Robert W. Brander

Wave Processes on Coral Reef Flats: Implications for Reef Geomorphology Using Australian Case Studies

Abstract Australian reef flats on the Cocos (Keeling) Islands atoll, Indian Ocean; Warraber Reef, Torres Strait; and Lady Elliot Island, Great Barrier Reef vary greatly in morphology (width, elevation) and hydrodynamic setting (wave and tidal regime). This study describes results from detailed wave and current measurements, under nonstorm conditions, along five reef flat transects on these reef systems and examines implications for surface geomorphic processes. Results show that wave frequency and transformation varies between reefs in a consistent manner dependent on tidal elevation, reef elevation, and reef width. A nondimensional reef energy window index (Ψ) is developed that incorporates these critical factors (water depth at spring high tide and reef width). A statistically significant relation (95% confidence interval) between Ψ and the proportion of time that wave energy propagates across reefs illustrates the index ability to characterise the wave process regime of reef flats and provide a physically meaningful descriptor of the efficacy of geomorphic processes on reefs. High values of Ψ indicate narrow and low-elevation reef flats, which are exposed to high wave energy and are geomorphically active. Low values reflect wide and high-elevation reef flats associated with less active wave and geomorphic processes. Results show that while incident energy is undoubtedly an important factor for reef geomorphology, the nature of wave modification across reef flats is equally important in governing levels of geomorphic activity that control development of surface geomorphic features on reef platforms.

Field observations on the morphodynamic evolution of a low-energy rip current system

Abstract A field experiment was conducted at Palm Beach, NSW, Australia to investigate morphodynamic relationships operating within a rip current system during a near complete sequence of beach state evolution under decreasing energy conditions. Observations and measurements were obtained from nearshore surveys, multiple dye releases, oblique photographs, and the deployment of various combinations of nine ducted impellor flow meters and five pressure sensors over a seven-day period in rip and feeder channels. The dataset provides a unique opportunity to assess and quantify aspects of the accretionary model of Wright and Short (Mar. Geol., 56, 1984, 93–118) for intermediate beaches. The morphology of the main rip channel exhibited distinct sequential adjustments with a narrowing of channel width, an increase in channel depth, a reduction in the cross-sectional area available for rip flow, and an increase in morphological relief. Rip flow velocity increased during the observed evolution and was also modulated by the tide, experiencing maximums at low tide and minimums at high tide. Morphologic and kinematic adjustments of the rip system were most pronounced during the transition to a transverse bar and rip state. The existence of a dominant feeder in a twin feeder and rip system contributed to the migration and establishment of a rip-head over the seaward slope of the opposite longshore bar. Net erosion in the rip channel was balanced by net deposition in the feeder channels and bar crests and qualitative observations suggest that this rip-head bar both receives transported sediments from the rip channel and contributes sediments to the ongoing accretion of the beach system. A direct relationship was found to exist between rip morphology and flow with rip velocity increasing as cross-sectional rip channel area decreases and these results are incorporated in a conceptual modification of the model of Wright and Short. The results of this study are restricted to a subset of commonly occurring beaches, but show that accretionary beach state evolution on intermediate beaches is indeed characterised by an increase in rip current velocity, the nature of which can be linked to morphologic control. In addition, the rip-head bar is identified as a potentially key component of low-energy rip system evolution.

Flow velocity and sediment transport in the swash zone of a steep beach

Detailed measurements of flow velocity and total sediment load were obtained in the swash zone on a steep beach. Swash motion was measured using ducted impeller flow meters and capacitance water level probes. During wave uprush, the onshore flow increased almost instantaneously from zero to its maximum velocity after the arrival of the leading edge of the swash lens and subsequently decreased gradually to zero for the remainder of the uprush. During backwash, the offshore flow increased steadily from zero to its maximum towards the end of the backwash and dropped rapidly to zero as the beach fell “dry”. The duration of backwash was typically longer than that of uprush and maximum water depth on the beach was attained just prior to the end of the uprush. The total sediment load was measured for 35 individual wave uprush events using a sediment trap. The amount of sediment transported by a single uprush was typically two to three orders of magnitude greater than the net transport per swash cycle (difference between uprush and backwash) inferred from surveys of beach profile change. The measured immersed weight total load transport rate displayed a strong relationship with the time-averaged velocity cubed, which is consistent with equations for both bedload transport and total load transport under sheet flow conditions. The Bagnold (1963, 1966) bedload transport model was tested against our field data and yielded Ib = ku3Tu(tanφ + tanβ), where Ib is the immersed weight of bedload transported during the entire uprush (kg m−1), k is a coefficient (kg m−4 s2), u is the time-averaged flow velocity for the uprush (m s−1), Tu is the uprush duration (s), φ is the friction angle of the sediment and β is the beach slope. The empirically determined value for the coefficient k was 1.37 ± 0.17.

