Assessing Coastal Vulnerability Index to Climate Change
Content
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Addo
Assessing Coastal Vulnerability Index to Climate Change: the Case of
Accra – Ghana
Kwasi Appeaning Addo
Department of Marine and Fisheries
Sciences
University of Ghana
P.O.Box Lg 99
Legon-Accra.
Email: [email protected]
www.cerf-jcr.org
ABSTRACT
Appeaning Addo, K., 2013. Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana,
Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue
No. 65, pp. 1892-1897, ISSN 0749-0208.
www.JCRonline.org
Coastal zones are under severe threat from climate change and its associated sea-level rise. Areas with relatively low
elevations will experience either temporal or permanent flooding, while other areas will experience increased coastal
erosion. Several factors within the coastal environment combine to drive coastal erosion. Identifying these variables and
quantifying their risk levels enable the vulnerability index of a particular location to be estimated. This study divided
the coast of Accra into three sections based on the geomorphology. The vulnerability index was estimated for the three
sections by determining their relative risk factors. The ‘square root of product mean’ (CVI 5) method was adopted for
this study. The results indicate that the coastal vulnerability index for the entire coast of Accra is 7.7, which falls within
the moderate risk category. The western section is more vulnerable to sea-level rise followed by the eastern and the
central sections. Inundation in the western section will result in displacement of the local population, destroy their
source of livelihood and flood the Densu wetlands – a RAMSAR site.
ADDITIONAL INDEX WORDS: Vulnerability index, Accra coast, risk factors, climate change, sea-level rise.
INTRODUCTION
The dynamic and complex multi-functional coastal environment
is constantly changing due to natural and anthropogenic driving
factors. These changes affect a number of often conflicting human
socio-economic activities that occur in these areas. The human
induced influences generally tend to exacerbate the natural stress
from wave and tide climates on coastal resources. Climate change,
as a result of the shifts in the mean state of the climate or in its
variability, has led to a rise in the earth's average surface
temperature (Gornitz, 2000). The resultant thermal expansion of
the ocean and the increased melting of the glaciers have facilitated
sea-level rise at a rate of about 1-2 mm/yr (IPCC, 2007).
Accelerated sea-level rise represents a significant coastal
management challenge to coastal nations, especially in developing
nations where there is scarcity of geospatial data. Sea-level rise
has resulted in increased erosion, inundation of vulnerable areas
and more frequent storms in Accra (Amoani et al., 2012). These
developments threaten coastal life and properties. There is
therefore the need to critically assess coastal vulnerability to
climate change.
Vulnerability assessment of the coastal zone takes into account
several factors that drive changes in the coastal zone. These
factors when identified, combined and quantified determine the
resilience of the coastal environment to these forces. Information
on the effectiveness of the factors driving erosion enables the
vulnerable state of a coastal area to be quantified. Vulnerability
assessments are specific to a given location, sector or group and it
depends on the local ecological and socio-economic
____________________
DOI: 10.2112/SI65-320.1 received 07 December 2012; accepted 06
March 2013.
© Coastal Education & Research Foundation 2013
characteristics (Hinkel and Klein, 2007). The vulnerability of
coastal systems to sea-level rise and to other drivers of change is
determined by their sensitivity, exposure and adaptive capacity
(Nicholls and Klein, 2005). A more detailed vulnerability
assessment at the local scale enables understanding into the
complexities of the coastal system at a localized scale in a specific
area.
Shoreline erosion in Accra has been reported by previous
studies (Anokwa et al., 2005; Campbell, 2006; Appeaning Addo
et al., 2011). It is estimated that about 80% of the shoreline is
threatened by erosion, while the remaining 20% is either stable or
accreting (Appeaning Addo et al., 2008). Coastal erosion has
affected the social and economic life of the local population,
threatened cultural heritage and hindered coastal tourism
development (Sagoe-Addy and Appeaning Addo, 2012).
According to Boateng (2012), substantial amount of houses has
been lost to coastal erosion in the past and the trend continues in
some areas along the coast. In the western part of the Accra coast,
17 coastal inhabitants in two coastal communities have lost their
buildings to coastal erosion over 26 years (Campbell, 2006). A
study by Appeaning Addo et al. (2011) estimates that about 85
houses will be lost to erosion in three communities in the western
part of Accra by 2025 under increasing sea-level rise, which will
displace over 2000 coastal inhabitants.
Causes of erosion in Accra are as a result of natural and
anthropogenic forcing factors (Appeaning Addo et al., 2011).
These driving forces of varying intensity and energy interact
within the Accra coastal area to initiate or exacerbate erosion.
Anokwa et al. (2005) identified the varying geological
constituents along the coast as a major driver of erosion. Beach
sand/gravel mining and general lack of enforcement of laws
banning beach sand mining have been identified by Appeaning
Journal of Coastal Research, Special Issue No. 65, 2013
Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana
Addo et al. (2008) as a major cause of increased erosion in Accra.
Appeaning Addo (2012) identified the unplanned physical
infrastructure development, population increase and increasing
tourism development in the coastal zone as a cause of increased
erosion. Coastal vegetation is cleared and wetlands are drained for
infrastructural development (Alvarado, 2003).
The Coastal Vulnerability Index (CVI) is one of the most
commonly used and simple methods to assess coastal vulnerability
to sea-level rise, in particular due to erosion and/or inundation
(Gornitz et al., 1991). CVI provides numerical basis for ranking
sections of coastline in terms of their potential for change as a
result of several factors such as sea-level rise, geology, wave
climate and geomorphology. It enables coastal managers to
identify regions where risk may be relatively high and develop
appropriate management strategies.
