ORI GIN AL PA PER

Overwash vulnerability assessment based on long-termwashover evolution

Tiago Garcia • Oscar Ferreira • Ana Matias • Joao Alveirinho Dias

Received: 30 May 2008 / Accepted: 17 September 2009 / Published online: 2 October 2009� Springer Science+Business Media B.V. 2009

Abstract An integrated methodology for evaluation of overwash vulnerability is

developed with respect to the historical washover evolution of a barrier island system.

Three different aspects of overwash are addressed in the vulnerability indices developed:

overwashed shoreline ratio, maximum overwash intrusion recurrence, and complete barrier

overwash. The indices were applied to the barriers in the Ria Formosa system in Southern

Portugal using an aerial photography catalogue covering the period 1947–2001. Historical

trends of washover evolution were observed to be different between the barriers analysed,

but generally, there was a decrease in washover number and dimensions throughout the

analysed period. The final overwash diagnostic obtained allowed an integrated overwash

vulnerability rating to be defined for each barrier, with vulnerabilities ranging from low to

extreme. The methodology has produced results that may assist coastal managers with

information concerning barrier island system overwash hazard, define the temporal and

geographical distributions of overwash, and provide indications as to where overwash is

most likely to occur in the future.

Keywords Overwash vulnerability � Washover evolution � Aerial photography �Barrier islands � Ria Formosa

1 Introduction

Overwash is defined as the continuation of the uprush over the crest of the most landward

berm (Shepard 1973) or frontal dune line, during a storm or an unusually high tide (Fisher

et al. 1974). The importance of studying overwash processes stems from the need for rec-

ognition of ancient sedimentary environments, understanding the dynamics of natural barrier

systems, and obtaining scientific background information to inform coastal engineering

practices (Schwartz 1975).

T. Garcia (&) � O. Ferreira � A. Matias � J. A. DiasCIMA, Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugale-mail: tgarcia@ualg.ptURL: www.cima.ualg.pt

123

Nat Hazards (2010) 54:225–244DOI 10.1007/s11069-009-9463-3

From a coastal management viewpoint, overwash is a significant phenomenon given the

range and extent of the consequences of the inundation. Landward sediment transport and

wave attack accompanying overwash may cause (USACE 2004): loss of, or damage to,

property or loss of access to property; damage to roads and other infrastructures (including

their burial); intrusion of sediment into navigation channels; requirement to remove

washover deposits from public and private property to regain functionality of the property;

loss of protection to the mainland afforded by protective barriers or dunes if they are

lowered by overwash; changes to the backshore environment; shoreline recession and

barrier island migration; and increased susceptibility to breaching (USACE 2004). Given

these consequences, coastal areas that experience frequent overwash must account for

washover processes in long-term management plans (decadal to century). Predictive

capability is required to prevent or prevent for overwash (USACE 2004), while

the application of overwash studies is necessary for the establishment of set-back lines for

housing and road construction (Leatherman 1977).

Sediment deposits and morphological features originating from overwash processes,

termed ‘washovers’, are easily identifiable features in high-resolution aerial photography

(Leatherman 1988), which makes remote sensing techniques an obvious choice for long-

term—decadal—washover evolution monitoring. A number of studies have been performed

(e.g. Schwartz 1975; Ritchie and Penland 1988; Hequette and Ruz 1991; Fletcher et al. 1995;

Morton et al. 2003; Morton and Sallenger 2003), in which overwash occurrence and its

associated consequences were inferred after analysis of a series of aerial photographs.

However, the majority of the overwash studies have been based on evaluations of post-storm

landscapes, consisting of reconnaissance-level investigations that include descriptions of

dramatic changes in landforms, inventories of damage to buildings and infrastructure, and

recommendations to prevent damage from subsequent storms (Nordstrom and Jackson

1995). The inclusion of long-term observations, as achieved by Hansen and Sallenger (2007),

is herein assumed to benefit the reliability of the information produced as it may diminish

misjudged conclusions regarding overwash assessment and forecast. This study seeks to

assist overwash vulnerability management. The proposed methodology may be utilised

along any overwash-prone barrier system worldwide and is based both on present conditions

and (perhaps more importantly) on the overwash history of the coast. In order to achieve that

purpose, three overwash vulnerability indices were developed and tested on the Ria Formosa

barrier island system in Southern Portugal.

2 Test case: Ria Formosa barrier island system

Ria Formosa is a coastal lagoon system located in the Algarve, the southernmost region of

Portugal. The lagoon is separated from the Atlantic Ocean by a barrier island system that

currently comprises two peninsulas, five islands, and six tidal inlets (Fig. 1). The regional

wave climate is dominated by two main incident wave directions, W-SW and E-SE, which

occur on average for 71 and 23% of the time, respectively (Costa et al. 2001). Annual mean

offshore significant wave height is 0.92 m (Costa et al. 2001). The triangular shape of the

system produces two areas in terms of exposure to wave action. The western flank is more

energetic, being under the direct influence of the dominant wave conditions, while the

eastern flank is directly exposed only to the E-SE conditions (Vila-Concejo et al. 2002).

For example, in 1997 and 1998, offshore significant wave height was about 1.0 m, whereas

the mean breaking significant wave height was 0.6 m at Cacela Peninsula (Matias 2000).

The most powerful storms originate from the SW (Costa et al. 2001), strengthening the

226 Nat Hazards (2010) 54:225–244

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wave energy gradient between the flanks of the system. Such a regional wave climate

consequently generates an eastward net littoral drift. Tides are semi-diurnal with an

average range varying between 1.3 and 2.8 m during neap and spring tides, respectively,

and the maximum astronomical tidal level can reach 1.8 m above mean sea level.

The evolution of the Ria Formosa barrier island system is characterised by shoreline

retreat, longshore drift, overwash, vegetated dune formation, tidal delta incorporation, inlet

migration, and backbarrier erosion (Pilkey et al. 1989). The barriers exhibit peculiar indi-

vidual characteristics since both destructive and constructive processes act in conjunction to

drive barrier evolution (Dias 1988). Table 1 summarises the broad morphological aspects of

the barriers pertinent to the application of the overwash vulnerability indices.

In the Ria Formosa barrier island system, overwash has been reported as being associated

with shoreline retreat, property damage, and barrier breaching (Pilkey et al. 1989; Andrade

1990a, b; Andrade et al. 1998; Matias 2000, 2006; Garcia et al. 2002; Matias et al. 2007;

Carrasco et al. 2007). The Ria Formosa lagoon and its surrounding margins and barriers were

legally constituted as a Natural Reserve in 1978 and as a Natural Park in 1987. Since its

existence, the Park Authority has put into practice several management plans in order to

diminish the vulnerability of the frontal dune ridge to coastal erosion, to overwash, and to

new tidal inlet formation. Soft engineering interventions have involved beach nourishment

with sediment dredged from the lagoon, dune reconstruction assisted by sand fences and

dune vegetation plantation, and tidal inlet relocation. These interventions have been moni-

tored, and the results obtained have been presented and discussed in detail in several prior

publications (e.g. Ramos and Dias 2000; Dias et al. 2003; Matias et al. 2004, 2005).

