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Distribution patterns of rip frequency and intensity in Algoa Bay, South Africa
Marine Geology, 76 (1987) 319-324 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
319
Letter Section
DISTRIBUTION PATTERNS OF RIP FREQUENCY AND INTENSITY IN ALGOA BAY, SOUTH AFRICA M.M.B. T A L B O T and G.C. B A T E Department of Botany, Institute for Coastal Research, University of Port Elizabeth, P.O. Box 1600, Port Elizabeth (South Africa)
(ReceivedJuly 7, 1986; Acceptedfor publication January 16, 1987)
Abstract Talbot, M.M.B. and Bate, G.C., 1987.Distribution patterns of rip frequencyand intensity in AlgoaBay, South Africa. Mar. Geol., 76: 319-324. A new technique of rip-current recording has been employed to provide quantitative data on rip frequency and intensity during twelve aerial surveys along the shores of Algoa Bay, South Africa. Distinct alongshore gradients in surfzone width and beach morphodynamic state were recorded.These gradients, themselves related to the geography of the bay, wind direction and the angle of deep-sea swells, result in a gradient of rip frequency and intensity in the bay. Rip currents always appear to be present along the Sundays River Beach (central and northern shores ), but t h e i r presence along the southwest shore was restricted to easterly wind-wave conditions. Both rip frequencyand intensity were positively and significantly correlated with surfzone width and beach state over the range of sea conditions encountered. Beach state accounted for 62% of the alongshore variability in rip frequencyand 35% of the variability in intensity.
Introduction T h e work t h a t has been carried out on t he distribution p a t t e r n s of rip currents has generally inferred the presence of rip currents from topographical features such as rip channels and low-tide e m b a y m e n t s ( H u n t e r et al., 1979; Short, 1985). While such data are useful for u n d e r s t a n d i n g the mechanics of rip currents, they have little application in quantifying the role of rip currents in o n s h o r e - o f f s h o r e exchanges of water across the breaker line. This is because such recordings do not take into account the pulsing n a t u r e of rip currents. Further, rip channels do not necessarily imply the presence of active rip systems.
0025-3227/87/$03.50
T h e study reported here concerns the first phase of an a t t e m p t to quantify the advective processes within a beach/ surfzone ecosystem, involving the determination of the temporal and spatial frequency of rip currents under a variety of surf conditions. T hi s study differs from previous work on rip-current frequency in t h a t rips were recorded by air and represent a snap-shot image of rip frequency at time of overflight. As a result only actively pulsing rips have been recorded.
The study environment Algoa Bay is the largest and best formed of several logarithmic-spiral bays along the Cape
© 1987 Elsevier Science Publishers B.V.
320
south coast (Bremner, 1983) and has a maximum depth of 49 fathoms ( 73 m) (Harris, 1978; see Fig. 1). Occasional rocky stretches occur between the Swartkops and Sundays rivers, but the shoreline between the Sundays River and Woody Cape is sandy. The topography of Algoa Bay is regular to the 20 m depth contour ( South African Naval Chart 126). The tides are semidiurnal with a mean spring amplitude of 1.6 m, and a maximum amplitude of 2.1 m. The beach is best classified as mesotidal according to the classification of Hayes (1979). The wind climate is directionally bimodal, with southwest and east being the modal vectors (Illenberger, 1986). Deep-water swell characteristics provided by Voluntary Observing Ships (VOS) along the southeast coast of South Africa (compiled by Roussouw, 1984) indicates a dominant southwest component. In summer, swells approaching from the east are also important, but have a percentage occurrence of 10% as compared to 25% for southwesterlies.
between the Swartkops River mouth and Woody Cape during 12 flights carried out at intervals between May 1985 and January 1986. Rip currents in Algoa Bay can best be recorded from the air at altitudes of about 500 m. The heavy silt load of actively pulsing rips and their resultant rip heads allow the observer to clearly establish the presence and location of rip systems which are actively discharging water across the breaker line at time of observation. Such systems are referred to as active rips. These hydrodynamic features were recorded in place of the common practice of recording rip channels (Hunter et al., 1979). In this study a rip channel remained merely a geological feature until it had a recognisable net seaward current (active rip). Furthermore, the area of the rip current and its rip head was measured to yield a semi-quantitative measure of rip intensity. Although rip intensity could not be quantified in terms of velocity and flux it provides a useful variable for comparative analyses, e.g. determining alongshore variability in rip strength.