This paper assesses and estimates the vulnerability index of the
Accra coast by identifying the various factors that can be
influenced by climate change to drive changes in the coastal
environment. The study involves two phases. The first phase
involves creating a database of geologic and environmental
variables. The variables included in this database are geology,
geomorphology, elevation, relative sea-level rise rates, shoreline
recession rates, tide range and mean wave height (after Gornitz
and White, 1992). The second phase of the study was conducted in
two parts. The first part entails assessing the potential impacts on
the shoreline due to these variables, while the second part
quantified the relative vulnerability of the different Accra coastal
environment to sea-level rise.
STUDY AREA
Accra lies along the Gulf of Guinea (Figure 1) and it is the
political and economic capital of Ghana. It is part of the Greater
Accra region of Ghana and the shoreline is about 40 km long. The
region is the smallest in Ghana but densely populated with about
3,909,764 inhabitants based on the 2010 population and housing
census figures (GSS, 2011). Accra is located at latitude 5.626 0 N
and longitude 0.10140 W, which influences the climatic conditions
that prevail along the coast.
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Efa, 2005). Along the shoreline, sandy platforms are associated
with lagoonal inlets and river deltas. Figure 2 shows the underlain
rocky beach and the devastating effect of coastal erosion along
part of the western section in the study area.
Figure 2. Coastal erosion along portion of western Accra coast.
The coastal area experiences significant differences in the
amount and seasonal distribution of precipitation. It has two rainy
seasons with the major season between April and July, and the
minor one between September and November. Sediment transport
to the littoral zone is high during the rainy season as the rivers
discharge their sediment from the upland catchment areas into the
sea (AESC, 1980). Inversely, sediment transport reduces during
the dry season when temperatures are over 300 C resulting in the
drying up of most of the rivers.
Waves approach the open coast from the south-southwest
direction. Currents that transport sediment include the longshore
current, the Guinea current that can measure up to 0.5 m/s during
raining season but is weak most of the year and the weak tidal
current (Wellens-Mensah et al., 2002). Various sizes of lagoons
exist along the coast with some associated with rivers and rivulets.
Some of the lagoons remain closed until opened by high river
flows due to heavy rains. The vegetation found along the coast of
Accra includes grasses, herbs, shrubs and different kinds of
mangroves usually located around the lagoons. Clearing of coastal
vegetation to put up shelter to accommodate the increasing
population and the emerging coastal tourism business has resulted
in a ‘land squeeze’ situation (Appeaning Addo, 2012).
This study adopted the division by Appeaning Addo et al. (2008)
where the study area was divided into three (3) sections based on
the geomorphology. These include the western (19.1 km) which
extends from Bortianor to Jamestown; Central (14.4 km) covering
the distance between Jamestown and Teshie; and the Eastern (14.6
km) which starts from Teshie to the Sakumo lagoon (refer to
results and Figure 3 for detailed description of the divisions).
METHODOLOGY
Figure 1. Location of the study area along the Ghana coast (adapted from
Survey and Mapping division of Ghana Lands Commission).
The study area is generally a low lying area (elevation ranges
between 2 and 12 m) with successions of ridges, slopes and
occasional rocky headlands. The coastal zone is underlain by a
gentle, mature topography that slopes towards the shore (Muff and
In order to develop a database for a local-scale assessment of
coastal vulnerability for the Accra coast, relevant data were
gathered from various sources. The sources include government
agencies such as the Geological Survey Department and the
Survey and Mapping Division of the Ghana Lands Commission as
well as journal articles. The compilation of this data set is essential
to mapping potential coastal changes due to climate change.
According to Gornitz et al. (1991), a vulnerable coastline is
characterized by low coastal relief, subsidence, extensive
Journal of Coastal Research, Special Issue No. 65, 2013
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shoreline retreat, and high wave/tide energies. The manipulation
process was simplified by classifying the geologic and
environmental data variables into new "risk" variables. Each risk
variable ranges in value from 1 to 5 and indicates the region’s
relative risk to erosion or inundation. The risk variables were
based on ETC-ACC (2011). The risk assignments for mean
elevation, mean shoreline displacement, local subsidence trend,
mean tidal range, and maximum significant wave height adopted
for this study are given in Table 1 (after Gornitz et al., 1991). The
risk assignments for geology and geomorphology are also given in
Tables 2 and 3 respectively where using the Likert scale with 1
being very low to 5 being very high.
Based on the information in Tables 1, 2 and 3, the risk factors
for the variables identified along the Accra coast in the three
sections are presented in Table 4. The maximum significant wave
height was obtained from AESC (1980) that gives indication of
the wave energy in transporting sediment; mean tidal range was
obtained from Wellens-Mensah et al. (2002); local subsidence rate
was derived from Takoradi tide gauge station data obtained from
the Survey and Mapping Division of the Lands Commission of
Ghana; mean shoreline displacement was obtained from
Appeaning Addo et al. (2008); and relative elevations was
obtained from Appeaning Addo (2009). The geomorphology
variable, which expresses the relative erodibility of different
landform types, were derived from 2005 orthophoto maps
obtained from the Survey and Mapping Division of the Lands
Commission of Ghana and information on the geology that shows
the different types of rocks along the Accra coast was obtained
from Muff and Efa (2005).
Table 1: Relative risk factors for elevation, shoreline displacement, local subsidence trend, tidal range, and wave height (after Gornitz et
al., 1991).
Variable
1
2
3
4
5
Mean elevation (m)
>30
>20 and ≤ 30
>10 and ≤ 20
>5 and ≤ 10
≥0≤5
Mean shoreline displacement (m/yr)
> 2.0 accretion
>1 and ≤ 2
>-1 and ≤ +1
> -2 and ≤ -1
≥ -2 and erosion
Local subsidence trend (mm/yr)
˂ -1 land rising
≥ -1 and ≤ 1
> 1 and ≤ 2
> 2 and ≤ 4
> 4.0 land sinking
Mean tidal range (m)
Maximum significant wave height (m)
˂ 1.0 microtidal
≥ 0 and ˂ 3
≥ 1 and ˂ 2
≥ 3 and ˂ 5
Table 2: Risk factors for geology (after Gornitz et al., 1991).