Geological effects of tsunami on mid-ocean atoll islands: The Maldives before and after the Sumatran tsunami

Low-lying coral islands are fragile landforms susceptible to long-term sea-level rise and extreme events, such as hurricanes and tsunamis. The Sumatran earthquake of 26 December 2004 generated waves that reached the Maldives 2500 km away. Observations of the effects of the tsunami are presented here, based on pre- and post-tsunami topographic and planform surveys of 13 uninhabited Maldivian islands. The surveys showed there was no substantial island erosion and no significant reduction in island area. Rather, the tsunami accentuated predictable seasonal oscillations in shoreline change, including localized retreat of exposed island scarps by up to 6 m, deposition of cuspate spits to leeward, and vertical island building through overwash deposition of sand sheets up to 0.3 m thick, covering up to 17% of island area. These results have implications for island stability indicating that low-lying reef islands are physically robust and the geological signature of tsunamis on atoll island development is minor.

Flow kinematics of low-energy rip current systems

Measurements of hydrodynamics and topography were obtained during two separate field experiments at Palm Beach, NSW, Australia in order to assess flow characteristics within rip current systems during accretionary conditions. At an evolutionary time scale (days-weeks), feeder channel flow decreased and rip-neck flow velocity increased during a cycle of intermediate beach state evolution. At shorter time scales (hours), rip current velocity is inversely related to changes in water depth and is clearly modulated by the tide. Spatially, rip current velocity increases progressively both longshore from the feeders to the rip-neck and offshore within the rip-neck itself. Flow velocity is not constant across the rip channel and increases towards the middle and deeper section of the rip channel. Vertical velocity profiles within the rip-neck and feeder channels show an initial increase in flow magnitude above the bed, with maxima towards the middle of the flow and then a subsequent decrease towards the surface. Evidence of pulsatory rip flow behaviour was found at infragravity frequencies in both rip-feeder and rip-neck environments. Infragravity energy was dominant in the feeder channels during the observed beach state evolution and increased in the rip-neck as the system evolved towards transverse bar and low tide terrace and rip beach states. There is evidence to suggest that topography may exert direct controls on rip flow. The results of this study provide both new quantitative information on the flow kinematics of rip currents and confirm findings of previous studies.

Beachgoers’ beliefs and behaviours in relation to beach flags and rip currents

BACKGROUND Coastal drownings claim on average 82 people per year in Australia. Beach flags are a primary safety strategy used on beaches. They are located away from rip currents, which are the main beach hazard affecting swimmers. Little is known about the behavioural and motivational factors associated with people choosing where to swim in relation to flags and rips. AIMS To assess the beliefs and behaviours of beachgoers in relation to beach flags and rip currents. METHODS Beachgoers at beaches in NSW were interviewed about their swimming beliefs and behaviours. They were asked to indicate on pictures depicting beach scenarios involving beach flags and fixed rip currents, where they would and would not swim. Logistic regression analysis was undertaken to determine predictors of correct and incorrect swimming behaviour. RESULTS Beachgoers who are aged from 30 to 49 years (OR 0.34, 95% CI 0.16, 0.74, p=0.006) are less likely to choose to swim between the flags than other swimmers. In addition, beachgoers who are at the beach with children are significantly more likely to choose to swim between the flags (OR 2.74, 95% CI 1.39, 5.40, p=0.004). Beachgoers with basic knowledge about rip currents are significantly more likely to swim away from the rip (OR 11.59, 95% CI 5.89, 22.81, p<0.001). DISCUSSION AND CONCLUSIONS Ocean swimmers aged from 30 to 49 years may choose to swim outside the flags, though they may not necessarily be swimming in the rip. Swimming outside of the flags may be linked with experience. The flags appear to be attractive to parents and carers of children. Whilst the flags indicate a relatively safe area of the beach, it is still vitally important for parents and carers to supervise children in this area. Basic rip current knowledge is an essential component in developing national interventions aimed at reducing coastal drowning. Beachgoers clearly need to know what a rip looks like in order to actively avoid swimming in it.