Methods
Results
Records of rip frequency and intensity were made along the 70 km stretch of coastline
Studies on the alongshore distribution of wave energy in Algoa Bay have been restricted
~
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Fig. 1. Location map of Algoa Bay showing bathymetric features (redrawn from South African Naval Chart No. 126 ).
321
to the shadow region of the Cape Recife peninsula, i.e. the area stretching from Cape Recife to ca. 5 km north of the Swartkops River (Van Wyk et al., 1970). Two pertinent findings were ( a ) that under a south and southwesterly wave approach, the energy distribution showed a pronounced increase from Cape Recife to the Swartkops River mouth, and (b) that the irregular topography of Riy Bank resulted in zones of wave convergence and divergence along the shoreline between Cape Recife and the Swartkops River. Further north similar effects could be expected to arise from the influence of the four islands within the bay, namely Jaheel Is., St. Croix Is., Brandon Is. and Bird Is. Although not directly equatable to wave energy, surfzone width was recorded in this study in order to obtain some indication of the wave energy reaching the shore along Algoa Bay. Two general conclusions arise from this consideration. Under southwesterly wind and wave conditions a distinct increase in surfzone width occurred from southwest to northeast, with a maximum some distance east of the Sundays River mouth. During easterly conditions the surfzone width was at a minimum at the eastern and western extremities of the bay, and the maximum was recorded in the vicinity of the Swartkops River. The gradient in wave energy along the shores of Algoa Bay provided a wide spectrum of environmental (and therefore rip) conditions within the 70 km long stretch. The study area was divided into 10 km strips which produced a total of 84 data points from the 12 flights. Regressing rip condition against both surfzone width and beach states indicated that both the frequency of rip occurrence and their intensity increased with increasing energy. Beach state accounted for 62 and 35% of the observed variance in rip frequency and intensity, respectively. Other factors such as tides, angle of wave incidence and antecedent beach morphology are also expected to play a role in determining rip conditions. There are several other reports of similar increases in rip intensity with rising waves (Cook, 1970; Eliot, 1973; Short, 1985;
Cowell, 1986). In contrast, most work carried out on the spacing of rip currents report a decrease in frequency with increasing energy. This discrepancy arises as a result of differences in the manner in which rip currents were recorded. We believe that the use of the "snapshot" method employed here ensures that only actively pulsing rips (almost at a given moment in time) are counted, and the data set is, therefore, not directly comparable to that provided in the literature. During low- and mediumenergy conditions, where rip channels are numerous (Hino, 1975; Sasaki and Horikawa, 1975), on-the-ground methods of rip-current recording tends to include non-active rip channels as rip currents thereby overestimating rip frequency. The frequency of rip channel were recorded in addition to active rips during two of the 12 runs during this study. The number of channels exceeded active rips by a factor of 2.8 and 2.0. Fully dissipative conditions were not encountered during the 12 runs. Consequently the regressions between rip characteristics and environmental conditions obtained above only hold for reflective and intermediate beach states (or surfzone widths < 300 m). From personal observations, it is expected that the rip frequency and wave energy would deviate from linearity at surfzone widths in excess of 300 m. Under dissipative conditions, Short (1985) reported the presence of large-scale but infrequent offshore currents ( "mega-rips" ). During dissipative conditions the three-dimensional character of the hydrology and morphology is lost and the water movements revert to a horizontally uniform flow with predominantly shoreward flow at the water surface and seaward flow at the bed (Longuet-Higgins, 1983). This model explains the decrease in rip frequency with increasing surfzone width during highenergy conditions. Rip activity appears to be a permanent feature of the hydrology of the Sundays River Beach. This is not the case south of the Swartkops River mouth, were rip occurrence is dependent on the direction of wave approach,
322
being restricted to easterly conditions. Distinct alongshore gradients of rip frequency and intensity were observed. Although not ammenable to statistical treatment, the gradients appeared to be controlled by the prevailing angle of wave incidence and antecedent wind direction. When both wind and waves approached from the east, rip frequency and intensity increased from east to west. Under such conditions maximum frequency and intensity was recorded in the vicinity of the Sundays River mouth (Fig. 2a). Rip activity and intensity were more evenly distributed when the wave train was from the south, or from both south west and east (Fig. 2b). The most pronounced alongshore gradient occurred under southwesterly conditions. Figure 2c illustrates rip frequencies and intensities within Algoa Bay after three days of westerly winds, coinciding with southwesterly deep-sea swells. Under such conditions rips were not active in the southwest corner. First signs of rip activity appeared near the Sundays River. Surfzone width followed a similar pattern to rip frequency. A steady increase in width from the southwest to the northeast, with the maximum occurred width occurring midway between the Sundays River and Woody Cape. In addition to alongshore variability, rip condition was temporally variable. Along the Sundays River Beach the total number of rips varied from a minimum of 31 to a maximum of 173. In terms of total rip activity a forty-fold difference was found between the minimum and maximum values recorded. Rips were generally linked to beach and surfzone topography, being associated with megacusp embayments or rip channels. Transient rips operating across shoaling areas {welded bars) that separate rip channels were occasionally observed. During longshore bartrough and rhythmic bar-and-beach states, two distinct types of rips were found, viz.: (a) "beach-terrace" rips, found localized to the inner breaker zone and discharging into the longshore trough, and (b) "outer-bar" rips which operate across the outer breaker zone,
~401 a 20~
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~ - . .