Variable
1
2
Sandstone
Unconsolidated soil
Lagoon/fluvial sediment
Metamorphic
Sand
Clay
Gravel
Table 3: Risk factors for geomorphology (after Gornitz et al., 1991).
Variable
1
2
Beach poorly developed
Marine with wave erosion
Non-marine (land erosion)
Non-glacial irregular coast
Barrier coast
Delta coast
Embayed non-rocky coast
≥ 2 and ≤ 4
≥ 5 and ˂ 6
3
> 4 and ≤ 6
≥ 6 and ˂ 6.9
4
5
3
4
Table 4: Risk factors for identified variables in the three geomorphic sections.
Variable
Western section
Central section
Mean elevation (m)
5
4
Mean shoreline displacement (m/yr)
3
3
Local subsidence trend (mm/yr)
2
2
Mean tidal range (m)
1
1
Maximum significant wave height (m)
1
1
Geomorphology
5
3
Geology
4
2
The relative risk variables contained within the database created
in GIS environment were used to formulate a coastal vulnerability
index (CVI). CVI may be used to identify areas that are at risk to
erosion or inundation. Gornitz and White (1992) and Gornitz et al.
(1997) proposed and tested (in terms of sensitivity analysis)
different formulas (considering 7 key variables) for the derivation
of the final CVI. The studies (Gornitz and White, 1992 and
Gornitz et al., 1997) identified that the methods were adequate for
> 6.0 macrotidal
≥ 6.9
5
Eastern section
5
4
2
1
1
3
3
the task when the number of risk factors that are missing data, for
a given location, are less than three. These formulas were adopted
for this study.
Product mean:
CV1
Square root of product mean:
Where: n = variables present
X2 = local subsidence trend
Journal of Coastal Research, Special Issue No. 65, 2013
CV5 = [ CV1]1/2
x1 = mean elevation
x3 = geology
Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana
X4 = geomorphology
X6 = maximum wave height
x5 = mean shoreline
displacement
x7 = mean tidal range
The methods have been discussed extensively in ETC-ACC
(2011). The CVI5 formulation was used to determine the CVI for
the Accra coast because it has been widely used in other
applications at the local, regional and supra-regional level (Thieler
et al., 2002; Thieler and Hammar-Klose, 1999; Gornitz et al.,
1991, Gornitz, 1990; Gornitz, 1991a).
The results were mapped through a GIS system that enabled the
most vulnerable areas to be identified. The data values of the CVI 5
calculated for the three sections were used to construct a
histogram for this study. Based on the histogram, three risk classes
were developed (i.e., low-, moderate-, and high-risk based on 33
percentile ranges) based on ETC CAA (2011). Low risk class
values are those values less than 6, moderate risk values range
from 6 to 9, and high risk values are greater than 9.
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The landforms identified from the 2005 orthophoto maps
included those formed by erosion and those formed by deposition.
Several geomorphic features occur in all the three sections (Figure
5). The varied types of rocks in the study area and the prevailing
geomorphic features informed the selection of these maps over the
others.
RESULTS
The three sections of the Accra coast (Figure 3) have similar
geological and physical features. Most of these features that have
been formed as a result of erosion are now also enhancing the
effectiveness of erosion.
Figure 5: Geomorphic features along Accra coast.
Transects along the shoreline at 100 m intervals were used by
Appeaning Addo et al. (2008) to estimate the historic rates of
erosion (Figure 6). Four shoreline positions of 1904, 1974, 1989
and 2002 were used to determine the historic shoreline rates of
change. The lateral movement of the different date shoreline
positions along Accra coast revealed the erosion pattern in each of
the three sections.
Figure 3: Three sections with similar features (based on Appeaning
Addo et al., 2008).
The geology of the Accra coastal environment is made up of
various types of rocks. They include unconsolidated sediments,
sandstones, quartz and lagoonal sediment (Figure 4). The western
part is predominantly unconsolidated soil while the eastern part is
more of quartz and little portions of lagoonal sediment. The
central part is mainly sandstone and a little portion of quartz.
Figure 6: Orthogonal transects for rates of change estimations
(Appeaning Addo et al., 2008).
The western and eastern sections are eroding more relative to
the central section (Figure 7). The rate of erosion for Accra is
estimated as 1.13 m/yr ± 0.17 m/yr (Appeaning Addo et al., 2008).
The western part is eroding at a rate of 1.30 ± 0.17 m/yr, the
central part is eroding at a rate of 0.40 ± 0.17 m/yr and the eastern
part is eroding at a rate of 1.50 ± 0.17 m/yr. This trend is expected
to continue and possibly increase under influence of increasing
sea-level rise.
Figure 4: Geology distribution along Accra coast (based on Muff
and Efa, 2005).
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1
A
B
C
D
E
F
0.5
Rate of change (m/yr)
0
-0.5
-1
-1.5
-2
-2.5
Western
-3
0
5
Central
10
Eastern
15
20
25
Distance along shore (km)
30
35
40
Figure 7: Historic rates of shoreline change in Accra (Appeaning
Addo et al., 2008).
The vulnerability index values computed for the Accra coast
range between 4.5 and 12.0 and they vary along the coast. The
average CVI5 for the three sections are: western section 9.0,
central section 4.8 and eastern section 7.3. The mean value for the
Accra coast is 7.7 which fall within the moderate risk category.
Based on the CVI5 values computed, the high risk area to sea-level
rise is the western section; the eastern section is classified as a
moderate risk area to sea-level rise while the central section is low
risk area to increasing sea-level rise (Figure 8).
Figure 8: Vulnerable areas along the Accra coast.
DISCUSSION
Climate change and accelerated sea-level rise will affect the
western, central and eastern coastal sections of Accra differently.
