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Bar morphology of dissipative beaches: An empirical model
Marine Geology, 51 (1983) 1 5 - 3 3 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
15
B A R MORPHOLOGY OF DISSIPATIVE B E A C H E S : A N E M P I R I C A L MODEL
DAN BOWMAN and VICTOR GOLDSMITH
Dept. o f Geography, Ben-Gurion University o f the Negev, Beer-Sheva 84120 (Israel) Israel National Oceanographic Institute, P.O. Box 8030, Haifa (Israel) and Earth Sciences and Resources Institute, University o f South Carolina, Columbia, SC 29208 (U.S.A.) (Received January 8, 1982; revised and accepted July 1, 1982) ABSTRACT
Bowman, D. and Goldsmith, V., 1983. Bar morphology of dissipative beaches: an empirical model. Mar. Geol., 51: 15--33. This study of 150 examples of nearshore bar morphology along the highly dissipative beaches (e > 33) of the southeast Mediterranean shoreline employs 32 years of aerial photography and wave data, aided by detailed short-term field studies. Three major bar types were delineated: non-rhythmic parallel/meandering bars, inner single-crescentic bars and double-crescentic bars. Each of these bar families includes variations. The bars were related to daily ship and shore-wave data, including wave spectra, from the day of observation back to 60 days prior to the observation. The increase in total bar occurrence during summer is related to crossing of a major wave-energy threshold in the spring, when significant wave heights < 1 m sharply increase to 70--85% in April--May. The bar morphology/wave comparisons further indicate that as the significant wave heights decrease and remain below 1 m, non-rhythmic bars form within 7--10 days, single-crescentic bars require 15 days, and double-crescentic bars require 20--30 days. This adjustment period of the bars to wave power causes a delayed response which accounts for lack of coincidence between wave energy and bar occurrence. The formation of the initial double-crescentic bar, and its transformation to the mature double-crescentic type, requires a short pulse of wave energy (0.5 < H,j 3 < 1.5 m). Some bar families occur throughout the year. The aseasonal occurrence is best shown by the mature double-crescentic type, which apparently is the final stage in the crescenticbar development sequence. However, other bar families show a tendency for a seasonal distribution which reflects their sensitivity to wave energy. Inner single-crescentic and initial double-crescentic bars are largely restricted to the calmest wave months of May/ April to October/November. There is an antiphase relationship between the frequencies of non-rhythmic and crescentic bars. INTRODUCTION P a t t e r n s o f i n s h o r e b a r s a n d s h o a l s are q u i t e v a r i a b l e , a n d e x p l a n a t i o n s o f t h e i r m o r p h o l o g i e s a r e far f r o m c o m p l e t e . E v e n d e f i n i t i o n s a n d c a t e g o r i z a t i o n o f b a r t y p e s l a c k s t a n d a r d i z a t i o n . A s e a r l y as 1 9 4 9 , K i n g a n d W i l l i a m s sugg e s t e d t h e f o l l o w i n g c l a s s i f i c a t i o n f o r b a r s : (a) p a r a l l e l t y p e c o n s i s t i n g o f u p t o f o u r s t r a i g h t b a r s ; a n d ( b ) c r e s c e n t i c s y s t e m s . K o m a r ( 1 9 7 6 , p . 2 7 4 ) illustrated three types of rhythmic nearshore topography, including crescentic
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© 1983 Elsevier Scientific Publishing Company
16 bars, inner anvil-shaped bars~ and the two types combined. Sonu (1972, 1973) described shoreward and longshore displacements of rhythmic systems, including inner and outer crescentic bars. Short (1978, 1979) suggested a three 2.5 m producing shore-parallel bars, breakers between 1 and 2.5 m producing crescentic bars, and waves <1 m forming barless reflective beaches. Wright et al. (1979) studied the high-energy coast of New South Wales and suggested a morphodynamic model of six bar stages reflecting transformation from dissipative to reflective wave conditions with an accretional-bar cycle similar to that of Short (1979). A bar model based on morphology and process evidence was suggested as a tentative global classification b y Greenwood and Davidson-Arnott (1979, table 1). They described six categories including ridge and runnel, cusp, multiple parallel, transverse and straight bars, as well as sinuous to crescentic bars. A similar approach was taken b y Chappell and Eliot (1979) in New South Wales, Australia. They coded segments of inshore morphology for statistical analysis and suggested a typology of seven morphologymirculation patterns. Along the Mediterranean coast of Israel, Striem (1974) described the temporal relief variations of the outer bar near Ashdod. Eitam et al. (1978) investigated temporal variations of subaerial and subaqueous profiles and the granulometric properties of the sand. Neither study attempted to relate the bars to concomitant wave energy. Goldsmith et al. (1982) studied bar development and concomitant wave energy at HaHoterim, northern Israel, and suggested a model of sequential bar development (Table I). The regional aerial photography and wave data indicate that the sequential stage developments and bar/wave relationships delineated at HaHoterim, might be representative of the entire region of the eastern flank of the Nile including the Israeli coast. This study is based on a much broader data sample b o t h in time (1949--1980) and space than that applied in our HaHoterim study. The first concern of this study is to delineate the wide range of nearshore bars and shoal patterns along the southeastern Mediterranean microtidal coastline. Emphasis is placed on the use of a large sample of 150 bar occurrences. The second aim is to widen our knowledge of environmental conditions of the different bar types b y relating them both to the immediate and antecedent wave conditions. The resulting empirically based process-response model is aimed at predicting the occurrence of distinctive bar types. METHODS The present study includes all available air photos of the southeast Mediterranean taken in the period 1949--1980. Additional data were derived b y 22 direct reconnaissance overflights conducted b y the authors from
17
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18
November 1978 to June 1979 over HaHoterim to Haifa Bay, along the coast of Nahariya, and from two flights which ran along the entire Mediterranean coast of Israel (Fig.l). Accordingly, 123 flights showing 150 clear coastal segments were available for bar pattern recognition. This assemblage of air photos showed a reasonable monthly distribution throughout the years, encompassing the full range of seasonal changes. However, although intensive, this air-photo survey does not depict sequential-bar development, due to the rapidity of bar changes. The air photos were compared with concomitant wave data including those of measured and observed waves at Ashdod collected by the Coastal Survey Group of the Ports Authority. Additional visual wave observations conducted at various coastal locations (Nahariya, Haifa, Tel-Aviv, Ashdod and Gaza) since 1949 were also used, as well as shipboard wave observations from adjacent coastal waters. The efficacy of observed significant wave heights, and their representativeness, has been shown in several studies (Goldsmith, 1979; Jardine, 1979; Bowman, 1980). The applicability of each of these wave data sources for the entire southeastern Mediterranean coast up to Haifa is based on Migniot (1974), Rosen and Vajda (1979) and Goldsmith and Golik (1980}.
