Papers by Gerben Ruessink
Ebb-tidal deltas are shallow features seaward of tidal inlets, acting as a wave filter for the ne... more Ebb-tidal deltas are shallow features seaward of tidal inlets, acting as a wave filter for the nearby barrier island and a source of sediment for the landward tidal basin. On many ebb-tidal deltas, channels rotate and shoals periodically attach to the downdrift island. This cyclic behavior can also include an alternation between one-and two-channel inlet configurations. The effect of the long-term (> years) cyclic behavior on the short-term patterns of waves, tidal currents, and sediment transport is unknown. Here, we use Delft3D/SWAN models to simulate the Dutch Ameland tidal inlet during four phases of the cycle to show that many of the physical processes on the ebb-tidal delta and in the entire tidal system are affected by the cyclic evolution of channels and shoals. In particular, the periodic variations in the channel positions appear to significantly influence the tidal asymmetry in the inlet and mean flow characteristics. As a result, the net sediment exchange between basin and sea is cyclic and follows the periodicity of the one-and two-channel inlet configuration. Moreover, we find that the wave energy dissipation on the ebb-tidal delta is enhanced by a shallow shoal or an updrift-oriented ebb-channel, which shields the coast from the incoming waves. Our results demonstrate how the cyclic channel-shoal dynamics at natural tidal inlets is likely to affect the safety functions of the ebb-tidal deltas, varying the offshore wave energy dissipation as well as adjusting the sediment pathways on the ebb-tidal delta.
— Shore-based remote sensing platforms are increasingly used to frequently (∼daily) obtain bathym... more — Shore-based remote sensing platforms are increasingly used to frequently (∼daily) obtain bathymetric information of large (∼km 2) nearshore regions over many years. With recorded wave frequency and wavenumber k (and hence wave phase speed c = /k), bed elevation z b can be derived using a model that relates and k to water depth. However, the accuracy of z b as a function of the sensor and the method of − k retrieval is not well known, especially not under low-period waves. Here, we assess the accuracy of z b , based on two sensors with their own method of phase speed retrieval, in a dynamic, kilometer-scale environment (Sand Engine, The Netherlands). Bias in z b is systematic. A fast Fourier transform (FFT) method on X-band radar imagery produced z b too shallow by 1.0 m for −15 m ≤ z b ≤ −9 m, and too deep by 2.3 m for z b ≥ −6 m. A cross-spectral method on optical video imagery produced z b too shallow by 0.59 m for −10 m ≤ z b ≤ −5 m, and too deep by 0.92 m for z b ≥ −1 m. Intermediate depths had negligible bias, −0.02 m for the radar-FFT approach and −0.01 m for the video-CS approach. The collapse of the FFT method in shallow water may be explained by the inhomogeneity of the wave field in the 960 m × 960 m analysis windows. A shoreward limit of the FFT method is proposed that depends on z b in the analysis windows.
Accretionary downstate beach sequence is a key element of observed nearshore morphological variab... more Accretionary downstate beach sequence is a key element of observed nearshore morphological variability along sandy coasts. We present and analyze the first numerical simulation of such a sequence using a process-based morphodynamic model that solves the coupling between waves, depth-integrated currents, and sediment transport. The simulation evolves from an alongshore uniform barred beach (storm profile) to an almost featureless shore-welded terrace (summer profile) through the highly alongshore variable detached crescentic bar and transverse bar/rip system states. A global analysis of the full sequence allows determining the varying contributions of the different hydro-sedimentary processes. Sediment transport driven by orbital velocity skewness is critical to the overall onshore sandbar migration, while gravitational downslope sediment transport acts as a damping term inhibiting further channel growth enforced by rip flow circulation. Accurate morphological diffusivity and inclusion of orbital velocity skewness opens new perspectives in terms of morphodynamic modeling of real beaches.
