St. Clair Beach Wave Dynamics

Early research focusing on beach profile changes within California (Shepard & La Fond, 1940; Shepard, 1950; Bascom, 1954) established a correlation between wave characteristics and beach morphology. Low, flat summer waves led to beach accretion and the formation of a prominent berm. Conversely, high, steep winter storm waves caused erosion, transporting sand offshore to form a longshore bar. This led to the terminology 'winter profile' (erosional) and 'summer profile' (accretional), reflecting seasonal variations. However, further research across various global locations showed that similar beach profile changes occurred, but not necessarily on a strict seasonal basis, highlighting the complex nature of beach dynamics and the limitations of solely relying on seasonal classifications. The initial findings emphasized the relationship between wave energy and sediment transport, with calmer conditions resulting in accretion and storm events driving erosion. The work highlighted the importance of wave energy and sediment movement in shaping beach profiles and laid the groundwork for more extensive investigation.

Subsequent research expanded on the initial California studies, demonstrating that the observed patterns weren't universally seasonal. Komar (1976) introduced a more generalized framework, classifying beach profiles as either 'storm profiles' (associated with high-energy wave events and erosion) or 'swell profiles' (linked to lower-energy wave conditions and accretion). This shift broadened the understanding of beach profile variability beyond simple seasonal cycles, emphasizing the influence of storm events regardless of the time of year. The new terminology better captured the broader spectrum of beach responses to varying wave climates. The study illustrates how understanding beach changes needs to move beyond simple seasonal patterns, considering the impact of storm waves irrespective of the time of year. This underscores the need for more comprehensive models that take into account the diverse range of wave conditions and their effects on coastal sediment dynamics.

Field studies revealed discrepancies between laboratory findings (wave tank experiments) and real-world observations. A key difference was that critical wave steepness, a crucial factor in determining beach erosion and accretion, tends to be lower in natural environments than predicted by two-dimensional wave tank experiments (Komar, 1960). This difference highlights the limitations of using solely laboratory-based models to predict beach behavior in complex, three-dimensional natural settings. Komar (1976) noted the difficulty in obtaining satisfactory wave data for accurately determining critical wave steepness due to the irregularity of profile changes and the possible greater importance of wave height variations compared to wave period variations. King (1972) further pointed out that critical wave steepness varies with the initial beach gradient, with gentler gradients having lower critical wave steepness values. The inherent complexities of field observations (numerous uncontrollable variables) complicate the task of creating a universally applicable model. Wright and Short (1984) supported this idea, demonstrating that the mechanisms causing beach change and the wave energy required depend heavily on the current state of the beach itself. The text emphasizes the dynamic equilibrium that beaches strive to maintain with their environment, and that predicting changes requires understanding the beach's initial state. The conclusion emphasizes the need for further field studies to bridge the gap between theoretical models and observed behavior.

The document discusses the use of predictive models, specifically referencing the work of Allen (1985) and Komar (1976). Allen's model, while using a different constant (2 vs 1.7), showed reasonable success in predicting onshore-offshore sediment transport in the majority of cases (87.5%). However, when comparing the predicted critical wave steepness to observed beach profile changes above low tide, Allen's model exhibited high accuracy for erosional events (98%) but lower accuracy for depositional events (45%). This discrepancy was attributed to a time lag in the appearance of accretional changes on the foreshore profiles. Despite this limitation, the study suggests that such models, while not perfect, could be beneficial in local planning and engineering design, particularly in identifying wave parameters most likely to lead to beach erosion. The discussion emphasizes the practical applications of these models, despite their limitations, for coastal management and engineering projects. The importance of using field data for model validation and refinement is highlighted, alongside the complexities involved in predicting both erosional and depositional events accurately.