A comparison of attitudes and knowledge of beach safety in Australia for beachgoers, rural residents and international tourists

Objectives: To compare attitudes and knowledge of beach safety in Australia of beachgoers, rural inland residents and international tourists.

Regional variations in rip density

An extensive dataset combining information obtained from aerial photographs of selected Australian beaches and studies in the literature from a wide variety of sites in Europe, the United States, Japan, South Africa and New Zealand is used to investigate the relationship between rip current spacing on intermediate beaches and regional wave climate. A new parameter termed rip density (RD) is introduced which defines the number of rips per kilometer of beach and is defined as the relationship y r /L r where y r is rip spacing and L r is a nominal length of beach. The variation in rip density was examined for five different regional wave environments termed west coast swell (WCS), east coast swell (ECS), fetch-limited wind wave with strong (SWS) and moderate (MWS) winds, and fetch-limited bays (SWB). Patterns of rip density were extremely consistent between the grouped wave climate environments with WCS beaches characterised by the lowest RD of 2 rips/km and SWB and MWS beaches having the highest RD with values ranging from 11-13 rips/km. ECS beaches have a RD of 5 with SWS lying in between the range for WCS and ECS beaches at approximately 3 rips/km. The variation in rip density between environments exhibits distinct scaling relationships with RD on SWB and MWS beaches being approximately 5 times greater than on WCS beaches and twice as great than on ECS beaches. ECS beaches also have 2.5 times the number of rips on WCS beaches. Based on measurements and estimates of rip channel and surf zone width, there is evidence to suggest that these scaling factors may also be applied to the variation in two-dimensional planform morphology between the environments. The results of this study also indicate that rip density decreases with increasing wave height, wave period, surf zone width, wave energy, and wave power, thus providing quantitative links between observed rip density and regional wave climate.

The experiences of weak and non-swimmers caught in rip currents at Australian beaches.

ABSTRACT Beach rip currents present a risk to weak and non-swimmers, whose limited open water swim competencies may render them incapable of swimming out of these common and strong offshore flows. Here, the aim has been to explore this groups vulnerability by identifying how the limits of their swimming competencies influence their abilities to successfully execute a rip current exit strategy. It provides empirical detail on their knowledge of rip currents, their behavioural responses when caught in the rip current, the advice they recalled and the educational messages they recommend to other swimmers. A secondary aim has been to begin a dialogue that recognises the capacity of the Australian beach cultures—as places of relaxation and natural beauty—to influence peoples decisions to enter the water. This cultural contextualisation may help explain why some self-identified weak and non-swimmers still enter the water despite their limited competencies. Through a synthesis of survey and interview data, we argue that weak and non-swimmers need to take the threat of rip currents seriously. Pre-emptive safety advice should be sought, especially if people intend to swim regardless of their swimming ability.

An Introduction to Rip Currents Based on Field Observations

Abstract Rip currents are fascinating, natural, surf zone phenomena that occur daily on many beaches throughout the world. My colleagues, students, advisors, and I have been studying rip currents for more than 10 years and have performed more than 10 comprehensive field experiments on various beaches throughout the world using different observational techniques and model simulations to improve our understanding and prediction of rip currents. We have written a series of scientific articles describing the intricacies and complexities of rip current behavior using statistical and mathematical equations. These manuscripts are typically published in professional journals, which often do not communicate our results to those who would benefit from the information—the beachgoing public and ocean swimmers. Herein, we summarize our findings to help people of all ages gain a better understanding of currents at the coast.