Journal
Journalof
ofCoastal
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Research
SI 64
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ICS2011
ICS2011 (Proceedings)
Poland
ISSN 0749-0208
An application of Dolan and Davis (1992) classification to coastal storms
in SW Spanish littoral
N. Rangel-Buitrago† and G. Anfuso†
†Departamento de Ciencias de la Tierra.
Facultad de Ciencias del Mar y Ambientales.
Polígono Río San Pedro s/n, 11510 Puerto Real (Cádiz), Spain.
E-mail: [email protected]
[email protected]
ABSTRACT
RANGEL-BUITRAGO, N. and ANFUSO, G., 2011. An application of Dolan and Davis (1992) classification to coastal
storms in SW Spanish littoral. Journal of Coastal Research, SI 64 (Proceedings of the 11th International Coastal
Symposium), – . Szczecin, Poland, ISBN 0749-0208.
Classification schemes for distinct meteorological and climatic phenomena, provide beneficial information
useful in evaluating their impacts on socio-economic activities and natural habitats. This work deals with storm
classification in the Atlantic side of Andalusia Region (SW Spain) for the 1958-2001 period, by means of the
index of Dolan and Davis (1992). Wave data from January 1958 to November 2001 were obtained from five
prediction locations (Huelva, Chipiona, Cadiz, Conil and Bolonia) of the HIPOCAS network. The Dolan and
Davis (1992) Storm Power Index was used to classify coastal storms into five classes, from weak to extreme.
The Index was calculated according to the formulation Hs2td, with Hs being the significant wave height and td
the storm duration in hours. Huelva and Chipiona locations respectively recorded 137 and 189 storms, Cadiz and
Conil respectively recorded 377 and 369 storms approaching from the third (mostly) and secondarily from the
forth quadrant. Bolonia location recorded most elevated number of storms (422), they approached principally
from the third quadrant (303 events) and secondarily from the second (59) and forth (60) quadrants. The
distribution of storm classes was very similar at all locations. Classes I (weak) and II (moderate) respectively
accounted for 60% and 23% of events included in the data set. Class III (significant), recorded 10% of the events
and Classes IV (severe) and V (extreme) accounted for 5% and 2%, respectively. Return period for Class V
events ranged from 2 to 12 yrs, with average values of 7-8 years and values from 1 to 3 yrs were observed for
Classes I to IV. Preliminary analysis on beach morphological changes and coastal structures damage pointed out
as in autumn important beach morphological changes were associated with the impact of Classes I to II events
which easily eroded well developed steep summer beach profiles. In winter, successive energetic events (Classes
III to V) produced less significant beach changes, because beaches already showed dissipative profiles, but
heavily impacted on dunes and coastal structures.
ADDITIONAL INDEX WORDS: Storms, Power Index, Erosion, Cadiz, Spain.
INTRODUCTION
Over the past decades several great storms and hurricanes
have caused important economic losses and scores of deaths
along the coastlines of the world (Bacon and Carter, 1991).
Environmental and economic impacts of aforementioned events
will be significant in future years because coastal development
is continually expanding and the climate change, which
predicts an increase of mean sea level and extreme storm surge
events (Brown and McLachlan, 2002; Phillips and Crisp, 2010).
In this sense, there is a need to characterize wave climate and
especially storms, in a way that accounts for their temporal
patterns and characteristics.
For the past 40 years, coastal scientists and the general public
have used the Saffir-Simpson Scale to compare tropical
cyclones. Concerning winter storms, several indexes have been
proposed. Allen (1981) proposed a storm index based on
prevailing onshore wind velocity which reflects storm energy.
Halsey (1986) proposed a ranking for Northeast Atlantic
coastal storms (northeasters or nor’easters) into five classes
based on a damage potential index.
Dolan and Davis (1992) used an index based on wave height
and storm duration, discriminating 1,347 nor’easters in 5
classes ranging from weak to extreme. Orford et al. (1992) and
Orford and Carter (1995) partially incorporated the role of
storm tides in a new storm index. Zhang et al. (2001) proposed
a storm erosion potential index which took into account storm
tides, wave energy and duration, this way reflecting the erosion
potential of large storms. Concerning recent studies on the
Spanish coasts, Rodríguez et al. (2003), Menéndez et al.
(2004), Mendoza and Jimenez (2008) and Mosso et al. (2009)
characterized wave height extreme values and recent storm
distribution in Huelva area and the Mediterranean coast.
The present work deals with storm classification in the
Atlantic side of the Andalusia Region for the 1958-2001
period, by means of the index of Dolan and Davis (1992).
