Effects of surfzone wave transformation on swash dynamics

Swash oscillations on two natural beaches were measured to show that the shape and magnitude of energy spectra can be largely dependent on processes occurring inside the surfzone. The observations took place on a steep, intermediate beach on the east coast (Tairua Beach), and a low-sloping, dissipative beach located on the west coast of New Zealand (Ngarunui Beach, Raglan), and aimed at improving the understanding of the effects of wave breaking, beach slope, and nonlinear wave interactions on the swash oscillations. These problems were addressed by analysing datasets obtained from field experiments undertaken at these two sites. A field experiment at Tairua Beach showed that swash oscillations were critically dependent on the stage of the tide which controlled the degree of wave energy dissipation over the sandbar crest. Under mild, near-constant offshore wave conditions, the presence of a sandbar and the tidally-controlled water depth over its crest determined whether most of the incoming waves broke before reaching the shoreline. This forced a change in the pattern of wave energy dissipation across the surfzone between low and high tide, which was reflected by changes to swash elevation (runup) on time-scales of a few hours. Significant runup height Rs, defined as 4 times the standard deviation of the waterline time series, varied by a factor of 2 between low tide, when most of the waves were breaking over the sandbar and high tide, when the waves were barely breaking. The increase in wave energy dissipation during low tide was associated with changes in swash maxima distribution, decrease in mean swash period and increasing energy at infragravity frequencies (0.004–0.05 Hz). Bispectral analysis suggested this infragravity modulation might be connected with the presence of secondary waves at the shoreline. Swash oscillations at Tairua were not homogeneous along the beach. Alongshore variability in Rs of up to 78% was observed and was mainly driven by changes in the sea-swell (0.05–0.4 Hz) band of the swash. This variability was predominantly controlled by alongshore changes in beach face slope, although alongshore patterning in wave breaking over the sandbar caused alongshore changes in wave dissipation and also resulted in alongshore swash variation in the sea-swell bandwidth. At infragravity frequencies, alongshore swash variability was not well associated either with changes in beach slope or wave breaking and was possibly linked to the presence of low-mode edge waves, observed from frequency-wavenumber spectra of the swash time series. A final experiment was conducted to understand the surfzone control on incident and infragravity runup on a gently-sloping beach. The observations showed that runup saturation at infragravity frequencies can occur under mild offshore energy conditions if the beach slope is sufficiently gentle. Infragravity saturation was observed for higher-frequency (> 0.025–0.035 Hz) infragravity waves, where typically less than 5% of the (linear) energy flux was reflected from the beach and where, similar to the sea-swell band, the swash energy was independent of offshore wave energy. The infragravity frequency range of saturation was determined by the tide, with saturation extending to lower frequencies at low tide when the local beach face slope over the concave-shaped profile was gentler. Runup was strongly dominated by infragravity frequencies, which accounted on average for 96% of the runup variance, and its energy levels were entirely consistent with strong infragravity wave dissipation observed in the surfzone, particularly when including the nonlinear contributions to the wave energy fluxes. Our observations show evidence of nonlinear interactions involving infragravity and high-frequency, harmonic waves, and suggest that these harmonics could play a role in the wave energy balance near the shoreline on low-sloping, dissipative beaches

Observations of storm morphodynamics using Coastal Lidar and Radar Imaging System (CLARIS): Importance of wave refraction and dissipation over complex surf-zone morphology at a shoreline erosional hotspot