The western section has a CVI value of 9.0, which makes it the
highest risk area to climate change compared to the eastern (7.3)
and central (4.8) sections. Although the entire Accra coast is
considered moderate in terms of risk to climate change according
to this study, the relatively high mean CVI value of 7.7 is
considerably high. The value is not consistent with other parts of
the world due to differences in the risk factor of variables used to
estimate CVI. The level of resistivity of the geomorphological and
geological features within the three sections to oceanic forcing
account for the CVI values obtained.
The sea level, which is rising at a rate of over 2 mm/yr (SagoeAddy and Appeaning Addo, 2012) will facilitate waves to break
closer nearshore and tides to flood low lying areas during very
high tides. Although the risk factors for wave (significant wave
height 1.4 m) and tide (tidal range 1 m) activities are very low
(refer to Table 4), they are significant in driving sediment along
and across shore (AESC, 1980). The longshore current moves
along the coast from west to east transporting sediment that
shapes the shoreline configuration. The resulting shoreline
orientation influences the breaking pattern of waves and how they
impact the shoreline. This in part explains the different CVI
values obtained for the three sections.
The western section of Accra coast is highly vulnerable to
climate change. Several factors account for the CVI value
obtained for the western part. The area has a barrier coast, poorly
developed beaches (refer to Table 3) and low mean elevation
(refer to Table 1) relative to the other sections. The very low lying
areas get flooded during very high tides and whenever there is a
storm surge. Recent incidence of flooding in some coastal
communities in the western section (e.g. Gleefe) can be attributed
to sea-level rise as a result of climate change (Amoani et al.,
2012). This section of the coast is also dominated by soft rock and
unconsolidated sediment that are easily erodible. The shoreline in
this section is eroding at a rate of about 1.30 m/yr, which is
relatively high. The observed historic erosion trend is expected to
continue under increasing sea-level rise as has been observed by
Appeaning Addo (2009). These factors account for the high CVI
value estimated for the western section of the Accra coast.
The central and eastern sections are dominated by soft and hard
rocks that influence shoreline morphological change. The central
section is eroding at a rate of about 0.4 m/yr which is relatively
low and the eastern part is eroding at a significantly high rate of
about 1.50 m/yr. The relatively deep bathymetry in the nearshore
area of the eastern section (Appeaning Addo, 2009) enables waves
to break closer to the shore and attack the shoreline, thereby
eroding the softer part of the shoreline. Although the historic
erosion rate is high in the eastern section, the presence of
relatively hard rocks will slow the rates down. This in part
explains the CVI value estimated for the eastern section. Nonmarine erosion as a result of rain water runoff into streams and
lagoons are dominant along the portions of the central and eastern
coast (refer to Figure 5). The channels created enable the sea to
move more inland during very high tide and contaminate the fresh
water system. This has the tendency to affect fresh water fishing
activities in the lagoons and streams. They can also affect crop
farming in the coastal zone. Continuous widening of the channels
will result to increased erosion problems which will lead to loss of
coastal land and properties. The relatively low risk factor for the
geomorphic features in the central and eastern sections partly
account for the CVI values obtained for these two sections
respectively.
The high risk state of the western section requires pragmatic
measures to manage the coastal environment and resources.
Although Oteng-Ababio et al. (2011) suggested relocating the
inhabitants as a measure to manage the erosion problem in the
western section, the residents are resolved to maintain their
occupancy due to their strong cultural and social ties to the
communities. The government has adopted constructing hard
engineering structures, such as groynes and revetments, to manage
coastal erosion in the critical areas along the coast. This approach
is not sustainable as the erosion problem is simply transferred
down-drift of the coast. There is therefore the need to explore the
option of using soft engineering measures, such as beach
nourishment which facilitate managing with nature, to manage the
erosion problems in the western section. The measures if adopted
Journal of Coastal Research, Special Issue No. 65, 2013
Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana
would preserve the source of livelihood of the inhabitants as well
as their social life.
CONCLUSION
The study has demonstrated that the coast of Accra is vulnerable
to climate change and its associated sea-level rise. The risk level
for the entire coastal area can be categorized as moderate. The
most vulnerable area is the western section where sea-level rise
will result in increased erosion and inundation of the low lying
areas, especially the Densu wetlands area. This confirms studies
by Appeaning Addo et al. (2008) and Anokwa et al. (2005) that
identified the western section is erosion prone. Increased erosion
and flooding in the Densu wetlands will affect the habitats of
migratory birds and destroy the ecology. The study has also
revealed that the combined effect of several factors within the
coastal domain places the risk factor to climate change at a higher
level. Geology, geomorphology and relatively low elevation are
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ACKNOWLEDGEMENT
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Nicholls R.J. and Klein R.J.T. (2005). Climate change and coastal
management on Europe's coast. In Vermaat J., Bouwer L., Turner K.
and Salomons W. (eds.), 2005. Managing European Coasts: Past,
Present and Future. Germany, Spinger.
Oteng-Ababio, M., Owusu, K. and Appeaning Addo, K. (2011). The
Vulnerable State of the Ghana Coast: The Case of Faana-Bortianor.
JAMBA: Journal of Disaster Risk Studies, Vol. 3, No.2, 429 – 442.
Sagoe-Addy K. and Appeaning Addo K. (2012). Effect of predicted sea
lev el rise on tourism facilities along Ghana’s Accra coast. Journal of
Coastal Conservation. DOI 10.1007/s11852-012-0227-y
Thieler E.R. and Hammar-Klose E. (1999). National assessment of coastal
vulnerability to sea-level rise. Preliminary results for U.S. Atlantic
Coast. Open-file report 99-593. U.S. GeologicalSurvey, Reston, VA, 1
sheet. Available at: http://pubs.usgs.gov/of/1999/of99-593/ (Assessed
on 14/03/2012).
Thieler E.R., Williams S.J. and Beavers R. (2002). Vulnerability of U.S.