Wright and Short (1985) highlighted the dependence of beach processes on the beach's existing state. They stated that the mechanisms driving beach cut and fill, along with the necessary wave energy, vary significantly depending on this pre-existing state. The contributions of incident waves, net surf zone circulations, and resultant sand transport are also influenced by the beach's condition. The modal beach state, according to their research, is a direct response to the modal breaker characteristics. The range of beach morphologies observed is, therefore, directly related to the range of wave conditions relative to the size of the beach sediment. Fine-grained sediments exposed to high wave energy tend to maintain highly dissipative states due to low sediment mobility, while coarse-grained beaches with low wave steepness exhibit highly reflective states. The highest mobility is associated with intermediate yet highly changeable wave conditions and medium-grained sediments, leading to beaches alternating between intermediate states. This emphasizes that a beach's response to wave action isn't uniform but depends crucially on its current morphological state and sediment composition. Understanding this dynamic interaction is key to accurate modeling of beach evolution and predicting future changes.

While beach profile variations are most easily visualized in two dimensions, Komar (1976) stressed that the reality of beach profile change is three-dimensional. This means that variations in longshore transport direction must also be considered when studying and modeling beach dynamics. King (1972) further emphasized the significance of longshore transport as a cause of coastal erosion. Waves acting perpendicular to the coast tend to only move material a short distance offshore, where much of it can be returned during calmer periods. However, sediment transport along the shore, moving material out of a specific area, often leads to more permanent erosion. Areas down-drift of groynes and headlands are particularly vulnerable to this type of erosion, as sediment is transported away from these structures. This highlights the need to incorporate three-dimensional aspects, particularly longshore sediment transport, into any comprehensive model or assessment of beach profile change and coastal erosion. A two-dimensional approach is insufficient to capture the complete picture of coastal sediment dynamics.

The study area's geology significantly influences the coastal features. The Otago Peninsula's formation involves the Dunedin Volcano, which had four main eruptive phases starting in the upper Miocene. The initial phase, centered on Portobello, involved the eruption of feldspathic trachytic lavas and fragmental rocks (Benson, 1959). This likely occurred before the sea's complete regression (Coombs et al., 1986), with ash from early eruptions found in the shallow-water Waipuna Bay formation (Suggate, 1978; Coombs et al., 1960). The initial eruptions might have been offshore, but were mostly subaerial, potentially forming a volcanic archipelago or peninsular system at the seaward edge of Cretaceous and Tertiary sediments. The underlying bedrock is formed by the Haast Schists, which outcrop west of the Dunedin Volcano and south along the Taieri to Brighton coastline. These schists, extending westward to Central Otago, are unconformably overlain by terrestrial deposits of the Henley Breccia and the Taratu Formation (Cretaceous). A Late Cretaceous to Upper Miocene marine transgression deposited a sequence of marine sediments that form the basement of the Dunedin volcanic complex (Coombs, 1965). The Dunedin Volcano's location at the Taieri Graben's northern end suggests a link to extensional tectonics. Though the current topography shows compressional forces, evidence suggests extensional stresses during volcanism (Coombs et al., 1986). Volcanism likely ceased with the Kaikoura Orogeny around 10 Ma, concluding the Otago Peninsula's volcanic formation.

The study highlights the importance of sediment sources and littoral transport in shaping the Otago coastline. Bardsley (1977) demonstrated that Catlins coast beach sands south of the Clutha River are rich in minerals from the Foveaux Strait-Western Province (hypersthene and hornblende). However, north of Nugget Point, near the Clutha River mouth, Haast Schist-derived quartzo-feldspathic sands become dominant. These sands extend northward, forming the dominant beach and dune deposits north of the Otago Peninsula. The northward and northeastward littoral transport is further confirmed by the presence of pyroxenes and olivine from the Dunedin Volcanics in sand deposits 15km northeast of the initial coastal volcanic outcrops. This demonstrates a clear northward sediment transport pattern, with the Clutha River acting as a major source of quartzo-feldspathic sands for the Otago Peninsula's beaches. This information is crucial in understanding the current composition of the beaches and the long-term sediment dynamics of the region. The mineralogical composition of the sands provides insights into the source materials and dominant sediment transport pathways along the coast.