Dune management along developed coasts has traditionally focussed on the suppression of the geomo... more Dune management along developed coasts has traditionally focussed on the suppression of the geomor-phic dynamics of the foredune to improve its role in sea defence. Because a stabilized foredune acts as an almost total barrier to aeolian transport from the beach, the habitat diversity in the more landward dunes has degraded. With the overarching objective to mitigate this undesirable loss in biodiversity, dune management projects nowadays increasingly intend to restore aeolian dynamics by reconnecting the beach-dune system with notches excavated through the foredune. Here, we use repeat topographic survey data to examine the geomorphic response of a coastal dune system in the Dutch National Park Zuid-Kennemerland to five notches excavated in 2012–2013 within an 850-m stretch of the 20-m high established foredune. The notches were dug in a V-shape (viewed onshore), with a width between approximately 50 and 100 m at the top, a (cross-dune) length between 100 and 200 m, and excavation depths between 9 and 12.5 m. The 1 Â 1 m digital terrain models, acquired with airborne Lidar and UAV photogrammetry, illustrate that during the 3-year survey period the notches developed into a U-shape because of wall deflation, and that up to 8-m thick and 150-m long depositional lobes formed landward of the notches. Sand budget computations showed that the sand volume of the entire study area increased by about 22,750 m 3 /year, which, given the 850-m width of the study area, corresponds to an aeolian input from the beach of approximately 26.5 m 3 /m/year. Between 2006 and 2012 all wind-blown beach sand deposited on the seaward side of the foredune; since 2013, the notches have caused 75% of the sand to be deposited landward of the foredune. This highlights that the notches are highly effective conduits for aeolian transport into the back dunes. Future monitoring is required to determine for how long the notches will stimulate aeolian dynamics and if (and when) vegetation eventually starts to regrow and enforces the degeneration of the notches.
[1] Long-term (>years) bathymetric data sets collected in six multiple near-shore sandbar systems... more [1] Long-term (>years) bathymetric data sets collected in six multiple near-shore sandbar systems were analyzed with complex empirical orthogonal function analysis to quantify intersite differences and similarities in cyclic offshore progressive bar behavior. The observations came from a 37-year annually sampled data set of four regions along the Dutch coast (spanning 70 km of coastline), an 18-year fortnightly to monthly sampled data set at Duck, North Carolina (alongshore extent $1 km), and a 7-year daily sampled data set of a single cross-shore profile at the Hasaki coast of Japan. The first complex mode, typically representing 50–70% of the total depth variance, described the long-term offshore progressive behavior and allowed for an objective separation of the barred part of the profile from the shoreward-and seaward-located nonbarred parts by considering a threshold bar amplitude below which the spatial results from the first mode were not considered reliable. The sandbars at the six examined sites share common lengths and nondimensional amplitude characteristics, which can be described by a negatively skewed Gaussian function. The absolute amplitude dimensions and the cycle return intervals differ, however, considerably between the sites. The key geometric parameters that steer this intersite variation are the time-averaged mean depths at the shoreward and seaward side of the bar zone (d shore and d sea , respectively) as well as their difference d bz. The degree to which intersite differences in d shore , d sea , and d bz are related linearly to intersite differences in bulk statistics of external forcings (wave, tide, sediment, and bed profile characteristics) is inconclusive.
Since its introduction in 1978, the Battjes and Janssen model has proven to be a popular framewor... more Since its introduction in 1978, the Battjes and Janssen model has proven to be a popular framework for estimating the cross-shore root-mean-square wave height H r ms transformation of random breaking waves in shallow water. Previous model tests have shown that wave heights in the bar trough of single bar systems and in the inner troughs of multiple bar systems are overpredicted by up to 60% when standard settings for the free model parameter c (a wave height-to-depth ratio) are used. In this paper, a new functional form for c is derived empirically by an inverse modelling of c from a high-resolution (in the cross-shore) 300-h H r ms data set collected at Duck, NC, USA. We find that, in contrast to the standard setting, c is not cross-shore constant, but depends systematically on the product of the local wavenumber k and water depth h. Model verification with other data at Duck, and data collected at Egmond and Terschelling (Netherlands), spanning a total of about 1600 h, shows that cross-shore H r ms profiles modelled with the locally varying c are indeed in better agreement with measurements than model predictions using the cross-shore constant c. In particular, model accuracy in inner bar troughs increases by up to 80%. Additional verifications with data collected on planar laboratory beaches show the new functional form of c to be applicable to non-barred beaches as well. Our optimum c cannot be compared directly to field and laboratory measurements of height-to-depth ratios and we do not know of a physical mechanism why c should depend positively on kh.