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19 Daily significant wave heights (H1/3) and periods for up to 60 days prior to each air photo date were retrieved from the wave data. Synoptic conditions in the eastern Mediterranean predating each bar observation were obtained from the detailed monthly synoptic reports of the Israel Meteorological Service. Energy spectrum was computed twice per day from the measured wave data using a Fast Fourier Transform algorithm described in Clairbout (1976). Wave statistics were computed based on Rayligh distribution and the equations of Goda (1974) were used. The approach was to analyze each air photo for its bar t y p e defined b y its plan view geometry and orientation. Subsequently, the bar types were arranged into discrete bar families and related, as response elements, to the wave and climatic conditions predating the day of observation. The length of the very calm period (H1/3 ~ 0.3 m) as well as number of days of H~/3 ~ 0.5 m, H~/3 < 1.0 m, H1/3 < 1.5 m and H~/3 < 2.5 m predating each bar-pattern recognition were also recorded. These wave heights were previously determined as being important in bar sequential development (Goldsmith et al., 1982). Therefore, a seasonal distribution of some bar types should be expected. THE SOUTHEASTERN MEDITERRANEAN COAST The Mediterranean coastal study area is 270 km long, extending from Rosh Hanikra in southern Lebanon to E1-Arish in Sinai (Fig.l). It is a relatively smooth coast, Haifa Bay being the only irregularity. The Sinai beaches are wide. However, the beaches of central and northern Israel are usually narrow, from a few tens of meters up to a maximum width of 100--200 m. In northern Israel, beaches may narrow to a few meters width and sometimes even be non-existent. Eolianite is located intermittently along the coast, forming low ridges or retreating coastal cliffs up to 40 m high. Submerged and eroded eolianite remnants occasionally form rocky outcrops in the inshore and along the water line. Beach sediments are often underlain by an abrasional plateau composed of lithified eolianite. Beachrock occurs along the waterline in central and northern Israel. Sand dunes penetrate over 30 km landward in the EI-Arish area, but only up to 2--5 km in southern and central Israel. Northward, they become restricted to river outlets, Haifa Bay being an exception. The coastal dunes are partly vegetated and stable. Haifa Bay is the northern border of the quartz province of the Nile delta. The Nile source is indicated b y mineral composition and grain size (Emery and Neev, 1960; Pomerancblum, 1966; Nir, 1973). Well-sorted medium beach sand decreases in size from Egypt eastwards. Calcium carbonate, present in only minor amounts (~ 2%) in the eastern Nile delta, increases to 6--8% in southern Israel, and up to 60% south of Haifa. North of Haifa Bay, the coarse sand is mainly biogenic (>90%), and consists of shell fragments which originate from local relict sands on the shelf. The wave climate of the southeastern Mediterranean has been thoroughly summarized and analyzed by Goldsmith and Sofer (in press), employing
20
57,326 observations and measurements collected during the years 1948--1980. There are three wave seasons: t he highest wave m ont hs are in winter, December- March {with m o n t h l y mean H~/3 = 1.0--1.5 m); the lowest wave mo n th s are May, S e p t e m b e r - - O c t o b e r (H~/3 = 0 . 6 - 0 . 8 m); and the intermediate wave m o n t h s are June--August. Wave periods also show a seasonal distribution with T1/3 = 7.2 s in the winter m o n t h s and TI/3 = 6 s in May. The winter wave m o n t h s are d o m i n a t e d by passing storm fronts at an average of 5--7 days interval. Maximum winter significant waves reach 8 m and 13 s, although only a b o u t 10% of H~/3 are >~2 m high. With respect to direction, smaller waves are d o m i n a n t l y f r o m the northwest, and southwest waves b e c o m e mo re i m por t ant with increasing wave size. Highest waves come from between west and west-northwest, t he longest fetch direction. Tide is semidiurnal with a low-tide range of 30--60 cm. There is an antiphase relationship between the main high-tide period (July--August) and the most s to r my period (November--May), which moderates beach dynamics
(Bowman, 1980). The nearshore shelf up to 30 m depth is of low slope gradient -- 0.5 to 1.0 ° (IOLR, 1971). Its bathymetry is basically parallel and simple, thus refraction does not markedly change the smooth energy distribution along the coast (Goldsmith and Golik, 1980). The Mediterranean coast of Israel possesses the prime prerequisites favoring rhythmic bar systems (Sonu, 1973), namely, gentle offshore slopes, abundance of nearshore sand reserve, small tidal range and limited fetch. SURF-ZONE DISSIPATION
It has been shown theoretically and in the laboratory (Guza and Inman, 1975), and applied to field studies (Wright et al., 1979) t hat the wave-breaker characteristics, degree of inshore resonance and t herefore bar t ype, are d e p e n d e n t on the reflectivity or surf-scaling parameter e given by: 6 = a i 03 ~ / g
tan 2/3
where: a i = incident wave amplitude near the point of wave breaking; cot = 27r/T; T = wave period; g = acceleration of gravity; and/3 = inshore/foreshore slope. Guza and Inman (1975) indicated t hat when e < 1, waves are com pl et el y reflected and wave-energy dissipation is negligible, whereas 1 < e < 2.5 indicates strong reflection and resonance, and low dissipation. Under these conditions, wave breakers will be surging and runup will be m axi m um , resulting in waves breaking on the beach-face. Under such conditions, the beach profile is steep and linear, while nearshore r h y t h m i c t o p o g r a p h y is com pl et el y absent (Wright et al., 1979). As e increases f r om 2.5 to 33, due to either decreasing nearshore gradient or increasing wave steepness, wave-energy dissipation increases and wave breakers tend to plunge across the surf zone. At e > 3a, complete dissipation occurs, and wave breakers tend to spill (Galvin, 1972). Under these
21 conditions, the surf zone is quite wide and of low gradient and a large variety of nearshore bar systems is common. Accordingly, e was calculated for the coast of Israel. The foreshore/inshore slope, ~, was compiled from bathymetric charts (IOLR, 1971; Hall and Bakler, 1975; Neev et al., 1976; Spar, 1976; Eitam et al., 1978) and from our data at HaHoterim. All sources indicate that the same medium slope gradient of 0 ° 20'--1 ° dominates the nearshore shelf out from the shoreline. However, the slope gradients of the offshore facing bar flanks are considerably steeper, ranging from I to 5 ° . The Israeli coast, because of its high wave steepness and low mean beach gradients, falls mainly within the dissipative regime with e >> 33 (Fig.2). The extremely dissipative character is indicated here b y the relatively wide (100--300 m) barred inshore, segmented into longshore sub-regions of troughs and shoals, and b y the rhythmic beaches, abundant bar types and dominant spilling breakers. When steep sloping bars ((3 = 5 ° ) dominate the inshore with low ai (< 0.5) and relative long (T > 9 s) waves, e values become as low as 4. Thus, reflection can also occur here in response to the local bar morphology, and is similar to the "inner reflection" domain of Wright et al. (1979). 9-
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Fig.2. The dissipativeness of the Israeli coast.
30
40
22 RESULTS
Bar morphology Three major bar groupings, or "families", were observed along the southeastern Mediterranean coast, with each family composed of several bar types: A. Non-rhythmic parallel~meandering. During storms, bar patterns are hidden under the surf. However, the one, two or multiple breaker lines may be in parallel or meandering forms. When viewed from above the surf reflects the shape of the bars. During post-storm conditions (i.e. H1/3 < 1.5 m), onshore-migrating multiple parallel bars were observed, mainly after the severest storms (Fig.3A). Multiple parallel bars were also indicated by Short (1975) as associated with high wave energy and low angle of wave approach. The single parallel-meandering bar type sometimes occurs (Fig. 3B) and was also described b y Chappell and Eliot (1979) and Sonu {1973). It indicates, according to Short (1979), the nearly final erosional stage. For all the abovementioned bar types, the shoreline usually lacks cyclicity or rip channels, suggesting negligible inshore circulation, as mentioned b y Greenwood and Davidson-Arnott (1979) and Wright {1980). These non-rhythmic parallel-meandering patterns may also include oblique bars alternately attached to the beach and to the shallowest of 2--3 meandering bars (Fig.3C), as also reported b y Sonu (1973}, Hands (1976) and Hunter et al. (1979). B. Single crescentic morphologies. The initial stage of this bar family is dominated by small, closely spaced transverse and oblique bars (Fig.3F) or by anvil-shaped bars (Fig.3D), as seen in plane view. In its advanced stage single inshore crescentic-shaped bars dominate, extending up to 80 m offshore (Fig.3E), taking either a shore-normal or skewed form. The crescents may also be shore attached via transverse or oblique bars. The beach is usually in-phase with bar cyclicity, forming a dominantly cuspoidal shoreline. Smaller-scale cusps, rips, or mega-cusps may also be present; however, beaches lacking a rhythmic shoreline were also observed. Short {1979) defined such crescentic bars as "welded bars". Clos-Arceduc {1962, 1964) concluded from observations on the Algerian coast that this pattern is typical of enclosed beaches. Our findings show that single-crescentic bars are frequent along open coasts as well. C. Double-crescentic morphologies. The outer bar of the initial doublecrescentic system is 200--400 m off the beach in parallel, meandering or in crescentic shape with a wavelength of 175--300 m. The advanced stage is shown in Fig.3G and the initial stage in Fig.3H. This outer bar may be either partly connected to, or detached from, the inner-bar system. Thus, an inshore channel may be either well defined or non-existent. Phase correlation between the inner- and outer-bar systems is usually not evident. The inner system in the initial stage consists of embryonic crescents extending across the seaward end of recurring small rip channels (Fig.3H). In the advanced stage, these shoals have been completely reshaped into a continuous cyclic system of
23 small crescents occupying 1/3 of the inshore width (Fig.3G). Both systems may be either shore normal or skewed. The inner-bar system is directly associated with the highly cuspoidal shoreline. However, linear shorelines have also been observed. Breakers often act selectively on the inner bars and on the horns of the outer ones, resulting in multiple breaking. The initial double-crescentic bar system may change laterally or become less distinct. Fading o u t of the inner bar alone has also been observed. The inner-bar system may sometimes take the form of a parallel bar with regularly recurring crescents (Fig.3I). This rare bar type seems to indicate a stage of transformation from the parallel/meandering family to the crescentic pattern, as mentioned b y Greenwood and Davidson-Arnott (1975). The mature double-crescentic type (Fig. 3J) is defined as an inner crescentic system extending > 2 / 3 of inshore width. The outer bar forms a parallel, slightly meandering or crescentic shape, and is located 150--270 m off the beach. A continuous trough stretching up to 100 m offshore separates the outer and inner systems. The shoreline shows a prominent cuspoidal form, but straight shorelines were also observed. A merging triple bar t y p e (Fig. 3K) was also observed. Double-crescentic systems were described b y Sonu (1973) and by Greenwood and Davidson-Arnott (1975, 1979), who emphasized that inner systems were often skewed and usually in-phase with shoreline rhythms. They also indicated that the outer-bar systems are predominantly symmetrical, stable and unrelated to shoreline rhythms. Rips were shown to flow seaward only a short distance b e y o n d the inner bar and then dissipate rapidly or merge with the longshore current. The triple-bar t y p e has also been observed by these authors. Bars usually do not show well~leveloped patterns in front of eolianite rock exposures in the inshore, b u t rather become disordered shoals. However, as sand is abundant in the study area, eolianite outcrops do not completely hinder bar development. Thus, outer meandering and crescentic systems have been observed even in such conditions. Groins and other coastal structures interfere mainly with the inner bar systems, whereas the outer bars often remain continuous and undisturbed. However, the t o m b o l o of the detached breakwater of the Haifa southern beach proved an obstacle for b o t h outer and inner bar systems. Simultaneity of bar patterns along the study area was not consistent. Some days the entire coast showed mainly one bar type, but in many cases spatial variations of bar types were manifested. The lack of simultaneity of bar patterns cannot stem here from a different wave climate or grain size as suggested by Komar (1976) for the Oregon coast. Wave climate (Goldsmith and Golik, 1980), tidal range and grain size are nearly homogeneous along the coastal segments. Therefore, local b o t t o m topography, which interferes with the nearshore circulation, best explains the lack of simultaneity. However, the clear seasonal trends (Fig.7) indicate that local variations in environmental parameters are of relatively minor importance along this coast.