Elevated water levels and large waves during storms cause beach erosion, overwash, and coastal flooding, particularly along barrier island coastlines. While predictions of storm tracks have greatly improved over the last decade, predictions of maximum water levels and variations in the extent of damage along a coastline need improvement. In particular, physics based models still cannot explain why some regions along a relatively straight coastline may experience significant erosion and overwash during a storm, while nearby locations remain seemingly unchanged. Correct predictions of both the timing of erosion and variations in the magnitude of erosion along the coast will be useful to both emergency managers and homeowners preparing for an approaching storm. Unfortunately, research on the impact of a storm to the beach has mainly been derived from pre and post storm surveys of beach topography and nearshore bathymetry during calm conditions. This has created a lack of data during storms from which to ground-truth model predictions and test hypotheses that explain variations in erosion along a coastline. We have developed Coastal Lidar and Radar Imaging System (CLARIS), a mobile system that combines a terrestrial scanning laser and an X-band marine radar system using precise motion and location information. CLARIS can operate during storms, measuring beach topography, nearshore bathymetry (from radar-derived wave speed measurements), surf-zone wave parameters, and maximum water levels remotely. In this dissertation, we present details on the development, design, and testing of CLARIS and then use CLARIS to observe a 10 km section of coastline in Kitty Hawk and Kill Devil Hills on the Outer Banks of North Carolina every 12 hours during a Nor\u27Easter (peak wave height in 8 m of water depth = 3.4 m). High decadal rates of shoreline change as well as heightened erosion during storms have previously been documented to occur within the field site. In addition, complex bathymetric features that traverse the surf-zone into the nearshore are present along the southern six kilometers of the field site. In addition to the CLARIS observations, we model wave propagation over the complex nearshore bathymetry for the same storm event. Data reveal that the complex nearshore bathymetry is mirrored by kilometer scale undulations in the shoreline, and that both morphologies persist during storms, contrary to common observations of shoreline and surf-zone linearization by large storm waves. We hypothesize that wave refraction over the complex nearshore bathymetry forces flow patterns which may enhance or stabilize the shoreline and surf-zone morphology during storms. In addition, our semi-daily surveys of the beach indicate that spatial and temporal patterns of erosion are strongly correlated to the steepness of the waves. Along more than half the study site, fifty percent or more of the erosion that occurred during the first 12 hours of the storm was recovered within 24 hours of the peak of the storm as waves remained large (\u3e2.5 m), but transitioned to long period swell. In addition, spatial variations in the amount of beach volume change during the building portion of the storm were strongly correlated with observed wave dissipation within the inner surf zone, as opposed to predicted inundation elevations or alongshore variations in wave height

Determination of the high water mark and its location along a coastline

The High Water Mark (HWM) is an important cadastral boundary that separates land and water. It is also used as a baseline to facilitate coastal hazard management, from which land and infrastructure development is offset to ensure the protection of property from storm surge and sea level rise. However, the location of the HWM is difficult to define accurately due to the ambulatory nature of water and coastal morphology variations. Contemporary research has failed to develop an accurate method for HWM determination because continual changes in tidal levels, together with unimpeded wave runup and the erosion and accretion of shorelines, make it difficult to determine a unique position of the HWM. While traditional surveying techniques are accurate, they selectively record data at a given point in time, and surveying is expensive, not readily repeatable and may not take into account all relevant variables such as erosion and accretion.In this research, a consistent and robust methodology is developed for the determination of the HWM over space and time. The methodology includes two main parts: determination of the HWM by integrating both water and land information, and assessment of HWM indicators in one evaluation system. It takes into account dynamic coastal processes, and the effect of swash or tide probability on the HWM. The methodology is validated using two coastal case study sites in Western Australia. These sites were selected to test the robustness of the methodology in two distinctly different coastal environments

Object-Based Coastal Morphological Change Analysis Based on LiDAR and Hurricane Events

