Barbuda Inland Erosion Hazard Map

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1. Simple empirical models were used to produce hazard scores for each land unit, defined by geology, soil and other environmental "elements". The models integrate the estimated effects of the "elements," which cause or influence the hazard and produce a score. The score is an estimate of the relative likelihood of the hazard at the land unit, but it has no physical meaning, like ‘days per year’ or ‘tons per acre’, etc.
2. To make the scores useable, they were classified. For each individual hazard estimate, the scores have been divided into 5 equal area classes, using the frequency distribution of the scores. Since the distribution of these hazard scores is usually "L-shaped" rather than uniform or "normal", the range of values in each class group is not uniform, (eg: 0-3; 3-17; 17- 62; etc). In the case of Barbuda because the number of discrete land units was small the scores have been classified using the Antigua distribution.
3. If we try to combine the effect of different types of hazard we are faced with the problem of adding apples and pears, and we have no common element like calorific value to use. Adding scores for land units would produce some odd results, but adding classes reduces this problem, rather like standardising data. Composite maps for combined hazards were produced by adding classes and then reclassifying.

Models used

1) Sheetwash and Rilling Hazard = K . R . S^1.55 . LU

where K = soil erodibility, R is a rainfall erosivity factor, S is the tangent of the slope angle in degrees and LU is a factor indicating effect of land use

2) Wind erosion hazard rating = I x DRY

where: I = T + St + Pt + Ps (and T = soil texture; St = stony surface; Pt = ped type; Ps = ped strength) and DRY = Number of months with less than 2.4 inches rain, using long term averages

3) Landslide hazard = (Soil-rock combination susceptibility) . (Slope factor) . (Land use factor)

where: Soil types are classified as: 0, unlikely to slide; 1, slide, given an underlying slip plane; 2, susceptible of internal slips

Rock types are classified as: 0, unsupportive of landslides; 1, supportive of soil above sliding, rock falls; 2, likely to slide with soil.

These factors multiplied together produce a combined susceptibility score of 1,2 or 4

Slope factor is related to the tangent of the slope angle, and doubled for angles above 30 degrees, reflecting the approach to the angle of repose of loose materials,

Rainfall factor: no critical threshold is evident so the factor was omitted.

Land Use factor: an estimated generalised figure for the effect of vegetation and other land uses relative to bare land. The effect of road cutting is however excluded.

Data used, other than field observation

Martin-Kaye, P. H. A., Reports on the geology of the Leeward and British Virgin Islands*, Leeward Islands Gov’t., 1959

Vernon, K. C. and D. M. Lang, Soil and land use surveys No. 19b, Barbuda* UWI, St Augustine, 1966

Erosion hazards are most commonly defined in relation to open land either under agricultural use or intended for an agricultural use. The application to development sites where some kind of construction is intended is related to localised incidents rather than a general picture. This emphasises the need for data at a more appropriate scale, not available at present, but more importantly, the need for field examination of sites after using this map for an initial classification, especially where comparisons need to be made.

Sheet and rill erosion are generally of little importance at a building development site. Gullying be dangerous particularly where the site lies in the path of a gully working back inland from a stream or ghaut (eroding head wards). The problem of deposition of materials from sheet and rill erosion is of low impact. Some inconvenience but little possibility of damage. That from gullies can be more of a nuisance. Overall, rill and sheet erosion are of little importance in assessing possible impacts on building development sites, while gullying is important.

Wind erosion, may cause problems in certain circumstances, and since other types of erosion are of less importance in Barbuda because of the low rainfall and mostly gentle relief it is included.

Mass movement, both the site from which the movement takes place and the destination of material, can produce major difficulties for building development. The only obvious danger in Barbuda is found in coastal and some inland limestone cliffs.

Consequently the chosen mapping combines mass movement (landslides and rock falls) and wind erosion. This is done by adding together the land unit by land unit values for mass movement and wind erosion. It should be noted that in the recent past in Barbuda, more damage was caused to soil, vegetation and some buildings by incursion of the sea, than by events originating on land.

1. The maps are composed of overlays of up to six different maps. In some cases the origin of the base on to which the thematic material– the modelling data– was recorded is not known and the fit between maps is only fairly good. When you have located your approximate position on a topographic map you can find the corresponding position by measurement on the maps. Unless you are very close to a boundary this will give you a good indication of the hazard. However, because some aspects of the hazard are impossible to model at this scale you should look to see whether you are at the foot of a slope. If it is a long slope you should add one class to the hazard you see.
2. This map will give you a good indication for most initial planning purposes, for selection of suitable sites for some purposes etc. When you have chosen an area, examination of the differences between specific sites may necessitate observing the factors in the model and comparing scores.
3. In general the level of erosion hazard is not high, especially under a dense vegetative cover, but where road cuts and other excavations have been made it will be higher than the indicated figure.

Projection Transverse Mercator
Datum NAD 1927
Delta WGS84 -11 175 188
Spheroid Clarke 1880
Major semi-axis 6378206
Minor semi-axis 6356584
Unit of Measurement metre
Meridian of origin 63 degrees West of Greenwich
Latitude of origin Equator
Scale factor at origin 0.9996
False Coordinates of origin 500 000 m Easting 0 m Northing