Ian L. Turner

Shoreline Definition and Detection: A Review

Abstract Analysis of shoreline variability and shoreline erosion-accretion trends is fundamental to a broad range of investigations undertaken by coastal scientists, coastal engineers, and coastal managers. Though strictly defined as the intersection of water and land surfaces, for practical purposes, the dynamic nature of this boundary and its dependence on the temporal and spatial scale at which it is being considered results in the use of a range of shoreline indicators. These proxies are generally one of two types: either a feature that is visibly discernible in coastal imagery (e.g., high-water line [HWL]) or the intersection of a tidal datum with the coastal profile (e.g., mean high water [MHW]). Recently, a third category of shoreline indicator has begun to be reported in the literature, based on the application of image-processing techniques to extract proxy shoreline features from digital coastal images that are not necessarily visible to the human eye. Potential data sources for shoreline investigation include historical photographs, coastal maps and charts, aerial photography, beach surveys, in situ geographic positioning system shorelines, and a range of digital elevation or image data derived from remote sensing platforms. The identification of a “shoreline” involves two stages: the first requires the selection and definition of a shoreline indicator feature, and the second is the detection of the chosen shoreline feature within the available data source. To date, the most common shoreline detection technique has been subjective visual interpretation. Recent photogrammetry, topographic data collection, and digital image-processing techniques now make it possible for the coastal investigator to use objective shoreline detection methods. The remaining challenge is to improve the quantitative and process-based understanding of these shoreline indicator features and their spatial relationship relative to the physical land–water boundary.

Swash infiltration-exfiltration and sediment transport

Field measurements of vertical pore-pressure gradients within the bed are used to quantify instantaneous (8 Hz) rates of swash infiltration-exfiltration across the beach face. Cyclic infiltration-exfiltration is associated with individual swash events, with observed vertical flow rates O(10−3) m/s. Rates of net swash-groundwater exchange (i.e., through-bed flow integrated over several swash cycles) are two orders of magnitude smaller. At the timescale of individual swashes, vertical pore-pressure gradients within the beach face are much greater than horizontal pore-pressure gradients. This permits application of the numerical solution of Darcys law for one-dimensional vertical flow to model fluctuating pore pressures (and hence vertical through-bed flow). Vertical flow through a porous bed modifies sediment mobility in (at least) two ways: (1) Seepage forces change the effective weight of surficial sediments, and (2) boundary layer “thickening” or “thinning” result in altered bed shear stresses. By considering these two (opposing) effects separately, a new Shields parameter is derived that incorporates terms for through-bed flow. Simulation of time-varying seepage force and bed stress effects over an uprush-backwash cycle suggests that the effect of altered bed stresses dominates over the change in effective weight and that infiltration-exfiltration effects are most important during uprush. Simulated transport rates are increased by up to 40% of the peak transport rate during uprush and reduced by 10% during backrush. In summary, swash infiltration-exfiltration across a saturated beach face enhances the net upslope transport of sediment.

A video-based technique for mapping intertidal beach bathymetry

Abstract Measuring the location of the shoreline and monitoring foreshore changes through time are core tasks carried out by coastal engineers for a wide range of research, monitoring and design applications. With the advent of digital imaging technology, shore-based video systems provide continuous and automated data collection, encompassing a much greater range of time and spatial scales than were previously possible using field survey methods. A new video-based technique is presented that utilises full-colour image information, which overcomes problems associated with previous grey-scale methods, which work well at steep (reflective) sites, but are less successful at flatter (dissipative) sites. Identification of the shoreline feature is achieved by the automated clustering of sub-aqueous and sub-aerial pixels in ‘Hue–Saturation–Value’ (HSV) colour space, and applying an objective discriminator function to define their boundary (i.e., ‘shoreline’) within a time-series of consecutive geo-referenced images. The elevation corresponding to the detected shoreline features is calculated on the basis of concurrent tide and wave information, which is incorporated in a model that combines the effects of wave set-up and swash, at both incident and infragravity frequencies. Validation of the technique is achieved by comparison with DGPS survey results, to assess the accuracy of the detection and elevation methods both separately and together. The uncertainties associated with the two sub-components of the model tend to compensate for each other. The mean difference between image-based and surveyed shoreline elevations was less than 15 cm along 85% of the 2-km study region, which corresponded to an horizontal offset of 6 m. The application of the intertidal bathymetry mapping technique in support of CZM objectives is briefly illustrated at two sites in The Netherlands and Australia.

