†,
‡
†
†Faculty of Geosciences ‡ Forschungszentrum Küste.
University of Szczecin
70-383, Poland. Email: geodabsi@gmail.com
Merkurstraße 11, 30419 Hannover,
Deutschland. Email: vousdoukas@gmail.com
ABSTRACT
†Faculty of Geosciences
University of Szczecin
70-383, Poland. Email: teref@univ.szczecin.pl
Wziatek, D., Vousdoukas, M.V. and Terefenko, P., 2011. Wave-cut notches along the Algarve coast, S. Portugal:
Characteristics and formation mechanisms. Journal of Coastal Research, SI 64 (Proceedings of the 11th
International Coastal Symposium), – . Szczecin, Poland, ISSN 0749-0208
Marine notches are undercut or groove forms, developing in vertical cliff profiles due to sea corrosion. The present contribution aims to study the spatial distribution and characteristics of notches on the Algarve rocky coast, combining in situ observations, rock sampling, laboratory analyses of chemical resistance and along-shore wave-power flux estimations. A total of 244 tidal notches was identified in 169 cliff profiles, along a 28-km coastal stretch. Tidal notches were more common in sheltered cliffs, while surf notches prevailed in exposed and moderately exposed areas. 58% of the tidal notches were U-shaped , and were linked to energetic wave conditions, which also favoured the formation of single notches. 40% of all tidal notches were V-shaped, mostly found on sheltered sections of the coast. The W combination was found to be dominant, being more common in sheltered cliff sides, while combinations of surf notches appeared in 84% of all cases, linked to increased wave action. The spatial distribution of tidal notches appeared to follow vertical variation of chemical resistance; however, that trend was weaker for the most energetic sections of the coastline, where wave forcing appeared to dominate. Similarly, the effect of chemical resistance was less prominent for abrasion notches; while, for notches forming above abrasion platforms, CaCO
3
absorption from fresh water was also an acting mechanism.
ADDITIONAL INDEX WORDS: marine notches, rocky shoreline, Algarve coast
Marine notches are undercut or groove forms that develop in vertical cliff-face profiles as a result of sea corrosion (Pirazzoli,
1986) and are often used as an indicator for the present and past mean sea-level position (Benac, Juracic and Bakran-Petricioli,
2004; Kershaw and Guo, 2001).
Marine notches can be divided into 4 genetic types according to their development (Pirazzoli, 1986): 1) tidal notches, appearing in the intertidal zone as a result of sea corrosion; 2) surf notches, formed above high tide level; 3) abrasion notches, generated by mechanical action of sand and pebbles entrained by waves and usually occurring at the boundary between hard cliffs and loose sediments; 4) structural notches, related to the presence of weaker lithological layers favoring local sea erosion or weathering processes (Kershaw and Guo, 2001).
Pirazzoli (1986) classified two basic shapes of tidal notches: Ushaped and V-shaped . Both shapes are symmetrical with respect to retreating point/zone which is related to the mean sea level (MSL).
The main difference between the V- and U-shaped notches is that the former have a very distinct retreating point; while the latter ones are characterized by retreat along a wider area of the cliff.
Given the complexity and spatio-temporal variability of sea levels and wave forcing, as well as other physical, chemical and biological processes (Benac, Juracic and Bakran-Petricioli, 2004;
Moura et al.
, 2006; Pirazzoli, 1986), notches tend to be complex formations that are often difficult to classify.
While Moura et al . (2006) reported the occurrence marine notches on the Algarve coast, the majority of the literature from this region concerns the Mediterranean coast, which is an environment characterized by mild wave and micro-tidal conditions. The present contribution is the first effort to study the spatial distribution and characteristics of notches on the Algarve rocky coast and, according to our knowledge, on the west Atlantic coast in general.
The study area is located in the central part of the Algarve region, southern Portugal, covering a coastline stretch of 28 km and extending between the cities of Galé and Portimão (Figure
1b). This meso-tidal area is part of the Lagos-Portimão Formation
(LP), consisting of limestone, marls and calcarenites and various karst phenomena (Antunes et al ., 1981). Coastal paleokarsts have been fossilized by detrital sediments from the Pliocene and
Pleistocene. Bioclastic limestones and sandstones of Lagos-
Portimão Formation are the most evident Miocene units in the western Algarve, overlying Carboniferous, Jurassic and
Cretaceous units. In some places a very clear angular unconformity can be observed (Pais et al ., 2000). Several morphological features of varied scale, such as coastal cliffs, shore platforms, zeta bays, pocket beaches, marine caves, marine notches, exist in the study area (Moura et al.
