Chris Blenkinsopp

Comprehensive Field Study of Swash-Zone Processes. II: Sheet Flow Sediment Concentrations during Quasi-Steady Backwash

AbstractSheet flow sediment concentration profiles were measured in natural conditions for the first time as part of a comprehensive field study on swash-zone hydrodynamics and sediment transport. Three conductivity concentration profilers (CCPs) measured the sediment concentration profile in the sheet flow layer with a 1-mm resolution in the swash zone of a dissipative beach. This paper focuses on sheet flow during quasi-steady backwash events generated by infragravity motion when the effects of phase lags, surface-generated turbulence, and accelerations are small. The sheet flow sediment concentration profile has a linear shape in the lower section of the profile and a power-law shape in the upper section, with the transition occurring at sediment volume fractions of 0.20–0.30. The shape of the concentration profile is self-similar for measured sheet flow layer thicknesses ranging from 6 to 18 mm. Because of the self-similarity, a single concentration profile curve can be used to describe the normalized...

Comprehensive Field Study of Swash-Zone Processes. I: Experimental Design with Examples of Hydrodynamic and Sediment Transport Measurements

AbstractA comprehensive study of swash-zone hydrodynamics and sediment transport was conducted on a macrotidal beach in Perranporth, United Kingdom. The unique study is the first to simultaneously measure suspended sediment and sheet flow sediment concentrations, water depth, near-bed velocity profiles, and high-resolution swash surface and bed-level changes on a natural beach. Data collected during the study are used to quantify the vertical profile of cross-shore and alongshore velocities and the importance of sheet flow sediment processes in the swash zone. The swash-zone boundary layer for cross-shore velocities is observed to generally occur over at least the lower 0.06 m of the water column. Alongshore velocities are often the same order of magnitude as the cross-shore velocities and are dominant near cross-shore flow reversal. Flows are often logarithmic in profile, but the instantaneous nature of the measurements renders application of the logarithmic model difficult. When valid, the logarithmic m...

Overwash experiment on a sandy barrier

ABSTRACT Matias, A., Masselink, G., Kroon, A., Blenkinsopp, C., and Turner, I.L., 2013. Overwash experiment on a sandy barrier This paper uses results obtained from the large-scale BARDEX II experiment undertaken in the Delta flume to investigate the morphological response of a prototype sandy barrier to wave and tidal forcing during overwash conditions. Since overwash processes are known to control short-term barrier dynamics and long-term barrier migration, the development of a robust quantitative method to define the critical conditions leading to barrier overwash is important both for scientific and practical management purposes. The Overwash Potential (OP), defined as the difference between the wave runup and the barrier elevation is used to define the overwash threshold condition, and to predict the morphological outcome of a particular overwash event. When OP is negative, wave runup is lower than the barrier crest and insignificant morphological changes are noticed at the barrier crest. When OP is positive, overwash occurs because predicted runup elevation is higher than the barrier crest. When OP is close to zero, overtop is expected with limited intrusion of water across the top of the barrier crest. To make effective use of OP it is necessary to identify a reliable runup predictor. Twelve runup equations were tested for this purpose, and the results were compared with the ones obtained using data from BARDEX experiment on a gravel barrier. A most reliable approach for the determination of OP for sandy barrier was similar to gravel barrier overwash experiments, with runup predictions provided by the equation of Stockdon et al. [Stockdon, H.F., Holman, R.A., Howd, P.A.,Sallenger, A.H., 2006. Empirical parameterization of setup, swash, and runup. Coast. Eng., 53, 573–588]. This is striking, since different runup predictors would have been expected because beach slope, hydraulic conductivity, grain-size, amongst other factors, differ for both types of barriers. Nevertheless, the two main morphologic characteristics for the computation of OP are beach slope and the barrier crest elevation, both accounted for in the proposed equation. The use of OP values provides a practical means by which to identify potential coastal hazards associated with barrier overwash processes and is considered to have a range of practical coastal management applications.