The influence of swash infiltration–exfiltration on beach face sediment transport: onshore or offshore?

Abstract Measurements were obtained from the swash-zone of a high-energy macrotidal dissipative beach of pore-pressure at four levels below the bed, and cross-shore velocity at a single height above the bed. Time-series from relatively high (Hs≈2.0 m) energy conditions were chosen for analysis from the mid-swash-zone at high tide. Calculation of upwards-directed pore-pressure gradients shows that, in this case, fluidisation of the upper layer of sediment, leading to enhanced backwash transport, is unlikely. An apparent conflict exists in the literature regarding the net effect of infiltration–exfiltration on the sediment transport, through the combined effects of stabilisation–destabilisation and boundary layer modification. This is examined for the data collected using a modified Shields parameter. Inferred instantaneous transport rates integrated over a single swash cycle show a decrease in uprush transport of 10.5% and an increase in backwash transport of 4.5%. Sensitivity tests suggest that there is a critical grain size at which the influence of infiltration–exfiltration changes from offshore to onshore. The exact value of this grain size is highly sensitive to the method used to estimate the friction factor.

Rapid water table fluctuations within the beach face: Implications for swash zone sediment mobility?

Abstract To incorporate groundwater infiltration/exfiltration in the description of swash zone sediment transport, it is required that the details of groundwater dynamics within the beach face be clarified. Field measurements of the vertical pore-pressure structure within the bed identify capillarity effects as the primary mechanism driving rapid and relatively large magnitude water table fluctuations within the swash zone. When the upper extent of the fully saturated capillary fringe coincides with the beach face surface, wave runup produces near-instantaneous increase in pore-pressures across the capillary fringe, corresponding to a rapid rise of the phreatic surface (i.e. the surface where pore-pressure = atmospheric pressure) to the sand surface. Therefore, counter to previous conclusions in the literature, the rapid rise and fall of the water table under the swash zone do not equate to regions of the beach face alternating between states that favor sediment deposition (unsaturated) and erosion (saturated). Similar reports that rapid fluctuations of the water table within the beach face correspond to rapid rates of vertical flow and hence bed fluidization, are also a misinterpretation. Field measurements, and a careful consideration of saturation and pore-pressure characteristics within the beach face, demonstrate that rapid water table rise is associated with minute (downwards) swash infiltration, rather than rapid (upwards) groundwater exfiltration. The cause of the pressure fluctuations and phreatic surface oscillations is the alternating appearance and disappearance of meniscuses at the sand surface, which generate pressure head fluctuations of decimeters due to the addition of millimeters of water.

Water table outcropping on macro-tidal beaches: a simulation model

Abstract During the fall of the tide the coastal water table within a beach may become decoupled from the ocean, resulting in the formation of a seepage face where water outcrops on the inter-tidal profile. A simulation model (called seep ) is developed to simulate the point of decoupling, and the subsequent motion of the seepage face as it continues to fall across the inter-tidal profile. The model is based entirely on the dynamics of an isolated water particle on the seepage face, i.e. the pressure distribution within the beach is ignored. Sensitivity analysis of the seep model to beach face slope and permeability characteristics suggests that small variations to these parameters can result in significantly differing rates of exit point motion and hence seepage face extent. To test the predictive capability of the seep model, results are presented from a field investigation of exit point dynamics on a macro-tidal beach, North Queensland Australia. The simulation model is found to accurately reproduce the observed seepage face motion at this field site. In addition, a re-analysis of seepage face extent reported in Nielsen (1990) from a protected, micro-tidal environment, shows similarly strong agreement between the reported and modelled exit point elevation.

The Performance of Shoreline Detection Models Applied to Video Imagery

Abstract Digital images of the intertidal region were used to map shorelines and the intertidal bathymetry along four geo-morphically and hydrodynamically distinct coastlines in the United States, United Kingdom, The Netherlands, and Australia. Mapping methods, each of which was originally designed to perform well at only one of the four sites, were applied to all four sites, and the results were compared to direct topographic surveys. It was determined that the rms errors of image-derived versus directly surveyed elevations depended on the prevailing hydrodynamic conditions as well as differences in each of the four different mapping methods. Before these differences were accounted for, rms errors ranged from 0.3 to 0.7 m. An empirical correction model that computed local estimates of setup, swash, and surf beat amplitudes reduced errors by about 50%, with residual rms errors ranging between 0.1 and 0.4 m. The model required tuning only one parameter that determined how each method was affected by swash at infragravity and incident wave frequencies. In environments where all methods successfully identify shorelines, the methods can be used somewhat interchangeably. The diversity of methods is advantageous in situations where one or more methods are likely to fail (e.g., lack of color imagery, high degree of alongshore variability). This remote sensing methodology can be applied to the shoreline and intertidal mapping problem across diverse nearshore environments.