Storms are considered one of the rapid climatic events that have a dramatic impact on coastal morphology, hence they require further investigation and quantifying of coastal changes and responses. Light detection and ranging (LiDAR) is the most advanced technology to be widely used by researchers for coastal geomorphological studies. The purpose of this study is to apply an object-based approach using repeated LiDAR surveys to understand the short-term morphological changes that occurred on Santa Rosa Island, Florida after category 3 hurricanes Ivan (2004) and Dennis (2005), making it the first study to apply this method, as opposed to previous studies’ commonly used field-based approaches. The first analysis was conducted using a coastal morphology analysis (CMA) tool. In the second analysis, the extracted mean elevation change values were linked to three factors—mean vegetation, mean slope, and mean elevation—to demonstrate their contribution to the change using ordinary least square (OLS) analysis. The third analysis was carried out using the classification and regression tree (CART) analysis. Of the study area, 18.64% encountered erosional processes and 11.35% with depositional processes during Hurricane Ivan, whereas during Hurricane Dennis, 5.91% faced erosional processes and 8.18% was affected by depositional processes. Both hurricanes resulted in a net sediment loss; 283,167 m3 during Hurricane Ivan and 52,440 m3 during Hurricane Dennis. Generally, objects tended to be irregular, asymmetrical, and shaped with smooth boundaries. Along the coast, most objects tended to have an elongated shape, but inland the shapes were more irregular. The overall OLS model during Hurricane Ivan yielded statistically significant results for the three factors, with a confidence level of 0.00 and an adjusted r-square of 0.40; and during Hurricane Dennis, the mean vegetation and mean elevation results yielded significant statistical results (p-value 0.00), while slope did not show significance and had an adjusted r-square of 0.47. CART analysis of both hurricanes ranked the mean elevation as the most important factor in predicting the mean elevation change, followed by the mean slope and finally the mean vegetation variable

Wave run-up on beaches and coastal structures

Wave run-up is an important design criterion for coastal structures and beach nourishment projects. Coastal engineers commonly use empirical formulae to predict this parameter. These formulae generally include the effect of berms, roughness and angle of wave attack, but neglect the influence of parameters such as hydraulic conductivity and beach groundwater levels. This thesis presents a laboratory and numerical study aimed to improve the predictive capability of existing formulae as well as to enhance our understanding of the swash hydrodynamics and their interaction with permeable beaches. In particular, it investigates the influence of hydraulic conductivity, roughness and beach groundwater on wave run-up and swash flows. Most of the data presented in this study were obtained from wave flume experiments performed on smooth-impermeable, rough-impermeable and rough-permeable slopes. The influence of hydraulic conductivity on swash hydrodynamics was quantified by means of a novel experimental setup consisting of non-deformable permeable structures, in which the influence of the surface roughness was isolated. A procedure based on the development of time-stack images provided accurate measurement of run-up and swash depths, while pressure transducers were used to measure the water table elevations inside the permeable structures. Laser Doppler velocimetry, a technique that does not disturb the flow, was used to measure the velocity profile of the uprush and backwash flows. In addition to the laboratory experiments, simulations using a Volume-Averaged Reynolds-Averaged Navier-Stokes (VARANS) model, validated against experimental results, were used to investigate the influence of hydraulic conductivity on the near-bed flow velocities and to obtain larger datasets of run-up on impermeable slopes. Analysis indicated that existing formulae adequately predict run-up from breaking waves on impermeable slopes. However, no previous formulae gave reliable predictions of run-up from non-breaking waves. Therefore, new empirical formulae were derived for non-breaking waves on impermeable slopes. These give good predictions when compared with the present data and data available in literature. The beach groundwater levels were found to have negligible influence on wave run-up. In contrast, hydraulic conductivity was shown to have a significant effect on wave-structure interaction parameters such as wave run-up, wave-induced water table elevation, swash depths, and swash flow velocities. As a result, new prediction formulae for breaking and non-breaking waves on permeable slopes were developed; these formulae include the influence of surface roughness and hydraulic conductivity through a new non-dimensional parameter. Moreover, flow velocity measurements in the swash zone showed that infiltration enhances onshore flow and time asymmetries. This is expected to promote onshore sediment transport inside the swash zone. The near-bed velocity measurements were also used to estimate bed shear stresses using the log-law method. The results showed that infiltration directly increases the bed shear stresses during the uprush phase, mainly due to the change in the boundary layer thickness. However, infiltration was also shown to indirectly reduce the bed shear stresses during the backwash phase by significantly reducing the backwash flow depths and velocities (continuity effect). Video observations of the breaking processes showed that hydraulic conductivity alters the shape of waves breaking on the slope. However, the change in shape is small and in all cases, the breaker type remained the same. Hydraulic conductivity was also shown to decrease the breaking point distance of plunging waves. The video analysis was also used to validate a new criterion presented in this study to determine whether or not waves will break on the slope; this criterion was shown to give better predictions of the transition between breaking and non-breaking waves than existing breaking criteria. This is one of the first studies to include the influence of hydraulic conductivity on run-up prediction formulae. If the porosity or hydraulic conductivity of a coastal structure or beach is known, these formulae in combination with the reduction factors suggested by EurOtop (2007) can lead to more accurate predictions of wave run-up and wave overtopping on permeable slopes. The improved understanding of the influence of hydraulic conductivity on the wave-induced water table elevation and on the swash hydrodynamic processes will benefit the modelling and management of coastal aquifers as well as the prediction of sediment transport in the swash zone