The South Otago continental shelf shows distinct sedimentological features. A northeast-southwest trending belt of relict terrigenous gravel facies on the middle shelf comprises well-rounded quartz clasts, largely derived from the Haast Schists via the Clutha River (Andrews, 1973). Debate surrounding this interpretation (Schofield, 1976, 1977; Cullen, 1976; Andrews, 1976; Probert, 1977) questioned if this facies represents a relict feature or a seascape in equilibrium with present-day sea level. Schofield (1976) suggested it could be a lag deposit in partial equilibrium with the Southland Current, where the current hydraulic regime removes finer materials. Further debate surrounded the origin of a submarine sand spit off the Otago Peninsula, with Andrews (1976) suggesting a formation during an 8-9 ka sea level standstill followed by drowning, while Schofield (1976) argued for modern formation through deposition from north-flowing currents. The inshore modern terrigenous sand facies forms a seaward-thinning wedge of fine grey sands, mainly originating from the Clutha River mouth. This wedge is widest and thickest offshore of the Clutha River and narrows substantially north of Brighton, continuing as a narrower ribbon around the Otago Peninsula. This reduction may reflect the extent of modern northward sand transport or a response to local hydraulic conditions, possibly current reversal against the Peninsula. This facies can be further divided into two shore-parallel suites based on mineralogy, reflecting different sediment sources.

The Clutha River, a major sediment source for the Otago coast, is now dammed. The impact of these dams (Roxburgh since 1961, Clyde since 1992) on the sediment budget is significant. The Roxburgh Dam, for instance, traps approximately 50% of the Clutha River's bedload at a rate of 0.61 Mt/yr (Jowett and Hicks, 1981 in Carter, 1986). With the Clyde Dam adding to this, the reduction in sediment input is likely even greater. It is uncertain if the shelf and coastal sediment systems have had sufficient time to respond to this decreased sediment input. The long-term consequences of this reduced sediment supply remain unclear and require further research. This reduced sediment supply could have future implications for beach processes, potentially leading to increased coastal erosion in the Otago area. The long-term impact of damming on sediment transport, and subsequent effects on beach morphology and coastal erosion, remain a key area of future investigation.

The Otago region's climate, summarized from de Lisle and Brown (1968), is dominated by mid-latitude westerly winds and the eastward passage of anticyclones at roughly 6-7 day intervals. Otago typically sits within a disturbed westerly airstream, influenced by the passage of these anticyclones and intervening low-pressure troughs. These troughs, frequently bringing rain and strong cold southwesterly winds, are often blocked by the Southern Alps, leading to strong north to northwest flows. These northerly flows precede the southwesterly winds and are associated with warm, dry weather in coastal Otago. Depressions east of the South Island often result in rain-bearing easterly to southerly winds on the Otago coast. During the late spring and summer months, sea breezes from an easterly direction frequently affect the coast under weak gradient flows. This pattern of alternating high and low-pressure systems, coupled with the orographic effects of the Southern Alps and the presence of sea breezes, creates a dynamic and variable wind regime. This variability is crucial in understanding how the wind influences wave conditions and subsequently beach processes.

Wave tank experiments (King & Williams, 1949; King, 1959) have shown that strong onshore winds cause offshore sand transport, even under wave conditions that would normally lead to onshore sand movement in the absence of wind. Field studies (Shepard & La Fond, 1940) also associated periods of strong onshore winds with erosional phases in beach profiles. The shoreward movement of surface waters caused by onshore winds is counteracted by a seaward return current at depth, and vice-versa with offshore winds. However, Komar (1976) notes the difficulty in separating the effects of these wind-induced currents from the wind's impact on wave characteristics. Onshore winds tend to generate short, steep waves that contribute to beach erosion, while offshore winds often create flatter waves, leading to accretion, masking the wind current's individual effect. The complex interaction between wind and waves makes it challenging to isolate the specific contribution of wind-driven currents on sediment transport. A clear understanding of the combined effects is crucial in accurate modeling of coastal processes.

The study collected wind data from December 14, 1993, to June 16, 1994, using a Lambrecht instrument. Unforeseen circumstances led to earlier than planned cessation of data collection. Mechanical issues resulted in a sporadic dataset with approximately 28 days of missing data. The gradient wind direction for these missing days was estimated visually using New Zealand Meteorological Services synoptic charts. The analysis of the wind data showed a conspicuous absence of northeasterly winds, particularly during the study period. However, northeast winds are a recognised part of the coastal Otago wind regime. The study concludes that local topographic influences at the data collection site may have led to the recording of northeast winds as northerly winds. The northerly wind direction was dominant in all wind roses, except December (where seven days of northerly data are missing), though speeds generally were less than 4 m/s. This observation is consistent with long-term Taiaroa Head wind records. Therefore, while a north to northeast wind is the most dominant wind direction, its intensity tends to be relatively low. This data has implications for evaluating the influence of wind on beach processes.