Simulating the influence of groundwater seepage on sediment transported by the sweep of the swash zone across macro-tidal beaches

A numerical model is developed to simulate the influence of groundwater seepage on swash zone sediment transport across macro-tidal beaches. The emergence of equilibrium profile morphology is examined in response to varying tide, sediment and initial profile characteristics. Due to the unresolved complexities of boundary layer flow and corresponding shear stresses in the swash zone, sediment transport across the beach face is modelled by incorporating an uprush phase described by the shallow water wave equations, within an heuristic model of equilibrium beach face slope. A new parameter, termed the “M” net transport parameter, is defined to parameterize the net flux of sediment per uprush-backrush cycle. The definition of differing equilibrium gradients across saturated versus unsaturated regions of the intertidal profile permits the saturation characteristics of the bed to be incorporated. The dynamics of the watertable exit point on the beach face is central to the scheme, as this defines the time-varying extent of saturated and unsaturated regions of the intertidal profile. The sensitivity of coastal seepage face development to tide, profile and sediment characteristics is highlighted through the derivation of a new seepage face parameter. The simulation model is found to successfully reproduce key aspects of observed profile adjustment and resulting equilibrium morphology. Complex morphodynamic feedback is observed between: the tidal stage, net transport per uprush-backrush cycle, profile slope, and the dynamics of the watertable outcrop. Net upper-profile steepening and lower-profile lowering is simulated on beaches composed of medium to coarse sand. This results in the emergence of a distinctive break in slope within the intertidal profile. Such morphology is commonly observed on macro-tidal coasts.

A generalized equilibrium model for predicting daily to interannual shoreline response

Coastal zone management requires the ability to predict coastline response to storms and longer-term seasonal to interannual variability in regional wave climate. Shoreline models typically rely on extensive historical observations to derive site-specific calibration. To circumvent the challenge that suitable data sets are rarely available, this contribution utilizes twelve 5+ year shoreline data sets from around the world to develop a generalized model for shoreline response. The shared dependency of model coefficients on local wave and sediment characteristics is investigated, enabling the model to be recast in terms of these more readily measurable quantities. Study sites range from microtidal to macrotidal coastlines, spanning moderate- to high-energy beaches. The equilibrium model adopted here includes time varying terms describing both the magnitude and direction of shoreline response as a result of onshore/offshore sediment transport between the surf zone and the beach face. The model contains two coefficients linked to wave-driven processes: (1) the response factor (φ) that describes the “memory” of a beach to antecedent conditions and (2) the rate parameter (c) that describes the efficiency with which sand is transported between the beach face and surf zone. Across all study sites these coefficients are shown to depend in a predictable manner on the dimensionless fall velocity (Ω), that in turn is a simple function of local wave conditions and sediment grain size. When tested on an unseen data set, the new equilibrium model with generalized forms of φ and c exhibited high skill (Brier Skills Score, BSS = 0.85).

CZM Applications of Argus Coastal Imaging at the Gold Coast, Australia

Abstract A video-based ARGUS coastal imaging system is being used at the northern Gold Coast, Australia to monitor and quantify the regional-scale coastal response to sand nourishment and construction of the world-first Gold Coast artificial (surfing) reef. This automated monitoring system is used to obtain hourly daylight images from four cameras that combined provide continuous coverage of 4.5 km of the coast. Digital image processing techniques are then applied on a routine (weekly to monthly) basis to extract a range of CZM information from the growing image database. Analyses include: the mapping of changing shoreline position (and hence beach width); the measurement of three-dimensional inter-tidal morphology and resulting changes in subaerial sand volume; and the comparison of wave breaking frequency at the reef and adjacent nearshore bars, to quantify enhanced recreational surfing opportunities at the reef site. Based upon the results of image analysis, to date (January 2003) an additional 20–30 m of net beach width was achieved along the approximately 2 km of nourished coastline, relative to the adjacent unnourished beaches to the north and south. Due to a regional net accretionary trend during this same period, in January 2003 the nourished beach at Surfers Paradise was some 50–80 m wider than at the commencement of the video monitoring program in mid 1999.