Observations of the swash zone on a gravel beach during a storm using a laser-scanner (Lidar)

ABSTRACT Almeida, L.P., Masselink, G., Russell, P.E., Davidson, M.A., Poate, T.G., McCall, R.T., Blenkinsopp, C.E. and Turner, I.L. 2013. Observations of the swash zone on a gravel beach during a storm using a laser-scanner The collection of detailed field measurements from the swash zone during storms is an extremely challenging task which is difficult to execute with traditional in-situ deployments (e.g., scaffold rigs with instruments). The levels of difficulty increase for gravel beaches where the wave energy reaches the beach face with almost no loss of energy, leading to violent plunging wave breaking on the beach face that can produce large vertical morphological changes and extremely strong uprushes that can easily and rapidly damage, bury or detach instrumentation. Remote-sensing techniques emerge as the most appropriate solution to perform field measurements under such adverse conditions since they have the ability to perform measurements without being deployed in-situ. A mid-range (~ 50 m) Laser-scanner mounted on a tower (~ 7 m high) in the mid beach face of a gravel beach (Loe Bar - SW England) was used to measure bed-level changes and runup at a sampling rate of 2 Hz along one beach profile during a storm. The results from the comparison of this system with other state-of-the-art instruments (e.g., ultrasonic bed level sensors, GPS and video cameras) indicate that the quality of the measurements obtained is within the accuracy of the standard methods. The advantages of this system is the reduced logistical infrastructure required for the deployment, the capability to perform surveys with high spatial and vertical resolution, during day and night, and to reach areas of the swash zone where no other instrument can be deployed safely. Measurements performed with a laser-scanner on a gravel beach (Loe Bar) show complex and fast-changing morphology on the gravel beach, which appears to be a form of negative morphodynamic feedback to controls the hydrodynamic evolution in the swash zone.

Bubble Size Measurements in Breaking Waves Using Optical Fiber Phase Detection Probes

Knowledge of the two-phase flow generated by breaking waves is desirable if we are to fully understand their effect on environmental processes such as air-sea gas exchange and production of sea-salt aerosol. However, due to the difficulties in making measurements in the high void fraction bubble plumes generated immediately after breaking, little detailed information is available. This paper describes laboratory measurements of the size of bubbles entrained in the dense plumes generated by wave breaking made using a pair of highly sensitive optical fiber phase detection probes. The results compare well with those of previous authors in the low void fraction parts of the flow, and go on to include data from within the highly aerated region present in the period shortly after breaking, close to the area of most active air entrainment. The data highlights the spatial and temporal evolution of the bubble sizes within breaker generated bubble plumes and demonstrates that some large air cavities with diameters of tens of millimeters are initially entrained. It is observed that the bubbles resident within the plume rapidly decrease in size with time and distance away from the point of primary entrainment as the large cavities initially entrained are broken up into smaller bubbles.

Monitoring Individual Wave Characteristics in the Inner Surf with a 2-Dimensional Laser Scanner (LiDAR)

This paper presents an investigation into the use of a 2-dimensional laser scanner (LiDAR) to obtain measurements of wave processes in the inner surf and swash zones of a microtidal beach (Rousty, Camargue, France). The bed is extracted at the wave-by-wave timescale using a variance threshold method on the time series. Individual wave properties were then retrieved from a local extrema analysis. Finally, individual and averaged wave celerities are obtained using a crest-tracking method and cross-correlation technique, respectively, and compared with common wave celerity predictors. Very good agreement was found between the individual wave properties and the wave spectrum analysis, showing the great potential of the scanner to be used in the surf and swash zone for studies of nearshore waves at the wave-by-wave timescale.

High Water Mark Determination Based on the Principle of Spatial Continuity of the Swash Probability