3 Overwash vulnerability indices

3.1 Development of washover inventory

The creation of an inventory of historical washovers for the Ria Formosa barriers following

an extensive analysis of aerial photograph sequences (see Matias 2006; Matias et al. 2008)

8º00' 7º40'

37º10'

0 5 km

Ria FormosaLagoon

10

60

40

20

200100

80

400

N

Cabanas

Cacela

Tavira

Ancão

Atl

anti

c O

cean

ALGARVE

FaroOlhão

Fuzeta

Tavira

PO

RT

UG

AL

W B

arre

ta

E B

arre

ta

W C

ulatraE

Culatra

E Arm

ona

W A

rmona

Ancão

Inle

t

Far

o-O

lhão

Inle

t

Armona Inlet

Fuzeta Inlet

Tavira InletLacém

Inlet

Fig. 1 The Ria Formosa barrier island system

Nat Hazards (2010) 54:225–244 227

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Table 1 Morphological features of the Ria Formosa barriers and their expected future evolution based onrecent and present conditions

Barrier Length(m)a

Width(m)b

Present conditionof the frontaldunec

Expected futureshoreline behaviourc

Auxiliary informationd

Ancao 8,540 96 Generally highand continuousexcept inoccupied areas

Retreat in the Westgraduallychanging toadvance in theEast

Intense development (Praia deFaro) in the centre of the barrier

WesternBarreta

5,231 141 Frontal ridgeabsent. Lowbarrier ridge

Retreat Historical migration path of AncaoInlet. Largest washover terracesof the whole system(length [ 1,000 m)

EasternBarreta

3,673 438 Generally highand continuous

Advance Sediment accretion (220 m in12 years) induced by thestabilization of the Faro-OlhaoInlet

WesternCulatra

1,758 479 Generally highand continuousexcept inoccupied areas

Retreat Intense development (Praia doFarol) in the western tip. Coastalerosion (5.8 m/year in 1958–1976) due to updrift Faro-OlhaoInlet

EasternCulatra

3,959 387 Interrupted ridgesconnected totidal channels

Advance Barrier length grew [3,000 mbetween 1947 and 2001.Formation of succession ofcurved ridges

WesternArmona

5,342 387 Generally highand continuous

Dynamicequilibrium

Westward growth due to a locallittoral drift inversion at theArmona Inlet ebbdelta. Largebarrier width resulted fromincorporation of flood-tidaldeltas

EasternArmona

3,294 73 Generally low orabsent inoccupied areas

Retreat Moderate development (Praia daFuzeta). Historical migrationpath of Fuzeta Inlet

Tavira 10,407 394 Generally highand continuousexcept inoccupied areas

Retreat in the Westgraduallychanging toadvance in theEast

Flood-delta incorporationgenerated a wide barrier.Stabilization of Tavira Inletinduced sediment accretion inthe eastern tip

Cabanas 6,117 123 Generally low andinterrupted

Retreat Barrier reformed after 1961 majorstorm. Evolution controlled byLacem Inlet

Cacela 2,890 113 Generally low andinterrupted

Retreat Lacem Inlet induces erosion andbarrier breaching

a Barrier length made on the 2001 aerial photographs following the criteria described in Sect. 2.1b Mean barrier width measured on the entire photography catalogue following the criteria described inSect. 2.1c Evaluation based on the analysis and comparison of aerial photographs and on the review of the literaturecited belowd Information extracted from Weinholtz (1964), Esaguy (1986), Dias (1988), Pilkey et al. (1989), Andrade(1990b), Matias (2000), Garcia et al. (2002), Vila-Concejo et al. (2002), Pacheco et al. (2004), Ferreira et al.(2006), and Matias (2006)

228 Nat Hazards (2010) 54:225–244

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motivated three vulnerability indices to be developed that encapsulate the following

aspects of overwash phenomenon: location of occurrence along the shoreline; inland

intrusion potential; and capability of causing barrier breaching. These simple indices

should be applicable to virtually any sandy barrier and can be measured by monitoring

morphological features including washover opening, overwash intrusion, barrier width, and

shoreline extent (Fig. 2).

The concept of shoreline adopted here is the landward limit of long-term wave action

upon the beach profile, being interpreted on aerial photographs as the contact between the

non-vegetated beaches and the adjacent vegetated dunes. This conception of shoreline can

be adopted for oceanic coastlines as well as for lagoon/bay beaches. As portrayed on

Fig. 2, the shoreline as defined does not include the washover contour within the frontal

dunes. The washover opening (ab in Fig. 2) is defined as the length of the shoreline section

between the dunes that limit the mouth of the washover. Both overwash intrusion (cd in

Fig. 2) and barrier width (ce) measured normal to this line and represent the maximum

excursion of the overwash sediment and the cross-shore distance between the oceanic and

the lagoon/bay shorelines, respectively. These features can be measured for any washover

Lagoon

Ocean

Ocean Beach

Lagoon Beach

aaa

cb b

c c

d

b

de

eDune

e

d

c

e

d

Ocean

Lagoon

Lagoon

Ocean

d

c

e

A

B

A B

Fig. 2 Scheme used to measure the morphological parameters needed for calculating the overwashvulnerability indices. Washover opening corresponds to ab; overwash intrusion corresponds to cd; andbarrier width corresponds to ce. It was assumed that if cd � ce then, for the purpose of calculating theindices, cd ¼ ce

Nat Hazards (2010) 54:225–244 229

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by analysis of aerial photographs if the washover cuts through an otherwise vegetated

barrier.

For the Ria Formosa washovers, work involved photo interpretation, remote sensing, and

digital mapping techniques which were managed using a GIS. Managing data within a GIS

was especially important for geospatial analysis including aerial photo registration, photo

analysis and comparison, measurement and mapping of features, and database development.

Due to the number—a total of 422 washovers were identified during the analysed period—

and diversity of washovers identified, it was necessary to establish photo-interpretation

criteria on which the identification of morphological features would be based. The criteria

used followed the work of Matias (2006) and Matias et al. (2008) and were based mainly on

the absence of dune vegetation and on the appearance of sandy surfaces, both parameters

being highly dependent on the resolution, colour mode, scale, and quality of the photographs

analysed. Additional criteria were needed to establish whether a washover should be

classified as active or inactive, and what could constitute complete overwash of the barrier.