STUDY AREA
Investigated littoral is located in the Gulf of Cadiz and faces
the Atlantic Ocean on the southwest coast of Spain. From an
Journal of Coastal Research, Special Issue 64, 2011
1891
Storm Impacts
administrative point of view it belongs to the Andalusia Region
and includes Huelva and Cadiz Provinces (Figure 1).
Region in nourishment works during the 1990s was 18 millions
of US $, which allowed the injection of about 23 millions of
cubic meters of sediments (Muñoz et al., 2001).
METHODS
Wave data from January 1958 to November 2001 were
obtained from five prediction locations (Huelva, Chipiona,
Cadiz, Conil and Bolonia, Figure 1) of the HIPOCAS network,
an atmospheric hindcast performed in the Atlantic Ocean and
Mediterranean Sea with horizontal resolution of about 20 km.
Each one of the used time series contains 128,518 data
collected with a frequency of 3 hours.
The Dolan and Davis (1992) Storm Power Index was used to
classify coastal storms. The Index was calculated according to
the formulation:
∫ Hs
Figure 1. Study area with location of the five used locations of
the HIPOCAS network.
The area corresponds with 230 km of a coastline broadly
northwest – southeast oriented and characterized by a great
diversity of coastal landforms and environments including sand
spits, beaches, dunes, saltmarshes, cliffs and rocky-shore
platforms. Most beaches are composed by fine-medium sand
essentially consisting of quartz and carbonates. Dune ridges
and rapidly migrating dunes are respectively observed in the
southern part of Huelva Province and close to Gibraltar Strait.
Cliffed sectors are essentially developed in the southern part of
Cadiz Province. The area includes mesotidal (Huelva Province
and central and northern part of Cadiz Province) and microtidal
southern part of Cadiz Province), semidiurnal environments.
The littoral is affected by western and eastern winds. Western
winds are related to Atlantic low pressure systems and blow
from WNW to WSW directions with a mean annual velocity of
16 km/h and a frequency of 13%. East winds, blowing from E
to SE directions, with an annual frequency of 20% and a mean
velocity of 28 km/h, are originally formed in the Mediterranean
Sea and greatly increase their velocity because canalized
through the Gibraltar Strait. Due to coastline orientation,
western winds give rise to both sea-type and swell waves and
eastern winds have no important fetch giving principally rise to
sea waves; main longshore drift flows south-eastward.
In the investigated area, winter storms constitute diffuse and
weak low Atlantic pressure systems and can continue for
several days and affect large areas, producing severe damages
to coastal structures and erosion of beaches and dunes.
In fact, the area is characterized by extensive sandy beaches
of great tourism interest that, in the past decade, have
undergone important erosion with locally recorded values
greater than 1 myr-1, essentially associated with storm events.
Since 1983, more than 600 fills and refills have been performed
along the Spanish coast. In Cadiz and Huelva Provinces,
numerous tourist beaches were nourished during the past
decade with the aim of balancing coastal retreat trends and,
especially, of making beaches more attractive by enlarging dry
beach width. Investment in the Atlantic face of Andalucía
2
td
(1)
with Hs, being the significant wave height and td the storm
duration in hours.
Calculations were carried out considering a threshold of 2.5
m because it represented rare events with only 10% of total
wave heights in the 44 years (following the methodology of
Dorsch et al., 2008) and it reflected the wave height at which
erosion started to affect Cadiz beaches according to ongoing
studies based on accurate 3D beach surveys (Plomaritis et al.,
2009, Del Rio et al., 2010).
Taking into account that tide in the investigated area is
semidiurnal, the minimum storm duration was fixed in 12 hours
- in this way the storm affected coast at least during a complete
tidal cycle. Concerning the interstorm period, it was arbitrarily
set at 1 day in order to create a set of declustered, independent
storm events (Morton et al., 1997; Dorsch et al., 2008).
Once storms were recognized and characterized, five
different classes were obtained by means of the natural breaks
function (Jenks and Caspall, 1971) that determines the best
arrangement of values into classes by iteratively comparing the
sum of squared differences among observed values within each
class and class average values.
Frequency analysis was applied to estimate storm power
return periods. This was calculated using the Generalized
Extreme Value (GEV) distribution (An and Pandy, 2005;
Rajabi and Modares, 2008) according to the formulation:
F ( x) = exp{− exp[−( x − u ) / α ]1 / k } (2)
Where x is the random variable and u, α and k are
respectively location, scale and shape parameters that should be
estimated for each sample. For k= 0, equation (2) reduces to an
extreme value corresponding to the Type I (or Gumbel)
distribution; with k>0, equation acquires an extreme value
corresponding to the Type III (or Weibull) distribution and
when k<0, it acquires an extreme value, i.e. Type II. Extreme
value Type I (Gumbel) distribution is:
F ( x) = exp{− exp[−( x − u ) / α ] }
(3)
Methods concerning parameter estimation for each
distribution are discussed in details in Rao and Hammed
(2000). The maximum likelihood and the method of moments
were used in this work to estimate distribution of parameters.