"/
: oL:L Swartk°ps RT'# i.~
//~/
_
'
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r
b
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I
~ / ~ Swartkops R .~1
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~"
oJ ,-"
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AlgoaBay
E,~abe~ 40., C
1
..j:-/" Sunday;R
_ /
___J /
./.., .
/
//
WoodyCp
,,~r.'r~ Swa~kops
Algoa Bay
por~
Elizabeth..~ ~
Fig. 2. Three examples of alongshore rip distribution, illustrating the difference in gradient under easterly ( a ), mixed (b) and westerly (c) wind-wave regimes. Rip frequency (solid line) and intensity (broken line) along each 10 km stretch of coastline is presented as a percentage of the total value for the 70 km study area. The surfzone width is represented by the number of stylized waves (one wave=50 m). The direction of dominant wave crest is given.
323 discharging into the nearshore. These currents usually operated independently of each other. During one of the exercises "beach-terrace" rips were found to exceed "outer bar" rips by a terrace factor of three. In terms of rip intensity, however, the "outer bar" rips were on average twice as active. There have been several attempts at classifying rip currents. Eliot (1973) recognized two different types of rips depending on their activity, viz., low-energy and high-energy rips. Short (1985), from observing 3513 rips, proposed a classification scheme based on the prevailing morphodynamic character of the beach and whether the beach was accreting or eroding. Three types of rips were suggested; "erosion", "mega", and "accretion" rips. Erosion and accretion rips accompany erosional and accretional stages of beach evolution respectively. Mega-rips are large-scale ( > 1 km) erosional rips. Such rips may result from the interaction between breaking wave-induced alongshore currents and the bedrock headlands in deeply embayed beaches (Cowell, 1986). It is clear from this study that rip currents should also be distinguished on the basis of their position within the surfzone. This is very much the case at the Sundays River Beach where the morphodynamic state is often longshore bar-trough or rhythmic bar-and-beach state and rips may either be found on the beach terrace or the outer bar. The "beach-terrace rips" do not involve any exchange of water between the surfzone and the nearshore since their discharge is restricted to the longshore trough. These rips therefore fail to drain the surfzone. Conversely, "outer-bar" rips have little direct influence on the beach terrace water, but are involved in exchanging material between the longshore trough and the nearshore. These rips were not referred to by Winter (1983) in his theoretical model of surfzone circulation, nor in the rip current classification of Short (1985) who worked in a onebar surfzone. No model t h a t considers the integrity of the surfzone as an ecological entity can ignore these features.
Conclusions The use of an aircraft to survey rip activity offers several key advantages over on-theground techniques. The observer is provided with a clearer idea of the intensity and morphology of the rip and its position with respect to the beach/surfzone topography. It provides a rapid "snap-shot" record of rips along extensive lengths of coastline. In Algoa Bay, for instance, similar runs, but using land-based transport would have taken several hours, thereby rendering the data unsuitable for comparative interpretations. The aerial method provides a unique opportunity to understanding spatial variability in rip condition by removing the meso- and long-term temporal variability in rip activity. Furthermore, the "snap-shot" technique provides some indication of the exchange brought about by rip currents in contrast to the method of counting rip channels. The suitability of this method could be further enhanced by using photogrammetric methods for more quantitative results regarding rip intensity.