Thus, after an initial qualitative analysis, the washovers were classified as active when the

appearance of its sandy surface was accompanied by an absence of vegetation. A lack of

vegetation suggests that the overwash occurred within the last several months to a year.

Complete overwash was considered to have taken place when the overwash flow crossed the

entire width of a barrier island, creating large washovers that terminate in the lagoon or bay

behind the barrier system. Complete overwash carries significance for barrier evolution

because if scouring of the barrier proceeds to an elevation below mean sea level, a new tidal

inlet may form (Leatherman 1988). Overtopping of artificial structures was excluded from

the analysis, because it deals with impermeable surfaces, with subsequent artificial

displacement or total removal of sediments (Matias et al. 2008).

The identification of a washover based on dune vegetation differences is complicated in

areas where the vegetation is sparse. Such is the case of many tidal inlet margins, where

washovers tend to be barely identifiable in airborne imagery. Hence, this study excluded

washovers located along the non-vegetated margin areas in the vicinity of tidal inlets.

Washovers in occupied areas tend to be quickly changed and sometimes eliminated by

human actions. Washover identification through aerial photography analysis as presented

here is not as reliable as for natural areas. Areas with intense development were therefore

excluded from the present study.

3.2 Overwash vulnerability indices

The three indices developed were designated as overwashed shoreline ratio (OSR); max-imum overwash intrusion recurrence (MOIR); and complete barrier overwash (CBO). OSRrefers to the overwash occurrence along the shoreline and indicates the relative fraction of

the entire extent of shoreline that consists of active washovers (i.e. evidenced by recent

(\12 months) overwash). This index was defined as

OSR ¼P

washover opening

shoreline extent� 100 ð1Þ

and the results obtained after its application may vary between 0% (minimum vulnera-

bility; absence of washovers along the entire shoreline) and 100% (maximum vulnera-

bility; uninterrupted washover along the entire shoreline).

MOIR refers to the inland intrusion history of overwash and represents the recurrence of

maximum overwash intrusion. Recurrence, or return period, is defined as the time in which

there is a significant probability that a phenomenon of a particular magnitude will occur.

230 Nat Hazards (2010) 54:225–244

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With respect to maximum overwash intrusion, the return period of a given event (i.e.

MOIR) can be determined, for designated overwash intrusion classes (e.g. 0–10, 11–20,

21–30 m), by the equation:

MOIR ¼ N þ 1P

number of active washovers with maximum intrusion greater than Xð2Þ

where N corresponds to the number of years of the analysed period and X corresponds to

the inferior limit of each overwash intrusion class (in metres). The maximum value of

MOIR is then dependent on the length (in years) of the period analysed and will achieve

values inferior to 1 whenever the number of washovers identified is greater than N ? 1.

Hence, regarding overwash intrusion, for the same value of X, maximum vulnerability will

correspond to the smallest MOIR value obtained. This index can also be employed to

construct forecasts of overwash intrusion, because it allows the determination of the the-

oretical overwash intrusion (X) associated with any overwash return period (e.g. 10, 25,

50 years).

CBO refers to the capacity of the overwash to cause barrier breaching and evaluates the

vulnerability to complete overwash in barrier islands. This third index can be calculated,

for each washover identified, using the following equation:

CBO ¼ overwash intrusion

barrier widthð3Þ

Due to the criteria used in the photo interpretation and mapping phases, overwash

intrusion has a maximum value corresponding to the barrier width. This implies that there

is no difference between an overwash that reaches the lagoon/bay shoreline and a higher

magnitude event as it assumes that both represent complete barrier-breaching scenarios

(see Fig. 2). Therefore, CBO results have a maximum value of 1, which represents a

situation of maximum vulnerability to breaching by overwash. CBO can be applied to

individual washovers, to sections of shoreline, or to a given barrier by integrating all

washovers in the inventory created for the barrier in order to obtain an overall potential

barrier-breaching evaluation.

3.3 Sub-indices

The application of the indices allows an examination of the history of overwash occur-

rence. Throughout the period of interest, overwash may have been irregularly distributed

through time or may have occurred in just a few years. In both cases, vulnerability

diagnostics should, from a coastal management viewpoint, represent worst case scenarios.

With respect to overwash, this means that a diagnostic should provide information about

the maximum magnitude of the phenomenon, even if its occurrence is rare. In that sense,

the following sub-indices may be determined to add to the information obtained from the

analysis: OSRM, the maximum OSR value registered during the analysed period (in

metres); MOI25, the maximum overwash intrusion with a 25 year return period (in metres);

and CBO2/3, the percentage of washovers with CBO greater than 2/3 of the barrier width.

Since each of these sub-indices address a specific aspect of overwash, they should be

interpreted in combination and sometimes weighted differentially in order to achieve a

final, integrated vulnerability evaluation. In this respect, some aspects of overwash may be

locally more relevant than others depending on the processes and morphology of the area

under investigation. More weight may be given, for example, to OSRM where overwash is a

frequent occurrence, and human settlements or features of natural significance are

Nat Hazards (2010) 54:225–244 231

123

significantly distant from the shoreline. It may also be appropriate to attribute more weight

to MOI25 where overwash is not common along the coast but where the potential exists for

severe damage or great morphological changes at locally vulnerable sites.

3.4 Aerial photography analysis and measurements

Aerial photographs from 1947, 1976, 1989, and 2001 were scanned at 1 m pixel size

resolution and georeferenced. The resulting georeferenced imagery was assembled, using

ER Mapper� software, to create digital mosaics. Due to the incompleteness of the 1989

flight, a 1985 flight was used for the non-covered areas including Barreta and Culatra

islands. The scale of the photography used varied from 1:8,000 to 1:30,000. Twenty-seven

mosaics were created: four mosaics for each of the seven barriers of the system, except for

Cabanas Island as it had not yet formed in 1947.

The photo-interpretation criteria were used to map shorelines, dunes, and washovers

with ArcGIS software. Direct and stereoscopic photo interpretation of barrier morphology,

publications concerning the regional coastal morphology, and in situ observations of

morphological and vegetation characteristics were taken into consideration during the

mapping phase. Shoreline extent, overwash intrusion, washover opening, and barrier width

were measured to determine rates of morphological evolution and to calculate the over-

wash vulnerability indices. The indices developed were applied to the ten morphological

sectors of the Ria Formosa barrier island system represented in Fig. 1 (i.e. the seven

barriers, three of which are divided into two distinct morphological sectors). Georefer-

encing accuracy was determined by overlapping mosaics and measuring detachments of

several well-known features. As the maximum distances registered were less than 10 m,

any morphological change smaller than 10 m was disregarded.