A considerable portion of the variability in nearshore sandbars is related to changes in the plan... more A considerable portion of the variability in nearshore sandbars is related to changes in the plan shape of quasi-rhythmic alongshore non-uniform features, such as rip channels and crescentic shapes. These changes may include changes in their alongshore length, cross-shore amplitude and alongshore position. Here, we use complex empirical orthogonal eigenfunction analysis to quantify these changes from a 3.4-year data set of almost daily time-exposure images of the double-barred coast at Noordwijk (Netherlands). The observed alongshore non-uniform features had alongshore lengths between 380 and approximately 3000 m and lifetimes in the order of months, considerably longer than the characteristic time scale of individual wave events. Transitions from one feature to another were mostly gradual, resulting from an alongshore differential growth in amplitude. Abrupt transitions, that is, the existing features disappeared entirely and were subsequently replaced by different features, were barely observed and did not always take place during high-energy wave events. The amplitude of the non-uniform features varied between 0 and 30 m on a weekly to monthly scale, unrelated to variations in the wave height. In addition, the features migrated back and forth along the shore with typical rates of Oð10 m=dayÞ on weekly scales with the rates increasing with an increase in the alongshore component of the wave power. On the whole our observations suggest that alongshore non-uniform sandbar variability is governed by free behaviour rather than by the direct forcing of the prevailing wave conditions.
Changes in nearshore sandbar morphology comprise of an alongshore uniform and non-uniform compone... more Changes in nearshore sandbar morphology comprise of an alongshore uniform and non-uniform component. The former reflects the overall on/offshore migration, while the latter expresses changes in quasi-rhythmic non-uniformities, such as crescentic plan shapes. Here we focus on the alongshore-uniform component, quantified from a 3.4-year data set of daily time-exposure video images of the double barred nearshore at Noordwijk, Netherlands. The high temporal resolution and the long duration of the data set allowed us to quantify the cross-shore bar migration at weekly, seasonal and inter-annual time scales and, accordingly, to compare the contribution of all three components to the total variability in cross-shore bar position. The maximum observed offshore-directed weekly, seasonal and inter-annual bar migration rates were 10, 0.5 and 0:2 m=day: Maximum onshore rates at weekly and seasonal scales were 8 and 0:5 m=day; while onshore migration at the inter-annual scale was not observed. The inter-annual bar migration dominated the bar crest variability over time spans longer than 10–13 months, whereas changes on the weekly scale were the dominant source of variability on time spans shorter than 7–10 months. Seasonal bar migration only dominated the bar crest variability at the outer bar on time spans between 7 and 13 months. In general, Noordwijk appears to be a site with a strong inter-annual signal, with limited seasonal variability, and with fluctuations at weekly scales that are long compared to the characteristic time scale of individual events, suggesting a response to sequences of events rather than to individual events.
Remote sensing of wave breaker patterns, clearly visible as high-intensity bands in time exposure... more Remote sensing of wave breaker patterns, clearly visible as high-intensity bands in time exposure video images, has become a powerful tool to obtain large-scale (kilometers) and long-term (years) time series of nearshore sandbar position. However, intensity-based bar crest positions x i differ from directly measured positions x • by a time-varying distance fix, which is of O(10 m) and depends on the offshore wave height H 0, the water level r/0, and the bathymetry itself. The effect of these parameters on fix was investigated from simultaneous video observations and bathymetric surveys, obtained in the double-barred system at Egmond aan Zee, Netherlands, and from wave model predictions, assuming that the roller energy represents image intensity. When the wave field over a bar was predicted to be nonsaturated, x i was observed and predicted to move offshore as either r/0 decreased or H 0 increased. Under saturated conditions, fix only responded to changes in r/0. Additional model investigations showed that an increase in outer bar crest depth, similar to that observed during interannual bar behavior, significantly reduced the fix variability at the outer bar and increased the fix variability at the inner bar. Implications of our observational and model findings for studying sandbar position from video imagery are outlined.