The earliest documentation of the Otago wave environment comes from Elliot (1958), based on personal observations rather than a formal data set. Elliot noted that swells generally ran from south to southeast, with wave periods ranging from 10.3 to 13 seconds. These waves were primarily considered swell waves modified by local winds, not waves generated locally. The bimodal distribution of local Otago winds was seen as a crucial factor influencing the nature of the dominant southerly swell waves reaching the coast. Wave heights varied considerably depending on wind direction; reaching 1.8–2.4m under fresh southwesterly winds but only 0.3–0.9m under north to northeasterly winds. Wave periods were relatively similar under both wind conditions, except at extremes where strong southwesterlies increased and strong north-to-northeasterlies decreased the periods. Elliot observed that southwesterly winds favored steeper, high-energy waves, while north to northeasterly winds produced flatter, low-energy waves. The study noted that breakers on the Otago coast were mainly plunging under all conditions. This initial work established a basic understanding of the wave climate and the influence of wind direction on wave characteristics, although its reliance on subjective observation limited its quantitative value.

Hodgson (1966) provided shore-based wave observations from Cape Saunders on the Otago Peninsula's south coast. Hodgson reported that southerly swell dominated (57%), followed by southeast swells (26%). However, the presentation of this data shows an inconsistency; either the bar length for southerly or southeasterly swells must be incorrect. This inconsistency affects the calculated frequencies of other swell directions (northeast, east, and southwest), making precise interpretation challenging. The present author uses a compromise distribution of these data for comparison purposes. The observed swell directions were southwest, south, southeast, east and northeast. The difference between Hodgson's data and other data sets highlights the variability in wave characteristics even within a relatively localized area. It points to the need for more precise and comprehensive data collection to fully understand the Otago wave climate, and to account for possible errors in earlier observations.

This study employed the U.S. Army Corps of Engineers Littoral Environment Observation (LEO) program (Schneider, 1981) for data collection. Daily shore-based observations were conducted from December 11, 1993, to May 31, 1994, at the eastern end of Tomahawk Beach. These visual observations, deemed appropriate for long-term assessment (Smith and Wagner, 1991; Plant and Griggs, 1992), recorded swell direction, significant wave height at the outermost breakpoint, wave type, wave period, and inshore sea state, along with wind direction. The significant wave height, the average height of the highest one-third of waves, was visually estimated, with surfer presence in the summer months aiding in accurate estimations. During autumn, estimations were entirely visual. The accuracy of these visual estimations is supported by previous studies (Schneider and Weigel, 1980), demonstrating that shore-based estimates provide a reasonable representation of prevailing breaker heights. The data from this study can be compared with previous work (Hodgson, 1966), allowing for assessment of the evolution of the Otago wave environment over time. The study demonstrates a methodology for collecting long-term wave data, highlighting the importance of visual observations in understanding long-term wave climates.

The study compared the Tomahawk Beach wave data with New Zealand Meteorological Service ship report data and Hodgson's (1966) Cape Saunders data. The ship report data included swell directions (west, northwest, north) not present in other datasets, reflecting the broader geographical area covered. The Otago coastal area is characterized by southwest, south, southeast, east, and northeast swell directions, primarily. Differences between the Cape Saunders (Hodgson, 1966) and Tomahawk datasets in swell directions (e.g., east swells observed at Cape Saunders but not Tomahawk) are explained by locational differences. Cape Saunders’ location on the eastern tip of the Otago Peninsula makes it more open to easterly swells, whereas Tomahawk might be more sheltered. The study hypothesizes that longer-period northeast swells at Cape Saunders may be refracted around to the east before recording, while some east swells might be recorded as southeast swells. The comparison of various datasets highlights the spatial variation of wave climate and the importance of location in wave data interpretation. The discrepancy in observed swell directions reveals the complex refraction patterns around the Otago Peninsula and the need for site-specific wave data.

