|Environment and development
in coastal regions and in small islands
CSI info 4
3.1 General Concepts
Based on the coastal geomorphology of the small Caribbean Islands, four major coastal types can be identified:
The following setback guidelines are based on geomorphological, geological and oceanographic characteristics as well as observed rates of change and socio-economic factors.
Geological composition and wave processes are major factors determining cliff retreat. "Hard" rock cliffs composed of volcanic and limestone rocks will generally erode much slower than cliffs composed of "soft" rocks such as clays and sandstones, where erosion rates may be as high as several metres a year. Cliff retreat rates are generally higher on windward coasts where wind and wave action is more intense. Cliff erosion is usually not a gradual process, but a sudden one as large blocks collapse especially in fractured rocks such as limestone.
Most of the cliffs in the Eastern Caribbean Islands are composed of limestone or volcanic rocks and generally cliff retreat rates are low, although as indicated above, cliff collapse is often a sudden process. A setback of 15 m from the cliff edge is adequate for the economic life (30 years) of most development projects.
|On volcanic or limestone cliffs, all new developments should be set back 15 m (50 ft) from the cliff edge.|
3.1.2 Low Rocky Shores
These shores are usually composed of limestone in the Eastern Caribbean Islands. They may be composed of ancient coral reefs marking previous higher sea level stands. Generally they show low levels of retreat, however, development in these areas is vulnerable to seawater inundation during tropical storms and hurricanes, thus a setback of 30 m from the vegetation line has been suggested.
|On low rocky shores, all new development should be set back 30 m (100 ft) from the natural vegetation line.|
3.1.3 Small Sandy Offshore Cays
Small sandy offshore cays exist in several countries e.g. Anguilla, the British Virgin Islands and the Grenadines. These cays are usually sandy, or they may consist of sand over a rock base. They often cover less that 10,000 square metres (100 m by 100 m), and they are usually anchored by coral reefs. Experience during Hurricane Luis in Anguilla (Cambers, 1996b) showed that these cays may completely disappear during such a catastrophic event. Furthermore, they may reform after the hurricane in a different location. For these reasons it is recommended that if development is permitted on these cays, then it should be of a temporary nature with wooden piles and all wooden construction. Actual setback distances should be determined using the methodology developed for beaches, see Section 3.1.4, appropriately modified for the small size of the particular cay.
|On small sandy offshore cays, if new development is permitted, it should be of a temporary nature with wooden piles and with all wooden construction.|
3.1.4 Sand and Stone Beaches
Due to the complexity of beaches and their changes, and their importance for tourism, recreation and development, it is recommended that setbacks be determined individually on a beach by beach basis. This will allow for greater setbacks on eroding beaches, which will in turn allow for conservation of the beaches, protection of beachfront property and the reduction of erosion caused by certain beach protection structures.
It is recommended that the line of "permanent" vegetation be used as the baseline for measurement. This is the tree line or scrub line and can be easily defined and agreed by different observers, also it shows only slight change apart from the relatively rare tropical storms and hurricanes. Features such as high water mark vary according to the tidal cycle and are very subjective especially when used by untrained observers. In addition, the use of the vegetation line as a baseline provides for the preservation of the most seaward sand dunes (where such dunes exist), these provide the first line of natural defence during a major storm event.
|The line of permanent vegetation should be used as the baseline for setback determination for beaches.|
Setbacks are developed for each beach based on the following parameters:
- Historical changes in the coastline position using the aerial photographs dating back to the 1960's when available.
- Recent beach changes using the beach monitoring databases developed within the Coast and Beach Stability in the Lesser Antilles project (COSALC).
- Changes in the position of the dune line/coastline likely to occur during a major (category 4) hurricane.
- Changes in coastline position likely to occur as a result of the predicted rise in sea level.
- Offshore features and changes.
- Coastal geomorphological features such as exposed beachrock and anthropogenic factors such as dune mining.
- Planning considerations such as lot size, national park designations.