24
25
Fig.3. Examples of bar families and bar types commonly observed along the study area. I. Non-rhythmic parallel~meandering. A. Multiple parallel bars. B. Single parallel bar with shoals welding onto the beach. C. Parallel-meandering with inshore oblique bars. (Eolianite patches are seen in the offshore.) H. Single-crescentic. D. Anvil-shaped bars. E. Inner single crescentic bars with eolianite exposures at the waterline. F. Transverse bars. III. Double-crescentic. G. Initial double-crescentic bars -- advanced stage. The far right side indicates a lateral transformation to the mature double crescentic stage. H. Initial double crescentic bars--initial stage. Only the inner system is shown. I. Double crescentic with an inner parallel system showing regular recurring crescents. J. Mature doublecrescentic bars. K. Triple-crescentic bar system.
26
Bar and wave seasonality Although the 150 observations of bars during the period 1948--1980 were spread throughout all months of the year, there is a definite tendency for more bars to occur during the six lowest wave months, June through November (62%), than the remaining six months (Fig.4A). This tendency is explained b y the m o n t h l y frequency of low waves {H~ n ~< 1 m) during the period 1948--1978 (Fig.4B). The parallelism between months of low waves and the annual distributions of bars is quite striking. Most notable is the increase in bars related to a major wave-energy threshold in the spring, when the frequency of Hit3 ~< 1 m sharply increases from 58% in the winter months to 70% in April and 85% in May. The calmer and barred summer composes an uninterrupted period of lower waves, explaining the observed seasonal stability of some of the bar types. Summer shows two peaks of low wave activity (Fig.4B). May to June composes one low wave peak which, when one allows for the lag time needed for bar formation, is clearly related to the high occurrence o f bars in June. J u l y is stormier, and therefore b o t h July and August have fewer occurrences of bars. August and September compose a second summer low wave peak, and this is mainly related to the peak of bar occurrence in September. Thus peaks of bar occurrence and wave energy do not match perfectly, b u t rather indicate a lag in the adjustment of bars to wave power, also observed b y Short (1979). Our field study of the sequential development of bars at HaHoterim, northern Israel, carried o u t during 1978--1979, also indicated that the transformation from stormy wave activity to the bar-formation period coincided with the energy threshold of April--May (Fig.4C, D). During this seven-month field study, the total frequency of bar occurrence, as welt as the frequency of crescentic bars, increased sharply during April, coincident with a decrease in the frequency of high waves (H~/3 >~ 1.5 m) from 10% to none. Wave regime predating bar observations The average daily significant wave heights preceding the occurrence of four bar types are shown in Fig.5. The 1--2-month period predating the bar observations clearly indicates a calming~lown tendency starting from storms with 2.5 m wave heights. Short (1979) similarly reported on accretional cycles; the fastest extended over 42 days. Thus, there is a time lag in bar changes, following the changes in wave climate. All bar types show, on an average, an antecedent low wave regime, with H~/3 ~< 1 m, continuously for 10---25 days prior to observation, and Hu3 ~ 0.5 m for 2--7 days before observation. Fig.4. A. Monthly frequency of total nearshore bar systems (1948--1980) from aerial photographs (N = 150). B. Average monthly frequency of the significant wave heights (~<1 m) from 32,735 ship wave observations in the southeastern Mediterranean (1948-1978). C. Frequency of five bar types observed during the 1978--1979 field season (see Table I). D. Frequency distribution of significant wave heights during the 1 9 7 8 - 1 9 7 9 field study at HaHoterim.
27
Bar occurrence 1948-1980
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Figure 5 indicates that for the crescentic-bar family, wave heights decrease to, and stay below, 0.5 m, on an average, for up to 8 days preceding bar observation. However, a close inspection of the data from the preceding week (Fig.6) shows that in most cases an energy "pulse", represented by waves of 0.5 < H~/3 ~< 1.0 m, occurs for a short length of time. This energy pulse is shown in Fig.6 by values >1 m2/frequency up to four days preceding the observation of both initial double-crescentic and mature double-crescentic bars. Our field observations suggest that this energy pulse triggers the changes from single-crescentic to initial double-crescentic, and again from initial double-crescentic to mature double-crescentic. Further, the higher energetic pulses in the days preceding the observation of the mature double-crescentic bring it to an equilibrium stage with the winter conditions. Thus, the mature double-crescentic bars occur throughout the year, lacking seasonality (Fig. 7). The single-crescentic and the initial double-crescentic bars are the most energy-sensitive types, being non,existent during winter but abundant from May to November (Fig.7). The oblique and transverse bar type is typified by
29
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30
dominance of higher energy, with H1/3 ~< 1 m for only 10 days prior to bar observation (Fig.5). This fits well with our former finding at HaHoterim (Goldsmith et al., 1982) where the oblique bar type was sequentially the first to follow storms. Non-rhythmic morphologies seem to occur most of the year (Fig.7). The major exception are the months of May to July, when the crescentic families dominate. Because July has higher waves, August shows the reappearance of non~yclic systems. This antiphase relationship between non-rhythmic and crescentic bar families continues in September/October. The occurrence of each of the other bar types is low, and no conclusions regarding their behavioral patterns should be drawn. Weather pat terns
The monthly weather reports of the Israel Meteorological Service were examined for the days preceding bar observations in order to delineate the typical synoptic conditions related to each bar family. The synoptic conditions which dominate during summer are related to the low pressure NE of Cyprus with increasing pressure toward the west, resulting in predominant weak westerly winds. The winter season is dominated by the passage of cyclonic lows eastward. Inner single~rescentic bars were observed during cyclonic conditions in the eastern Mediterranean (4 October, 1975) as well as during the passage of cold fronts from a depression in central Europe, while a monsoonal Persian surface trough predominated in the low altitudes (28 Aug., 1972;25 Aug., 1973; 1 Sept., 1973). These bars were also observed when a warm ridge dominated the eastern Mediterranean (2 May, 1971) and when an inactive cold front passed Israel (12--14 September, 1973). Mature double-crescentic bars were observed during prevalence of anticyclones and ridges (15 February, 1975), during a surface high with stable clear weather and southerly warm and dry air flow (15 March, 1975), as well as during a monsoonal trough with anticyclonic flow (30 July, 1971). It seems that no unique general synoptic condition can be related to each bar family. DISCUSSION
The coast of Israel shows strong dissipativeness, encouraging bar formation throughout the year. The bars are most frequent at, and right after, the calmest wave months (June, September/October) and least abundant in the November--March stormy period. Long-term examination of the bar occurrences, together with wave-height data, provide insight into the different energy levels of the various bar families and their seasonal sensitivity. Energyinsensitive bar types occur throughout the year. The threshold for initiation and destruction of the energy-sensitive bar types is frequently crossed. The inshore along the southeast Mediterranean is dominated by both singleand double-crescentic bar families, which form 71% of the total bar types. Thus, this coast is different from the Australian coast studied by Short (1979), where the inner single-crescentic bars, defined as megacusps and
31
welded bars, dominated. The single-crescentic and the initial double-crescentic bar types are the most seasonally-sensitive. Their seasonal occurrence thus significantly differs from the observations of Greenwood and Davidson-Arnott (1979), who reported a lack of seasonality and continuous dominance of crescentic bars. This seems to be related to a lower wave energy in their study area, i.e. below threshold conditions. An energy pulse, which initiates the formation of the initial and the mature double-crescentic stages, is clearly shown by the daily peak spectral energy data prior to the bar observation. The mature double~rescentic bars compose an energetic, aseasonal type occurring throughout the year. These observations indicate that nearshore bars are not always simply destroyed during storms and regenerated during low waves as suggested by Bernard et al. (1959), Davis and Fox (1972) and Coakley et al. (1973). The overall calming down interval of 40--60 days before bar observation represents the recovery of the inshore following storms. In the beach cycle observed by Hayes (1972) along the New England beaches, well
This study emanated from a project on nearshore bar systems supported by the Geography Programs, Office of Naval Research, Contract No. N0001478-C-0645, NP 388-148. V. Goldsmith, Principal Investigator. Wave data from Ashdod were kindly provided by D. Divon of the Coastal Study Division, Israel Ports Authority, and analyzed as part of a project funded by the Sea Grant International Program, National Sea Grant, N.O.A.A., Grant No. 04-8-M01-162. Weather and directional wave data at Tel-Shikmona were provided by H. Arbel and I. Matasaru, Israel Meteorological Station at Tel-Shikmona. Special thanks go to K. Kfley, Virginia, to Mrs. N. Peer, Ben-Gurion University, for the drafting, to the Israel Meteorological Service --the Archive, for providing wave climate data, to Mr. S. Sofer of the Israel National Oceanographic Institute, Haifa, for data processing, and to Y. Daube for assisting in many ways.
