This portion of the
site is very much under construction so Please bare with me. there could be a
few typos and/or broken links all of which will be ironed out in time.
Thanks
University College Cork
Colαiste na hOliscoile Corcaigh
Guileen Cliff
Erosion Study
FINAL REPORT
REPORT PREPARED FOR:
GUILEEN AND FINURE COMMUNITY GROUP
REPORT PREPARED BY:
HYDRAULICS
AND MARITIME RESEARCH CENTRE (UCC) (DR. J. MURPHY)
NOVEMBER 2004
Guileen Cliff Erosion
Study
Table of Contents
1.0 Introduction 2
2.0 Shoreline Features 3
2.1. Cliff Erosion 6
2.1.1. Historical Erosion 13
2.2. Conclusion 14
3.0 Environmental Loading 18
3.1. Wave Conditions 18
3.2. Water Levels 18
4.0 Proposed Cliff Stabilisation Measures 19
4.1. Concrete Retaining Wall 19
4.2. Rock Revetment 19
4.3. Gabion Wall 20
5.0 Effects on Adjacent Landmasses 21
6.0 Appendix 1: Sea Cliff Erosion 22
1.0 Introduction
This study undertaken by the Hydraulics and Maritime Research Centre (HMRC) of
UCC involves an assessment of the coastline fronting an important coastal walk
and a number of cottages in Guileen, Co.
2.0 Shoreline Features
The area of interest is about 13Dm in length and is bounded on the eastern end by a concrete parking/boat storage area that has a 3/4 tier gabion wall protecting the cliff face and on the western end by an RC retaining wall which was constructed by one of the house owners (Plate I). The shoreline consists of various sandstone rock outcrops dipping diagonally in a seaward direction (Plate 2). Generally there is no sediment present on the exposed foreshow apart from the areas between the outcrops and at the base of the cliff (Plate 3). This material consists of coarse granular sediment varying in size from large boulders to fine gravel. The rock outcrops provides a relatively high level of protection to the cliff for the majority of sea conditions as its level at the cliff toe approximately corresponds to high water levels of extreme spring tides. Although a few areas of cliff are more vulnerable to wave attack given the gaps in the rock.
Figure 11997 Ordnance Survey Map
The most critical storm condition for this
location would be from directions in the S to SSE sector at a time of high
spring tides. Waves from easterly directions are largely blocked by Power Head
but could still cause damage at Guileen if accompanied by high water levels. It
should be noted that it is high water levels as much as large waves that cause
damage to soft cliffs.
The cliff varies in height between about 6-9m and is composed of a agglomerate
of different sized sediment embedded within a fine silty/clayey material. When
a section of the cliff collapses sediment is released into the littoral zone.
It is likely that clay and fine sand sediment is carried offshore where it is
deposited in deeper water whilst coarse sands and gravels would remain closer
to the shoreline and may periodically wash up on nearby beaches.
Plate I Aerial photograph of Cliff showing proposed
line of coastal protection
structure
Plate 2 Rock Outcrops fronting cliffs
Plate 3 Base of Cliff (between outcrops) after recent storms
Plate 4 Top of Cliff showing different sediment types
2.1.
Cliff Erosion
A discussion on the nature of soft cliff erosion is provided in appendix I and
should be referred to prior to reading this section.
Erosion of the soft cliffs at Guileen will have serious consequences for the
village and the area. Not only will a section of an important coastal walk be
lost but also a number of cottages, which are an intrinsic part of the village
and of important historical significance will be destroyed. Evidence of erosion
and its implications is obvious from visiting the site but the detail of this
erosion will now be examined
The residents of Guileen have been familiar with the problem for a number of
years. A road that once fronted the cottages is now a narrow pathway that in
itself is in danger of being lost. Prior to the recent storms on Oct 27-28 2004
the last major erosional event took place in January of 1990. It was a similar
combination of storm surge and spring tide that caused collapse and recession
of the cliff face. The Evening Echo of 15th
January 1991 ran an article on storm
damage in
Guileen long
suffering from erosion was also hit but without serious damage to property. But
a roadway leading to 12 private houses, once
able to take motor
traffic, is now only a narrow pathway at the edge of the
Erosion of the cliff is evident by
examining the nature of the cliff face. Generally if the cliff front is well vegetated it indicates
that erosion is not severe whereas a lack of vegetation and mounds of material
at the base are indicative of erosion. As one walks in a westerly direction
along the shoreline, a combination of these two situations is evident. As
discussed in the appendix I a cliff face does not erode uniformly so some areas
are relatively well vegetated and others have had recent falls. Often after a
fall the cliff adopts a more stable profile (particularly if it is a rotational
type failure) and with the buffer of material at its base it is unlikely to
fail again for a number of years. It would seem that since the storms of 1990,
recession has not been significant and up until recently many areas of the
cliff were well vegetated. In some places vegetation had even established at
the base of the cliff Relatively small failures have occurred since 1990 but
they could be attributed to terrestrial loading and possibly also to several
drains which exit at the cliff face. Plate 9 shows a recent photograph of the
cliff whilst Plate 10 shows a similar photograph taken in 1990. It can be seen
from examining the drain pipes (circled in red) that the position of the cliff
has not changed significantly in the time period between the two photographs.