, 2006).
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Wave cut notches at the Algarve coast
Wave power P at each position along each notch was equal to
P=E·C g
·cos
θ exp
; where E is wave energy E =0.125·
ρ
·g·H 2 and
θ exp is the wave incidence angle on the notch, considering its local orientation. Thus integration of P for the alongshore notch length provided the total power, while division by length resulted in the total power per notch length . The latter was considered to be the final indicator of exposure to wave forcing.
The range of the estimated values was separated into 5 distinct classes according to Jenks’ Natural Breaks Optimization (Jenks,
1967) and notches were classified accordingly (see also Table 1).
Calculations were made for each notch separately in batch mode, using specially developed MATLAB scripts.
Figure 1. a) Map of southern Portugal showing the location of the study area as well as the location of the Portuguese Hydrographic
Institute’s wave buoy ; b) Satellite image of the study area showing the locations of the sampled profiles
Wave and tide data for the period from May 1995 until the present were obtained from a wave buoy deployed by the
Portuguese Hydrographic Institute ( www.hidrografico.pt
), south of the city of Faro. The data were used to derive a set of representative annual conditions, as described below.
In situ notch identification/classification took place during field surveys (September 2009–June 2010) at low tide, for optimal notch visibility from land and/or from sea when direct access was not possible. Locations were recorded using a Differential GPS and the 73 datum. When notches were inaccessible, locations were derived from 1:25,000 scale topographic maps (Portuguese
Military Maps) and aerial photos at 1:10000 scale. Notch dimensions (vertical elevation/extent anddepth), were measured using digital laser rangefinder equipment. All information from the field surveys as well as the sample processing (to follow) was collected and analyzed in an ArcGIS database.
Since the wave measurements were obtained offshore at a depth of around 120 m, Snell’s Law was used to obtain wave height and direction estimates close to the coast (US Army Corps of
Engineers, 2002): sin
C
sin
C
0
0
1 where 0 denotes off-shore conditions,
θ corresponds to the wave angle and C to the wave celerity C = L / T ; where T is the wave period and L is the wavelength expressed by the wave dispersion equation (US Army Corps of Engineers, 2002). While Snell’s Law provides the wave direction at a given depth (at the present case d =10 m), the wave height is given by the equation H=H where K s
and respectively:
K r
0
·K s
·K r
;
are the shoaling and refraction coefficients, where C g
K r

1


1

sin 2 sin
( )
2
0




1 / 4
K s
C g 0 is the group velocity, equal to C n =
C

0 .
5

1


+
4 π d
L sinh 4 π d
L



 g
=n·C with n equal to:
2
3
The average wave conditions for all the data acquisition period and the average coastline orientation of the study area (113 o ) were considered in all the wave refraction estimations.
Based on the wave incidence angle at the coast, the exposure angle and percentage of exposed notch length were estimated.
Figure 2. Basic notch shapes (a) and notch combinations (b), HT: high tide, LT: low tide, R point: retreating point, R zone: retreating zone. Classification follows Pizzaroli (1986).
Notch classification was based on the definition by Pirazzoli
(1986). Wave-cut notches were distinguished from caves based on a threshold notch depth of 2.5 meters. Only active tidal, surf and abrasion notches have been classified. Notches were considered as active if they were present between the spring low tide and the upper limit of the spray zone (see also Table 2).
Table 1 Notch classification on the grounds of the estimated wave power per notch length P/
λ
.
Very exposed
Exposed
Moderately exposed
Moderately sheltered
Sheltered
P/
λ
(W/m)
2934-
3783
2065-
2934
1244-2065 441-1244 0-441
Notches were classified to genetic type according to three factors (Table 2): a) elevation relatively to the low and high tide; b) shape; and c) texture (see also Pirazzoli, 1986). Since Pirazzoli
(1986) introduced elevation limits suitable for micro-tidal conditions, we adapted these for the Algarve coast (Table 2).
