Gerben Ruessink

Measuring and modeling the effect of surface moisture on the spectral reflectance of coastal beach sand

Surface moisture is an important supply limiting factor for aeolian sand transport, which is the primary driver of coastal dune development. As such, it is critical to account for the control of surface moisture on available sand for dune building. Optical remote sensing has the potential to measure surface moisture at a high spatio-temporal resolution. It is based on the principle that wet sand appears darker than dry sand: it is less reflective. The goals of this study are (1) to measure and model reflectance under controlled laboratory conditions as function of wavelength () and surface moisture () over the optical domain of 350–2500 nm, and (2) to explore the implications of our laboratory findings for accurately mapping the distribution of surface moisture under natural conditions. A laboratory spectroscopy experiment was conducted to measure spectral reflectance (1 nm interval) under different surface moisture conditions using beach sand. A non-linear increase of reflectance upon drying was observed over the full range of wavelengths. Two models were developed and tested. The first model is grounded in optics and describes the proportional contribution of scattering and absorption of light by pore water in an unsaturated sand matrix. The second model is grounded in soil physics and links the hydraulic behaviour of pore water in an unsaturated sand matrix to its optical properties. The optical model performed well for volumetric moisture content 24% ( 0.97), but underestimated reflectance for between 24–30% ( 0.92), most notable around the 1940 nm water absorption peak. The soil-physical model performed very well ( 0.99) but is limited to 4% 24%. Results from a field experiment show that a short-wave infrared terrestrial laser scanner ( = 1550 nm) can accurately relate surface moisture to reflectance (standard error 2.6%), demonstrating its potential to derive spatially extensive surface moisture maps of a natural coastal beach.

Evolution of the early Antarctic ice ages

Significance The Antarctic ice cap waxed and waned on astronomical time scales throughout the Oligo-Miocene time interval. We quantify geometries of Antarctic ice age cycles, as expressed in a new climate record from the South Atlantic Ocean, to track changing dynamics of the unipolar icehouse climate state. We document numerous ∼110-thousand-year-long oscillations between a near-fully glaciated and deglaciated Antarctica that transitioned from being symmetric in the Oligocene to asymmetric in the Miocene. We infer that distinctly asymmetric ice age cycles are not unique to the Late Pleistocene or to extremely large continental ice sheets. The patterns of long-term change in Antarctic climate interpreted from this record are not readily reconciled with existing CO2 records. Understanding the stability of the early Antarctic ice cap in the geological past is of societal interest because present-day atmospheric CO2 concentrations have reached values comparable to those estimated for the Oligocene and the Early Miocene epochs. Here we analyze a new high-resolution deep-sea oxygen isotope (δ18O) record from the South Atlantic Ocean spanning an interval between 30.1 My and 17.1 My ago. The record displays major oscillations in deep-sea temperature and Antarctic ice volume in response to the ∼110-ky eccentricity modulation of precession. Conservative minimum ice volume estimates show that waxing and waning of at least ∼85 to 110% of the volume of the present East Antarctic Ice Sheet is required to explain many of the ∼110-ky cycles. Antarctic ice sheets were typically largest during repeated glacial cycles of the mid-Oligocene (∼28.0 My to ∼26.3 My ago) and across the Oligocene−Miocene Transition (∼23.0 My ago). However, the high-amplitude glacial−interglacial cycles of the mid-Oligocene are highly symmetrical, indicating a more direct response to eccentricity modulation of precession than their Early Miocene counterparts, which are distinctly asymmetrical—indicative of prolonged ice buildup and delayed, but rapid, glacial terminations. We hypothesize that the long-term transition to a warmer climate state with sawtooth-shaped glacial cycles in the Early Miocene was brought about by subsidence and glacial erosion in West Antarctica during the Late Oligocene and/or a change in the variability of atmospheric CO2 levels on astronomical time scales that is not yet captured in existing proxy reconstructions.