The recent storms of October 2004 have removed any loose material at the base
and also caused additional erosion. The cliff face is now practically vertical
with its base now subject to wave action during large storm events. The next
major failure, which could occur at any stage given the right combination of
events, will result in the loss of the coastal walk and possibly also to some
of the cottages. As it is the coastal walk is potentially dangerous as parts of
it axe overhanging the cliff edge and in addition a section of stone wall is in
imminent danger of collapse (plates 11 and 13). Various photographs have been
included below that show the current state of the cliff.
It should be noted that the concrete retaining wall which acts as the western
boundary of the area of interest has performed its function well since its
construction. However, it is probable that it has not been severely tested to
date.
Plate 5 Section of cliff immediately west of
slipway showing a number of
areas that
suffered recent erosion
I
Plate 6 Erosion of cliff base
Plate 7 Western section of cliff
Plate 8 Undermining of Stone Wall
Plate 9 Western Boundary of study area showing
retaining wall and drainage pipes
(circled)
Plate 10 Western Boundary of study area in 1990 showing drainage pipes (circled)
Plate 11 Proximity of Coastal Walk to Cliff Edge
Plate 12 New Fencing set back from cliff edge
Plate 13 Partial collapse of existing fencing
2.1.1. Historical Erosion
Examining changes in shoreline position
as indicated by ordnance survey maps and aerial photographs is a standard
method of estimating historical erosion rates. It is required that such
information spans over a wide time frame such that reasonable estimates of
erosion rates can be determined. For this study the following information was
available,
Ordnance Survey Map (Surveyed 1842 and revised and levelled in 1930-1986)
(Figure 2)
Ordnance Survey Map (Surveyed 1997) (Figure 1)
Aerial Photograph (1951) (Plate 14)
Aerial Photograph (2000) (Plate 15)
Figures 3, 4 and 5 show the comparison between the 2000 aerial photograph and
the 1997 OS map, the 1951 aerial photograph and the 1842 OS map respectively.
To produce these drawings the maps and photographs had to be magnified
digitized, resealed and rotated such that they could be fitted on one another.
Given the nature of the maps and photographs it was not possible to get a
perfect fit and indeed some of the trends observed tend to be inconsistent but
overall some conclusions can be made regarding the changes in cliff position.
If figure 3 is first considered it can be seen that the 1997 and 2000 cliff top
position are very similar. This would be expected given the long intervals
between erosion events as was previously discussed. In the area immediately
east of the retaining wall the cliff position is unchanged whilst more towards
the slipway the trend is uncertain with the 2000 position seeming to be more seaward
than the 1997 map. It is believed that the cliff position is substantially
unchanged between the 19972000 time period and that any differences can be
attributed to difficulties in locating the cliff top from the aerial
photograph. As a final comment the most seaward line correspond to the HWM and
show that it is at least 10m from the current base of the cliff
Figure 4 compares the two aerial photographs. It indicates a significant change
in the cliff top position in the 50 year time period. Recession of the order of
a few meters has occurred and sections of the roadway (third blue line from
seaward direction) have been lost. This plot gives the best representation of
the erosion that has occurred.
It is difficult to make any real conclusion about the changes to the shoreline
position from the 1842 survey map (figure 5). It is probable that the cliff was
not thoroughly surveyed so some of the detail is missing. It is also not
certain that what has been interpreted as the top of the cliff is the actual
top. The outermost red line represents the HWM and is in a similar position to
that in the 1997 map. The second line may represent the base of the cliff and
it can be seen that it is considerably seaward of the current position (same as
cliff top). The third line, marked in figure 3 as the cliff top,
has a position approximately similar to
the 2000 position and in some locations it is more landward. It is likely that
the cliff top was not accurately marked in this survey but that the cliff base
was properly surveyed. Therefore more can to inferred from the change in the
cliff base position, which shows a recession of more than 5m in places.