Finally, the hook-shape surf notch is term we have adopted to cover a range of the present observations. Hook-shape notches were found a few meters above high tide (see Table 2), were active and were formed by wave-induced seaspray. Apart from their inclined, skewed parabola shapes, hook-shape surf notches
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Wziatek et al . are characterized by a distinguishable roof, consisting of a more resistant rock layer.
Table 2. Notch genesis classification criteria
Elevation
Shape
Tidal
4 m above low tide level
Relatively symmetrical U,
V, or ripple
Notch type
Surf
1.5 m above high tide level
Asymmetrical or hook-like shape
Abrasion
-
Symmetrical common combination was W -shape, followed by 2-step and 3-step combinations, including one surf notch.
The total number of surf notches was 38, most of them (84%) occurring in combination with tidal notches, either 2-step (59%), or 3-step (41%). Only 6% of all developed surf notches occurred as single notch in a vertical cliff profile. The total number of abrasion notches observed was 76, all of them occurring as single active notches on the sides of cliffs bordering sandy beaches.
Texture Porous - Smooth
Tidal notches were classified as U, V or rippleshaped (Figure
2). In each case the deeper cut point or zone was found around the
MSL, while its position also related to the resistance of the cliff layers. Ripple notches can also be considered as a combination of small notches (Pirazzoli, 1986), but were classified as a shape when a) the vertical dimensions and the spacing between individual ripples was below 20 cm and b) more than 3 individual notches of these dimensions were present along a vertical cliff profile (Figure 2).
When more than two distinct tidal or surf notches occurred in the same cliff profile, they were classified as a notch combination, according to the following 4 categories: 1) W : combination of two tidal notches (Pirazzoli,1986), 2) 3-W -combination of three tidal notches, 3) 2-step : combination of tidal notch and surf notch in a upper part (Moura et al ., 2006), 4) 3-step : combination of two tidal notches and surf notch (Figure 2).
Figure 3. Percentage of notches on the Algarve coast that are classified as a) tidal notches or b) surf notches; Occurrence % of the basic notch shapes classified as V, U and ripple (c) and notch combinations (d) among the various exposure classes describe as:
1; Very exposed 2; Exposed 3; Moderately exposed 4; Moderately sheltered 5; Sheltered
In order to obtain information on the chemical resistance of the notch formations in the study area, 27 rock samples were collected from eight vertical cliff profiles (Figure 1). Samples were collected from the lithological layers of notches, benches, platforms and vertical cliffs, and their CaCO
3
content was used as a proxy for chemical resistance (Leonitjew, Nikiforow and
Safjanow, 1982). Samples were analyzed in a Carlo Erba CN
Elemental Analyzer.
Analysis of the 14-year time-series of wave data showed that the wave climate is characterized by prevailing smooth and moderate sea states and by the occurrence of two different types of storms: from the southwest (70% of storm occurrences) and from the southeast (30% of storm occurrences). The largest waves are generated by deep low-pressure systems and come from the SW, which is also the direction of the average wave conditions, characterized by H s,mean
=1.5 m and T p,mean
=6.6 s. The tidal regime can be described as semidiurnal and meso-tidal with neap tides ranging from 1.36 m to 2.70 m and with spring tides from 0.64 m to 3.82 m.
A total of 244 tidal notches were identified in the study area on
169 cliff profiles, with U-shaped notches predominant (58%). The remainder were mostly V-shaped (40%) and rippleshaped (2%).
Most (69%) of the tidal notches were part of combinations and
31% were single formations on vertical cliff profiles. The most
Tidal notches occurred mostly in sheltered cliffs (32%), but there was also a significant number of formations on the exposed sides (25%). Of all the tidal notches, 16% were classified as ‘very exposed’ and ‘moderately exposed’ (see Table 1 for definitions and Figure 3a). Most surf notches (34%) were classified as
‘moderately exposed’, followed by 29% occurrence of ‘very exposed’, corresponding to 21% of the total occurrences classified as ‘exposed’ (Figure 3b). The maximum estimated wave power per notch length was around 3783 W/m, while the average value for all occurrences was 1561 W/m. It is interesting that values were estimated considering the dominant wave direction (SW), and, as a result, 21 eastward-facing notches produced zero values.