32 REFERENCES Bernard, H.A., Major, C.F. and Paxrott, B.S., 1959. The Galveston barrier Island and environ: A model for predicting reservoir occurrence and trend. Trans. Gulf Coast Assoc. Geol. Soc., 9: 221--224. Bowman, D., 1980. Validity of visually estimated wave parameters. Israel J. Earth Sci., 28: 94--99. Chappell, J. and Eliot, I.G., 1979. Surf beach dynamics in time and space -- an Australian case study, and elements of predictive model. Max. Geol., 32: 231--250. Clairbout, J.F., 1976. Fundamentals of Geophysical Data Processing with Application of Petroleum Prospecting. McGraw-Hill, New York, N.Y., 274 pp. Clos-Arceduc, A., 1962. Etude sur les rues aerienne des alluvions littorales d'allure periodique, cordons littoraux et festons. Soc. Ft. Photogramm. Bull., 4: 13--21. Clos-Arceduc, A., 1964. La photographie aerienne et l'~tude des depSts pr~littoraux : Etude de photo-interpretation. No. 1, Inst. Geogr. Nat., Paris. Coakley, J.P., Haras, W. and Freeman, N., 1973. The effect of storm surge on beach erosion, Point Pelee. Proc. 16th Conf., Great Lakes Res., pp.377--389. Davis, R.A. and Fox, W.T., 1972. Coastal processes and nearshore sand bars. J. Sediment. Petrol., 42: 401--412. Eitam, Y., Hecht, A. and Sass, E., 1978. Topographic and granulometric variations on the shore of Ma'agan Mikhael, Eastern Mediterranean. Israel J. Earth Sci., 27: 1--13. Emery, K.O. and Neev, D., 1960. Mediterranean beaches of Israel. Bull. Geol. Surv. Israel, 26 : 1--24. Galvin, C.J., 1972. Wave breaking in shallow water. In: R.E. Meyer (Editor), Waves on Beaches and Resulting Sediment Transport. Academic Press, New York, N.Y., pp.413--456. Goda, Y., 1974. Estimation of wave statistics from spectral information. In: W. Edge (Editor), Ocean Wave Measurement and Analysis, Vol. I. Am. Soc. Civ. Eng., New York, N.Y., pp. 320--337. Goldsmith, V., 1979. VIMS-BLM second order wave climate model and wave climatology of Baltimore Canyon Trough shelf area. SRAMSOE No. 203, Virginia Inst. of Mar. Sci., Gloucester Pt., Va., pp.15-1 to 15-77 + 3 app. Goldsmith, V. and Golik, A., 1980. Sediment transport model of the southeastern Mediterranean coast. Mar. Geol., 37: 147--175. Goldsmith, V. and Sofer, S., in press. Wave climatology of the Southeastern Mediterranean: An integrated approach. Israel J. Earth-Sci. Goldsmith, V., Bowman, D. and Kiley, K., 1982. Sequential stage development of crescentic bars: southeastern Mediterranean. J. Sediment. Petrol., 52: 233--249. Greenwood, B. and Davidson-Arnott, R.G.D., 1975. Marine bars and neaxshore sedimentary processes, Kouchibouguac Bay, New Brunswick. In: J. Hails and A. Cart (Editors), Nearshore Sediment Dynamics and Sedimentation. Wiley, New York, N.Y., pp.123-150. Greenwood, B. and Davidson-Arnott, R.G.D., 1979. Sedimentation and equilibrium in wave formed bars: a review and case study. Can. J. Earth-Sci., 16: 312--332. Guza, R.T. and Inman, D.L., 1975. Edgewaves and beach cusps. J. Geophys. Res., 80: 2997--3012. Hall, J.K. and Bakler, N., 1975. Detailed bathymetric and shallow seismic surveys at five locations along the Mediterranean coast of Israel. Geol. Surv. Israel Field Rep., 1/75, 21 pp. Hands, E.B., 1976. Observations of barred coastal profiles under the influence of rising water levels, eastern Lake Michigan, 1967--71. U.S. Army Corps of Eng., Coastal Eng. Res. Cent. Tech. Rep., 76-1, 113 pp. Hayes, M.O., 1972. Forms of sediment accumulation in the beach zone. In: R.E. Meyers (Editor), Waves on Beaches and Resulting Sediment Transport. Academic Press, New York, N.Y., pp.297--356.
33 Hunter, R.E., Clifton, A.E. and Phillip, R.L., 1979. Depositional processes, sedimentary structures and predicted vertical sequences in barred nearshore systems, Southern Oregon coast. J. Sediment. Petrol., 49: 711--726. IOLR, 1971. Bathymetric charts of the Israel Mediterranean shelf and upper slope in three 1:100,000 scale sheets. Jardine, T.P., 1979. The reliability of visually observed wave heights. Coastal Eng. J., 3: 33--38. King, C.A.M. and Williams, W.W., 1949. The formation and movement of sand bars by wave action. Geogr. J., 113: 70--85. Komar, P.D., 1976. Beach Processes and Sedimentation. Prentice-Hall, Englewood Cliffs, New Jersey, 429 pp. Migniot, C., 1974. Creation of a nuclear power station north of Hadera -- Natural phenomena study. Report by Laboratoire Central d'Hydraulique de France for Israel Electric Co., 34 pp. Neev, D., Almagor, G., Arad, A., Ginzburg, A. and Hall, J.K., 1976. The geology of the southeastern Mediterranean Sea. Geol. Surv. Israel Bull., 68, 51 pp. Nir, Y., 1973. Geological history of recent and subrecent sediments of the Israel Mediterranean shelf and slope. Geol. Surv. Israel Rep., MG/73/2,179 pp. Pomerancblum, M., 1966. The distribution of heavy minerals and their hydraulic equivalent in sediments of the Mediterranean continental shelf of Israel. J. Sediment. Petrol., 36: 162--179. Rosen, D.S. and Vajda, M., 1979. Hadera wind and wave climate. Coastal Mar. Eng. Res. Inst., Technion City, Haifa, Israel, P.N. 47/79, 7 pp. Short, A.D., 1975. Offshore bars along the Alaskan Arctic coast. J. Geol., 83: 209--221. Short, A.D., 1978. Characteristic beach morphodynamics on the southeastern Australian coast. 4th Aust. Conf. on Coastal Eng., Inst. of Eng., Adelaide, pp.148--152. Short, A.D., 1979. Three dimensional beach stage model. J. Geol., 87: 553--571. Short, A.D., 1980. Beach response to variations in breaker height. Coastal Studies Unit Tech. Rep., 80/2, Dept. of Geogr., Univ. of Sydney, N.S.W. Sonu, C.J., 1972. Field observation of nearshore circulation and meandering currents. J. Geophys. Res., 77 : 3232--3247. Sonu, C.J., 1973. Three dimensional beach changes. J. Geol., 81: 42--64. Spar, S.M., 1976. Sedimentological Behavior of the Beach Sand in the Vicinity of the Netanya Breakwaters. M.Sc. Thesis, Hebrew Univ., Jerusalem, 58 pp. Striem, H.L., 1974. The offshore bars at Ashdod, their topography and seasonal behaviour and their indicative ratios. Israel Atomic Energy Commission, IAo1299, 34 pp. Wright, L.D., 1980. Beach cut in relation to surf zone morphodynamics. Coastal Studies Unit Tech. Rep., 80/2, Dept. of Geogr., Univ. of Sydney, N.S.W. Wright, L.D., Chappell, J., Thom, B.G., Bradshaw, M.P. and Cowell, P., 1979. Morphodynamics of reflective and dissipative beach and inshore systems; southeastern Australia. Mar. Geol., 32: 105--140.