The study investigated the relationship between wind direction and wave characteristics. Southerly winds (southwest, south, southeast) favored higher-energy, shorter, steeper waves with increased height and lower periods, while northerly winds (northwest, north, northeast) favored lower-energy, flatter waves with decreased height and increased periods. These findings suggest that onshore southerly winds should be associated with beach erosion, while offshore northerly winds should be associated with accretion (consistent with Elliot, 1958). The observed breaker types also correlated with wind direction. Purely plunging breakers were predominantly observed under offshore northerly winds, while spill-plunge breakers were more common, and onshore southerly winds primarily led to spilling breakers. Although local winds influence breaker type, nearshore slope, swell height, and period also play a role (Pethick, 1984), explaining the observed variability in breaker types. This analysis shows the combined influence of wind and other factors on wave characteristics and beach processes. The interaction between wind, wave height, wave period and the resultant breaker type influence the overall coastal energy and related sediment transport.

The study analyzed reported sea states from the New Zealand Meteorological Service, noting a discrepancy between reported swell heights and sea states. Swells over 4m were reported at only 0.8%, while a significant portion of rough and very rough sea states were associated with waves in the slight and moderate swell height groupings. This indicates a possible overestimation of sea state relative to swell height. The study suggests that the reported sea state is directly linked to the highest forecast wind speed (excluding outlook wind speeds) for the coastal forecast area. For example, forecasts of high wind speeds ('southwest 20 knots, rising to 40 knots') correlate with sea state forecasts like 'seas becoming Very Rough'. This highlights a potential bias in sea state reporting, influenced more by forecast wind speed than by the actual swell heights. The study concludes that the reported sea state appears to be an overestimation of sea surface conditions compared to the reported swell heights. However this is consistent with the hydrographic office description of the relationship between local wind speeds and the heights of locally generated waves.

Nicholson (1979) conducted monthly beach profile surveys on the Otago Peninsula's north coast (KaiKai, Murdering, Long, and Purak:anui beaches). Significant profile variations were observed among the beaches, lacking distinct seasonal patterns. This study didn't attempt to link the profile changes to the wave and wind environments. The north coast beaches are geographically distinct from the south coast beaches studied here, with the two areas subject to significantly different wave climates. This underscores the regional variability in wave dynamics and its importance in shaping coastal morphology. The comparison of Nicholson’s north coast study and the present south coast study highlights the complexity of beach processes and the importance of site-specific investigations in the Otago region. The lack of clear seasonal patterns in the north suggests that a generalized approach might not be applicable to all parts of the Otago coastline.

The study involved beach profile surveys conducted at five different locations along St. Clair-Ocean and Tomahawk beaches on the Otago Peninsula. The surveys aimed to measure beach profile changes over a six-month period (December 11, 1993 - May 31, 1994). Data collection utilized the U.S. Army Corps of Engineers Littoral Environment Observation (LEO) program's methodology (Schneider, 1981), incorporating visual observations deemed appropriate for long-term wave climate assessment (Smith and Wagner, 1991; Plant and Griggs, 1992). Daily observations were taken around 8:00 am from the headland at the eastern end of Tomahawk Beach. Recorded parameters included swell direction, significant wave height at the outermost breakpoint, wave type, wave period, and inshore sea state, along with concurrent wind direction. Significant wave height, representing the average height of the highest one-third of waves, was visually estimated, using surfer observations in summer months to aid accuracy. In the autumn months, estimations relied solely on visual assessment. The consistency and reliability of visually estimated wave heights were supported by previous research, indicating a reasonable correlation with gauge measurements (Schneider and Weigel, 1980). This consistent data collection methodology allowed for comprehensive analysis of beach profile changes in relation to wave and wind conditions.