Acknowledgements We t h a n k the South African National Committee for Oceanographic Research of the CSIR and the Department of E n v i r o n m e n t Affairs and Tourism for funding this research. We are indebted to Professor A. McLachlan and Dr A.D. Short for critically reading the manuscript.
References Bremner, J.M., 1983. Properties of logarithmic spiral beaches with particular reference to AlgoaBay. In: A. McLachlan and T. Erasmus (Editors), Sandy Beaches as Ecosystems.Junk, The Hague,pp. 97-113. Cook, D.O., 1970. The occurrenceand geologicwork of rip currents of southern California. Mar. Geol.,9:173-186. Cowell,P.J., 1986.Australian"megarip" study.EOS,Trans. Am. Geophys.Union, 67: p.442. Eliot, I.G., 1973. The persistence of rip current patterns on sandy beaches. Proc. 1st Australian Conf. on Coastal Engineering, Inst. Engineers,pp. 29-34.
324 Gunst, R.F. and Mason, R.L., 1980. Regression Analysis and its Application. A Data-Orientated Approach. (Statistics: Textbooks and Monographs, Vol. 34) Dekker, New York, N.Y., 402 pp. Harris, T.F.W., 1978. Review of coastal currents in southern African Waters. S. Afr. Natl. Sci. Progr. Rep. No. 30. Hayes, M.O., 1979. Barrier island morphology as a function of tidal and wave regime. In: S.P. Leatherman (Editor), Barrier Islands. Academic Press, New York, N.Y., pp. 1-27. Hino, M., 1975. Theory of the formation of rip-current and cuspidal coast. Proc. 14th Coastal Eng. Conf., Copenhagen, A.S.C.E., pp. 901-919. Hunter, R.E., Clifton, H.E. and Phillips, L., 1979. Depositional processes, sedimentary structures and predicted vertical sequences in barred nearshore systems, southern Oregon coast. J. Sediment. Petrol., 49: 711-726. Illenberger, W., 1986. Sand budget and sedimentology of the Alexandria coastal dunefield. M.Sc. thesis, Univ. of Port Elizabeth, South Africa.
Longuet-Higgins, M.S., 1983. Wave set-up. Percolation and undertow in the surf zone. Proc. R. Soc. London, Ser. A, 390: 283-291. Roussouw, J., 1984. Review of existing wave data, wave climate and design waves for South African and South West African ( Namibian ) coastal waters. CSIR Rep. C/SEA 8401. Sasaki, T.O. and Horikawa, K., 1975. Nearshore current system on a gently sloping bottom. Coastal Eng. Jpn., 18: 123-142. Short, A.D., 1985. Rip current type, spacing and persistence, Narrabeen Beach, Australia. Mar. Geol., 65: 47-71. Van Wyk, W., Groot, C.N. and Swanepoel, J.C.K., 1970. Algoa Bay Coastal Erosion Investigation Hydraulics Research Unit Report. South African Council for Scientific and Industrial Research. Winter, D.F., 1983. A theoretical model of surf zone circulation and diatom growth. In: A. McLachlan and T. Erasmus (Editors), Sandy Beaches as Ecosystems. Junk, The Hague, pp. 157-167.
319
Letter Section
DISTRIBUTION PATTERNS OF RIP FREQUENCY AND INTENSITY IN ALGOA BAY, SOUTH AFRICA M.M.B. T A L B O T and G.C. B A T E Department of Botany, Institute for Coastal Research, University of Port Elizabeth, P.O. Box 1600, Port Elizabeth (South Africa)
(ReceivedJuly 7, 1986; Acceptedfor publication January 16, 1987)
Abstract Talbot, M.M.B. and Bate, G.C., 1987.Distribution patterns of rip frequencyand intensity in AlgoaBay, South Africa. Mar. Geol., 76: 319-324. A new technique of rip-current recording has been employed to provide quantitative data on rip frequency and intensity during twelve aerial surveys along the shores of Algoa Bay, South Africa. Distinct alongshore gradients in surfzone width and beach morphodynamic state were recorded.These gradients, themselves related to the geography of the bay, wind direction and the angle of deep-sea swells, result in a gradient of rip frequency and intensity in the bay. Rip currents always appear to be present along the Sundays River Beach (central and northern shores ), but t h e i r presence along the southwest shore was restricted to easterly wind-wave conditions. Both rip frequencyand intensity were positively and significantly correlated with surfzone width and beach state over the range of sea conditions encountered. Beach state accounted for 62% of the alongshore variability in rip frequencyand 35% of the variability in intensity.