15
B A R MORPHOLOGY OF DISSIPATIVE B E A C H E S : A N E M P I R I C A L MODEL
DAN BOWMAN and VICTOR GOLDSMITH
Dept. o f Geography, Ben-Gurion University o f the Negev, Beer-Sheva 84120 (Israel) Israel National Oceanographic Institute, P.O. Box 8030, Haifa (Israel) and Earth Sciences and Resources Institute, University o f South Carolina, Columbia, SC 29208 (U.S.A.) (Received January 8, 1982; revised and accepted July 1, 1982) ABSTRACT
Bowman, D. and Goldsmith, V., 1983. Bar morphology of dissipative beaches: an empirical model. Mar. Geol., 51: 15--33. This study of 150 examples of nearshore bar morphology along the highly dissipative beaches (e > 33) of the southeast Mediterranean shoreline employs 32 years of aerial photography and wave data, aided by detailed short-term field studies. Three major bar types were delineated: non-rhythmic parallel/meandering bars, inner single-crescentic bars and double-crescentic bars. Each of these bar families includes variations. The bars were related to daily ship and shore-wave data, including wave spectra, from the day of observation back to 60 days prior to the observation. The increase in total bar occurrence during summer is related to crossing of a major wave-energy threshold in the spring, when significant wave heights < 1 m sharply increase to 70--85% in April--May. The bar morphology/wave comparisons further indicate that as the significant wave heights decrease and remain below 1 m, non-rhythmic bars form within 7--10 days, single-crescentic bars require 15 days, and double-crescentic bars require 20--30 days. This adjustment period of the bars to wave power causes a delayed response which accounts for lack of coincidence between wave energy and bar occurrence. The formation of the initial double-crescentic bar, and its transformation to the mature double-crescentic type, requires a short pulse of wave energy (0.5 < H,j 3 < 1.5 m). Some bar families occur throughout the year. The aseasonal occurrence is best shown by the mature double-crescentic type, which apparently is the final stage in the crescenticbar development sequence. However, other bar families show a tendency for a seasonal distribution which reflects their sensitivity to wave energy. Inner single-crescentic and initial double-crescentic bars are largely restricted to the calmest wave months of May/ April to October/November. There is an antiphase relationship between the frequencies of non-rhythmic and crescentic bars. INTRODUCTION P a t t e r n s o f i n s h o r e b a r s a n d s h o a l s are q u i t e v a r i a b l e , a n d e x p l a n a t i o n s o f t h e i r m o r p h o l o g i e s a r e far f r o m c o m p l e t e . E v e n d e f i n i t i o n s a n d c a t e g o r i z a t i o n o f b a r t y p e s l a c k s t a n d a r d i z a t i o n . A s e a r l y as 1 9 4 9 , K i n g a n d W i l l i a m s sugg e s t e d t h e f o l l o w i n g c l a s s i f i c a t i o n f o r b a r s : (a) p a r a l l e l t y p e c o n s i s t i n g o f u p t o f o u r s t r a i g h t b a r s ; a n d ( b ) c r e s c e n t i c s y s t e m s . K o m a r ( 1 9 7 6 , p . 2 7 4 ) illustrated three types of rhythmic nearshore topography, including crescentic
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16 bars, inner anvil-shaped bars~ and the two types combined. Sonu (1972, 1973) described shoreward and longshore displacements of rhythmic systems, including inner and outer crescentic bars. Short (1978, 1979) suggested a three
17
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18
November 1978 to June 1979 over HaHoterim to Haifa Bay, along the coast of Nahariya, and from two flights which ran along the entire Mediterranean coast of Israel (Fig.l). Accordingly, 123 flights showing 150 clear coastal segments were available for bar pattern recognition. This assemblage of air photos showed a reasonable monthly distribution throughout the years, encompassing the full range of seasonal changes. However, although intensive, this air-photo survey does not depict sequential-bar development, due to the rapidity of bar changes. The air photos were compared with concomitant wave data including those of measured and observed waves at Ashdod collected by the Coastal Survey Group of the Ports Authority. Additional visual wave observations conducted at various coastal locations (Nahariya, Haifa, Tel-Aviv, Ashdod and Gaza) since 1949 were also used, as well as shipboard wave observations from adjacent coastal waters. The efficacy of observed significant wave heights, and their representativeness, has been shown in several studies (Goldsmith, 1979; Jardine, 1979; Bowman, 1980). The applicability of each of these wave data sources for the entire southeastern Mediterranean coast up to Haifa is based on Migniot (1974), Rosen and Vajda (1979) and Goldsmith and Golik (1980}.
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19 Daily significant wave heights (H1/3) and periods for up to 60 days prior to each air photo date were retrieved from the wave data. Synoptic conditions in the eastern Mediterranean predating each bar observation were obtained from the detailed monthly synoptic reports of the Israel Meteorological Service. Energy spectrum was computed twice per day from the measured wave data using a Fast Fourier Transform algorithm described in Clairbout (1976). Wave statistics were computed based on Rayligh distribution and the equations of Goda (1974) were used. The approach was to analyze each air photo for its bar t y p e defined b y its plan view geometry and orientation. Subsequently, the bar types were arranged into discrete bar families and related, as response elements, to the wave and climatic conditions predating the day of observation. The length of the very calm period (H1/3 ~ 0.3 m) as well as number of days of H~/3 ~ 0.5 m, H~/3 < 1.0 m, H1/3 < 1.5 m and H~/3 < 2.5 m predating each bar-pattern recognition were also recorded. These wave heights were previously determined as being important in bar sequential development (Goldsmith et al., 1982). Therefore, a seasonal distribution of some bar types should be expected. THE SOUTHEASTERN MEDITERRANEAN COAST The Mediterranean coastal study area is 270 km long, extending from Rosh Hanikra in southern Lebanon to E1-Arish in Sinai (Fig.l). It is a relatively smooth coast, Haifa Bay being the only irregularity. The Sinai beaches are wide. However, the beaches of central and northern Israel are usually narrow, from a few tens of meters up to a maximum width of 100--200 m. In northern Israel, beaches may narrow to a few meters width and sometimes even be non-existent. Eolianite is located intermittently along the coast, forming low ridges or retreating coastal cliffs up to 40 m high. Submerged and eroded eolianite remnants occasionally form rocky outcrops in the inshore and along the water line. Beach sediments are often underlain by an abrasional plateau composed of lithified eolianite. Beachrock occurs along the waterline in central and northern Israel. Sand dunes penetrate over 30 km landward in the EI-Arish area, but only up to 2--5 km in southern and central Israel. Northward, they become restricted to river outlets, Haifa Bay being an exception. The coastal dunes are partly vegetated and stable. Haifa Bay is the northern border of the quartz province of the Nile delta. The Nile source is indicated b y mineral composition and grain size (Emery and Neev, 1960; Pomerancblum, 1966; Nir, 1973). Well-sorted medium beach sand decreases in size from Egypt eastwards. Calcium carbonate, present in only minor amounts (~ 2%) in the eastern Nile delta, increases to 6--8% in southern Israel, and up to 60% south of Haifa. North of Haifa Bay, the coarse sand is mainly biogenic (>90%), and consists of shell fragments which originate from local relict sands on the shelf. The wave climate of the southeastern Mediterranean has been thoroughly summarized and analyzed by Goldsmith and Sofer (in press), employing
20
57,326 observations and measurements collected during the years 1948--1980. There are three wave seasons: t he highest wave m ont hs are in winter, December- March {with m o n t h l y mean H~/3 = 1.0--1.5 m); the lowest wave mo n th s are May, S e p t e m b e r - - O c t o b e r (H~/3 = 0 . 6 - 0 . 8 m); and the intermediate wave m o n t h s are June--August. Wave periods also show a seasonal distribution with T1/3 = 7.2 s in the winter m o n t h s and TI/3 = 6 s in May. The winter wave m o n t h s are d o m i n a t e d by passing storm fronts at an average of 5--7 days interval. Maximum winter significant waves reach 8 m and 13 s, although only a b o u t 10% of H~/3 are >~2 m high. With respect to direction, smaller waves are d o m i n a n t l y f r o m the northwest, and southwest waves b e c o m e mo re i m por t ant with increasing wave size. Highest waves come from between west and west-northwest, t he longest fetch direction. Tide is semidiurnal with a low-tide range of 30--60 cm. There is an antiphase relationship between the main high-tide period (July--August) and the most s to r my period (November--May), which moderates beach dynamics