Spatial and temporal variability in the ridge and runnel morphology along the North Lincolnshire coast

Ridges and runnels are low-amplitude, shore-parallel bars and troughs in the intertidal zone of macrotidal sandy beaches. Ridge and runnel beaches are very common in the United Kingdom, particularly in the vicinity of large river outlets. Hence, an understanding of its dynamics will increase our understanding of British coastal processes, which may be useful in national coastal management plans. A number of studies have focussed on the morphodynamics of ridges and runnels, however, the main shortcoming of these previous studies is that the morphodynamics have generally been considered at limited spatial and temporal scales. This research investigates the ridges and runnels on a variety of scales and is innovative in the sense that small-scale morphodynamic behaviour is attempted to be linked to large-scale and long-term dynamics. The study area is the north Lincolnshire coast, east England, where generally 3–5 well-developed ridges and runnels are present. [Continues.

Swash zone dynamics of coarse-grained beaches during energetic wave conditions

Coarse-grained beaches, such as pure gravel (PG), mixed sand-gravel (MSG) and composite (CSG) beaches, can be considered as one of the most resilient non-cohesive morpho-sedimentary coastal environments to energetic wave forcing (e.g., storms). The hydraulically-rough and permeable nature of gravel (D50 > 2 mm), together with the steep (reflective) beach face, provide efficient mechanisms of wave energy dissipation in the swash zone and provide a natural means of coastal defence. Despite their potential for shore protection very little is known about the response of these environments during high energetic wave conditions. Field measurements of sediment transport and hydrodynamics on coarse-grained beaches are difficult, because there are few instruments capable of taking direct measurements in an energetic swash zone in which large clasts are moving, and significant morphological changes occur within a short period of time. Remote sensing methods emerge in this context as the most appropriate solution for these types of field measurement. A new remote sensing method, based around a mid-range (~ 50 m) 2D laser-scanner was developed, which allows the collection of swash zone hydrodynamics (e.g., vertical and horizontal runup position, swash depth and velocity) and bed changes on wave-by-wave time scale. This instrument allowed the complete coverage of the swash zone on several coarse-grained beaches with a vertical accuracy of approximately 0.015 m and an average horizontal resolution of 0.07 m. The measurements performed with this new methodology are within the accuracy of traditional field techniques (e.g. video cameras, ultrasonic bed-level sensors or dGPS). Seven field experiments were performed between March 2012 and January 2014 on six different coarse-grained beaches (Loe Bar, Chesil, Slapton, Hayling Island, Westward Ho! and Seascale), with each deployment comprising the 2D laser-scanner together with complementary in-situ instrumentation (e.g., pressure transducer, ADV current meter). These datasets were used to explore the hydrodynamics and morphological response of the swash zone of these different environments under different energetic hydrodynamic regimes, ranging from positive, to zero, to negative freeboard regimes. With reference to the swash zone dynamics under storms with positive freeboard regimes (when runup was confined to the foreshore) it was found that extreme runup has an inverse relationship with the surf scaling parameter (=2Hs /gTptan2). The highest vertical runup excursions were found on the steepest beaches (PG beaches) and under long-period swell, while lower vertical runup excursions where linked to short-period waves and beaches with intermediate and dissipative surf zones, thus demonstrating that the contrasting degree of wave dissipation observed in the different types of surf zones is a key factor that control the