The remote sensing of the sea surface is commonly regarded as a logistically simple and cost-effe... more The remote sensing of the sea surface is commonly regarded as a logistically simple and cost-effective way of obtaining long-term (> years) time series of nearshore sandbar location. In this paper, we demonstrate that, similar to video imagery, time-exposure X-band (f 3 cm wave length) radar images contain alongshore, breaking-related high-intensity bands that can be used to compute the location of the underlying sandbar crests. Analysis of time-exposure X-band images and bathymetric surveys collected at Egmond aan Zee, Netherlands shows that the radar-based bar crest location, defined as the cross-shore location of maximum radar intensity, differs from the in situ surveyed crest location by a distance of O(10 m). This difference, Dx ¯radar, depends linearly on the offshore water level, consistent with model predictions in which the radar intensity is assumed to behave as the energy of the surface roller. The model additionally suggests a dependence of Dx ¯radar on the offshore wave height when the wave field across the bar is non-saturated. In general, such wave conditions did not result in radar images with sufficient contrast to determine the bar crest location and, accordingly, the wave-height dependence was not observed clearly in the field. Because of the dependence of Dx ¯radar on the offshore water level, time series of radar-based bar crest location show artificial variability when the images are collected at different water levels. Various techniques for correcting this variability are discussed. D
Shore-parallel sandbar systems often exhibit considerable quasi-regular alongshore variations, su... more Shore-parallel sandbar systems often exhibit considerable quasi-regular alongshore variations, such as crescentic plan shapes. Therefore, morphological change in such systems commonly consists of two-and three-dimensional variability. The former is related to overall on/offshore bar migration, whereas the latter is induced by the horizontal amplitude growth, migration, or length scale change of the quasi-regular topography. In this paper, we used a six-week data set of bathymetric surveys and video images of breaking-induced foam to quantify short-term (days–weeks) two-and three-dimensional variability in the bar-crest position of the double barred beach at Egmond aan Zee (Netherlands). The alongshore-uniform response was computed as the mean of each bar-crest line, whereas the alongshore non-uniform behaviour was characterised by the amplitude around each mean. A more sophisticated description of the inner-bar data set was provided by complex empirical orthogonal function (CEOF) analysis. The first complex mode, containing about 85% of the variance in the data set, corresponded to the amplitude growth and longshore migration of an approximately 600 m long alongshore non-uniformity. The longshore migration rate varied between 0 and 150 m/day and was found to be well related to the longshore component of the offshore wave energy flux. The second complex mode explained about 10% of the variance and largely described the along-shore-averaged cross-shore bar migration. The CEOF results suggest that short-term variability in bar-crest position is largely due to changes in the quasi-regular topography and not to alongshore-uniform on/offshore oriented behaviour.
The systematic contributions of short waves, infragravity waves and mean flows to the cross-shore... more The systematic contributions of short waves, infragravity waves and mean flows to the cross-shore sediment transport were studied with direct measurements of instantaneous near-bed sediment concentrations and velocities as well as with an energetics-based sediment transport model using measured time series of near-bed cross-shore flow as input. The study was conducted at four cross-shore positions in the multiple bar system of Terschelling, Netherlands during three field campaigns, each with a five-week duration. The data were collected in 3-to 9-m water depth during a wide range of conditions, from low-energy non-breaking conditions to almost fully saturated breakers. The height over depth ratio H s = h was applied at all four measuring stations as a local conditional parameter. All model predictions were grouped into H s = h classes with a width of 0.02 to highlight the generality of the data. The energetics approach predicted that the suspended load dominated over the bedload at all four stations. Both the sediment flux measurements as well as the model predictions showed that the largest contributors to the gross suspended sediment transport were made by the short waves and the undertow, inducing an onshore and offshore transport, respectively. Their contributions to the gross suspended transport were about 40 to 50% under surf zone conditions. Bound infragravity waves were observed and predicted to result in an offshore directed transport that was of subordinate magnitude compared to the observed and estimated magnitudes by the short waves and the undertow. However, as these transports almost cancelled out because of their about equal magnitude but opposite sign, the suspended transport by bound infragravity waves may not be ignored and may, rather paradoxically, have a relatively large influence on the net suspended sediment transport. Hydrodynamical processes that do not seem to be of importance to the onshore and offshore sediment transport in 3-to 9-m water depth in the long run are mean flows under non-breaking conditions and free infragravity motions. A direct comparison between measured sediment fluxes and the model predictions suggests that simple energetics models are suitable for predicting cross-shore sediment transport in 3-to 9-m water depth.