Introduction T h e work t h a t has been carried out on t he distribution p a t t e r n s of rip currents has generally inferred the presence of rip currents from topographical features such as rip channels and low-tide e m b a y m e n t s ( H u n t e r et al., 1979; Short, 1985). While such data are useful for u n d e r s t a n d i n g the mechanics of rip currents, they have little application in quantifying the role of rip currents in o n s h o r e - o f f s h o r e exchanges of water across the breaker line. This is because such recordings do not take into account the pulsing n a t u r e of rip currents. Further, rip channels do not necessarily imply the presence of active rip systems.
0025-3227/87/$03.50
T h e study reported here concerns the first phase of an a t t e m p t to quantify the advective processes within a beach/ surfzone ecosystem, involving the determination of the temporal and spatial frequency of rip currents under a variety of surf conditions. T hi s study differs from previous work on rip-current frequency in t h a t rips were recorded by air and represent a snap-shot image of rip frequency at time of overflight. As a result only actively pulsing rips have been recorded.
The study environment Algoa Bay is the largest and best formed of several logarithmic-spiral bays along the Cape
© 1987 Elsevier Science Publishers B.V.
320
south coast (Bremner, 1983) and has a maximum depth of 49 fathoms ( 73 m) (Harris, 1978; see Fig. 1). Occasional rocky stretches occur between the Swartkops and Sundays rivers, but the shoreline between the Sundays River and Woody Cape is sandy. The topography of Algoa Bay is regular to the 20 m depth contour ( South African Naval Chart 126). The tides are semidiurnal with a mean spring amplitude of 1.6 m, and a maximum amplitude of 2.1 m. The beach is best classified as mesotidal according to the classification of Hayes (1979). The wind climate is directionally bimodal, with southwest and east being the modal vectors (Illenberger, 1986). Deep-water swell characteristics provided by Voluntary Observing Ships (VOS) along the southeast coast of South Africa (compiled by Roussouw, 1984) indicates a dominant southwest component. In summer, swells approaching from the east are also important, but have a percentage occurrence of 10% as compared to 25% for southwesterlies.
between the Swartkops River mouth and Woody Cape during 12 flights carried out at intervals between May 1985 and January 1986. Rip currents in Algoa Bay can best be recorded from the air at altitudes of about 500 m. The heavy silt load of actively pulsing rips and their resultant rip heads allow the observer to clearly establish the presence and location of rip systems which are actively discharging water across the breaker line at time of observation. Such systems are referred to as active rips. These hydrodynamic features were recorded in place of the common practice of recording rip channels (Hunter et al., 1979). In this study a rip channel remained merely a geological feature until it had a recognisable net seaward current (active rip). Furthermore, the area of the rip current and its rip head was measured to yield a semi-quantitative measure of rip intensity. Although rip intensity could not be quantified in terms of velocity and flux it provides a useful variable for comparative analyses, e.g. determining alongshore variability in rip strength.
Methods
Results
Records of rip frequency and intensity were made along the 70 km stretch of coastline
Studies on the alongshore distribution of wave energy in Algoa Bay have been restricted
~
281
i ......... i
i .....FR,CA 24
°'~i~
l l l a b e28 th
,"
26%
a2E
O~
,~'"
rm 0
---
.
.~'" _ . F"- - "2.'.'.'~.'~~ ""~" = - . . -. ,. . .
:'~ --~:_
fi'
""
ALGOA
;,
BAY
,f.-,,
"'ha°'
""
I
3,°s
....
0
5
10
N
15km
Scale
Fig. 1. Location map of Algoa Bay showing bathymetric features (redrawn from South African Naval Chart No. 126 ).