These parameters are combined to provide a setback value for each beach. This represents the setback for all permanent development, e.g. residences (wood and cement), hotels, villas, swimming pools, roads.
|No development should be permitted seaward of the baseline, that is the "permanent" vegetation line, with the obvious exceptions of jetties and docking facilities|
Obviously blanket setbacks, which cover all types of development are easiest to enforce. However, in some islands it may be desirable to make special cases for some types of development. For instance in Anguilla, a special provision was made for piled, wooden beach restaurants, on the grounds that their economic viability depended on their proximity to the beach. In the past they had sometimes been permitted on the beach itself, however, it was stipulated that they should be set back at least 8 m (25 feet) landwards of the vegetation line.
In most cases one setback value is calculated for each beach, however, there may be exceptions for very long beaches such as Shoal Bay in Anguilla, Pinney's Beach in Nevis, where because of their length and particular characteristics, the beach may be divided into more than one section for setback calculation.
3.2 Methodology for Beach Setback Calculation
Seven parameters are used in the setback determination for an individual beach, these are described in detail below.
Historical changes are determined for each beach using the aerial photographs. Stereoscopic pairs of photographs are studied and major changes regarding each beach recorded, e.g. the disappearance of a sand dune. Then reference points close to the beach such as buildings, road intersections are selected, these reference points have to be visible on each set of photographs. Measurements are made from these reference points to the vegetation line and the offshore step, this is the seaward toe of the beach. It is marked in the field by a vertical downwards step near the wave breakpoint and is a distinctive feature on some beaches. It can usually be picked out on aerial photographs and is a more constant parameter than a low or high water line.
The number of points per beach depends on the number of reference points that can be identified on the sets of aerial photographs, in some of the less developed areas there may only be one or two measurement points per beach. These measurements are then compared and changes in the position of the vegetation edge and the offshore step determined over time.
There are many errors involved in this technique e.g. distortion towards the edge of the photographs, difficulty in identifying fixed locations (reference points), and difficulty in fixing the position of the offshore step.
Besides possible errors in the measurements, there are other factors which must be considered when using aerial photographs for assessing coastal change. For most islands there will be no more than three sets of photographs for the period 1960's to the 1990's, these represent just three time series. Beaches change dramatically from week to week and also seasonally, especially during winter when measurements may vary considerably from one week to the next if a major winter swell or "groundsea" event occurs. Additionally, tidal variations also exist, although tidal range in the Eastern Caribbean is very low, and in these measurements the offshore step was used rather than a particular water line.
Based on the foregoing, the assessment of shoreline change using aerial photograph measurements provides only an estimation of the actual change. Usually major changes can be picked out with this method. Figure 1 shows a comparison of the 1954 and 1982 shorelines at Crane Beach on the southeast coast of Barbados (Proctor & Redfern, 1982), here major changes have taken place. However, in this paper, historical shoreline change represents only one of several factors included in the setback calculation.
Figure 1. Comparison of the
1954 and 1982 Shorelines at Crane Beach, Barbados
(Proctor & Redfern, 1982).
The historical changes may also include hurricane changes if a hurricane passed over a particular island during the period of historical record. This may represent an over-estimation in the setback calculation, since hurricanes are factored in separately, see (iii) on the following page. Adjustments for this can be made in the overall setback calculation.
In most of the Eastern Caribbean Islands beach profiles have been surveyed as part of the COSALC project every three months since the early 1990's or before. These data are detailed and far more accurate than those described in (i) above. However, they only cover a short time period, less than ten years, and do not always include all the beaches in a particular island. Figure 2 shows the changes in beach width that have taken place on a west coast beach in Nevis, here there has been dramatic erosion over a seven year period.
The beach profile database is used to determine trends and changes in beach width, which is used as an indicator of short term shoreline change.
In view of predictions for more intense and more frequent hurricanes over the next two decades, it is likely that each island in the Eastern Caribbean will be impacted by a major hurricane passing directly over or nearby in the next 30 years, thus it is especially important to build this factor into the setback determination. The actual retreat of the dune edge or land edge resulting
Figure 2. Changes in Beach Width on the West Coast of Nevis
from a category 4 hurricane has been documented for Hurricane Hugo in 1989 and for Hurricane Luis in 1995 (Cambers, 1996c). This is viewed as a "permanent" change, since sand dunes and land take decades to form. Figure 3 shows the type of changes in the seaward sand dunes that were recorded after Hurricane Luis in 1995. Data for dune/land edge changes in 1989 (Hurricane Hugo) exist for Dominica and Nevis, and in 1995 (Hurricane Luis) for Anguilla, Antigua-Barbuda, Dominica, Montserrat, Nevis and St. Kitts. For these islands this factor can be determined on a beach by beach basis.