Video measurements of swash were made at the low-sloping beach of the multiple bar system at Ters... more Video measurements of swash were made at the low-sloping beach of the multiple bar system at Terschelling, Netherlands. The majority of the measurements were conducted under highly dissipative conditions with Iribarren numbers • 0 (the ratio of beach slope to the square root of offshore wave steepness) less than 0.2. Infragravity (0.004-0.05 Hz) waves dominated the swash with an average ratio of infragravity and total swash height Rig/R of 0.85. Using linear regression we investigated the dependence of swash parameters on environmental conditions such as shortwave height, period, and local beach slope. On average, Rig was about 30% of the offshore wave height H0; the slope in the linear H0 dependence of Rig amounted to only 0.18, considerably smaller than that observed on steeper beaches. The data set shows evidence for saturation of the higher infragravity frequencies for • 0 less than, roughly, 0.27. In our opinion, this saturation caused the constant of proportionality in the linear relationship between Rig/H o and • 0 to be significantly larger than that observed under higher Iribarren number regimes. The saturated tails of the swash spectra had an approximate f-3 roll-off (where f is frequency), whereas, in general, the nonsaturated parts were white. This lack of significant peaks casts doubt on the causality between infragravity waves and nearshore bars.
Mean alongshore currents observed on two barred beaches are compared with predictions based on th... more Mean alongshore currents observed on two barred beaches are compared with predictions based on the one-dimensional, time-and depth-averaged alongshore momentum balance between forcing (by breaking waves, wind, and 10-100 km scale alongshore surface slopes), bottom stress, and lateral mixing. The observations span 500 hours at Egmond, Netherlands, and 1000 hours at Duck, North Carolina, and include a wide range of conditions with maximum mean currents of 1.4 m/s. Including rollers in the wave forcing results in improved predictions of the observed alongshore-current structure by shifting the predicted velocity maxima shoreward and increasing the velocity in the bar trough compared with model predictions without rollers. For these data, wave forcing balances the bottom stress within the surfzone, with the other terms of secondary importance. The good agreement between observations and predictions implies that the one-dimensional assumption holds for the range of conditions examined, despite the presence of small alongshore bathymetric nonuniformities. With stronger bathymetric variations the model skill deteriorates, particularly in the bar trough, consistent with earlier modeling and laboratory studies.
An energetics-based expression for the depth-averaged cross-shore suspended sediment transport ra... more An energetics-based expression for the depth-averaged cross-shore suspended sediment transport rate due to infra-gravity (frequencies 0.004-0.04 Hz) waves is proposed by parameterizing moments of the near-bed flow. The moments represent the sediment stirring by short (frequencies 0.04-0.33 Hz) waves and the subsequent transport by the cross-shore motion of the infragravity waves phase-coupled to the incident groups. The data were gathered at three positions in 3-to 9-m depth in the multiple-bar system ofTerschelling <Netherlands). In these depths bound infragravity waves are the ?ole. co~tributors to t~e infragravity-induced sediment transport, causing it to be offshore directed. The pa-rametem:atlon ts formulated 10 terms of the local Urse\1 number Ur and is valid for Ur s 2.5; in addition, it contains spatial breaking information to account for the observed reduction in the moments by the breaking-induced destruction of the incident wave groups. The proposed expression was successfully cross-validated against two other data sets.
Recordings of near-bottom pressure obtained at three positions in a gemly sloping multiple-bar sy... more Recordings of near-bottom pressure obtained at three positions in a gemly sloping multiple-bar system were analyzed with a bispectral technique to quamify the relative importance of forced motions to the total infragravity (0.004-0.04 Hz) wave field under breaking and nonbreaking conditions, both on a temporal and spatial scale. The temporal variations in the ratio of bound to total infragravity energy E • nd/E • were well associated with the local relative wave height H.,.,/h (where H,., is the significam sea-swell wave height and h is water depth). E • na/Ei • increased with higher H,.,/h values, reaching maximum values of up to 0.8 at the onset of shortwave breaking. With an increasing imensity of breaking the coupling between incidem and infragravity waves reduced to negligible values in a saturated sea-swell wave field. The strong phase coupling at the breakpoint with a phase relationship close to the relationship predicted by bound-wave theory strongly supports the hypothesis that forced waves are the main source of free infragravity motions. In the cross-shore direction, E • na was largest at the breakpoim. Wave breaking was associated with a rapid decrease in the ratio Ebna/Eig in the onshore direction and, consequently, with a rapid increase in the comribution of free infragravity energy to the total infragravity field. The data set indicated that free infragra-vity energy may also be generated in the absence of breaking waves; the reason for this is not understood.