4 Results

4.1 Long-term evolution of washovers

The washovers identified for the barriers of the Ria Formosa during the period 1947–2001

are portrayed in Fig. 3. The total number of washovers identified in the entire system was

100 in 1947, 152 in 1976, 129 in 1985/89, and 41 in 2001. Both washover opening and

overwash intrusion dimensions also experienced an overall decrease over the whole period

(Matias et al. 2008). Measured values of individual washover opening and overwash

intrusion varied from a few metres (within the uncertainty limit) to 2,071 m and to 254 m,

respectively.

4.2 Overwashed shoreline ratio (OSR)

The results obtained for OSR show a general reduction in the longshore extent of over-

washed areas in the system (Fig. 4). In most cases, the largest overwashed shoreline extent

was registered in 1947 (e.g. Ancao) or in 1976 (e.g. Eastern Armona). Although OSRevolution for the system as a whole was marked by a general reducing trend, some barriers

experienced a more marked reduction (e.g. Cacela Peninsula and Cabanas Island after its

formation in 1976; and Culatra Island and Eastern Armona, after 1976). For the remaining

barriers, the reduction over time in OSR was relatively smoother and characterised by

232 Nat Hazards (2010) 54:225–244

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smaller variations. In 2001, only Western Barreta had a high OSR (44%) while the

remaining barriers exhibited OSR values of \6%.

4.3 Maximum overwash intrusion recurrence (MOIR)

Between 1947 and 2001, the greatest maximum overwash intrusion was observed in

Western Barreta (254 m) while the smallest was recorded in Western Culatra (66 m)

(Fig. 5). MOIR was calculated for each 10 m class from the uncertainty limit (10 m) to the

maximum value of each sector. Following the methodology of Teixeira (2006), the results

0 2 km

1-Ancão1947

1976

1989

2001

0 2 km

3-Culatra

1947

1976

1985

20010 2 km

2-Barreta

1947

1976

1985

2001

Non Vegetated Areas

Vegetated Areas

Washovers

1

2

3

4

5

67

Ria Formosa BarrierIslands System

(Algarve, Portugal)

0 2 km

5-Tavira

1947

1976

1989

2001

0

2 km

6-Cabanas

0

1976

1989

2001

0 2 km

7-Cacela

1947

1976

1989

2001

0 2 km

4-Armona

1947

1976

1989

2001

A

B

Fig. 3 Evolution of the washover distribution for each barrier of the Ria Formosa between 1947 and 2001.Note that the scales vary between the barriers. The western tip of the 1947 Cacela Peninsula drawing is astylised representation due to incomplete coverage of that area by the available photography

Nat Hazards (2010) 54:225–244 233

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were plotted on a logarithmic graph of maximum overwash intrusion versus recurrence

(Fig. 5). Logarithmic regression was applied to the graphic of each barrier to obtain curves

that reasonably adjusted to each data series. The curve equation of each barrier was later

used to project maximum overwash intrusion for events with 10, 25, and 50 years return

periods (Table 2). In addition, MOIR results were employed to estimate average frequency

of barrier overwash. The results show that Cabanas is expected to experience overwash

with a maximum intrusion exceeding 10 m on an annual basis, while Ancao, Culatra,

Eastern Armona, Tavira, and Cacela should experience similar events on an annual to

biannual basis. For Barreta and Western Armona, a similar event is more likely to have a

greater return period (3–4 years).

4.4 Complete barrier overwash (CBO)

CBO was determined for each washover identified within the analysed period. The results

obtained were used to calculate the average CBO and CBO2/3 value for each barrier

(Table 3). Five of the barriers (Ancao, Western Barreta, Eastern Armona, Cabanas, and

Cacela) have an average CBO exceeding 0.5 (i.e. on average, their washovers occupy at

least half the barrier width). For the remaining barriers, average CBO is smaller. Wash-

overs with CBO2/3 were recorded broadly except in Eastern Barreta, Western Culatra, and

Western Armona where they were absent. Western Barreta (80%) and Eastern Armona

(58%) exhibited the highest relative number of washovers intruding more than 2/3 of the

barrier width and are also the barriers that obtained higher average values of CBO.

4.5 Integration of the indices

The results obtained for the sub-indices OSRM, MOI25, and CBO2/3 were divided into four

classes (Fig. 6), each corresponding to a vulnerability value ranging from 1 (lowest

0

10

20

30

40

50

60

70

80

90

100

1947 1976 1989 2001

t necer htiw enil er ohS

over

w)

%( hsa

Time (years)

Ria Formosa BarriersAncãoWestern BarretaEastern BarretaWestern CulatraEastern Culatra

Western ArmonaEastern ArmonaTaviraCabanasCacela

Fig. 4 Results obtained for overwashed shoreline ratio (OSR) for the Ria Formosa barriers for the period1947–2001

234 Nat Hazards (2010) 54:225–244

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vulnerability) to 4 (highest). Those values were summed for each barrier to obtain an

integrated final diagnostic potentially varying between 3 (if a barrier obtained the

lowest vulnerability value for each of the three sub-indices) and 12 (if a barrier

obtained the highest vulnerability value for each of the three sub-indices). The proce-

dure applied assessed and rated the vulnerability of the barriers as follows: Western

Barreta (11); Cacela (9); Cabanas (8); Eastern Culatra and Eastern Armona (7);

Ancao (6); Eastern Barreta, Western Culatra, and Tavira (5); and Western Armona (4).

The results classify each barrier from low to extreme vulnerability to overwash

(Table 4).

10

100

1000

0,1 1Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

WESTERN BARRETA

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

10

100

1000M

axim

um O

verw

ash

Intr

usio

n (m

)

Return Period (years)

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

10

100

1000

Max

imum

Ove

rwas

h In

trus

ion

(m)

Return Period (years)

Maximum = 129m

Maximum = 254m

Maximum = 189m

Maximum = 66m

Maximum = 172m

Maximum = 113m

Maximum = 93m

Maximum = 137m

Maximum = 130m

Maximum = 142m

WESTERN ARMONAANCÃO

EASTERN BARRETA

WESTERN CULATRA

EASTERN CULATRA

EASTERN ARMONA

TAVIRA

CABANAS

CACELA

10 100 0,1 1 10 100

0,1 1 10 1000,1 1 10 100

0,1 1 10 100 0,1 1 10 100

0,1 1 10 1000,1 1 10 100

0,1 1 10 100 0,1 1 10 100

Fig. 5 Results for maximum overwash intrusion recurrence (MOIR) for the Ria Formosa barrier islandsystem

Nat Hazards (2010) 54:225–244 235

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5 Discussion

5.1 Data acquisition procedures

The ability to perform real-time coastal mapping has greatly increased in recent decades

with the development of technologies such as GPS, LIDAR, and ortho-imagery. Historical

data can often be limited or of poor quality, periodicity, or accuracy, which may result in

level of detail not useful in the evaluation of coastal hazards. This present work took

advantage of a 54-year catalogue of aerial photographs, which proved to be a good data

source for the purpose of overwash hazard assessment. However, due to the nature of

coastal change, the usual caveat applies to future forecasts, such that future scenarios are

valid only for the period during which forcing mechanisms are similar to those observed

historically and at present. In the context of accelerating sea-level rise and the potential for

increased storminess in the future, the uncertainty regarding predicted scenarios becomes

even greater.