National Parks to sea-level rise and coastal change. U.S. Geological
Survey fact sheet FS 095-02. [U.S. Geological Survey, Reston, VA], 2
pp. Available at: http://pubs.usgs.gov/fs/fs095-02/ (Assessed on
27/03/2012).
Wellens-Mensah, J., Armah, A. K., Amlalo, D. S., Tetteh, K., (2002).
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Protection of the Coastal and Marine Environment in Sub-Saharan
Africa. Accra, Ghana.
Journal of Coastal Research, Special Issue No. 65, 2013
Addo
Assessing Coastal Vulnerability Index to Climate Change: the Case of
Accra – Ghana
Kwasi Appeaning Addo
Department of Marine and Fisheries
Sciences
University of Ghana
P.O.Box Lg 99
Legon-Accra.
Email: [email protected]
www.cerf-jcr.org
ABSTRACT
Appeaning Addo, K., 2013. Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana,
Proceedings 12th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue
No. 65, pp. 1892-1897, ISSN 0749-0208.
www.JCRonline.org
Coastal zones are under severe threat from climate change and its associated sea-level rise. Areas with relatively low
elevations will experience either temporal or permanent flooding, while other areas will experience increased coastal
erosion. Several factors within the coastal environment combine to drive coastal erosion. Identifying these variables and
quantifying their risk levels enable the vulnerability index of a particular location to be estimated. This study divided
the coast of Accra into three sections based on the geomorphology. The vulnerability index was estimated for the three
sections by determining their relative risk factors. The ‘square root of product mean’ (CVI 5) method was adopted for
this study. The results indicate that the coastal vulnerability index for the entire coast of Accra is 7.7, which falls within
the moderate risk category. The western section is more vulnerable to sea-level rise followed by the eastern and the
central sections. Inundation in the western section will result in displacement of the local population, destroy their
source of livelihood and flood the Densu wetlands – a RAMSAR site.
ADDITIONAL INDEX WORDS: Vulnerability index, Accra coast, risk factors, climate change, sea-level rise.
INTRODUCTION
The dynamic and complex multi-functional coastal environment
is constantly changing due to natural and anthropogenic driving
factors. These changes affect a number of often conflicting human
socio-economic activities that occur in these areas. The human
induced influences generally tend to exacerbate the natural stress
from wave and tide climates on coastal resources. Climate change,
as a result of the shifts in the mean state of the climate or in its
variability, has led to a rise in the earth's average surface
temperature (Gornitz, 2000). The resultant thermal expansion of
the ocean and the increased melting of the glaciers have facilitated
sea-level rise at a rate of about 1-2 mm/yr (IPCC, 2007).
Accelerated sea-level rise represents a significant coastal
management challenge to coastal nations, especially in developing
nations where there is scarcity of geospatial data. Sea-level rise
has resulted in increased erosion, inundation of vulnerable areas
and more frequent storms in Accra (Amoani et al., 2012). These
developments threaten coastal life and properties. There is
therefore the need to critically assess coastal vulnerability to
climate change.
Vulnerability assessment of the coastal zone takes into account
several factors that drive changes in the coastal zone. These
factors when identified, combined and quantified determine the
resilience of the coastal environment to these forces. Information
on the effectiveness of the factors driving erosion enables the
vulnerable state of a coastal area to be quantified. Vulnerability
assessments are specific to a given location, sector or group and it
depends on the local ecological and socio-economic
____________________
DOI: 10.2112/SI65-320.1 received 07 December 2012; accepted 06
March 2013.
© Coastal Education & Research Foundation 2013
characteristics (Hinkel and Klein, 2007). The vulnerability of
coastal systems to sea-level rise and to other drivers of change is
determined by their sensitivity, exposure and adaptive capacity
(Nicholls and Klein, 2005). A more detailed vulnerability
assessment at the local scale enables understanding into the
complexities of the coastal system at a localized scale in a specific
area.
Shoreline erosion in Accra has been reported by previous
studies (Anokwa et al., 2005; Campbell, 2006; Appeaning Addo
et al., 2011). It is estimated that about 80% of the shoreline is
threatened by erosion, while the remaining 20% is either stable or
accreting (Appeaning Addo et al., 2008). Coastal erosion has
affected the social and economic life of the local population,
threatened cultural heritage and hindered coastal tourism
development (Sagoe-Addy and Appeaning Addo, 2012).
According to Boateng (2012), substantial amount of houses has
been lost to coastal erosion in the past and the trend continues in
some areas along the coast. In the western part of the Accra coast,
17 coastal inhabitants in two coastal communities have lost their
buildings to coastal erosion over 26 years (Campbell, 2006). A
study by Appeaning Addo et al. (2011) estimates that about 85
houses will be lost to erosion in three communities in the western
part of Accra by 2025 under increasing sea-level rise, which will
displace over 2000 coastal inhabitants.
Causes of erosion in Accra are as a result of natural and
anthropogenic forcing factors (Appeaning Addo et al., 2011).
These driving forces of varying intensity and energy interact
within the Accra coastal area to initiate or exacerbate erosion.
Anokwa et al. (2005) identified the varying geological
constituents along the coast as a major driver of erosion. Beach
sand/gravel mining and general lack of enforcement of laws
banning beach sand mining have been identified by Appeaning
Journal of Coastal Research, Special Issue No. 65, 2013
Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana
Addo et al. (2008) as a major cause of increased erosion in Accra.
Appeaning Addo (2012) identified the unplanned physical
infrastructure development, population increase and increasing
tourism development in the coastal zone as a cause of increased
erosion. Coastal vegetation is cleared and wetlands are drained for
infrastructural development (Alvarado, 2003).
The Coastal Vulnerability Index (CVI) is one of the most
commonly used and simple methods to assess coastal vulnerability
to sea-level rise, in particular due to erosion and/or inundation
(Gornitz et al., 1991). CVI provides numerical basis for ranking
sections of coastline in terms of their potential for change as a
result of several factors such as sea-level rise, geology, wave
climate and geomorphology. It enables coastal managers to
identify regions where risk may be relatively high and develop
appropriate management strategies.
