SHORELINE AND PROPERTY PROTECTION OPTIONS
INTRODUCTION
The erosion problems that are commonly seen along shorelines often arise as a result of
failure to recognise that shorelines have always been areas of continuous and sometimes
dramatic change. This lack of understanding of shoreline processes has been catastrophic
for many property owners both private and public. The situation is not without remedies
and a variety of reasonably “low cost” alternatives are available. However it must be
cautioned that while appearing deceptively simple most of them require careful
judgement and design and can entail considerable investment to correctly implement.
Therefore before any action is taken it is important to recognize and understand the
natural forces at work in the general area of the proposed project.
AVAILABLE OPTIONS
Three options are available to combat erosion:
1. Take no action (although generally viewed by a property owner as unacceptable
this is used to help evaluate different alternatives. It is not advisable to react
immediately to an erosion problem since it might be caused by a temporary factor
e.g. unusual swell events and tropical storm waves).
2. Relocate endangered structures (involves moving them to a different area or
further from the water on the same lot. Moving a building is an expense which
could be wasted if it is not moved back far enough. It is therefore necessary to
investigate erosion rates and the likelihood that this rate will continue at or below
historic levels though the required life of the setback).
3. Take positive action to halt the erosion (includes devices that either armour the
shoreline, intercept or reduce wave energy offshore, or retain earth slopes against
sliding).
SHORELINE AND PROPERTY PROTECTION STRUCTURES
Most shore protection structures are built primarily to protect coastal development from
sea storm waves and erosion. The principal structures for shore protection used in
Barbados include seawall, revetments, groynes, and breakwaters. Each serves a special
purpose and each affects the shoreline in a different manner.
Seawalls, Bulkheads and Sheet Piling
These are functionally similar and consist of solid vertical walls (metal, concrete or
masonry) to protect the shoreline or fill land. A seawall is a solid barricade built at the
water’s edge to protect the shore and prevent inland flooding. Seawalls are expensive and
are suitable only for special situations since they often compound shoreline erosion
problems because in the long run they do not hold or protect the beach. In fact they often
accelerate the loss of beach sand. As the wall deflects the wave, it forces downward into
the beach deposit. This causes the sand to erode away seaward of the footing and the
beach to diminish or disappear. Often the seawall is undermined and collapses.
The advantage of vertical walls is that they require less material and take up less space.
The disadvantage is that reflected wave energy is maximized with a vertical facing wall
and hence these are only normally constructed along calmer shorelines (e.g. within
harbours and protected lagoons). Sloped or curving seawalls are often used at open sites.
Groynes
These are rocky protective structures which project seaward (perpendicular) of the
shoreline and are connected to the shoreline. A groyne is a dam of sand basically; a
structure built to interrupt longshore sand movement (littoral drift) and trap sand to
stabilize or widen a beach. Sand is trapped on the updrift side causing the shoreline to
accrete. However the downdrift side of groynes can cause shorelines to erode because
longshore transport is interrupted.
Groynes are effective only (a) when there is a significant volume of littoral drift, (b)
when the drift carries coarse material and (c) when the beach downstream from the
groyne can be sacrificed. Groynes are most effective when set up in groyne fields with
the spacing being two (2) or three (3) times their length. As part of this concept there is a
need for placing sand artificially to fill the area between the groynes thereby ensuring a
more or less uninterrupted sand supply to downdrift shores.
Revetments
These are heavy facing armour stones on a slope to protect it and the adjacent upland
against wave scour. They normally consist of sloping rock walls with progressively larger
sized stones being placed on top smaller stones. The outer most layer consists of “armour
stones” The outer basal extension of the structure (the toe) takes the brunt of wave action
striking the slope and the irregular surfaces and spaces between the stones are effective in
dissipating wave energy and minimizing reflective waves. This protection dissipates the
wave energy with less adverse effect on the beach than a seawall. But reflected waves
carry sand and other bottom material seaward and way from the shoreline and can
undermine shore protection structures unless they are properly designed and placed on a
solid rock foundation.
The advantage of a revetment is its flexibility which allows it to settle into underlying
soil or experience minor damage and still continue to function. Because of its rough
surface it experiences less wave run-up and overtopping. The disadvantage of the
structure is that it requires heavy construction equipment to be built. Like seawalls,
revetments protect only the land behind them and provide to adjacent areas or associated
beach areas. If a beach is to be retained adjacent to a revetment additional structures such
as groynes or breakwaters may be required.
Gabions
These are structures consisting of smaller rocks and rubble placed in baskets or cages.
These can be constructed manually and serve as an alternative where heavy equipment
access is inappropriate. The wire mesh should be PVC coated, the baskets tightly packed
and a filer cloth should be used beneath the structure to help control settlement. Gabions
should be used in collaboration with other revetment structures to protect the structure
from scour. Tight packing of the baskets is important in avoiding distortion of the baskets
under wave action. They should not be used in beach areas.
Breakwaters
These structures are aligned to the shoreline and separate from it. The seaward face of the
breakwater absorbs wave energy and caused currents and wave action shoreward of the
structure to be diminished. Thus shorelines opposite properly located breakwaters are less
prone to erosion but still allow longshore transport to occur thus reducing the likelihood
of erosion along adjacent shorelines to either side of the breakwater.
The low energy shadow zone on the landward side of the structure significantly reduces
the ability of waves to transport sediment. Sand is therefore gradually trapped behind the
structure and accumulates.
Beach Fills
(Beach nourishment) Beach fills are quantities of sand placed on the shoreline by
mechanical means e.g. dredging and pumping from offshore deposits, or overland hauling
and dumped by trucks. The resulting beach provides some protection to the area behind it
and also serves as a valuable recreational resource.
The beach fill functions as an eroding buffer zone. As large waves strike it, sand is
carried offshore and deposited in a bar. As the bar grows it cause these large incoming
waves to break further offshore. The useful life of a fill, which depends on how quickly it
erodes, can be completely eliminated in a short period of time by a rapid succession of
severe storms. Beach fills generally have a relatively low initial cost but a regular
maintenance cost of adding new fill (periodic renourishment).
SUMMARY
Every shoreline construction project affects natural habitats and ecosystems function.
Whether it is clearing, grading, landfilling, laying foundations, or erecting structures
there will be effects. Shore construction has complex effects. Often short term solutions
can caused serious long term problems (e.g. destabilization of beaches). Shoreline and
property protection options are often termed “low cost”. This does not mean they are
cheap since any properly implemented protection method is expensive. The term “low
cost” simply means that the various measures are commensurate with the value of
property being protected and they are among the lower priced options available.
Any alternative requires evaluation of the shore form, planned uses of the land, money
and time available, and other effects of the decision. The Coastal Zone Management Unit
often provided both formal and informal consultation to property owners that request
assistance, on some of the more appropriate options to help reduce erosion and potential
property damage along coastal segments of Barbados.
For additional information, contact:
The Director
Coastal Zone Management Unit
Bay Street
St. Michael
BARBADOS
Tel: (246) 228-5950/1/2/3
Fax: (246) 228-5956
Email: coastal@caribsurf.com