(Bowman, 1980). The nearshore shelf up to 30 m depth is of low slope gradient -- 0.5 to 1.0 ° (IOLR, 1971). Its bathymetry is basically parallel and simple, thus refraction does not markedly change the smooth energy distribution along the coast (Goldsmith and Golik, 1980). The Mediterranean coast of Israel possesses the prime prerequisites favoring rhythmic bar systems (Sonu, 1973), namely, gentle offshore slopes, abundance of nearshore sand reserve, small tidal range and limited fetch. SURF-ZONE DISSIPATION
It has been shown theoretically and in the laboratory (Guza and Inman, 1975), and applied to field studies (Wright et al., 1979) t hat the wave-breaker characteristics, degree of inshore resonance and t herefore bar t ype, are d e p e n d e n t on the reflectivity or surf-scaling parameter e given by: 6 = a i 03 ~ / g
tan 2/3
where: a i = incident wave amplitude near the point of wave breaking; cot = 27r/T; T = wave period; g = acceleration of gravity; and/3 = inshore/foreshore slope. Guza and Inman (1975) indicated t hat when e < 1, waves are com pl et el y reflected and wave-energy dissipation is negligible, whereas 1 < e < 2.5 indicates strong reflection and resonance, and low dissipation. Under these conditions, wave breakers will be surging and runup will be m axi m um , resulting in waves breaking on the beach-face. Under such conditions, the beach profile is steep and linear, while nearshore r h y t h m i c t o p o g r a p h y is com pl et el y absent (Wright et al., 1979). As e increases f r om 2.5 to 33, due to either decreasing nearshore gradient or increasing wave steepness, wave-energy dissipation increases and wave breakers tend to plunge across the surf zone. At e > 3a, complete dissipation occurs, and wave breakers tend to spill (Galvin, 1972). Under these
21 conditions, the surf zone is quite wide and of low gradient and a large variety of nearshore bar systems is common. Accordingly, e was calculated for the coast of Israel. The foreshore/inshore slope, ~, was compiled from bathymetric charts (IOLR, 1971; Hall and Bakler, 1975; Neev et al., 1976; Spar, 1976; Eitam et al., 1978) and from our data at HaHoterim. All sources indicate that the same medium slope gradient of 0 ° 20'--1 ° dominates the nearshore shelf out from the shoreline. However, the slope gradients of the offshore facing bar flanks are considerably steeper, ranging from I to 5 ° . The Israeli coast, because of its high wave steepness and low mean beach gradients, falls mainly within the dissipative regime with e >> 33 (Fig.2). The extremely dissipative character is indicated here b y the relatively wide (100--300 m) barred inshore, segmented into longshore sub-regions of troughs and shoals, and b y the rhythmic beaches, abundant bar types and dominant spilling breakers. When steep sloping bars ((3 = 5 ° ) dominate the inshore with low ai (< 0.5) and relative long (T > 9 s) waves, e values become as low as 4. Thus, reflection can also occur here in response to the local bar morphology, and is similar to the "inner reflection" domain of Wright et al. (1979). 9-
~o
F-
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5 ¸
LU
-r-
LU
"A
entire range
4-
J
o
3"
~main~rar~e(-80[x.) 2-
10
20 WAVE PERIOD(sec)
Fig.2. The dissipativeness of the Israeli coast.
30
40
22 RESULTS
Bar morphology Three major bar groupings, or "families", were observed along the southeastern Mediterranean coast, with each family composed of several bar types: A. Non-rhythmic parallel~meandering. During storms, bar patterns are hidden under the surf. However, the one, two or multiple breaker lines may be in parallel or meandering forms. When viewed from above the surf reflects the shape of the bars. During post-storm conditions (i.e. H1/3 < 1.5 m), onshore-migrating multiple parallel bars were observed, mainly after the severest storms (Fig.3A). Multiple parallel bars were also indicated by Short (1975) as associated with high wave energy and low angle of wave approach. The single parallel-meandering bar type sometimes occurs (Fig. 3B) and was also described b y Chappell and Eliot (1979) and Sonu {1973). It indicates, according to Short (1979), the nearly final erosional stage. For all the abovementioned bar types, the shoreline usually lacks cyclicity or rip channels, suggesting negligible inshore circulation, as mentioned b y Greenwood and Davidson-Arnott (1979) and Wright {1980). These non-rhythmic parallel-meandering patterns may also include oblique bars alternately attached to the beach and to the shallowest of 2--3 meandering bars (Fig.3C), as also reported b y Sonu (1973}, Hands (1976) and Hunter et al. (1979). B. Single crescentic morphologies. The initial stage of this bar family is dominated by small, closely spaced transverse and oblique bars (Fig.3F) or by anvil-shaped bars (Fig.3D), as seen in plane view. In its advanced stage single inshore crescentic-shaped bars dominate, extending up to 80 m offshore (Fig.3E), taking either a shore-normal or skewed form. The crescents may also be shore attached via transverse or oblique bars. The beach is usually in-phase with bar cyclicity, forming a dominantly cuspoidal shoreline. Smaller-scale cusps, rips, or mega-cusps may also be present; however, beaches lacking a rhythmic shoreline were also observed. Short {1979) defined such crescentic bars as "welded bars". Clos-Arceduc {1962, 1964) concluded from observations on the Algerian coast that this pattern is typical of enclosed beaches. Our findings show that single-crescentic bars are frequent along open coasts as well. C. Double-crescentic morphologies. The outer bar of the initial doublecrescentic system is 200--400 m off the beach in parallel, meandering or in crescentic shape with a wavelength of 175--300 m. The advanced stage is shown in Fig.3G and the initial stage in Fig.3H. This outer bar may be either partly connected to, or detached from, the inner-bar system. Thus, an inshore channel may be either well defined or non-existent. Phase correlation between the inner- and outer-bar systems is usually not evident. The inner system in the initial stage consists of embryonic crescents extending across the seaward end of recurring small rip channels (Fig.3H). In the advanced stage, these shoals have been completely reshaped into a continuous cyclic system of
23 small crescents occupying 1/3 of the inshore width (Fig.3G). Both systems may be either shore normal or skewed. The inner-bar system is directly associated with the highly cuspoidal shoreline. However, linear shorelines have also been observed. Breakers often act selectively on the inner bars and on the horns of the outer ones, resulting in multiple breaking. The initial double-crescentic bar system may change laterally or become less distinct. Fading o u t of the inner bar alone has also been observed. The inner-bar system may sometimes take the form of a parallel bar with regularly recurring crescents (Fig.3I). This rare bar type seems to indicate a stage of transformation from the parallel/meandering family to the crescentic pattern, as mentioned b y Greenwood and Davidson-Arnott (1975). The mature double-crescentic type (Fig. 3J) is defined as an inner crescentic system extending > 2 / 3 of inshore width. The outer bar forms a parallel, slightly meandering or crescentic shape, and is located 150--270 m off the beach. A continuous trough stretching up to 100 m offshore separates the outer and inner systems. The shoreline shows a prominent cuspoidal form, but straight shorelines were also observed. A merging triple bar t y p e (Fig. 3K) was also observed. Double-crescentic systems were described b y Sonu (1973) and by Greenwood and Davidson-Arnott (1975, 1979), who emphasized that inner systems were often skewed and usually in-phase with shoreline rhythms. They also indicated that the outer-bar systems are predominantly symmetrical, stable and unrelated to shoreline rhythms. Rips were shown to flow seaward only a short distance b e y o n d the inner bar and then dissipate rapidly or merge with the longshore current. The triple-bar t y p e has also been observed by these authors. Bars usually do not show well~leveloped patterns in front of eolianite rock exposures in the inshore, b u t rather become disordered shoals. However, as sand is abundant in the study area, eolianite outcrops do not completely hinder bar development. Thus, outer meandering and crescentic systems have been observed even in such conditions. Groins and other coastal structures interfere mainly with the inner bar systems, whereas the outer bars often remain continuous and undisturbed. However, the t o m b o l o of the detached breakwater of the Haifa southern beach proved an obstacle for b o t h outer and inner bar systems. Simultaneity of bar patterns along the study area was not consistent. Some days the entire coast showed mainly one bar type, but in many cases spatial variations of bar types were manifested. The lack of simultaneity of bar patterns cannot stem here from a different wave climate or grain size as suggested by Komar (1976) for the Oregon coast. Wave climate (Goldsmith and Golik, 1980), tidal range and grain size are nearly homogeneous along the coastal segments. Therefore, local b o t t o m topography, which interferes with the nearshore circulation, best explains the lack of simultaneity. However, the clear seasonal trends (Fig.7) indicate that local variations in environmental parameters are of relatively minor importance along this coast.