For each of the exposure classes, the majority of the notches were U-shaped , apart from the ‘moderately sheltered’ class for which the V-shape was more frequent (Figure 3c). The results show a connection between the formation of U-shaped notches and wave forcing, with most of the U-shaped notches classified as
‘very exposed’. Within the notches in intermediate exposure classes, the number of V-shaped notches was higher. Since only 6 ripple-shape notches have been identified, a link with exposure to wave power was not possible.
W notch combinations were found to be mostly ‘exposed’,
‘moderately exposed’ and ‘sheltered’, while the sole W-3 combination was classified as ‘sheltered’ (Figure 3d). The results show that increased wave forcing favors the formation of 2-step combinations, even though there are some exceptions, such as the identification of few ‘moderately exposed’ occurrences. 84% of 2step combinations were subject to important wave forcing (very exposed, exposed, moderately exposed). Similarly, the occurrence of 3-step combinations was found to be favored by increasing exposure to wave power and 77% of them were classified as
‘relatively exposed’.
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Wave cut notches at the Algarve coast
The average CaCO
3
content for all vertical cliff profile samples
(79.8%) indicated relatively low chemical resistance, with values tending to increase eastwards, from 58.2% to 81% (Figure 4). The cliff profile with the highest chemical resistance was Albandeira beach, with a CaCO
3
content up to 92%. The lowest value was measured at Carvoeiro beach (58.3%). Among the sampled profiles, four were located in sheltered cliff sides, while the other four profiles were classified as ‘exposed’ (Figure 4). sheltered areas. Pirazzoli (1986) and Moura et al . (2006) concluded that the size of the notches grows with exposure to wave power, and our findings suggest that intense wave action acting along the entire intertidal cliff section can curve the profile and even merge distinct formations into a single, larger tidal notch.
Figure 4. Average CaCO
3 along the Algarve coast.
content (%) in vertical cliff profiles
Given that most previous studies on wave-cut notches report findings from micro-tidal, moderate-energy areas like the
Mediterranean Sea (Andriani and Walsh, 2007; Benac, Juracic and
Bakran-Petricioli, 2004; De Waele, Mucedda and Montanaro,
2009; Kershaw and Guo, 2001; Nixon, Reinhardt and Rothaus,
2009), several existing terms and definitions had to be adapted to the conditions of the meso-tidal, high-energy Algarve coast. For example, Pirazzoli (1986) considered ripple and W notches to be shapes resulting from changes in the relative elevation of the MSL and the cliff. In the present study, these formations have been considered as combinations of different notches, introducing the new term ‘ W -combination’, since the occurrence of two distinct active notches in the same cliff profile is frequently observed along the Algarve coast (6a,c). Similarly, the hook-shape surf notch is term adopted to describe a particular notch morphology, which was frequently observed and distinct from the other formations.
Notches forming such combinations were found at the low and high tide mark, respectively, and their occurrence can be justified by the fact that, over a tidal cycle, the water level has increased
‘residence time’ around the low and high tide levels. As a result, the average wave forcing along the cliff profile peaks at those extremes and results in local retreat of the cliff. The dimensions of the separation zone was found to be related to a) vertical differences in rock resistance; and b) the overall intensity of wave forcing, which in some cases tended to merge the two notches.
Another factor for further investigation is bio-accretion, which may favor notch separation.
The present observations highlight mechanical wave forcing as one of the most important formation factors overall, in agreement with some previous studies (Pirazzoli 1986, Kershaw and Guo,
2001). Most of the single notches were found in cliffs subject to intense wave forcing, while they were less frequent in more
Figure 5. Aerial photographs indicating the location and exposure
(see color scale) of notches around several Algarve beaches between Galé and Carvoeiro.
On the other hand, chemical resistance appeared to be equally important, since in six of the eight analyzed profiles (Galé, Galé platforms, Sofitel, Tectonic, Albandeira, Marinha 2 Figure 5), notch occurrence was found occur where chemical resistance was reduced (Figure 6c). That was not the case in two of the analyzed profiles (Marinha 1 and Carvoeiro beach), where notches also occur in layers with the greatest chemical resistance (Figure 6a,b).