The results generated in this study are based on four available ‘snapshots’ of a dynamic

region experiencing a natural, episodic coastal process. From a coastal management

viewpoint, the diagnostics achieved must therefore be interpreted cautiously given that

Table 2 Estimated maximumoverwash intrusion (in metres)associated with different MOIR,for each barrier

Barrier MOIR

10 years 25 years 50 years

Ancao 97 137 157

Western Barreta 119 197 257

Eastern Barreta 51 128 186

Western Culatra 38 52 63

Eastern Culatra 95 131 159

Western Armona 51 82 105

Eastern Armona 65 87 104

Tavira 80 106 126

Cabanas 88 111 129

Cacela 95 127 151

Table 3 Average CBO and per-centage of washovers attainingCBO2/3 for each barrier analysed

Average CBO Washovers withCBO2/3 (%)

Ancao 0.5 34

Western Barreta 0.9 80

Eastern Barreta 0.2 0

Western Culatra 0.1 0

Eastern Culatra 0.3 5

Western Armona 0.1 0

Eastern Armona 0.7 58

Tavira 0.2 5

Cabanas 0.6 34

Cacela 0.5 28

236 Nat Hazards (2010) 54:225–244

123

variations in overwash processes within each selected period undoubtedly exist. Although

the overall period of observations should be determined in accordance with local washover

evolution rates and forcing mechanism changes, a minimum of 30 years is deemed

appropriate, especially for the determination of MOIR. Data series (flight intervals, in the

test case presented) should ideally be collected with a recurrence interval of no longer than

10 years. This allows for a representation of local coastal dynamics and the identification

of major interim events.

0 5 km

Faro

Olhão

Tavira

0-25%

25-50%

50-75%

75-100%

OSRM MOI25 CBO2/3

0-50m

50-100m

100-150m

150-200m

0-25%

25-50%

50-75%

75-100%

LegendAncão

Barreta

Culatra

Armona

Tavira

CabanasCacela

Ria Formosa Barrier Islands SystemOverwash Vulnerability

Fig. 6 Representation of the results obtained after integration of the sub-indices. In the legend, OSRM

corresponds to the maximum percentage of active washovers relative to the shoreline extent registeredbetween 1947 and 2001; MOI25 corresponds to the maximum overwash intrusion (in metres) associated witha 25-year return period event; and CBO2/3 corresponds to the percentage of washovers that intruded morethan 2/3 of the barrier width between 1947 and 2001

Table 4 Final overwash vulnerability classification after integration of the OSRM, MOI25, and CBO2/3results

Barriers

Vulnerability Classes

Low = 1-3 Medium = 4-6 High = 7-9 Extreme = 10-12

Ancão

Western Barreta

Eastern Barreta

Western Culatra

Eastern Culatra

Western Armona

Eastern Armona

Tavira

Cabanas

Cacela

Vulnerability classes are based on the sums of the individual index ratings (see Fig. 6)

Nat Hazards (2010) 54:225–244 237

123

Overwash in the vicinity of human-occupied areas is commonly responsible for damage

to roads and infrastructure, especially burial. Often, overwash sediments are artificially

removed to guarantee normal access to, and use of, an affected area. Therefore, it is likely

that some washover changes, particularly in developed areas, cannot be accurately mon-

itored using remote sensing techniques unless there is pre- and post-event monitoring.

Finally, washover identification based on vegetation changes using remote sensing

techniques as adopted here may be difficult when dune vegetation is rare and sparse, as is

the case in tidal inlet margins. Hence, washover data acquisition as proposed may not be

reliable for tidal inlet margins, and its application to such settings is not advised.

5.2 Evaluation of the indices

The indices developed for the evaluation of overwash vulnerability are new and need to be

evaluated in detail to fully determine their usefulness, however, based on this study the

potential does exist to provide coastal managers with likelihood scenarios for overwash

vulnerability. Historical OSR values can be used to detect evolutionary trends in overwash

occurrence along a barrier; that is, to detect whether overwash occurrence is decreasing,

steady, or increasing. OSRM sets the top limit for the occurrence along a barrier or par-

ticular section of shoreline. Based on recent historical trends, the data may also be used to

deduce whether an increase in the occurrence of overwash (OSR) is likely in the future,

and, if so, by how much (OSRM).

MOIR is useful in introducing a temporal scale to overwash hazard evaluation by

allowing the calculation of overwash intrusion for specific return periods. For instance,

MOIR can inform decisions about where to place infrastructure. For that purpose, MOI25

was calculated on the basis that, according to coastal changes observed in the Ria Formosa,

a smaller return period event (e.g. 10 years) would not be so significant, and a larger return

period (e.g. 50 years) may completely alter the barrier. In addition, a 25 year projection is

well supported by the long-term washover inventory developed in the study. When com-

bined with the information provided by OSRM, this sub-index provides a two-dimensional

(along- and cross-shore) evaluation of the area that may be affected by overwash.

Regarding CBO, caution must be taken when interpreting the diagnostic provided by

this index, because a washover classified as indicating maximum vulnerability may

actually have extended considerably beyond the lagoon/bay shoreline. Because of the

distribution of flight schedules over time, historical photography catalogues usually miss

long periods, and so there is little information on how the washovers have evolved between

flights. It is also likely that any washover terminus in contact with a lagoon/bay water body

experiences considerable amounts of reworking in the time between aerial photography

sets. A maximum vulnerability classification for CBO can therefore refer to a larger

overwash with more significant consequences. The development of the sub-index CBO2/3

results from the supposition that any washover intruding to 2/3 of the barrier width must be

acknowledged as a warning sign for several hazards (e.g. new inlet formation, and inun-

dation of occupied areas located along the bay/lagoon shoreline). Shorelines meeting the

CBO2/3 criteria should receive careful monitoring by relevant authorities and possibly

management measures. This sub-index could potentially be refined by using the critical

width of the barrier to replace the 2/3 in its formulation, which may improve the diagnostic

for management actions. The critical width of a barrier island is defined as the smallest

cross-shore dimension that minimises the net loss of sediment from the island over periods

from decades to centuries (Rosati and Stone 2007). Although the critical width may

provide a physically based distance measurement for the CBO sub-index, rather than the

238 Nat Hazards (2010) 54:225–244

123

2/3 ratio value used here, this would require knowledge of the unique critical width for

each different barrier in the study area.