Journal of Coastal Research, Special Issue 64, 2011
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Rangel-Buitrago and Anfuso
RESULTS AND DISCUSSION
Storm Determination and Characteristics
A total amount of 1,494 events was determined following the
use of the Storm Power Index (Dolan and Davis 1992).
Concerning the obtained results at the five investigated
locations, important differences in the number and approaching
directions of storms can be highlighted. Huelva and Chipiona
locations respectively recorded 137 and 189 storms
approaching from the third quadrant, Cadiz and Conil
respectively recorded 377 and 369 storms approaching from the
third (mostly) and secondarily from the forth quadrant, with
very few events approaching from the second quadrant.
Bolonia location recorded most elevated number of storms
(422), they approached principally from the third quadrant (303
events) and secondarily from the second and forth quadrants,
respectively with 59 and 60 events.
Distribution of storm events presented a clear log-normal
trend and was divided, using the natural breaks (Jenks and
Caspall, 1971), into five classes (Figure 2), i.e. Class I (weak),
Class II (moderate), Class III (significant), Class IV (severe)
and Class V (extreme).
Figure 2. Storm classes obtained for Bolonia locations using
the natural beaks function of Jenks and Caspall (1971).
Distribution of storm classes was very similar at all
locations (Figure 3). Classes I (weak) and II (moderate)
respectively accounted for 60% and 23% of events included in
the data set. Class III (significant), recorded 10% of the events
and Classes IV (severe) and V (extreme) accounted for 5% and
2%, respectively. Average wave height and storm duration
values presented important variations, wave period presenting
more constant values from 6.3 (Class I) to 9.8 (Class V).
Dealing with monthly distribution, Class I events were
observed during all the year (but July and August), Classes II
and III from October to March-May and Classes IV and V from
November to February, with maximum values in December
(Class V) and January (Class IV).
Distribution of number of storms and extreme events (i.e.,
maximum values of wave height and storm power) per year
were determined. An elevated number of storms (≥ 9) were
recorded in 1963 and 1996. Wave height values greater than
6.6 m (corresponding to Class V events) were recorded in
1958, 1966, 1973, 1977, 1981, 1982, 1989 and 2000.
Figure 3. Storm class distribution at studied locations.
Storm power values associated with Class V (extreme)
events were recorded in 1958, 1966, 1970, 1979, 1981, 1989,
1996 and 2000, while an elevated storm duration (≥ 150 hr) of
Class V events was recorded in 1958, 1970, 1979 and 1996.
A good correspondence was observed between previous data
and results obtained for Huelva littoral by Rodríguez et al.
(2003) which identified for the 1956-1996 interval, eight main
stormy periods. Seven of them coincided with years of high
storm power values (≥ 3,000 m2hr, Class IV) and five with
years characterized by a great number of storms (≥ 9). The six
calm periods recorded by Rodríguez et al. (2003) coincided
with years of low storm power values.
In a further step, storm trend during the 1958-2001 period
was analyzed following Komar and Allan (2008) which stated
that records between 25 to 35 years have sufficient lengths to
permit analyses of potential trends of increasing wave heights,
presence of climate-controlled cycles or annual variations due
to climate events.
Maximum values of storm power presented a cyclic behavior
more than a defined trend. Average values of recurrence period
for Class V events at five studied locations, ranged from 2 to 12
yrs, with mean values of 7-8 yrs (Figure 4). Similar values were
obtained using the Gumbel method and were in accordance
with the 6-7 year recurrence period for most important storms
proposed for Cadiz and Huelva areas by Rodriguez et al.
(2003) and Muñoz and Enríquez (1998).
Classes I to IV showed a period of recurrence ranging from 1
to 3 yrs. Storm occurrence probability was 98% for Class I (i.e.
almost 1 event per year) to 16% for Class V (Figure 4). Stormy
years were characterized by Classes III to V events and
numerous storms which summed great storm duration.