These two profiles have distinct characteristics and will be discussed separately below.
The notch formation at Carvailo beach showed alongshore variations in shape, orientation and exposure to wave action. On exposed cliff sides, V-shaped notches appeared at the toe of the cliff, right above the abrasion platform; while, in the most exposed areas, a surf notch was also found slightly above this, creating a 2step combination. While one would expect that an abrasion platform should have a higher chemical resistance than a layer containing notches, the results showed that the latter had ~13% less CaCO
3
than the platform and 20% more than the bench occurring above the notch. In that case, the formation could be attributed either to local variations of the wave power or to more acidic fresh water accumulating at the surface of the platform.
The profile at Marinha 1 Beach (Figure 6b) included an abrasion notch at the toe of a stack, which was located around 7 m shoreward of the MSL shoreline. Rock samples from the notch and the layer above had moderate and very similar chemical
Journal of Coastal Research , Special Issue 64, 2011
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Wziatek et al . resistance ( CaCO
3
content around 70%), which implies that mechanical action was the main factor driving notch formation.
In the present study increased CaCO
3
content has been used as a proxy of reduced chemical resistance. Gabriel et al.
(2009) have shown that, at the same time, higher CaCO
3
content can imply increased mechanical resistance. This can be an important factor on the wave-dominated Algarve coast, where mechanical erosion appears to be the predominant factor. Detailed assessment of mechnical resistance in the study area thus constitutes an important point for further research. of surf notches ( 2-step and 3-step ) were also very frequent, with
84% of all occurrences found to be very exposed to wave power.
The spatial distribution of the tidal notches appeared to follow the vertical variation of chemical resistance. However, that trend was weaker along the highest energy sections of the coastline, where wave forcing appeared to dominate. Similarly, the effect of chemical resistance was less prominent for abrasion notches, for which mechanical factors were again more important. Finally, tidal notches forming above abrasion platforms showed distinct behavior that could be linked to the presence of fresh water, with higher CaCO
3
absorption.
The fact that the cliffs of the study area are characterized by significant spatial variations in composition and structure constitutes an important difference from the majority of related literature and poses additional difficulties for the precise assessment of dominant formation factors.
Figure 6. Examples of cliff profiles where the notch occurrence does not correspond to vertical variations in chemical resistance: a) Carvailo beach, b) Marinha 1 beach and Tectonic beach c), where the opposite pattern is observed.
The present study comprises the first effort to study in detail the wave-cut notches of the Algarve coast, and one of the few studies on such formations in general. As a result, the present findings introduce questions for further research and suggest some improvements in terms of research approach. Apart from the difficulty inherent in classifying notch shapes and combinations, due to often vague definitions, future research should aim to deal with problems such as dating the material, and provide more elaborate notch shape definitions using innovative techniques like
LIDAR or stereo-vision. Such additional information would be invaluable for a better understanding of the dominant formation mechanisms and factors. Moreover, the accuracy of estimated wave power fluxes along the coastline could be further improved.
While all the above are part of plans for future research, the present findings constitute a significant improvement in our understanding of the characteristics and development processes of wave-cut notches in the area and in meso- to macro-tidal areas in general.
Extensive information about the shapes, locations, chemical resistance and wave forcing of wave-cut notches of the southern
Algarve coast was collected and analyzed for the first time. The study discusses observations from a total of 244 tidal notches, identified at 169 cliff profiles, found along a 28 km coastal stretch.
The results highlight mechanical wave action as an important erosional factor. Even though most notch shapes and combinations were found to be related to various levels of exposure to wave forcing, a trend of increasing occurrence with greater wave power was discerned. Tidal notches were more common in sheltered cliffs, while surf notches occurred more in exposed and moderately exposed areas.
The majority (58%) of the tidal notches were U-shaped , and were found to be favored by higher energy wave conditions.
Increased exposure to wave power was favored the formation of single notches. V-shaped notches constituted 40% of all tidal notches and were more common on sheltered sections of the coast.
In terms of combinations, W-shaped notches were the most prevalent, especially on the sheltered sides of cliffs. Combinations
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