Infragravity-wave dynamics in a barred coastal region, a numerical study

This paper presents a comprehensive numerical study into the infragravity-wave dynamics at a field site, characterized by a gently sloping barred beach. The nonhydrostatic wave-flow model SWASH was used to simulate the local wavefield for a range of wave conditions (including mild and storm conditions). The extensive spatial coverage of the model allowed us to analyze the infragravity-wave dynamics at spatial scales not often covered before. Overall, the model predicted a wavefield that was representative of the natural conditions, supporting the model application to analyze the wave dynamics. The infragravity-wave field was typically dominated by leaky waves, except near the outer bar where bar-trapped edge waves were observed. Relative contributions of bar-trapped waves peaked during mild conditions, when they explained up to 50% of the infragravity variance. Near the outer bar, the infragravity-wave growth was partly explained by nonlinear energy transfers from short waves. This growth was strongest for mild conditions, and decreased for more energetic conditions when short waves were breaking at the outer bar. Further shoreward, infragravity waves lost most of their energy, due to a combination of nonlinear transfers, bottom friction, and infragravity-wave breaking. Nonlinear transfers were only effective near the inner bar, whereas near the shoreline (where losses were strongest) the dissipation was caused by the combined effect of bottom friction and breaking. This study demonstrated the models potential to study wave dynamics at field scales not easily covered by in situ observations.

Parameterization of Velocity Skewness under Waves and its Effect on Cross-Shore Sediment Transport

Skewness of the near-bed cross-shore orbital velocity plays a central part in many beach equilibrium and sediment transport models. Although Boussinesq models can be used to predict the velocity skewness of shoaling and breaking waves, these models are computationally quite intensive. Herein, a large collection of data from a number of field environments are used to derive via genetic programming an analytical expression for velocity skewness under shoaling and breaking waves. The resulting expression provides a good fit to the data.The inclusion of skewness in a sediment transport model demonstrates its importance in predicting the shoreward transport of sediment during periods of beach recovery.

BARDEX II: Bringing the beach to the laboratory - again!

ABSTRACT Masselink, G, Turner, I.L., Conley, D.C., Ruessink, B.G., Matias, A., Thompson, C., Castelle, B. and Wolters, G., 2013. BARDEX II: Bringing the beach to the laboratory – again! Proto-type laboratory experiments are particularly useful in coastal research when forcing parameters are modified in a way that is impossible to achieve in the field, and where installation and maintenance of instrumentation requires absence of waves. In 2008, the Barrier Dynamics Experiment (BARDEX) took place in the Delta Flume, the Netherlands. This project, funded by Hydralab III, focused on the effect of varying wave, sea level and beach groundwater conditions on a gravel beach (D50 = 10 mm). In 2012, a similar project was carried out, referred to as BARDEX II, this time funded by Hydralab IV and on a sandy beach (D50 = 0.42 mm). During the experiment, a 4.5-m high and 70-m wide sandy barrier was constructed in the flume with a lagoon situated to the landward. The barrier was instrumented with a very large number (> 200) of instruments and subjected to a range of wave conditions (Hs = 0.8 m; Tp = 4–12 s) and varying sea and lagoon water levels. Five distinct test series were executed over a 20-day period: series A focused on beach response due to accretionary/erosive wave conditions and a high/low lagoon water level; series B investigated the effect of a lower sea level on nearshore bar dynamics; series C simulated tidal effects; series D addressed the swash/overtopping/overwash threshold; and during series E the beach-barrier system was subjected to an extended period of energetic overwash conditions. This paper will describe the experimental design and the test programme during BARDEX II.