Although the indices were developed for coastal barriers, OSR and MOIR may also be

applied with confidence to any sandy shore. Although CBO involves the barrier’s width in

its formulation, the concept behind CBO could be adapted to evaluation of vulnerability of

any inland features (e.g. dunes, buildings, agricultural fields, roads, and natural or human

heritage sites) exposed to overwash. In that respect, a rather simple adaptation of Eq. 3,

consisting of replacing the denominator with the cross-shore distance between the shore-

line and the feature of interest, would achieve that purpose.

The indices are useful for informing coastal management plans and actions, namely

through the identification of the areas where barrier breaching, new inlet formation, and

habitation/infrastructure damage by overwash is most likely to occur. The indices can also

be used to help predict where barrier roll-over mechanisms may occur, and to delimit set-

back lines for the location or relocation of development.

5.3 Test case analysis: Ria Formosa

The vulnerability rating obtained (Fig. 6) shows that overwash vulnerability varies sig-

nificantly along the Ria Formosa barrier system. Such heterogeneity was observed not only

in space but also in time. The trends observed for the number of washovers (Fig. 3) and

OSR results (Fig. 4) indicate that a general decrease in overwash activity took place

throughout the analysed period. In 2001, the number and size of active washovers at each

barrier was significantly smaller than those observed in 1947 (for 1976 in the case of

Cabanas). In fact, after 1976, overwash activity declined consistently in the Ria Formosa

barriers (Matias et al. 2008). As overwash events are usually associated with storms

(Morton et al. 2000), it is possible that the decline in overwash was associated with weaker

storm events. Many authors (e.g. Weinholtz 1964; Esaguy 1986; Andrade 1990b) refer to

extreme storms which impacted the barrier system in 1941, 1961, and 1969, causing

dramatic morphological changes (and damage to occupied areas). Hence, the system was

probably in a more vulnerable state until 1976 perhaps due to the action of such storms.

Subsequently, overwash resistance was gradually regained, and the system became more

robust. The peak in overwash occurrence (1947–1976) was related to the existence of

immature, low-lying tidal inlet margins formed as result of recent updrift accretion and

downdrift starvation of sediment (Matias et al. 2008). Since then, the dunes have been

recovering, benefiting, in some cases, from the various interventions undertaken by the

Natural Park Authority since 1987. For example, major dune nourishment interventions on

Cacela Peninsula between 1996 and 1998 dramatically changed the barrier morphology

and inhibited further overwash (Matias et al. 2005). However, all artificial interventions

undertaken on the barrier system since the 1940s have only accounted for 12% of overwash

activity (particularly, the artificial stabilization of Faro-Olhao Inlet) and also 12% of

washover cessation (58% of which by nourishment, Matias et al. 2008).

The most dramatic consequences of overwash tend to be associated with its intrusion

potential, which can be directly inferred from MOIR values. Besides being used to identify

what can be submerged, damaged, or buried by overwash, MOIR complements the OSRdiagnostic as it can identify situations where OSR values may be low but yet overwash can

still intrude considerably. Ancao Peninsula is one such case. Washovers on this long

peninsula tend to be infrequent (Fig. 3) and possess small openings due to the height of the

barrier’s frontal dune ridge (Table 1), but once overwash occurs it tends to be relatively

intrusive (Table 2).

Nat Hazards (2010) 54:225–244 239

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An adequate characterisation of overwash vulnerability must therefore include, at least,

the application of OSR and MOIR. The results obtained for CBO can also raise alerts that

would not otherwise be evident. Such is the case of Eastern Armona, where active

washovers were identified to be somewhat infrequent (Fig. 3) and of low intrusion (Fig. 5)

in relation to other more overwash-prone barriers of the system. However, its very narrow

width results in a situation of high vulnerability to complete overwash (Table 3; Fig. 6).

The entire list of indices and associated sub-indices must thus be fully integrated in order to

provide a complete vulnerability diagnostic. From the final vulnerability classification that

resulted from the integration of OSRM, MOI25, and CBO2/3, it was possible to assemble the

barriers in vulnerability classes (Table 4). Within the Ria Formosa system, Western Bar-

reta is the only barrier with extreme vulnerability; Cacela and Cabanas constitute the high

vulnerability group; Eastern Culatra, Eastern Armona, and Ancao have medium vulnera-

bility; and Eastern Barreta, Western Culatra, Tavira, and Western Armona comprise the

low vulnerability group.

Given the vulnerability classification produced, and although overwash with relevant

impacts can take place on the majority of the barriers, from a coastal management

viewpoint the results place the centre of concern on Western Barreta, Cacela, and Cabanas.

Western Barreta’s low-lying narrow morphology, gained as a consequence of the sediment

starvation induced by the updrift Ancao Inlet (Vila-Concejo et al. 2006a; Matias et al.

2007), is responsible for the high OSR and maximal MOI25 and CBO2/3 results (Fig. 6). In

the near future, so long as the sediment starvation induced by the Ancao Inlet continues,

Western Barreta should maintain or even increase its vulnerability to overwash. Under

those circumstances, a roll-over mechanism, similar to the one illustrated by Leatherman

(1988), has to be viewed as a likely scenario.

In 1976, Cabanas was an overwash-prone barrier where new inlet formation could take

place at numerous sites. This barrier is classified as highly vulnerable to overwash

(Table 4), although in 2001 washovers were almost absent on that barrier (Figs. 3, 4). This

apparent paradox is resolved by taking into consideration the large-scale beach nourish-

ment and dune restoration that was carried out in the late 1990s (Ramos and Dias 2000;

Dias et al. 2003). The actual condition of the barrier was exposed recently, in 2003, when

the impact of a strong SE storm caused complete barrier overwash and created a new

ephemeral tidal inlet.

In the last few decades, Cacela has experienced significant shoreline retreat and frequent

complete overwash (Matias 2000) resulting in the roll-over of the barrier. Consequently,

dune restoration and beach nourishment were carried out on this barrier in the late 1990s.

Sand nourishment over pre-existing washovers may have masked the real vulnerability of

the barrier (Figs. 3, 4). As with Cabanas, Cacela also experienced the formation of a new

tidal inlet as a result of the same 2003 SE storm.

The cases of Cabanas and Cacela appear to indicate the need to examine washover

histories when dealing with vulnerability evaluations along highly dynamic coasts. The

most important washover formation mechanism in the Ria Formosa system is tidal inlet

processes (Matias et al. 2008), and 45% of the system length corresponds to high tidal inlet

hazard areas (Vila-Concejo et al. 2006b). Evaluations of overwash vulnerability must

therefore cover the time-scales at which tidal inlet migration takes place locally. Diag-

nostics based on short-term observations are therefore not sufficient for the Ria Formosa

barriers and similar environments.