extreme runup on coarse-grained beaches. Contrasting morphological responses were observed on the different coarse-grained beaches as a result of the distinct swash\surf zone hydrodynamics. PG beaches with narrow surf zone presented an asymmetric morphological response during the tide cycle (accretion during the rising and erosion during the falling tide) as a result of beach step adjustments to the prevailing hydrodynamics. On dissipative MSG and CSG beaches the morphological response was limited due to the very dissipative surf zone, while on an intermediate CSG beach significant erosion of the beach face and berm was observed during the entire tide cycle as a result of the absence of moderate surf zone wave dissipation and beach step dynamics. Fundamental processes related to the link between the beach step dynamics and the asymmetrical morphological response during the tidal cycle were for the first time measured under energetic wave conditions. During the rising tide the onshore shift of the breaking point triggers the onshore translation of the step and favors accretion (step deposit development), while during the falling tide the offshore translation of the wave breaking point triggers retreat of the step and favours backwash sediment transport (erosion of the step deposit). Under zero and negative freeboard storm regimes (when runup exceeds the crest of the barrier or foredune), field measurements complimented by numerical modelling (Xbeach-G) provide clear evidence that the presence of a bimodal wave spectrum enhances the vertical runup and can increase the likelihood of the occurrence of overtopping and overwash events over a gravel barrier. Most runup equations (e.g., Stockdon et al., 2006) used to predict the thresholds for storm impact regime (e.g., swash, overtopping and overwash) on barriers lack adequate characterisation of the full wave spectra; therefore, they may miss important aspects of the incident wave field, such as wave bimodality. XBeach-G allows a full characterization of the incident wave field and is capable of predicting the effect of wave spectra bimodality on the runup, thus demonstrating that is a more appropriate tool for predicting the storm impact regimes on gravel barriers. Regarding the definition of storm impact regimes on gravel barriers, it was found that wave period and wave spectra bimodality are key parameters that can affect significantly the definition of the thresholds for these different regimes. While short-period waves dissipate most of their energy before reaching the swash zone (due to breaking) and produce short runup excursions, long-period waves arrive at the swash zone with enhanced heights (due to shoaling) and break at the edge of the swash, thus promoting large runup excursions. When offshore wave spectrum presents a bimodal shape, the wave transformation on shallow waters favours the long period peak (even if the short-period peak is the most energetic offshore) and large runup excursions occur. XBeach-G simulations show that the morphological response of fine gravel barriers is distinct from coarse gravel barriers under similar overtopping conditions. While on coarser barriers overtopping regimes are expected to increase the crest elevation and narrow the barrier, on fine barriers sedimentation occurs on the back of the barrier and in the lower beach face. Such different sedimentation patterns are attributed to the different hydraulic conductivity of the different sediment sizes which control the amount of flow dissipation (due to infiltration) and, therefore, the capacity of the flow to transport sediment across and over the barrier crest. The present findings have significantly improved our conceptual understanding of the response of coarse-grained beaches during storms. A new field technique to measure swash dynamics in the field was developed during this thesis and has great potential to become widely used in a variety of coastal applications.EPSR

Rip currents in Mediterranean environment: a case study along eastern Ligurian coast