This paper assesses and estimates the vulnerability index of the
Accra coast by identifying the various factors that can be
influenced by climate change to drive changes in the coastal
environment. The study involves two phases. The first phase
involves creating a database of geologic and environmental
variables. The variables included in this database are geology,
geomorphology, elevation, relative sea-level rise rates, shoreline
recession rates, tide range and mean wave height (after Gornitz
and White, 1992). The second phase of the study was conducted in
two parts. The first part entails assessing the potential impacts on
the shoreline due to these variables, while the second part
quantified the relative vulnerability of the different Accra coastal
environment to sea-level rise.
STUDY AREA
Accra lies along the Gulf of Guinea (Figure 1) and it is the
political and economic capital of Ghana. It is part of the Greater
Accra region of Ghana and the shoreline is about 40 km long. The
region is the smallest in Ghana but densely populated with about
3,909,764 inhabitants based on the 2010 population and housing
census figures (GSS, 2011). Accra is located at latitude 5.626 0 N
and longitude 0.10140 W, which influences the climatic conditions
that prevail along the coast.
1893
Efa, 2005). Along the shoreline, sandy platforms are associated
with lagoonal inlets and river deltas. Figure 2 shows the underlain
rocky beach and the devastating effect of coastal erosion along
part of the western section in the study area.
Figure 2. Coastal erosion along portion of western Accra coast.
The coastal area experiences significant differences in the
amount and seasonal distribution of precipitation. It has two rainy
seasons with the major season between April and July, and the
minor one between September and November. Sediment transport
to the littoral zone is high during the rainy season as the rivers
discharge their sediment from the upland catchment areas into the
sea (AESC, 1980). Inversely, sediment transport reduces during
the dry season when temperatures are over 300 C resulting in the
drying up of most of the rivers.
Waves approach the open coast from the south-southwest
direction. Currents that transport sediment include the longshore
current, the Guinea current that can measure up to 0.5 m/s during
raining season but is weak most of the year and the weak tidal
current (Wellens-Mensah et al., 2002). Various sizes of lagoons
exist along the coast with some associated with rivers and rivulets.
Some of the lagoons remain closed until opened by high river
flows due to heavy rains. The vegetation found along the coast of
Accra includes grasses, herbs, shrubs and different kinds of
mangroves usually located around the lagoons. Clearing of coastal
vegetation to put up shelter to accommodate the increasing
population and the emerging coastal tourism business has resulted
in a ‘land squeeze’ situation (Appeaning Addo, 2012).
This study adopted the division by Appeaning Addo et al. (2008)
where the study area was divided into three (3) sections based on
the geomorphology. These include the western (19.1 km) which
extends from Bortianor to Jamestown; Central (14.4 km) covering
the distance between Jamestown and Teshie; and the Eastern (14.6
km) which starts from Teshie to the Sakumo lagoon (refer to
results and Figure 3 for detailed description of the divisions).
METHODOLOGY
Figure 1. Location of the study area along the Ghana coast (adapted from
Survey and Mapping division of Ghana Lands Commission).
The study area is generally a low lying area (elevation ranges
between 2 and 12 m) with successions of ridges, slopes and
occasional rocky headlands. The coastal zone is underlain by a
gentle, mature topography that slopes towards the shore (Muff and
In order to develop a database for a local-scale assessment of
coastal vulnerability for the Accra coast, relevant data were
gathered from various sources. The sources include government
agencies such as the Geological Survey Department and the
Survey and Mapping Division of the Ghana Lands Commission as
well as journal articles. The compilation of this data set is essential
to mapping potential coastal changes due to climate change.
According to Gornitz et al. (1991), a vulnerable coastline is
characterized by low coastal relief, subsidence, extensive
Journal of Coastal Research, Special Issue No. 65, 2013
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Addo
shoreline retreat, and high wave/tide energies. The manipulation
process was simplified by classifying the geologic and
environmental data variables into new "risk" variables. Each risk
variable ranges in value from 1 to 5 and indicates the region’s
relative risk to erosion or inundation. The risk variables were
based on ETC-ACC (2011). The risk assignments for mean
elevation, mean shoreline displacement, local subsidence trend,
mean tidal range, and maximum significant wave height adopted
for this study are given in Table 1 (after Gornitz et al., 1991). The
risk assignments for geology and geomorphology are also given in
Tables 2 and 3 respectively where using the Likert scale with 1
being very low to 5 being very high.
Based on the information in Tables 1, 2 and 3, the risk factors
for the variables identified along the Accra coast in the three
sections are presented in Table 4. The maximum significant wave
height was obtained from AESC (1980) that gives indication of
the wave energy in transporting sediment; mean tidal range was
obtained from Wellens-Mensah et al. (2002); local subsidence rate
was derived from Takoradi tide gauge station data obtained from
the Survey and Mapping Division of the Lands Commission of
Ghana; mean shoreline displacement was obtained from
Appeaning Addo et al. (2008); and relative elevations was
obtained from Appeaning Addo (2009). The geomorphology
variable, which expresses the relative erodibility of different
landform types, were derived from 2005 orthophoto maps
obtained from the Survey and Mapping Division of the Lands
Commission of Ghana and information on the geology that shows
the different types of rocks along the Accra coast was obtained
from Muff and Efa (2005).
Table 1: Relative risk factors for elevation, shoreline displacement, local subsidence trend, tidal range, and wave height (after Gornitz et
al., 1991).