ABSTRACT Liu, X.; Xia, J. (C.); Blenkinsopp, C.; Arnold, L., and Wright, G., 2014. High water mark determination based on the principle of spatial continuity of the swash probability This study presents a model that determines the position of the high water mark (HWM) based on the spatial continuity of inundation probability due to swash for a range of HWM indicators. These indicators include mean high water (MHW), high water line (HWL), and a number of shoreline features, such as the vegetation line. HWM identifies the landward extent of the ocean and is required for cadastral boundary definition, land-use and infrastructure development along the foreshore ,and for planning associated with climate change adaptation. In this paper, shoreline indicators are extracted using an object-oriented image analysis (OOIA) approach. Ten-year hourly swash heights (shoreline excursion length) are fitted into a cumulative distribution function. The probability that swash will reach the various HWM indicators over a 10 y period is then estimated. The spatial continuity distances of the swash probability of HWM indicators are calculated using semivariogram models that measure similarity of swash probability. The spatial continuity distance is defined as the distance between the lower bound of sampling position (the most seaward HWM indicator) and the position where autocorrelation, or the similarity of swash probability of the various HWM indictors, approaches zero. The latter is considered as the HWM position in this study. This HWM determination method is evaluated at two study sites at different latitudes and with distinct coastal features.

FIELD MEASUREMENTS OF NET SEDIMENT FLUX FROM INDIVIDUAL SWASHES ON A SANDY BEACH

The shoreline along sandy beaches is located at a unique position on the earth’s surface where marine and terrestrial processes converge. The swash zone distinguishes the landward-most reach of wave action. Field observations from this shallow and highly energetic region reveal that individual waves regularly deposit or remove hundreds of kilograms of sand per meter width of beach. Such high rates of sand movement represent several centimeters of bedlevel change and far exceed the underlying pace of beach evolution. Relatively large morphological changes caused by single swashes might suggest that very rapid beach erosion or accretion is a common occurrence. The contrasting reality shown by these new observations is that beaches generally exhibit a state of dynamic equilibrium.

Surf-swash interactions on a low-tide terraced beach

ABSTRACT Almar, R.; Almeida, P.; Blenkinsopp, C., and Catalan, P., 2016. Surf-swash interactions on a low-tide terraced beach. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 348-352. Coconut Creek (Florida), ISSN 0749-0208. Through an integrated approach, this paper investigates the role of coupled surf-swash dynamics on outgoing waves using data collected at a low-tide terraced beach, Grand Popo, Benin (Gulf of Guinea, West Africa). Observed reflection is 8 %. Analyses are conducted from deep water directional wave spectra measurements, daily beach surveys and remote video measurements. Our results show that the swash can be a non-negligible component of the nearshore energy balance (14% of total dissipation) and is closely tied to reflection. Reflection thus depends on waves at swash inception (offshore waves and surf zone saturation), and shoreface slope varying with tide and morphological evolution. An outgoing cut-off frequency (shortest reflected waves) can be linked to swash saturation with a strong dependence on shoreface slope. A phase-resolving Boussinesq model was validated and used to investigate the influence of terrace width, upper shoreface slope and tidal elevation over the terrace. This papers puts forward the role of the coupled system surf-swash and underlines potential key interactions between a rapid shoreface evolution and surf zone hydro-morphodynamics.

High Frequency Field Measurements of an Undular Bore Using a 2D LiDAR Scanner

The secondary wave field associated with undular tidal bores (known as whelps) has been barely studied in field conditions: the wave field can be strongly non-hydrostatic, and the turbidity is generally high. In situ measurements based on pressure or acoustic signals can therefore be limited or inadequate. The intermittent nature of this process in the field and the complications encountered in the downscaling to laboratory conditions also render its study difficult. Here, we present a new methodology based on LiDAR technology to provide high spatial and temporal resolution measurements of the free surface of an undular tidal bore. A wave-by-wave analysis is performed on the whelps, and comparisons between LiDAR, acoustic and pressure-derived measurements are used to quantify the non-hydrostatic nature of this phenomenon. A correction based on linear wave theory applied on individual wave properties improves the results from the pressure transducer (Root mean square error, R M S E of 0 . 19 m against 0 . 38 m); however, more robust data is obtained from an upwards-looking acoustic sensor despite high turbidity during the passage of the whelps ( R M S E of 0 . 05 m). Finally, the LiDAR scanner provides the unique possibility to study the wave geometry: the distribution of measured wave height, period, celerity, steepness and wavelength are presented. It is found that the highest wave from the whelps can be steeper than the bore front, explaining why breaking events are sometimes observed in the secondary wave field of undular tidal bores.