Figure 3. Schematic Representation of Dune Retreat after Hurricane Luis
For other Eastern Caribbean Islands, where such information does not exist, the data can be extrapolated from the northern islands pairing islands with similar geographical and geomorphological characteristics e.g. data exist for Dominica and Nevis for the hurricanes of 1989 and 1995, this can be applied to the other mountainous volcanic islands such as Grenada, St. Vincent and St. Lucia. Similarly the 1995 hurricane data for low islands such as Anguilla, Antigua and Barbuda can be applied to other comparable locations e.g. the Turks and Caicos Islands and some of the cays in the British Virgin Islands. Factors which controlled the hurricane response variation between beaches, such as coastline orientation and indentation, the width of the offshore shelf and past history of anthropogenic alterations, can also be built into this prediction. Based on a particular island's hurricane record, it may be necessary to develop a worst case climate scenario, for it is possible in the light of hurricane predictions for the next two decades that a particular island may be subjected to more than one severe hurricane.
A causal relationship between the retreat of sandy shorelines and general global sea level rise has been established over the last century (Bird, 1976; National Research Council, 1991). This relationship was first formulated by Bruun (1962) and is known as the Bruun Rule. It is based on the concept of an equilibrium beach profile which is a statistical average profile that maintains its form apart from small fluctuations including seasonal effects. This shows that as sea level rises, material is eroded from the upper beach and deposited on the nearshore ocean bottom. Figure 4 shows a schematic representation of the Bruun Rule. Consequently the ocean moves landwards or in other words there is shoreline recession. However, it is a difficult concept to confirm, for while the beach erosion/recession can be relatively easily quantified, the offshore sedimentation may be spread over a very broad zone. Notwithstanding the above, the Bruun Rule has been tested with a fair level of success along the southeast coast of Florida and in the Great Lakes (National Research Council, 1991).
The shoreline recession resulting from predicted sea level rise over the next 30 years is factored into the setback calculation.
|Shoreline recession resulting from predicted sea level rise is calculated as follows: a rise of sea level of y metres causes a shoreline recession of y times 100 m.|
Based on a predicted sea level rise of 0.3 m by the year 2100 (this is one of the lower estimates), this translates to 0.1 m by the year 2030, the shoreline recession is 0.1 x 100 = 10 m. Most development has an economic life of 30 years, so this time period is used for the calculation.
Some of the tectonically active Caribbean Islands may actually be rising, so their coastal areas may not be affected by sea level rise. However, until tide gauge records can be established for each island, it is recommended that the shoreline recession as calculated here be applied.
Figure 4. Schematic Representation of the Bruun Rule
Offshore characteristics and changes are used qualitatively to weight the calculated setback. Coral reefs and wide shallow offshore shelves often provide protection to particular beaches. Evidence from Anguilla, Antigua and Nevis (Cambers, 1996c) has shown that bays protected by nearshore reefs often experienced less erosion during the 1995 hurricanes than those which were more exposed.
Coastline shape was another factor, e.g. in Antigua, which has a very indented coastline where there are small bays within larger bays, the more sheltered bays suffered less damage during Hurricane Luis (Black et al, 1996).
Changes in offshore ecosystems are another variable used qualitatively, to weight the calculated setback. In Anguilla many bays were protected by dead intact Acropora palmata reefs (elkhorn coral reefs) prior to Hurricane Luis. These reefs provide an important natural breakwater function.
However, many of these reefs were reduced to rubble by the hurricane, thus water depths may have increased providing the potential for higher wave action and beach erosion.
Certain coastal features are also used qualitatively to weight other factors. Features such as exposed beachrock provide indicators of long term erosion. Also practices such as sand dune mining remove protective barriers and sand reservoirs from the beach system.