MORPHOLOGICAL MONITORING OF A SHOREFACE NOURISHMENT NOURTEC EXPERIMENT AT TERSCHELLING, THE NETHERLANDS

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

Rapid adjustment of shoreline behavior to changing seasonality of storms: Observations and modelling at an open-coast beach†

An 8-year time series of weekly shoreline data collected at the Gold Coast, Australia, is used to examine the temporal evolution of a beach, focusing on the frequency response of the shoreline to time-varying wave height and period. Intriguingly, during 2005 the movement of the shoreline at this site changed from a seasonally-dominated mode (annual cycle) to a storm-dominated (~monthly) mode. This unexpected observation provides the opportunity to explore the drivers of the observed shoreline response. Utilizing the calibration of an equilibrium shoreline model to explore the time-scales of underlying beach behavior, the best-fit frequency response (days-1) is shown to be an order of magnitude higher post-2004, suggesting that a relatively subtle change in wave forcing can drive a significant change in shoreline response. Analysis of available wave data reveals a statistically significant change in the seasonality of storms, from predominantly occurring at the start of the year pre-2005 to being relatively consistent throughout the year after this time. The observed change from one mode of shoreline variability to another suggests that beaches can adapt relatively quickly to subtle changes in the intra-annual distribution of wave energy. This article is protected by copyright. All rights reserved.

Accuracy of Nearshore Bathymetry Inverted From

Shore-based remote sensing platforms are increasingly used to frequently (~daily) obtain bathymetric information of large (~km²) nearshore regions over many years. With recorded wave frequency Ω and wavenumber k (and hence wave phase speed c = Ω/k), bed elevation z_b can be derived using a model that relates Ω and k to water depth. However, the accuracy of z_b as a function of the sensor and the method of Ω-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 zb is systematic. A fast Fourier transform (FFT) method on X-band radar imagery produced zb 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 zb 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 videoCS 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.

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ABSTRACT Castelle, B., Dubarbier, B., Tissier, M., Bonneton, P., Conley, D.C., Ruessink, B.G. and Masselink, G., 2013. Testing numerical hydrodynamic and morphodynamic models against BARDEX II Experiment data sets. The hydrodynamics on barred beaches and mechanics of sediment transport related to sandbar migration, berm formation/destruction, barrier crest dynamics and washover deposition are extremely complex. At this time, process-based models encompassing all these processes are non-existent. Among other shortcomings, the lack of existing intensive high-frequency full-scale data limits the range improvement and validation of nearshore process-based models. In June 2012, the large-scale Barrier Dynamics Experiment (BARDEX II) was performed in the Delta Flume, providing new datasets for rigorous testing of existing hydrodynamic, groundwater and morphodynamic models and further assisting their development. Three types of models are expected to be applied and further improved: (1) the short-wave averaged surfzone beach profile evolution models 1DBeach and UNIBEST-TC, (2) the short-wave averaged and infragravity-wave resolving XBeach model that addresses morphological changes of the nearshore area, beaches, dunes and backbarrier during storms including cross-barrier groundwater fluxes and (3) the short-wave resolving hydrodynamic models SURF-GN and SWASH. In this contribution, we present the application of three of the five models. 1DBeach is applied to a morphological sequence characterised by onshore and subsequent rapid offshore sandbar migration for time-invariant wave forcing and falling tide. XBeach model is applied to a rising tide sequence characterized by a rapid shoreline retreated and overtopping and overwash processes. SURF-GN is applied to a high tide run with occasional overtoppings. All these model applications are described and model skills are qualitatively assessed. Guidelines for future model improvements and validation on BARDEX II dataset are further discussed.

-Band Radar and Optical Video Data

This chapter examines the impacts of climate change on the natural coastal ecosystems in the North Sea region. These comprise sandy shores and dunes and salt marshes in estuaries and along the coast. The chapter starts by describing the characteristic geomorphological features of these systems and the importance of sediment transport. Consideration is then given to the role of bioengineering organisms in feedback relationships with substrate, how changes in physical conditions such as embankments affect coastal systems, and the effects of livestock. The effects of climate change—principally accelerated sea-level rise, and changes in the wind climate, temperature and precipitation—on these factors affecting coastal ecosystems are then discussed. Although the focus of this chapter is on the interaction of abiotic conditions and the vegetation, the potential impacts of climate change on the distribution of plant species and on birds breeding in salt marshes is also addressed. Climate impacts on birds, mammals and fish species are covered in other chapters.