2.2. Conclusion
From the previous discussion it is clear that the cliffs are eroding but not on
a continual basis. Cliff falls and recession are associated with large storm
events and high water levels that may occur less than once a decade. However
the episodic nature of the erosion should not act as an excuse to postpone or
neglect the implementation of coastal protection works. Wear and tear on the
front face of the cliff will result in the loss of the coastal walk in the next
few years whilst for the right type of storm the cottages will be damaged or
lost. In addition future erosion rates could increase, particularly with the
rise in mean sea level (which is well documented) and secondly with an increase
in storminess due to climatic changes (not hilly proven yet). It is generally
accepted in the field of coastal engineering that soft coastlines (beaches, dunes,
glacial cliffs) will come under increasing threat in the future due to the
combined impact of these two effects.
Figure 2 1842 Ordnance Survey Map
Plate 14 1951 Aerial Photograph
Plate 15 2000 Aerial Photograph
Figure 3 Comparison Aerial Photograph (2000) with OS Map (1997)
Figure 4 Comparison Aerial Photograph (2000) with Aerial Photograph (1951)
Figure 5 Comparison Aerial Photograph (2000) with OS Map (1842)
3.0 Environmental Loading
3.1. Wave Conditions
No information is available on the
magnitude of the wave loading on the cliff face. However it would be expected
that waves generally do not reach the base of the cliff. Future studies for the
design of coastal protection structures would require a topographic survey to
vefl1 levels but it is not envisaged that a detailed wave climate study would
be necessary. The maximum unbroken wave that can reach the cliff can be
estimated empirically by using the ratio of the wave height to water depth
(H/d). Research shows that this value varies between 0.7 and unity depending on
the beach slope and wave frequency.
3.2. Water Levels
High water levels are regarded as the most critical factor in influencing cliff
recession. Normal spring tides do not generally affect the cliff which
indicates that the base Level should be in excess of 4.3m O.D. Poolbeg. This is
the MHWS level for
za =0.01(1013pa)
As an example of possible surge
levels, a storm which occurred on the 16/12/89 is considered. This storm caused
particular damage on the south coast and corresponded with high tide levels and
particularly low atmospheric pressures (944mbar). By using the formula given
above the storm would have resulted in a mean sea level rise of 0.69m. If wave
and wind set-up were added to this value then a total water level increase
above the predicted tides would be of the order of 1.5m.
Regarding sea level rise there is no general consensus as to the magnitudes of
rates but worst scenario predictions to 2100 estimate mean sea level increases
of up to a metre. A more realistic estimation would be about half this value
but would still have a considerable impact on soft coastlines.
4.0 Proposed Cliff Stabilisation Measures
In this section three options for coastal protection/cliff
stabilisation structures are proposed. Each option combats the most significant
cause of erosion (undercutting of the cliff base) and also secondary causes
(localised collapse due to heavy rainfall). Indicative costings are provided
for each option but it should be stated that these figures are best estimates.
More detailed study is required to determine all the design details the true
costs. In addition a 140m length of structure is assumed in all costing
calculations.
It should be stated that work at the site will be complicated by restrictive
and difficult access to the cliff. All access will have to take place from the
eastern side with possibly a haul road being built over the rock outcrops.
The three methods discussed below include a, concrete retaining wall, rock
revetment and gabion retaining
wall. Stabilisation by means of grading the slope of the cliff back from the
base is a technique that is often used but in this case it was not
considered, as there is no space available.
4.1. Concrete Retaining Wall
The most obvious protection option is to construct a concrete retaining wall at
the base of the cliff. This can be either reinforced concrete (MC) wall or a
mass concrete wall and should possibly stand to a height of 3m. The preferred
option would be to construct an RC wall as it would have a smaller footprint
and would require less pumping or hauling of concrete to the remoter sections
of the site. Although it should be stated that some contractors prefer mass
concrete walls as they are simpler to construct. The footing for the wall would
have to be laid on a prepared base and may require the use of rock dowels. Such
a requirement would be decided at design stage. Ideally the wall would be
offset from the cliff by about 3-5 meters such that the area behind can be
filled with granular material and a smooth slope created between the cliff edge
and top of wall. Existing drains could discharge into the granular fill and the
slope could be planted with suitable vegetation in order to prevent rain
induced damage. This slope would be similar in character to area at the crest
of the retaining wall on the western boundary of the cliffs.
Typical costs for this option would be
MC Wall @ 2,000/m
280,000
Granular Fill @
25/rn3 56,000
Approximate Total 336,000
4.2. Rock Revetment
A rock revetment constructed at the base of the cliff is another possible
option to
protect the cliff from further erosion. This structure would have a possible
height of
3-4m and a front slope of 1:1.5. However given the nature of the shoreline a
revetment may be difficult to construct. It would have a base width of
possibly 6m and the toe would need to be secured into the rock to prevent
slippage of the rock slope. Therefore it would be much more massive than a
retaining wall and would allow little space to stabilise the top of the cliff.