Variable
1
2
3
4
5
Mean elevation (m)
>30
>20 and ≤ 30
>10 and ≤ 20
>5 and ≤ 10
≥0≤5
Mean shoreline displacement (m/yr)
> 2.0 accretion
>1 and ≤ 2
>-1 and ≤ +1
> -2 and ≤ -1
≥ -2 and erosion
Local subsidence trend (mm/yr)
˂ -1 land rising
≥ -1 and ≤ 1
> 1 and ≤ 2
> 2 and ≤ 4
> 4.0 land sinking
Mean tidal range (m)
Maximum significant wave height (m)
˂ 1.0 microtidal
≥ 0 and ˂ 3
≥ 1 and ˂ 2
≥ 3 and ˂ 5
Table 2: Risk factors for geology (after Gornitz et al., 1991).
Variable
1
2
Sandstone
Unconsolidated soil
Lagoon/fluvial sediment
Metamorphic
Sand
Clay
Gravel
Table 3: Risk factors for geomorphology (after Gornitz et al., 1991).
Variable
1
2
Beach poorly developed
Marine with wave erosion
Non-marine (land erosion)
Non-glacial irregular coast
Barrier coast
Delta coast
Embayed non-rocky coast
≥ 2 and ≤ 4
≥ 5 and ˂ 6
3
> 4 and ≤ 6
≥ 6 and ˂ 6.9
4
5
3
4
Table 4: Risk factors for identified variables in the three geomorphic sections.
Variable
Western section
Central section
Mean elevation (m)
5
4
Mean shoreline displacement (m/yr)
3
3
Local subsidence trend (mm/yr)
2
2
Mean tidal range (m)
1
1
Maximum significant wave height (m)
1
1
Geomorphology
5
3
Geology
4
2
The relative risk variables contained within the database created
in GIS environment were used to formulate a coastal vulnerability
index (CVI). CVI may be used to identify areas that are at risk to
erosion or inundation. Gornitz and White (1992) and Gornitz et al.
(1997) proposed and tested (in terms of sensitivity analysis)
different formulas (considering 7 key variables) for the derivation
of the final CVI. The studies (Gornitz and White, 1992 and
Gornitz et al., 1997) identified that the methods were adequate for
> 6.0 macrotidal
≥ 6.9
5
Eastern section
5
4
2
1
1
3
3
the task when the number of risk factors that are missing data, for
a given location, are less than three. These formulas were adopted
for this study.
Product mean:
CV1
Square root of product mean:
Where: n = variables present
X2 = local subsidence trend
Journal of Coastal Research, Special Issue No. 65, 2013
CV5 = [ CV1]1/2
x1 = mean elevation
x3 = geology
Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana
X4 = geomorphology
X6 = maximum wave height
x5 = mean shoreline
displacement
x7 = mean tidal range
The methods have been discussed extensively in ETC-ACC
(2011). The CVI5 formulation was used to determine the CVI for
the Accra coast because it has been widely used in other
applications at the local, regional and supra-regional level (Thieler
et al., 2002; Thieler and Hammar-Klose, 1999; Gornitz et al.,
1991, Gornitz, 1990; Gornitz, 1991a).
The results were mapped through a GIS system that enabled the
most vulnerable areas to be identified. The data values of the CVI 5
calculated for the three sections were used to construct a
histogram for this study. Based on the histogram, three risk classes
were developed (i.e., low-, moderate-, and high-risk based on 33
percentile ranges) based on ETC CAA (2011). Low risk class
values are those values less than 6, moderate risk values range
from 6 to 9, and high risk values are greater than 9.
1895
The landforms identified from the 2005 orthophoto maps
included those formed by erosion and those formed by deposition.
Several geomorphic features occur in all the three sections (Figure
5). The varied types of rocks in the study area and the prevailing
geomorphic features informed the selection of these maps over the
others.
RESULTS
The three sections of the Accra coast (Figure 3) have similar
geological and physical features. Most of these features that have
been formed as a result of erosion are now also enhancing the
effectiveness of erosion.
Figure 5: Geomorphic features along Accra coast.
Transects along the shoreline at 100 m intervals were used by
Appeaning Addo et al. (2008) to estimate the historic rates of
erosion (Figure 6). Four shoreline positions of 1904, 1974, 1989
and 2002 were used to determine the historic shoreline rates of
change. The lateral movement of the different date shoreline
positions along Accra coast revealed the erosion pattern in each of
the three sections.
Figure 3: Three sections with similar features (based on Appeaning
Addo et al., 2008).
The geology of the Accra coastal environment is made up of
various types of rocks. They include unconsolidated sediments,
sandstones, quartz and lagoonal sediment (Figure 4). The western
part is predominantly unconsolidated soil while the eastern part is
more of quartz and little portions of lagoonal sediment. The
central part is mainly sandstone and a little portion of quartz.
Figure 6: Orthogonal transects for rates of change estimations
(Appeaning Addo et al., 2008).
The western and eastern sections are eroding more relative to
the central section (Figure 7). The rate of erosion for Accra is
estimated as 1.13 m/yr ± 0.17 m/yr (Appeaning Addo et al., 2008).
The western part is eroding at a rate of 1.30 ± 0.17 m/yr, the
central part is eroding at a rate of 0.40 ± 0.17 m/yr and the eastern
part is eroding at a rate of 1.50 ± 0.17 m/yr. This trend is expected
to continue and possibly increase under influence of increasing
sea-level rise.
Figure 4: Geology distribution along Accra coast (based on Muff
and Efa, 2005).
Journal of Coastal Research, Special Issue No. 65, 2013
1896
Addo
1
A
B
C
D
E
F
0.5
Rate of change (m/yr)
0
-0.5
-1
-1.5
-2
-2.5
Western
-3
0
5
Central
10
Eastern
15
20
25
Distance along shore (km)
30
35
40
Figure 7: Historic rates of shoreline change in Accra (Appeaning
Addo et al., 2008).
The vulnerability index values computed for the Accra coast
range between 4.5 and 12.0 and they vary along the coast. The
average CVI5 for the three sections are: western section 9.0,
central section 4.8 and eastern section 7.3. The mean value for the
Accra coast is 7.7 which fall within the moderate risk category.