Storm distribution was compared with the North Atlantic
Oscillation Index (NAO). In fact, storm generation and tracks
across Southern Europe are related to the NAO which
represents the differences of atmospheric pressures at sea level
between the Azores and Iceland (Rodwell et al., 1999). Within
this study, high storm power values and large storm durations
prevailed during negative values of NAO oscillations as
previously observed by Hurrell (1995) in Southern Europe and
Rodríguez et al. (2003) in the Gulf of Cadiz. During NAO
positive values, low cyclonic activity is recorded and winters
are dryer than normal because of the predominance of eastern
winds and the deviation towards higher latitudes of active
systems (Rodwell et al., 1999).
Journal of Coastal Research, Special Issue 64, 2011
1893
Storm Impacts
Figure 4. Storm recurrence and probability for the different
storm classes at Cadiz location. A) Storm occurrence
probability versus annual maximum Storm Power. B) Annual
maximum Storm Power versus the reduced value using the
Gumbel distribution.
Storm impact and coastal erosion
Despite the main aim of the present work is the
characterization of coastal storms, it was carried out an attempt
to analyze the relationship between storm events and littoral
erosion/coastal structure damage. The aforementioned
relationship depends on storm characteristics and coastal
morphology and behavior (Phillips, 2008; Thomas et al., 2011),
and it is often not very evident because damages prior to visible
failure often respond to a series of storms or a series of stormy
years. In this sense, Bryant (1988) and Ferreira (2005)
suggested that storm frequency is more important than wave
energy for generate beach damages. Zhang et al. (2001) and
Lozano et al. (2004) affirmed that one large storm often causes
much more severe beach erosion than the contribution of many
small storms, especially if they do not impact the dunes. Forbes
et al. (2004) and Dorsh et al. (2008) observed that shorter
recovery time between storms increased the vulnerability of
some coasts to further damage in less severe events.
According to the results of a beach topographic monitoring
program carried out in Cadiz littoral with a monthly periodicity
during the 1996-1998 period (Anfuso and Gracia 2005), it is
possible to state that in the investigated littoral, beach erosion
depends on very local conditions such as exposition, contouring
conditions, morphodynamic state and storm temporal
distribution (Anfuso et al., 2007). In this sense, dissipative
beaches recorded small morphological changes and were not
greatly affected by storm waves, the recovery period being of
several weeks. Steeper beaches presented most important
morphological changes but recovery took place in shorter time
as observed in the Northern part of Cadiz Gulf by Ferreira
(2005) which stated that 2 weeks was sufficient time to permit
beach recovery. Concerning storm temporal distribution, most
beaches recorded severe morphological changes in autumn
when Classes I and II storm events impacted on beaches which
showed a clear summer profile. Successive Classes III to V
events in December-February usually produced less important
morphological changes in beach profile but affected dunes
(Figure 5, Anfuso and Gracia, 2005).
Figure 5. Example of dune erosion at P. Candor Beach (north
of Cadiz) due to a Class III event on 5th February 1998.
CONCLUSIONS
Huelva and Cadiz littorals are particularly susceptible to
storm events, especially considering that many coastal sectors
are recording important erosion processes and are heavily
urbanized. Special attention must be devoted to beach surface
losses which will cause severe economic damages to coastal
tourism, the main economic activity for the investigated area.
In this study, a storm classification into five classes was
obtained at five locations for the HIPOCAS records of 19582001 period. Most powerful storms, i.e. Classes IV and V
events, approached from SW directions and took place from
November to February, with maximum values in December
(Class V) and January (Class IV).
Return period of Class V events ranged from 2 to 12 yrs,
with average values of 7-8 years and values from 1 to 3 yrs
were observed for Classes I to IV. Preliminary analysis on
beach morphological changes and coastal structures damage
pointed out as in autumn important beach morphological
changes were associated with the impact of Classes I to II
events which easily eroded well developed steep summer beach
profiles. In winter, successive energetic events (Classes III to
V) produced less significant beach changes, because beaches
already showed a dissipative profile, but heavily impacted on
dunes and coastal structures. Further studies are needed for
deeply understand beach response to specific storm events in
order to design an appropriate vulnerability assessment and
adaptation strategy at local and regional scale.
ACKNOWLEDGEMENTS
This work is a contribution to the RESISTE Research Project
(CGL2008-00458/BTE, supported by the Spanish Ministry of
Science & Technology and by European Funds for Regional
Development – F.E.D.E.R.) and to the Andalusia P.A.I.
Research Group no. RNM-328. Thanks go to Puertos del
Estado (Spanish Ministry of Public Works) for HIPOCAS
wave data records. This work has been partially developed at
the Centro Andaluz de Ciencia y Tecnología Marinas
(CACYTMAR), Puerto Real (Cadiz, Spain).
Journal of Coastal Research, Special Issue 64, 2011
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Rangel-Buitrago and Anfuso
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