Recordings of near-bottom pressure at six cross-shore positions in a gently sloping multiple bar ... more Recordings of near-bottom pressure at six cross-shore positions in a gently sloping multiple bar system were analysed to study the temporal and spatial variability of infragravity (0.004—0.04 Hz) energy. The temporal variations in infragravity levels at each position were, as expected, strongly related to those in the offshore incident wave energy. Furthermore, infra-gravity energy was better correlated to swell than to sea energy and, under non-breaking conditions, was tidally modulated with larger values during low water than during high water. During low-energy conditions infragravity energy was almost constant over the profile, suggesting a dominance of free long-wave motions; however, it rapidly grew in the onshore direction during more energetic, though non-breaking conditions, caused by significant bound-wave contributions to the total long-wave field. Infragravity energy was significantly damped under surf-zone conditions, even causing a decline in the onshore direction. The data indicates that the release of bound long waves during the breaking process of the incident sea and swell may be the main source of free infragravity motions.
The behaviour of a multiple bar system in the nearshore zone of the island of Terschelling, the N... more The behaviour of a multiple bar system in the nearshore zone of the island of Terschelling, the Netherlands, is investigated on the time scale of years using a data set of soundings. The data set covers the period from 1965 to 1993. The behaviour is analysed in terms of bar crest position and in terms of morphometric parameters, such as bar crest depth, height, width, and volume. Two or three breaker bars are permanently present in the study area. Each individual bar passes through three stages during its existence: (1) generation close to the shore, (2) seaward migration and (3) degeneration at the outer margin of the nearshore bar zone. The key factor controlling the behaviour of the inner bars is the crest depth of the outer bar. Three different couplings between the behaviour of the individual bars have been observed: (1) bar stage changes due to the degeneration of the outer bar, (2) prevention of the transition of an inner bar from the first to the second stage, caused by the appearance from alongshore of a new outer bar having a small crest depth and (3) difference in mean annual seaward migration rate caused by the possible presence of a more seaward positioned bar. It is hypothesized that the crest depth of the outer bar governs the relative importance of processes inducing shoreward and seaward migration in the inner nearshore bar zone and therefore controls the behaviour of the nearshore bar zone on the time scale of years.
Sand nourishments are a widely applied technique to increase beach width for recreation or coasta... more Sand nourishments are a widely applied technique to increase beach width for recreation or coastal safety. As the size of these nourishments increases, new questions arise on the adaptation of the coastal system after such large unnatural shapes have been implemented. This paper presents the initial morphological evolution after implementation of a mega-nourishment project at the Dutch coast intended to feed the surrounding beaches. In total 21.5 million m 3 dredged material was used for two shoreface nourishments and a large sandy peninsula. The Sand Engine peninsula, a highly concentrated nourishment of 17 million m 3 of sand in the shape of a sandy hook and protruding 1 km from shore, was measured intensively on a monthly scale in the first 18 months after completion. We examine the rapid bathymetric evolution with concurrent offshore wave forcing to investigate the feeding behaviour of the nourishment to the adjacent coast. Our observations show a large shoreline retreat of O (150 m) along the outer perimeter of the peninsula, with locally up to 300 m retreat. The majority (72%) of the volumetric losses in sediment on the peninsula (1.8 million m 3 ) were compensated by accretion on adjacent coastal sections and dunes, confirming the feeding property of the mega nourishment. Further analyses show that the morphological changes were most pronounced in the first 6 months while the planform curvature reduced and the surf zone slope flattened to pre-nourishment values. In the following 12 months the changes were more moderate. Overall, the feeding property was strongly correlated to incident wave forcing, such that months with high incoming waves resulted in more alongshore spreading. Months with small wave heights resulted in minimal change in sediment distribution alongshore and mostly cross-shore movement of sediment.
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Papers by Gerben Ruessink