So long as sediment supply and wave climate are fairly constant, the updrift areas of

tidal inlets within the study area should continue to experience sediment accretion and

consequent shoreline advance. Such a scenario is likely to be maintained for the eastern tip

240 Nat Hazards (2010) 54:225–244

123

of Ancao, for Eastern Barreta, for Eastern Culatra, and for the eastern tip of Tavira.

Overwash vulnerability for these barriers should decrease in the future. Conversely, where

sediment is lacking and the shoreline is consequently retreating, it is probable that

overwash will become more frequent with potentially greater magnitude in the future.

This scenario is likely for the remaining parts of the system, with the exception of

Western Armona. Significant changes are not expected for this barrier as the ebb delta of

the Armona Inlet provides some shelter from the dominant waves and assures an addi-

tional sediment supply to that sector, as described by Andrade (1990b). It should be noted

that this system consists of a number of diverse processes, both natural and anthropo-

genic, which control washover morphology and persistence (see Matias et al. 2008;

Matias 2006). Future human actions can, however, contribute to changes on the above

expected scenarios leading either to a smaller number of washovers (e.g. beach nour-

ishment, dune recovery) or to an increase on overwash (e.g. inlet relocation, jetties

realignment or construction).

The methods employed may not be completely applicable to densely occupied areas as

they are based on the photographic interpretation of sandy surfaces. The Ria Formosa

barriers do contain such occupied areas, which in most cases are located near, or within,

zones designated as vulnerable/hazardous. Such is the case of Praia de Faro (Ancao),

Praia do Farol (Western Culatra), Praia da Fuzeta (Eastern Armona), and Praia do Barril

(Tavira). An assessment of the risk associated with these developed areas is beyond the

scope of this study. However, the vulnerability evaluation presented here could be

employed as part of an exercise concerning integrated overwash risk assessment which

should also include components such as social and economic vulnerabilities.

Andrade (1990a) employed a single topo-hydrographic survey to define overwash

vulnerability, complete overwash vulnerability, and most likely washover type for each

barrier of the Ria Formosa. Although there is agreement concerning the general diagnostic

of overwash vulnerability, there are discrepancies between the results of that study and

ours regarding complete overwash vulnerability. According to Andrade, along Eastern

Culatra and Tavira there are areas where complete overwash is likely to occur, while our

results indicate that the hazard is unlikely to take place in those areas. The results of

Andrade (1990a) refer only to the immediate post-1976 period, and need to be placed in

the context of the longer-term decline in overwash activity as revealed from the aerial

photographic record and indices presented here. During the period of this study, from

1947 to 2001, overwash evolved from a dramatic initial situation to a stage of fewer

occurrences.

The results obtained have allowed the magnitude limits of overwash occurrence for the

Ria Formosa barriers to be constrained and have also provided reliable indications as to

where overwash is most likely to occur in the future. The current situation of the barrier

system is generally more robust than in the past as a consequence of natural and human

actions. However, changes in the robustness of the barriers (caused by variations in sed-

iment supply, vegetation development, and human intervention and occupation), and/or in

the storminess of the region, may require adjustments in the methodology proposed and the

acquisition of extra data in the future. Future overwash occurrence will allow the methods

presented to be validated and improved, by comparing predictions to monitored overwash

activity. In that respect, the forecasts made have been in agreement with in situ obser-

vations of overwash activity in the period 2001–2008, as barrier breaching has occurred

along the barriers that were classified as the most vulnerable, namely Western Barreta,

Cabanas, and Cacela.

Nat Hazards (2010) 54:225–244 241

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6 Conclusion

This study has presented three overwash vulnerability indices, formulated using measures

of washover morphology and distribution, and has applied them to the Ria Formosa barrier

system in Southern Portugal. The indices have been integrated to allow the construction of

an overwash vulnerability diagnostic that incorporates both along- and cross-shore over-

wash expressions. Being based on long-term observations, the indices are not only able to

track aspects of barrier washover evolution but are also of predictive value providing past/

present forcing mechanisms and modes of general barrier evolution remain relatively

constant in the future. Overwash vulnerability in the Ria Formosa ranged from extreme

(Barreta Island) to low (e.g. Tavira Island). Extreme vulnerability resulted from a com-

bination of a maximum extension of overwashed shoreline of 55%; a 25-year return

interval maximum overwash intrusion of about 200 m; and overwash intrudes more than

2/3 of the barrier width along 80% of the barrier. The areas of the barrier system in which

there is dense development were diagnosed to have low or medium vulnerability to

overwash. The results obtained for such locations require a caution interpretation as the

methodology presented was developed, and is more reliable, for sandy surfaces rather than

for developed areas in which direct evidence of overwash is generally removed by humans

or can be otherwise obscured.

The results obtained can ultimately be employed as environmental indicators for

establishing guidelines for coastal management actions. The applicability of the indices for

that purpose lies in identifying the areas where overwash is most likely to lead to barrier

breaching, new inlet formation, infrastructure damage, and danger to humans. The results

are also important for predicting where barrier roll-over mechanisms are expected to occur

or for delimiting set-back lines for human activities and developments.

Acknowledgments This study arose within the scope of the Portuguese project PRIMO (Prediction andImpacts of Runup and Overwashes) under the contract POCTI/CTA/39849/2001. Ana Matias was finan-cially supported by the Fundacao para a Ciencia e a Tecnologia, grant reference SFRH/BPD/18476/2004;and by the Centro de Investigacao Marinha e Ambiental da Universidade do Algarve, grant reference CIMA/GT1-01/2005. This work is also a contribution of the network TANGO (Red Iberoamericana en tele-deteccion aplicada a la prevencion de riesgos geologicos litorales). The authors thank the reviewers for theirconstructive comments.