321
to the shadow region of the Cape Recife peninsula, i.e. the area stretching from Cape Recife to ca. 5 km north of the Swartkops River (Van Wyk et al., 1970). Two pertinent findings were ( a ) that under a south and southwesterly wave approach, the energy distribution showed a pronounced increase from Cape Recife to the Swartkops River mouth, and (b) that the irregular topography of Riy Bank resulted in zones of wave convergence and divergence along the shoreline between Cape Recife and the Swartkops River. Further north similar effects could be expected to arise from the influence of the four islands within the bay, namely Jaheel Is., St. Croix Is., Brandon Is. and Bird Is. Although not directly equatable to wave energy, surfzone width was recorded in this study in order to obtain some indication of the wave energy reaching the shore along Algoa Bay. Two general conclusions arise from this consideration. Under southwesterly wind and wave conditions a distinct increase in surfzone width occurred from southwest to northeast, with a maximum some distance east of the Sundays River mouth. During easterly conditions the surfzone width was at a minimum at the eastern and western extremities of the bay, and the maximum was recorded in the vicinity of the Swartkops River. The gradient in wave energy along the shores of Algoa Bay provided a wide spectrum of environmental (and therefore rip) conditions within the 70 km long stretch. The study area was divided into 10 km strips which produced a total of 84 data points from the 12 flights. Regressing rip condition against both surfzone width and beach states indicated that both the frequency of rip occurrence and their intensity increased with increasing energy. Beach state accounted for 62 and 35% of the observed variance in rip frequency and intensity, respectively. Other factors such as tides, angle of wave incidence and antecedent beach morphology are also expected to play a role in determining rip conditions. There are several other reports of similar increases in rip intensity with rising waves (Cook, 1970; Eliot, 1973; Short, 1985;
Cowell, 1986). In contrast, most work carried out on the spacing of rip currents report a decrease in frequency with increasing energy. This discrepancy arises as a result of differences in the manner in which rip currents were recorded. We believe that the use of the "snapshot" method employed here ensures that only actively pulsing rips (almost at a given moment in time) are counted, and the data set is, therefore, not directly comparable to that provided in the literature. During low- and mediumenergy conditions, where rip channels are numerous (Hino, 1975; Sasaki and Horikawa, 1975), on-the-ground methods of rip-current recording tends to include non-active rip channels as rip currents thereby overestimating rip frequency. The frequency of rip channel were recorded in addition to active rips during two of the 12 runs during this study. The number of channels exceeded active rips by a factor of 2.8 and 2.0. Fully dissipative conditions were not encountered during the 12 runs. Consequently the regressions between rip characteristics and environmental conditions obtained above only hold for reflective and intermediate beach states (or surfzone widths < 300 m). From personal observations, it is expected that the rip frequency and wave energy would deviate from linearity at surfzone widths in excess of 300 m. Under dissipative conditions, Short (1985) reported the presence of large-scale but infrequent offshore currents ( "mega-rips" ). During dissipative conditions the three-dimensional character of the hydrology and morphology is lost and the water movements revert to a horizontally uniform flow with predominantly shoreward flow at the water surface and seaward flow at the bed (Longuet-Higgins, 1983). This model explains the decrease in rip frequency with increasing surfzone width during highenergy conditions. Rip activity appears to be a permanent feature of the hydrology of the Sundays River Beach. This is not the case south of the Swartkops River mouth, were rip occurrence is dependent on the direction of wave approach,
322
being restricted to easterly conditions. Distinct alongshore gradients of rip frequency and intensity were observed. Although not ammenable to statistical treatment, the gradients appeared to be controlled by the prevailing angle of wave incidence and antecedent wind direction. When both wind and waves approached from the east, rip frequency and intensity increased from east to west. Under such conditions maximum frequency and intensity was recorded in the vicinity of the Sundays River mouth (Fig. 2a). Rip activity and intensity were more evenly distributed when the wave train was from the south, or from both south west and east (Fig. 2b). The most pronounced alongshore gradient occurred under southwesterly conditions. Figure 2c illustrates rip frequencies and intensities within Algoa Bay after three days of westerly winds, coinciding with southwesterly deep-sea swells. Under such conditions rips were not active in the southwest corner. First signs of rip activity appeared near the Sundays River. Surfzone width followed a similar pattern to rip frequency. A steady increase in width from the southwest to the northeast, with the maximum occurred width occurring midway between the Sundays River and Woody Cape. In addition to alongshore variability, rip condition was temporally variable. Along the Sundays River Beach the total number of rips varied from a minimum of 31 to a maximum of 173. In terms of total rip activity a forty-fold difference was found between the minimum and maximum values recorded. Rips were generally linked to beach and surfzone topography, being associated with megacusp embayments or rip channels. Transient rips operating across shoaling areas {welded bars) that separate rip channels were occasionally observed. During longshore bartrough and rhythmic bar-and-beach states, two distinct types of rips were found, viz.: (a) "beach-terrace" rips, found localized to the inner breaker zone and discharging into the longshore trough, and (b) "outer-bar" rips which operate across the outer breaker zone,
~401 a 20~
I
~ - . .