The analysis of beach profile changes across the five survey locations revealed unexpected inconsistencies with existing theoretical models. Despite theoretical predictions suggesting continuous erosion throughout the study period, all five profiles showed phases of both accretion and erosion. This discrepancy highlighted the limitations of current theoretical models in accurately predicting the dynamic behavior of natural beaches, emphasizing the need for further refinement. The study used a mean fall velocity of 0.04 m/s, based on sediment analysis conducted on May 1st, yielding a mean sediment size of 1.75 phi. This slightly larger value than that found by Hodgson (1966) (1.96 phi) for St. Clair-Ocean Beach, was attributed to the post-erosional collection of coarser sediments. A smaller mean sediment size would have resulted in a lower fall velocity and thus a larger discrepancy between the observed and predicted data. This deviation from theoretical expectations emphasizes the inadequacy of simplistic models in capturing the full complexity of beach profile evolution. It underscores the need to integrate various factors and site-specific conditions to understand observed beach changes accurately.

The study tested the applicability of Dean's (1973) equation for critical wave steepness to the observed data, revealing significant limitations. The equation did not accurately predict the observed beach profile changes, with observed wave steepness consistently higher than the predicted values. The study notes that the Dean equation was found to fit the Allen (1985) data better, likely because Allen's study had significantly lower wave heights. This further suggests that the relationship between critical wave steepness and wave height may be non-linear and dependent on the scale of the waves relative to sediment size. Results indicated critical wave steepness values were higher than those predicted by Dean (1973), conflicting with Iwagaki and Noda's (1963) wave tank experiment findings. These inconsistencies emphasize the limitations of applying laboratory-based models directly to natural environments and highlight the need for more field investigations into this crucial parameter. It suggests that the relationship between critical wave steepness and factors like wave height and sediment size might not be universal across different natural coastal environments.

Analysis of the data linked specific wave characteristics to beach profile changes. The accretional period between December 20 and January 4, marked by the formation of a prominent berm, correlated with the presence of flat east-southeast and south swells with a mean wave steepness less than 0.009 (King, 1972; Komar, 1976). This supports the understanding that low-steepness waves promote berm building and beach accretion. The study also considered the influence of longshore transport on profile changes. The eastward longshore transport under easterly quarter swells explained the strong accretion at the western ends of the study beaches and contributed to the formation of the south-facing spits. In contrast, the main erosional events were linked to extended periods of strong southwesterly winds. These winds increased longshore velocities, enhancing the transport of sand eastward, away from the western end of St. Clair. The more direct approach of southerly waves, compared to the oblique approach of easterly quarter swells, was also considered in relation to the longshore impulse. The findings suggested that while southerly swells create eastward longshore velocities, strong southwesterly winds enhance these velocities considerably.

The study further examined the influence of wind on erosion and accretion events. Strong southwesterly winds were consistently associated with major erosional episodes. These winds contributed to erosion by increasing longshore velocities, promoting seaward bottom return currents, and generating larger, steeper waves with reduced periods (as discussed in Chapter 4). The association of these winds with deep low-pressure systems also led to higher water levels, further increasing erosional potential. These findings were consistent with Elliot's (1958) observations, establishing the importance of strong southwesterly winds in creating erosional conditions on the Otago coast. Conversely, easterly swells contributed to accretion, particularly at the western ends of the beaches. While wind enhancement of longshore velocities could benefit accretion in this area, the effect is less pronounced than for erosion due to the more oblique approach of easterly quarter swells. This comprehensive analysis illustrated the complex interplay between wind, wave, and sediment transport in determining the erosional and depositional patterns observed.

The strong accretion at the western ends of the beaches, caused by longshore sand transport under easterly quarter swells, is critical in explaining the south-facing spit tips on the Otago Peninsula. The inlets are primarily located at the southern corners of the beaches, where dominant southerly swells are most refracted and decayed—the lowest wave energy zones. Longshore velocities generated by northeast and east swells also contribute to the formation of south-facing spits and help maintain inlet openings in the southern corners. The dimensionless Dean parameter predicted the beaches to be predominantly in intermediate states. Reflective conditions, associated with low-height swells and the presence of prominent berms, were observed only briefly. This analysis, considered alongside Carter's (1988) morphodynamic indices, suggested that the beaches might be more dissipative than indicated by the Dean parameter, considering the prevalent number of spilling waves in the surf zone. The study integrated diverse factors—wave direction, wave steepness, wind, and longshore transport—to understand the observed beach morphology and dynamic equilibrium.