References

Andrade C (1990a) Estudo da susceptibilidade ao galgamento da Ria Formosa. Geolis, vol IV, Fasc. 1 e 2,69–76 (in Portuguese)

Andrade C (1990b) O ambiente de barreira da Ria Formosa, Algarve-Portugal. Tese de Doutoramento.Universidade de Lisboa. 627 p (in Portuguese)

Andrade C, Barata A, Teles M (1998) An analysis of the vulnerability to overwash of the Ria FormosaBarrier System (Portugal) using a simple multi-attribute rating technique (SMART) approach. In:proceedings of the 5th international conference littoral 1998, 511–519

Carrasco A, Ferreira O, Matias A, Dias J (2007) Backbarrier evolution and complete overwash occurrence.CD-ROM of the Coastal Sediments ‘07, ASCE, 13 p

Costa M, Silva R, Vitorino J (2001) Contribuicao para o estudo do clima de agitacao matıtima na costaPortuguesa. CD-ROM of the 2as Jornadas Portuguesas de Engenharia Costeira e Portuaria, PIANC, 20p (in Portuguese)

Dias J (1988) Aspectos geologicos do Litoral Algarvio. Geonovas 10:113–128 (in Portuguese)Dias J, Ferreira O, Matias A, Vila-Concejo A, Sa-Pires C (2003) Evaluation of soft protection techniques in

barrier islands by monitoring programs: case studies from Ria Formosa (Algarve-Portugal). J CoastRes SI 35:117–131

242 Nat Hazards (2010) 54:225–244

123

Esaguy A (1986) Ria de Faro. Ilha de Tavira. Evolucao. Direccao Geral de Portos, 8 p (in Portuguese)Ferreira O, Garcia T, Matias A, Taborda R, Dias J (2006) Integrated method for representation of set-back

lines for coastal erosion hazards at sandy shores. Cont Shelf Res 26:1030–1044Fisher J, Leatherman S, Perry F (1974) Overwash processes on Assateague Island. In: proceedings of the

14th International Conference on Coastal Engineering, ASCE, pp 1194–1212Fletcher C, Richmond B, Barnes G, Schroeder T (1995) Marine flooding on the coast of Kaua’i during

Hurricane Iniki: hindcasting inundation components and delineating washover. J Coast Res 11(1):188–204

Garcia T, Ferreira O, Matias A, Dias J (2002) Recent evolution of Culatra Island (Algarve, Portugal). In:proceedings of the 6th International Conference Littoral 2002, Eurocoast-Portugal Association (2)289–294

Hansen M, Sallenger A (2007) Barrier island vulnerability to breaching: a case stuffy on Dauphin Island,Alabama. CD-ROM of the Coastal Sediments ‘07, ASCE, 9 p

Hequette A, Ruz M (1991) Spit and barrier island migration in the Southeastern Canadian Beaufort Sea.J Coast Res 7(3):677–698

Leatherman SP (1977) Overwash hydraulics and sediment transport. Coastal Sediments ‘77, pp 135–148Leatherman SP (1988) Barrier Island handbook. Maryland Coastal Publications Series, Laboratory for

Coastal Research, University of Maryland, Baltimore, 92 pMatias A (2000) Estudo morfosedimentar da Penınsula de Cacela. MSc thesis, Universidade do Algarve 245

p (in Portuguese)Matias A (2006) Overwash sedimentary dynamics in the Ria Formosa barrier islands. Ph.D. thesis,

Universidade do AlgarveMatias A, Ferreira O, Dias J, Vila-Concejo A (2004) Development of indices for the evaluation of dune

recovery techniques. Coast Eng 51:261–276Matias A, Ferreira O, Mendes I, Dias J, Vila-Concejo A (2005) Artificial construction of dunes in the south

of Portugal. J Coast Res 21(3):472–481Matias A, Vila-Concejo A, Ferreira O, Morris B, Dias J (2007) Sediment transport patterns during overwash.

CD-ROM of the Coastal Sediments ‘07, ASCE, 13 pMatias A, Ferreira O, Vila-Concejo A, Garcia T, Dias J (2008) Classification of washover dynamics in

barrier islands. Geomorphology 97:655–674Morton R, Sallenger A (2003) Morphological impacts of extreme storms on sandy beaches and barriers.

J Coast Res 36(3):741–752Morton R, Gonzalez J, Lopez G, Correa I (2000) Frequent non-storm washover of Barrier Islands, Pacific

Coast of Colombia. J Coast Res 16(1):82–87Morton R, Guy K, Hill H, Pascoe T (2003) Regional morphological responses to the March 1962 ash

Wednesday storm. In: proceedings of the international conference on coastal sediments ‘03. CD-ROMpublished by World Scientific Press and East Meets West Productions, Corpus Christi, TX, 11 pp

Nordstrom K, Jackson N (1995) Temporal scales of landscape change following storms on a human-alteredcoast, New Jersey, USA. J Coastal Conserv 1:51–62

Pacheco A, Garcia T, Vila-Concejo A, Matias A, Ferreira O, Dias J (2004) Quantitative analysis of theevolution of a stabilized tidal inlet and the adjacent shoreline. A regional approach. 29th InternationalConference on Coastal Engineering, ASCE, paper no. 118

Pilkey O, Neal W, Monteiro J, Dias J (1989) Algarve barrier islands: a noncoastal-plain system in Portugal.J Coast Res 5(2):239–261

Ramos L, Dias J (2000) Atenuacao da vulnerabilidade a galgamentos oceanicos no sistema da Ria Formosamediante intervencoes suaves. In: proceedings of the 3rd Symposium on the Iberian Atlantic Margin,Faro, Portugal, pp 361–362 (in Portuguese)

Ritchie W, Penland S (1988) Rapid dune changes associated with overwash processes on the deltaic coast ofSouth Louisiana. Mar Geo 81:97–122

Rosati J, Stone G (2007) Critical width of barrier islands and implications for engineering design. CD-ROMof the Coastal Sediments ‘07, ASCE, 14 p

Schwartz R (1975) Nature and genesis of some storm washover deposits. Technical Memorandum 61,Coastal Engineering Research Center, 69 pp

Shepard F (1973) Submarine geology, 3rd edn. Harper and Row Publishers, New York, 517 ppTeixeira S (2006) Slope mass movements on rocky sea-cliffs: a power-law distributed natural hazard on the

Barlavento Coast, Algarve, Portugal. Cont Shelf Res 26:1077–1091USACE (2004) Coastal overwash. Part 1: Overview of Processes. Regional Sediment Management (RSM)

Demonstration Program Technical Note, 36 ppVila-Concejo A, Matias A, Ferreira O, Duarte C, Dias J (2002) Recent evolution of the natural inlets of a

barrier island system in Southern Portugal. J Coast Res, SI 36:745–752

Nat Hazards (2010) 54:225–244 243

123

Vila-Concejo A, Matias A, Ferreira O, Dias J (2006a) Inlet sediment bypassing to a downdrift washoverplain. J Coast Res, SI 39:401–405

Vila-Concejo A, Matias A, Pacheco A, Ferreira O, Dias J (2006b) Quantification of inlet-related hazards inbarrier island systems. An example from the Ria Formosa, Portugal. Cont Shelf Res 26:1045–1060

Weinholtz M (1964) Contribuicao para o estudo da evolucao das flechas de areia na costa sotavento doAlgarve (Ria de Faro). Internal Report, Direccao-Geral de Servicos Hidraulicos, Portugal (inPortuguese)

244 Nat Hazards (2010) 54:225–244

123