"/
: oL:L Swartk°ps RT'# i.~
//~/
_
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r
b
.,.. ~',o~ -'" . . ~ . . ' ~ , ~ - " I /-=--~-,~ .......
I /
I
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I
~"
oJ ,-"
WoodyCp - '
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E,~abe~ 40., C
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_ /
___J /
./.., .
/
//
WoodyCp
,,~r.'r~ Swa~kops
Algoa Bay
por~
Elizabeth..~ ~
Fig. 2. Three examples of alongshore rip distribution, illustrating the difference in gradient under easterly ( a ), mixed (b) and westerly (c) wind-wave regimes. Rip frequency (solid line) and intensity (broken line) along each 10 km stretch of coastline is presented as a percentage of the total value for the 70 km study area. The surfzone width is represented by the number of stylized waves (one wave=50 m). The direction of dominant wave crest is given.
323 discharging into the nearshore. These currents usually operated independently of each other. During one of the exercises "beach-terrace" rips were found to exceed "outer bar" rips by a terrace factor of three. In terms of rip intensity, however, the "outer bar" rips were on average twice as active. There have been several attempts at classifying rip currents. Eliot (1973) recognized two different types of rips depending on their activity, viz., low-energy and high-energy rips. Short (1985), from observing 3513 rips, proposed a classification scheme based on the prevailing morphodynamic character of the beach and whether the beach was accreting or eroding. Three types of rips were suggested; "erosion", "mega", and "accretion" rips. Erosion and accretion rips accompany erosional and accretional stages of beach evolution respectively. Mega-rips are large-scale ( > 1 km) erosional rips. Such rips may result from the interaction between breaking wave-induced alongshore currents and the bedrock headlands in deeply embayed beaches (Cowell, 1986). It is clear from this study that rip currents should also be distinguished on the basis of their position within the surfzone. This is very much the case at the Sundays River Beach where the morphodynamic state is often longshore bar-trough or rhythmic bar-and-beach state and rips may either be found on the beach terrace or the outer bar. The "beach-terrace rips" do not involve any exchange of water between the surfzone and the nearshore since their discharge is restricted to the longshore trough. These rips therefore fail to drain the surfzone. Conversely, "outer-bar" rips have little direct influence on the beach terrace water, but are involved in exchanging material between the longshore trough and the nearshore. These rips were not referred to by Winter (1983) in his theoretical model of surfzone circulation, nor in the rip current classification of Short (1985) who worked in a onebar surfzone. No model t h a t considers the integrity of the surfzone as an ecological entity can ignore these features.
Conclusions The use of an aircraft to survey rip activity offers several key advantages over on-theground techniques. The observer is provided with a clearer idea of the intensity and morphology of the rip and its position with respect to the beach/surfzone topography. It provides a rapid "snap-shot" record of rips along extensive lengths of coastline. In Algoa Bay, for instance, similar runs, but using land-based transport would have taken several hours, thereby rendering the data unsuitable for comparative interpretations. The aerial method provides a unique opportunity to understanding spatial variability in rip condition by removing the meso- and long-term temporal variability in rip activity. Furthermore, the "snap-shot" technique provides some indication of the exchange brought about by rip currents in contrast to the method of counting rip channels. The suitability of this method could be further enhanced by using photogrammetric methods for more quantitative results regarding rip intensity.
Acknowledgements We t h a n k the South African National Committee for Oceanographic Research of the CSIR and the Department of E n v i r o n m e n t Affairs and Tourism for funding this research. We are indebted to Professor A. McLachlan and Dr A.D. Short for critically reading the manuscript.
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