Based on the CVI5 values computed, the high risk area to sea-level
rise is the western section; the eastern section is classified as a
moderate risk area to sea-level rise while the central section is low
risk area to increasing sea-level rise (Figure 8).
Figure 8: Vulnerable areas along the Accra coast.
DISCUSSION
Climate change and accelerated sea-level rise will affect the
western, central and eastern coastal sections of Accra differently.
The western section has a CVI value of 9.0, which makes it the
highest risk area to climate change compared to the eastern (7.3)
and central (4.8) sections. Although the entire Accra coast is
considered moderate in terms of risk to climate change according
to this study, the relatively high mean CVI value of 7.7 is
considerably high. The value is not consistent with other parts of
the world due to differences in the risk factor of variables used to
estimate CVI. The level of resistivity of the geomorphological and
geological features within the three sections to oceanic forcing
account for the CVI values obtained.
The sea level, which is rising at a rate of over 2 mm/yr (SagoeAddy and Appeaning Addo, 2012) will facilitate waves to break
closer nearshore and tides to flood low lying areas during very
high tides. Although the risk factors for wave (significant wave
height 1.4 m) and tide (tidal range 1 m) activities are very low
(refer to Table 4), they are significant in driving sediment along
and across shore (AESC, 1980). The longshore current moves
along the coast from west to east transporting sediment that
shapes the shoreline configuration. The resulting shoreline
orientation influences the breaking pattern of waves and how they
impact the shoreline. This in part explains the different CVI
values obtained for the three sections.
The western section of Accra coast is highly vulnerable to
climate change. Several factors account for the CVI value
obtained for the western part. The area has a barrier coast, poorly
developed beaches (refer to Table 3) and low mean elevation
(refer to Table 1) relative to the other sections. The very low lying
areas get flooded during very high tides and whenever there is a
storm surge. Recent incidence of flooding in some coastal
communities in the western section (e.g. Gleefe) can be attributed
to sea-level rise as a result of climate change (Amoani et al.,
2012). This section of the coast is also dominated by soft rock and
unconsolidated sediment that are easily erodible. The shoreline in
this section is eroding at a rate of about 1.30 m/yr, which is
relatively high. The observed historic erosion trend is expected to
continue under increasing sea-level rise as has been observed by
Appeaning Addo (2009). These factors account for the high CVI
value estimated for the western section of the Accra coast.
The central and eastern sections are dominated by soft and hard
rocks that influence shoreline morphological change. The central
section is eroding at a rate of about 0.4 m/yr which is relatively
low and the eastern part is eroding at a significantly high rate of
about 1.50 m/yr. The relatively deep bathymetry in the nearshore
area of the eastern section (Appeaning Addo, 2009) enables waves
to break closer to the shore and attack the shoreline, thereby
eroding the softer part of the shoreline. Although the historic
erosion rate is high in the eastern section, the presence of
relatively hard rocks will slow the rates down. This in part
explains the CVI value estimated for the eastern section. Nonmarine erosion as a result of rain water runoff into streams and
lagoons are dominant along the portions of the central and eastern
coast (refer to Figure 5). The channels created enable the sea to
move more inland during very high tide and contaminate the fresh
water system. This has the tendency to affect fresh water fishing
activities in the lagoons and streams. They can also affect crop
farming in the coastal zone. Continuous widening of the channels
will result to increased erosion problems which will lead to loss of
coastal land and properties. The relatively low risk factor for the
geomorphic features in the central and eastern sections partly
account for the CVI values obtained for these two sections
respectively.
The high risk state of the western section requires pragmatic
measures to manage the coastal environment and resources.
Although Oteng-Ababio et al. (2011) suggested relocating the
inhabitants as a measure to manage the erosion problem in the
western section, the residents are resolved to maintain their
occupancy due to their strong cultural and social ties to the
communities. The government has adopted constructing hard
engineering structures, such as groynes and revetments, to manage
coastal erosion in the critical areas along the coast. This approach
is not sustainable as the erosion problem is simply transferred
down-drift of the coast. There is therefore the need to explore the
option of using soft engineering measures, such as beach
nourishment which facilitate managing with nature, to manage the
erosion problems in the western section. The measures if adopted
Journal of Coastal Research, Special Issue No. 65, 2013
Assessing Coastal Vulnerability Index to Climate Change: the Case of Accra – Ghana
would preserve the source of livelihood of the inhabitants as well
as their social life.
CONCLUSION
The study has demonstrated that the coast of Accra is vulnerable
to climate change and its associated sea-level rise. The risk level
for the entire coastal area can be categorized as moderate. The
most vulnerable area is the western section where sea-level rise
will result in increased erosion and inundation of the low lying
areas, especially the Densu wetlands area. This confirms studies
by Appeaning Addo et al. (2008) and Anokwa et al. (2005) that
identified the western section is erosion prone. Increased erosion
and flooding in the Densu wetlands will affect the habitats of
migratory birds and destroy the ecology. The study has also
revealed that the combined effect of several factors within the
coastal domain places the risk factor to climate change at a higher
level. Geology, geomorphology and relatively low elevation are
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the major factors that facilitate the high risk level in the western
section. Generally, the risk factor associated with wave and tide
actions will be minimal according to the standard risk categories
adopted (refer to Table 1) but based on the Accra coastal
conditions they will be significant. In the eastern section, the
presence of relatively deep bathymetry (Appeaning Addo et al.,
2008) will facilitate considerable impact from wave actions. This
will result in weakening the geology and lead to increased erosion
as the sea-level continue to rise. The low risk area to sea-level rise
along the Accra coast is the central section. This confirms model
simulations by Appeaning Addo et al. (2008).
ACKNOWLEDGEMENT
The author is grateful to the University of Ghana for supporting
the author to present this paper at the ICS2013 conference in
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Journal of Coastal Research, Special Issue No. 65, 2013