24
25
Fig.3. Examples of bar families and bar types commonly observed along the study area. I. Non-rhythmic parallel~meandering. A. Multiple parallel bars. B. Single parallel bar with shoals welding onto the beach. C. Parallel-meandering with inshore oblique bars. (Eolianite patches are seen in the offshore.) H. Single-crescentic. D. Anvil-shaped bars. E. Inner single crescentic bars with eolianite exposures at the waterline. F. Transverse bars. III. Double-crescentic. G. Initial double-crescentic bars -- advanced stage. The far right side indicates a lateral transformation to the mature double crescentic stage. H. Initial double crescentic bars--initial stage. Only the inner system is shown. I. Double crescentic with an inner parallel system showing regular recurring crescents. J. Mature doublecrescentic bars. K. Triple-crescentic bar system.
26
Bar and wave seasonality Although the 150 observations of bars during the period 1948--1980 were spread throughout all months of the year, there is a definite tendency for more bars to occur during the six lowest wave months, June through November (62%), than the remaining six months (Fig.4A). This tendency is explained b y the m o n t h l y frequency of low waves {H~ n ~< 1 m) during the period 1948--1978 (Fig.4B). The parallelism between months of low waves and the annual distributions of bars is quite striking. Most notable is the increase in bars related to a major wave-energy threshold in the spring, when the frequency of Hit3 ~< 1 m sharply increases from 58% in the winter months to 70% in April and 85% in May. The calmer and barred summer composes an uninterrupted period of lower waves, explaining the observed seasonal stability of some of the bar types. Summer shows two peaks of low wave activity (Fig.4B). May to June composes one low wave peak which, when one allows for the lag time needed for bar formation, is clearly related to the high occurrence o f bars in June. J u l y is stormier, and therefore b o t h July and August have fewer occurrences of bars. August and September compose a second summer low wave peak, and this is mainly related to the peak of bar occurrence in September. Thus peaks of bar occurrence and wave energy do not match perfectly, b u t rather indicate a lag in the adjustment of bars to wave power, also observed b y Short (1979). Our field study of the sequential development of bars at HaHoterim, northern Israel, carried o u t during 1978--1979, also indicated that the transformation from stormy wave activity to the bar-formation period coincided with the energy threshold of April--May (Fig.4C, D). During this seven-month field study, the total frequency of bar occurrence, as welt as the frequency of crescentic bars, increased sharply during April, coincident with a decrease in the frequency of high waves (H~/3 >~ 1.5 m) from 10% to none. Wave regime predating bar observations The average daily significant wave heights preceding the occurrence of four bar types are shown in Fig.5. The 1--2-month period predating the bar observations clearly indicates a calming~lown tendency starting from storms with 2.5 m wave heights. Short (1979) similarly reported on accretional cycles; the fastest extended over 42 days. Thus, there is a time lag in bar changes, following the changes in wave climate. All bar types show, on an average, an antecedent low wave regime, with H~/3 ~< 1 m, continuously for 10---25 days prior to observation, and Hu3 ~ 0.5 m for 2--7 days before observation. Fig.4. A. Monthly frequency of total nearshore bar systems (1948--1980) from aerial photographs (N = 150). B. Average monthly frequency of the significant wave heights (~<1 m) from 32,735 ship wave observations in the southeastern Mediterranean (1948-1978). C. Frequency of five bar types observed during the 1978--1979 field season (see Table I). D. Frequency distribution of significant wave heights during the 1 9 7 8 - 1 9 7 9 field study at HaHoterim.
27
Bar occurrence 1948-1980
& wave heights Aerial photos
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28
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Number of continuous days before
bar observation
Fig. 5. Mean number of continuous days preceding bar observations, for waves of indicated significant heights. The parentheses indicate the number of observations averaged for each bar type.
Figure 5 indicates that for the crescentic-bar family, wave heights decrease to, and stay below, 0.5 m, on an average, for up to 8 days preceding bar observation. However, a close inspection of the data from the preceding week (Fig.6) shows that in most cases an energy "pulse", represented by waves of 0.5 < H~/3 ~< 1.0 m, occurs for a short length of time. This energy pulse is shown in Fig.6 by values >1 m2/frequency up to four days preceding the observation of both initial double-crescentic and mature double-crescentic bars. Our field observations suggest that this energy pulse triggers the changes from single-crescentic to initial double-crescentic, and again from initial double-crescentic to mature double-crescentic. Further, the higher energetic pulses in the days preceding the observation of the mature double-crescentic bring it to an equilibrium stage with the winter conditions. Thus, the mature double-crescentic bars occur throughout the year, lacking seasonality (Fig. 7). The single-crescentic and the initial double-crescentic bars are the most energy-sensitive types, being non,existent during winter but abundant from May to November (Fig.7). The oblique and transverse bar type is typified by
29
Initial double crescentic
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Days before observation Fig.6. D a i l y peak spectra] w a v e e n e r g y m the w e e k p r e c e d i n g each bar o b s e r v a t i o n .
Merging
•
Mature ~ g l ~ crescentic
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Fig.7. M o n t h l y occurrence of bar types, 1948--1980, along the southeastern Mediterranean coast (N = 150).
30
dominance of higher energy, with H1/3 ~< 1 m for only 10 days prior to bar observation (Fig.5). This fits well with our former finding at HaHoterim (Goldsmith et al., 1982) where the oblique bar type was sequentially the first to follow storms. Non-rhythmic morphologies seem to occur most of the year (Fig.7). The major exception are the months of May to July, when the crescentic families dominate. Because July has higher waves, August shows the reappearance of non~yclic systems. This antiphase relationship between non-rhythmic and crescentic bar families continues in September/October. The occurrence of each of the other bar types is low, and no conclusions regarding their behavioral patterns should be drawn. Weather pat terns
The monthly weather reports of the Israel Meteorological Service were examined for the days preceding bar observations in order to delineate the typical synoptic conditions related to each bar family. The synoptic conditions which dominate during summer are related to the low pressure NE of Cyprus with increasing pressure toward the west, resulting in predominant weak westerly winds. The winter season is dominated by the passage of cyclonic lows eastward. Inner single~rescentic bars were observed during cyclonic conditions in the eastern Mediterranean (4 October, 1975) as well as during the passage of cold fronts from a depression in central Europe, while a monsoonal Persian surface trough predominated in the low altitudes (28 Aug., 1972;25 Aug., 1973; 1 Sept., 1973). These bars were also observed when a warm ridge dominated the eastern Mediterranean (2 May, 1971) and when an inactive cold front passed Israel (12--14 September, 1973). Mature double-crescentic bars were observed during prevalence of anticyclones and ridges (15 February, 1975), during a surface high with stable clear weather and southerly warm and dry air flow (15 March, 1975), as well as during a monsoonal trough with anticyclonic flow (30 July, 1971). It seems that no unique general synoptic condition can be related to each bar family. DISCUSSION
The coast of Israel shows strong dissipativeness, encouraging bar formation throughout the year. The bars are most frequent at, and right after, the calmest wave months (June, September/October) and least abundant in the November--March stormy period. Long-term examination of the bar occurrences, together with wave-height data, provide insight into the different energy levels of the various bar families and their seasonal sensitivity. Energyinsensitive bar types occur throughout the year. The threshold for initiation and destruction of the energy-sensitive bar types is frequently crossed. The inshore along the southeast Mediterranean is dominated by both singleand double-crescentic bar families, which form 71% of the total bar types. Thus, this coast is different from the Australian coast studied by Short (1979), where the inner single-crescentic bars, defined as megacusps and
31
welded bars, dominated. The single-crescentic and the initial double-crescentic bar types are the most seasonally-sensitive. Their seasonal occurrence thus significantly differs from the observations of Greenwood and Davidson-Arnott (1979), who reported a lack of seasonality and continuous dominance of crescentic bars. This seems to be related to a lower wave energy in their study area, i.e. below threshold conditions. An energy pulse, which initiates the formation of the initial and the mature double-crescentic stages, is clearly shown by the daily peak spectral energy data prior to the bar observation. The mature double~rescentic bars compose an energetic, aseasonal type occurring throughout the year. These observations indicate that nearshore bars are not always simply destroyed during storms and regenerated during low waves as suggested by Bernard et al. (1959), Davis and Fox (1972) and Coakley et al. (1973). The overall calming down interval of 40--60 days before bar observation represents the recovery of the inshore following storms. In the beach cycle observed by Hayes (1972) along the New England beaches, well
This study emanated from a project on nearshore bar systems supported by the Geography Programs, Office of Naval Research, Contract No. N0001478-C-0645, NP 388-148. V. Goldsmith, Principal Investigator. Wave data from Ashdod were kindly provided by D. Divon of the Coastal Study Division, Israel Ports Authority, and analyzed as part of a project funded by the Sea Grant International Program, National Sea Grant, N.O.A.A., Grant No. 04-8-M01-162. Weather and directional wave data at Tel-Shikmona were provided by H. Arbel and I. Matasaru, Israel Meteorological Station at Tel-Shikmona. Special thanks go to K. Kfley, Virginia, to Mrs. N. Peer, Ben-Gurion University, for the drafting, to the Israel Meteorological Service --the Archive, for providing wave climate data, to Mr. S. Sofer of the Israel National Oceanographic Institute, Haifa, for data processing, and to Y. Daube for assisting in many ways.