On the positive side a revetment is likely to be much cheaper provided that
suitable rock amour material can be readily obtained.
Projected Costs I500/m 210,000
4.3. Gabion Wall
A gabion wall has
already been constructed so
it is pertinent to discuss the possibility of extending it to the unprotected
section of cliff. Generally gabions can be used as retaining walls in coastal
applications if waves will not directly impact on them. In addition there is
uncertainty about the design life of gabions especially in coastal areas.
Gabions also require good quality fill but it is presumed that this is readily
available. For this application there will be some limited wave action so the
base baskets would likely need to be encased in concrete. The base baskets
should be I .5m in width and the height of the structure should be 4m (4
baskets). The gabion wall should be offset from the cliff face and anchored
into the backfill using a terramesh unit.
Projected Costs
Gabions (150/m3) 105,000
Granular Fill @
£25/m3 56.000
Total 6l, 000
Guileen
Cliff Erosion November 2004
5.0 Effects on Adjacent Landmasses
It is expected that the proposed
structure will not have a significant impact on adjacent landmasses. It would
be located above the MHW level so would not interfere with existing wave
propagation or currents along or outside the cliff area. Given the fact that it
is only a short section of cliff and is eroding rather slowly it does not
contribute any significant amount of sediment to nearby beaches.
Adjacent, unprotected, areas of shoreline will continue to erode at existing
rates but any structure will have no impact either positive or negative on
this. Flanking of the retaining wall (or revetment) is not possible in this
case provided the whole 140m length of structure is constructed. Flanking
refers to increased localised erosion that usually occurs around the edges of
the structure.
To conclude it can be said that the proposed structure is relatively
unobtrusive and should not have any significant detrimental effects on the
adjacent Landmasses or on the existing hydrodynamic regime of the area.
6.0 Appendix 1: Sea Cliff Erosion
Coastal cliff retreat can
be described as a long-term, Landward movement of the coast. Retreat
rates can vary considerably depending on the relative importance of a number of
variables such as cliff lithology, material strength, degree of wave exposure,
precipitation rate and groundwater seepage. The type of failures identified
include slumps (rotational slide with a coherent slide mass) blockfalls
(massive blocks with near vertical failure planes) and debris flows (chaotic,
internally mixed deposit). Past studies on soft sea cliff behaviour have
documented a variety of seasonally related degradational processes. Many cliffs
retreat rapidly during the stormy winter months when waves and heavy rainfall destabilise and erode them.
In contrast the cliffs tend to be relatively inactive during the dry summer
months, although they may suffer slight degradation due to sea spray.
Even though problems caused by cliff retreat are widespread there is a general
lack of understanding as to the long-term behaviour of this coastline type. It
is not known whether historical erosional trends will continue at the current
rate or accelerate/decelerate in the future. Furthermore soft cliffs are an
integral part of the overall coastal sediment circulation system inasmuch as
they can provide an important source of sediments for downdrift areas. Although
sea cliff erosion is normally quantified in terms of average annual retreat
rate most coastal retreat occurs rapidly and episodically during the passage of
severe storms. The fallen debris can buttress the cliff against further failure
but subsequent waves and currents can quickly remove it, leaving the cliffs
once again susceptible to failure. Depending on the nature of the sediment it
might reside on the beach for a considerable time or if very fine will be
carried away and have no beneficial effect. Current coastal process models do
not account for the episodic nature of cliff erosion and the impact such events
have on shoreline evolution.
Soft cliff materials are usually very weak and often fail through a combination
of both terrestrial and marine processes. Failure due to marine processes is
very evident with wave action undercutting the base of the cliff. Although
narrow high-energy winter/storm beaches can expose cliffs to direct wave
attack, larger waves generally break offshore and do not reach the cliff face.
Only during high spring tides and storm-elevated sea level do waves reach the
cliff. At these times there is erosion of cliff face material. The nature of
the cliff failure will depend on the cliff material type and is often
accelerated by cycles of freeze-thaw and wetting-drying. This type of erosion
although important is often not considered to be most influential in
determining the long-term retreat of the cliff face. Cliff failure that occurs
as a result of terrestrial processes such as precipitation, run-off and
groundwater seepage is often more significant. During periods of heavy rain,
water infiltrates the cliff and the land behind them. Such soaking especially
when aided by the force of groundwater as it seeps from the cliff face weakens
the sediment to a level that it cannot resist the downward pull of gravity. The
cliff can fail by means of rotational slip or translational slide depending on
a number of factors.
Sea cliffs do not erode uniformly in time and space and this can have an
influence on performance of a beach system. Failures are usually localised and
movement is usually precipitated by some variation in conditions such as a
change in temperature
Free Web Counters & Statistics |