These include factors such as lot size, existence of marine parks and designations such as pristine coastal areas and vary from country to country. Some coastal lots may be very narrow, less than 100 ft in depth. Setbacks may cause some of these lots to become unsuitable for development. Government acquisition may be a solution in some of these cases, but for economic reasons, it is rarely a feasible option in the Eastern Caribbean Islands. Thus setback guidelines must take such limitations into account.
By their very nature, these considerations are subjective and therefore difficult to generalize. Two examples from the Anguilla methodology can be used to show how such considerations are included (Cambers, 1996a).
Along the northern and central sections of Road Bay, the recommended setback was 29 m, however, parcel size and shape would prevent the implementation of this setback. So instead a setback of 18 m was recommended, a reduction of 11 m. In a second case, the initial recommendation in Anguilla was for all new development to be positioned at the established setbacks.
However, a special case was made for beach bars and restaurants, whose owners felt that unless they were as near to the beach as possible, their commercial viability would be endangered. Therefore an exception was made and all new beach bars or restaurants have to be positioned 8 m (26 feet) landward of the permanent vegetation line.
|The actual setback for a particular beach is
determined by adding three components :
Changes in coastline position (p), this includes the historical changes, described in Chapter (i), and the recent changes, described in Chapter (ii). Where these two measurements conflict, e.g. historically a site shows erosion but within recent years the beach profile data has shown accretion, local information is used to determine which time frame should take precedence. Usually this is the historical data because it covers a much longer time period. But if there has been a recent coastline alteration such as a groyne or breakwater, the more recent trend may take precedence.
Changes in the position of the dune line/coastline resulting from a major (category 4) hurricane (h), this is based on Chapter (iii).
Changes in the position of the coastline resulting from coastal recession as a result of predicted sea level rise over the next 30 years (s), this is based on Chapter (iv).
The setback is calculated as follows :
setback = (p) + (h) + (s)
The setback will always be measured landwards from the line of "permanent" vegetation or tree line.
Once the setback is calculated using the above equation, offshore features and changes, geomorphological and anthropogenic factors, and planning considerations are subjectively included in the calculation by multiplying the setback value by a certain factor. Appendix I contains a setback calculation for a sample beach.
3.3 Setback Applications
Once the setbacks have been determined for each beach, a table can be prepared listing the values. However, from a planning perspective this may provide too much detail and prove too complicated for a planning board or development control authority.
In Anguilla, where this methodology has been used and is being implemented, it was decided to group the beaches into several setback categories. Based on the data for individual beaches, four categories were determined:
- Category 1: Setback is 18 m (60 feet) landward of the vegetation line;
- Category 2: Setback is 30 m (100 feet) landward of the vegetation line;
- Category 3: Setback is 45 m (150 feet) landward of the vegetation line;
- Category 4: Setback is 92 m (300 feet) landward of the vegetation line.
Most of the beaches fell into category 2. A map was prepared showing the proposed setback categories for beaches, see Figure 5. These generalized categories will allow for ease of implementation by the Planning Board in Anguilla. Table 1 summarizes coastal development setbacks in Anguilla.
The detailed setback determination for each beach, proposed in this methodology, will provide planners with the necessary information and justification for some flexibility. This is essential for planners who have to deal with socio-economic factors as well as those of a physical nature. However, interpretation of the coastal setback guidelines should not be so flexible that they become meaningless. These setbacks, which can be fully justified and explained to developers, should facilitate future coastal development.
Table 1 Coastal Development Setback Guidelines in Anguilla
|Cliffs||12 m (40 feet) from the cliff edge|
|Low rocky shores||30 m (100 feet) from the vegetation line|
|Sandy cays||Development restricted to piled, wooden structures, actual setbacks as for beaches.|
|Sand and stone beaches||1. Beach bars and
restaurants 8 m (25 feet) from the vegetation line.
2. All other development categorized by beach, see Figure 5, as follows:
Category 1: 18 m (60 feet) landward of the vegetation line;
Category 2: 30 m (100 feet) landward of the vegetation line;
Category 3: 45 m (150 feet) landward of the vegetation line;
Category 4: 92 m (300 feet) landward of the vegetation line.
However, it must be emphasized that any setback policy must be combined with an education and awareness campaign so that members of the public as well as special interest groups such as architects, contractors and politicians, fully understand the need for such setbacks.