32 REFERENCES Bernard, H.A., Major, C.F. and Paxrott, B.S., 1959. The Galveston barrier Island and environ: A model for predicting reservoir occurrence and trend. Trans. Gulf Coast Assoc. Geol. Soc., 9: 221--224. Bowman, D., 1980. Validity of visually estimated wave parameters. Israel J. Earth Sci., 28: 94--99. Chappell, J. and Eliot, I.G., 1979. Surf beach dynamics in time and space -- an Australian case study, and elements of predictive model. Max. Geol., 32: 231--250. Clairbout, J.F., 1976. Fundamentals of Geophysical Data Processing with Application of Petroleum Prospecting. McGraw-Hill, New York, N.Y., 274 pp. Clos-Arceduc, A., 1962. Etude sur les rues aerienne des alluvions littorales d'allure periodique, cordons littoraux et festons. Soc. Ft. Photogramm. Bull., 4: 13--21. Clos-Arceduc, A., 1964. La photographie aerienne et l'~tude des depSts pr~littoraux : Etude de photo-interpretation. No. 1, Inst. Geogr. Nat., Paris. Coakley, J.P., Haras, W. and Freeman, N., 1973. The effect of storm surge on beach erosion, Point Pelee. Proc. 16th Conf., Great Lakes Res., pp.377--389. Davis, R.A. and Fox, W.T., 1972. Coastal processes and nearshore sand bars. J. Sediment. Petrol., 42: 401--412. Eitam, Y., Hecht, A. and Sass, E., 1978. Topographic and granulometric variations on the shore of Ma'agan Mikhael, Eastern Mediterranean. Israel J. Earth Sci., 27: 1--13. Emery, K.O. and Neev, D., 1960. Mediterranean beaches of Israel. Bull. Geol. Surv. Israel, 26 : 1--24. Galvin, C.J., 1972. Wave breaking in shallow water. In: R.E. Meyer (Editor), Waves on Beaches and Resulting Sediment Transport. Academic Press, New York, N.Y., pp.413--456. Goda, Y., 1974. Estimation of wave statistics from spectral information. In: W. Edge (Editor), Ocean Wave Measurement and Analysis, Vol. I. Am. Soc. Civ. Eng., New York, N.Y., pp. 320--337. Goldsmith, V., 1979. VIMS-BLM second order wave climate model and wave climatology of Baltimore Canyon Trough shelf area. SRAMSOE No. 203, Virginia Inst. of Mar. Sci., Gloucester Pt., Va., pp.15-1 to 15-77 + 3 app. Goldsmith, V. and Golik, A., 1980. Sediment transport model of the southeastern Mediterranean coast. Mar. Geol., 37: 147--175. Goldsmith, V. and Sofer, S., in press. Wave climatology of the Southeastern Mediterranean: An integrated approach. Israel J. Earth-Sci. Goldsmith, V., Bowman, D. and Kiley, K., 1982. Sequential stage development of crescentic bars: southeastern Mediterranean. J. Sediment. Petrol., 52: 233--249. Greenwood, B. and Davidson-Arnott, R.G.D., 1975. Marine bars and neaxshore sedimentary processes, Kouchibouguac Bay, New Brunswick. In: J. Hails and A. Cart (Editors), Nearshore Sediment Dynamics and Sedimentation. Wiley, New York, N.Y., pp.123-150. Greenwood, B. and Davidson-Arnott, R.G.D., 1979. Sedimentation and equilibrium in wave formed bars: a review and case study. Can. J. Earth-Sci., 16: 312--332. Guza, R.T. and Inman, D.L., 1975. Edgewaves and beach cusps. J. Geophys. Res., 80: 2997--3012. Hall, J.K. and Bakler, N., 1975. Detailed bathymetric and shallow seismic surveys at five locations along the Mediterranean coast of Israel. Geol. Surv. Israel Field Rep., 1/75, 21 pp. Hands, E.B., 1976. Observations of barred coastal profiles under the influence of rising water levels, eastern Lake Michigan, 1967--71. U.S. Army Corps of Eng., Coastal Eng. Res. Cent. Tech. Rep., 76-1, 113 pp. Hayes, M.O., 1972. Forms of sediment accumulation in the beach zone. In: R.E. Meyers (Editor), Waves on Beaches and Resulting Sediment Transport. Academic Press, New York, N.Y., pp.297--356.
33 Hunter, R.E., Clifton, A.E. and Phillip, R.L., 1979. Depositional processes, sedimentary structures and predicted vertical sequences in barred nearshore systems, Southern Oregon coast. J. Sediment. Petrol., 49: 711--726. IOLR, 1971. Bathymetric charts of the Israel Mediterranean shelf and upper slope in three 1:100,000 scale sheets. Jardine, T.P., 1979. The reliability of visually observed wave heights. Coastal Eng. J., 3: 33--38. King, C.A.M. and Williams, W.W., 1949. The formation and movement of sand bars by wave action. Geogr. J., 113: 70--85. Komar, P.D., 1976. Beach Processes and Sedimentation. Prentice-Hall, Englewood Cliffs, New Jersey, 429 pp. Migniot, C., 1974. Creation of a nuclear power station north of Hadera -- Natural phenomena study. Report by Laboratoire Central d'Hydraulique de France for Israel Electric Co., 34 pp. Neev, D., Almagor, G., Arad, A., Ginzburg, A. and Hall, J.K., 1976. The geology of the southeastern Mediterranean Sea. Geol. Surv. Israel Bull., 68, 51 pp. Nir, Y., 1973. Geological history of recent and subrecent sediments of the Israel Mediterranean shelf and slope. Geol. Surv. Israel Rep., MG/73/2,179 pp. Pomerancblum, M., 1966. The distribution of heavy minerals and their hydraulic equivalent in sediments of the Mediterranean continental shelf of Israel. J. Sediment. Petrol., 36: 162--179. Rosen, D.S. and Vajda, M., 1979. Hadera wind and wave climate. Coastal Mar. Eng. Res. Inst., Technion City, Haifa, Israel, P.N. 47/79, 7 pp. Short, A.D., 1975. Offshore bars along the Alaskan Arctic coast. J. Geol., 83: 209--221. Short, A.D., 1978. Characteristic beach morphodynamics on the southeastern Australian coast. 4th Aust. Conf. on Coastal Eng., Inst. of Eng., Adelaide, pp.148--152. Short, A.D., 1979. Three dimensional beach stage model. J. Geol., 87: 553--571. Short, A.D., 1980. Beach response to variations in breaker height. Coastal Studies Unit Tech. Rep., 80/2, Dept. of Geogr., Univ. of Sydney, N.S.W. Sonu, C.J., 1972. Field observation of nearshore circulation and meandering currents. J. Geophys. Res., 77 : 3232--3247. Sonu, C.J., 1973. Three dimensional beach changes. J. Geol., 81: 42--64. Spar, S.M., 1976. Sedimentological Behavior of the Beach Sand in the Vicinity of the Netanya Breakwaters. M.Sc. Thesis, Hebrew Univ., Jerusalem, 58 pp. Striem, H.L., 1974. The offshore bars at Ashdod, their topography and seasonal behaviour and their indicative ratios. Israel Atomic Energy Commission, IAo1299, 34 pp. Wright, L.D., 1980. Beach cut in relation to surf zone morphodynamics. Coastal Studies Unit Tech. Rep., 80/2, Dept. of Geogr., Univ. of Sydney, N.S.W. Wright, L.D., Chappell, J., Thom, B.G., Bradshaw, M.P. and Cowell, P., 1979. Morphodynamics of reflective and dissipative beach and inshore systems; southeastern Australia. Mar. Geol., 32: 105--140.