openThis thesis proposes a study on the rip currents development within a Mediterranean embayed beach. The rip (or cross-shore) currents are among the most investigated phenomena in the eld of coastal research, and their fame is due to their environmental and socio-economic implications. The coastal areas are considered as transition environments, where hydrosphere, lithosphere, biosphere, atmosphere and (often) anthroposphere meet. The rip currents are a crucial component of the coastal hydro-morphodynamic processes (hydrosphere and lithosphere) (Short, 1999; Castelle et al., 2016), play a role in larval recruitment processes (biosphere) (Shanks et al., 2010), and they are also well known as risks source for beachgoers (anthroposphere) (Short and Hogan, 1994; Austin et al., 2012). However, the rip currents role along the Mediterranean coasts is often neglected, and most of the literature concerns the rip currents in oceanic environments. The aim of this research is a detailed description of the rip currents behaviour along a Mediterranean embayed beach, also considering the possible sea-level rise implications. The study area was identied within Levanto bay, along the eastern Ligurian coast (NW Italy). The research activity has been conducted through an integrated application of several investigation methodologies, in order to obtain the best possible results in therm of phenomena description. The rip currents individuation is performed through a coastal video-monitoring system installed on the Levanto beach, and the collected data were processed through a dedicated software for coastal video-monitoring (Brignone et al., 2012). Several eld surveys were performed to obtain a full description of the geomorphological boundary conditions (topo-bathymetric surveys and sedimentological sampling). The rip currents description and evaluation were executed through the application of the XBeach model (Roelvink et al., 2009), which is a well-known tool for coastal modelling. Moreover, the modelling approach allowed the evaluation of the possible rip currents response under dierent sea-level rise scenarios (local sea-level projections to 2100) (Kopp et al., 2014). The obtained results show a detailed description of the rip currents phenomena, showing their essential role in the local coastal dynamics. The proposed research approach has proved to be reliable for the rip currents investigation in the Mediterranean environment, and it can be applied along any stretch of coast of the Mediterranean Sea. Moreover, the modelling results showed a signicant relation between sea-level rise and rip currents behaviour. The results of this study highlight the role of the rip currents in the Mediterranean environment and represent a rm basis for the rip currents investigation along the Mediterranean coasts.openXXXI CICLO - SCIENZE E TECNOLOGIE PER L'AMBIENTE E IL TERRITORIO (STAT) - Scienze della terraCarpi, Luc

Concerning Swash On Steep Beaches

This investigation focuses on the prediction of sediment transport and beach evolution in coarse-grained beaches. This includes observed morphological changes on both gravel and mixed beaches from experimental investigations at the Large Wave Flume (GWK) in Hanover. Germany. The recorded measurements show that the majority of morphology change took place adjacent to the zone of wave-breaking, close to the shoreline in both cases. Based on these observations, the discussions are carried out with psirticular regard to the observed tendency for onshore transport axid profile steepening in the swash zone. The aim is to identify the cross-shore hydrodynamics and sediment transport mechanisms involved, to advance understanding of this type of beach and to improve our qucintitative capabilities for predicting shoreline and morphological changes in this zone. With this in mind, this thesis includes a discussion of the physical processes related to swash hydrodynamics and sediment transport. It also introduces the description of the mathematical framework used to study wave hydrodynamics in the swash zone. Emphasis is given to the Boussinesq equations which have been found to be a suitable approach. For these equations an evaluation of the two available shoreline boundary conditions is carried out and it is shown that the moving shoreline accurately reproduces the velocity field in the swash zone. The profile evolution investigation is carried out evaluating the transport rates from a bed-load sediment transport formulation coupled with velocities calculated from a set of Boussinesq equations [Lynett et al 2002). Then the equation for conservation of sediment is solved to estimate the morphological changes as proposed by (Rakha et al 1997). It is shown that such an approach is useful to investigate the processes that control this evolution. A discussion on the influence of bottom friction on the predicted profiles is presented. Numerical results in both beaches show that the use of a higher friction factor f during uprush improves the simulations of morphological changes. However, the variation of friction by itself was not able to reproduce the measured profiles. A plausible reason to explain this is that further mechanisms other than friction play an important role in the overall response of coarsegrained beaches. For both beaches it is established that, if the efficiency factor (C) in the sediment transport equation and bottom friction are kept the same in the uprush and backwash, accurate representation of profile evolution is not possible. Indeed, the features of the predicted profiles are reversed. When the C parsimeter is set larger during the uprush than during the backwash, the predicted profiles are closer to the observations. Differences between the predicted profiles from setting non-identical C-values and friction factors for the swash phase, are believed to be linked to both the infiltration effects on the flow above the beachface, the bore collapse picking up sediment from the bed, and the accelerated flow in the uprush. The discussion is made with reference to main physical processes acting over the beachface for both the mixed and gravel beach