the island, around Charlestown could be used for hazard zones in the south.
The water elevation estimations were based on assumptions of the drainage capabilities of the zones, but they can be qualified with appropriate hydraulic analyses, such as was done for the areas mentioned below.
Detailed surveys were conducted at two sites and the information used to provide more accurate water level contours within the flood hazard zones. The two areas were: Lower Bath Ghaut and its surroundings; Camp River from the bridge at Nisbet Plantations Hotel to about 500 metres upstream. These water levels are shown in separate maps.
Owing to limitations in the availability in daily rainfall data and high resolution elevation data, the resulting map can only be considered preliminary. Nevertheless, it can be used for the following:
Ranking of areas on the island according to flood hazards.
Development of land use zoning plans.
Determination of areas for which detailed flood studies should be undertaken (or further developed).
The map MUST NOT be used for estimating water levels for various storm return periods.
The map DOES NOT provide any information about localized flooding due to flash flooding.
The map DOES NOT make any declaration of erosion hazard due to fast flowing water currents.
*** 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.
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 along Camps River were conducted by Dwight C. Francis, Saddle Village St. Kitts. Detailed surveys along Bath Ghaut and its surroundings were conducted by Anwar Deonanan and Associates, 3rd Street, Barataria, Trinidad, West Indies.
Controls points obtained from the Land and Surveys Department, St. Kitts/Nevis 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