Shaping The Beach: Cross-Shore Sand Transport in the Swash Zone

Conference Paper, Published Version

van der Werf, Jebbe; Dionísio António, Sara; Kranenborg, Joost; Vermeulen, Bart; Campmans, Geert; van der Zanden, Joep; Ribberink, Jan; Reniers, Ad; Hulscher, Suzanne

Shaping The Beach: Cross-Shore Sand Transport in the

Swash Zone

Verfügbar unter/Available at: https://hdl.handle.net/20.500.11970/106696 Vorgeschlagene Zitierweise/Suggested citation:

van der Werf, Jebbe; Dionísio António, Sara; Kranenborg, Joost; Vermeulen, Bart;

Campmans, Geert; van der Zanden, Joep; Ribberink, Jan; Reniers, Ad; Hulscher, Suzanne (2019): Shaping The Beach: Cross-Shore Sand Transport in the Swash Zone. In: Goseberg, Nils; Schlurmann, Torsten (Hg.): Coastal Structures 2019. Karlsruhe: Bundesanstalt für Wasserbau. S. 803-811. https://doi.org/10.18451/978-3-939230-64-9_080.

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Abstract: The swash zone is the highly dynamic region near the shoreline where waves run up and

down the beach. Swash processes determine whether sand is stored on the upper beach or is transported offshore, and they thus strongly affect shoreline evolution. The swash zone sand transport processes are not fully understood and no generally-valid numerical swash zone sand transport models are available. The Shaping The Beach (2018-2023) research projects aims to develop a new parameterization for sand transport in the swash zone, through a combination of detailed field-scale wave flume experiments and advanced numerical modeling (OpenFoam, XBeach). As a final step, the new parameterization will be implemented in practical morphological modelling systems (e.g. Delft3D) and validated against field cases. This is expected to result in a widely applicable practical morphological model that can reliably predict shoreline evolution.

Keywords: swash zone, sand transport, shoreline evolution, numerical modelling, wave flume experiment

1 Introduction

The swash zone is the part of the beach near the shoreline that is alternately inundated and exposed by flow uprush and backwash (Fig. 1). It is arguably the most dynamic nearshore zone with energetic and unsteady flows, high turbulence levels, large sand transport rates and rapid morphological change. Swash processes determine whether sand is stored on the upper beach or is returned to the surf zone, hence they strongly affect shoreline evolution.

Fig. 1. Backwash on the beach in Nantucket, Massachusetts, USA. © Versageek – Flickr.

Shaping The Beach: Cross-Shore Sand Transport in the Swash Zone

J. van der Werf

, S. Dionísio António

, J. Kranenborg

, B. Vermeulen

,

G. Campmans

, J. van der Zanden

, J. Ribberink

, A. Reniers

& S. Hulscher

1

Deltares, Delft, The Netherlands

2

University of Twente, Enschede, The Netherlands

3

MARIN, Wageningen, The Netherlands

4

Delft University of Technology, Delft, The Netherlands

Coastal Structures 2019 - Nils Goseberg, Torsten Schlurmann (eds) - © 2019 Bundesanstalt für Wasserbau ISBN 978-3-939230-64-9 (Online) - DOI: 10.18451/978-3-939230-64-9_080

Knowledge on swash zone sand transport is essential to understand, and cope with, coastal erosion and sedimentation by natural processes and human interferences. This knowledge is used to assess climate change impacts (increased storminess and rising sea levels) on beach stability, to design cost-effective beach protection measures such as sand nourishments, and to design coastal structures (harbors, breakwaters).

The swash zone sand transport processes are not fully understood and no generally-valid numerical swash zone sand transport models are available. This greatly hampers the prediction of beach and shoreline evolution, and the sediment exchange between the wet and dry part of the beach. This knowledge gap can largely be attributed to a lack of sand transport process data from controlled, field-scale wave flume experiments.

This paper describes the Shaping The Beach research project (2018-2023), aimed to develop a new parameterization for sand transport in the swash zone, through a combination of detailed laboratory experiments and advanced numerical modeling. In Section 2 we will review the state-of-the-art knowledge on swash zone sand transport processes. Section 3 presents the Shaping The Beach research plan. Section 4 shows preliminary results. The conclusions are presented in Section 5.

2 Current knowledge on swash zone sand transport 2.1 Process knowledge

The swash hydrodynamics depend on the incident-wave conditions and the beach morphology. This can be expressed using the surf similarity parameter or Iribarren number, ξo= β/√(Ho/Lo), where β, Ho and Lo are the beach slope, deep water wave height and deep water wavelength. Low-frequency (f <≈ 0.05 Hz) energy dominates swash oscillations at dissipative beaches with ξo <≈ 1, whereas high-frequency incident bores (f >≈ 0.05 Hz) tend to dominate at reflective beaches for higher Irribaren numbers (Elfrink & Baldock, 2002; Masselink & Puleo, 2006). A typical 5 s period, 1.0 m high wave at the Holland Coast with a ~1/50 beach slope results in ξo = 0.1, indicating predominantly low-frequency swash.

The incident waves interact with the swash of preceding waves. Cáceres & Alsina (2012) characterized three different wave-swash interactions: (i) wave capture, where the second wave captures the previous one during both uprush stages, (ii) weak wave-backwash interaction, when the incident wave overrides a preceding backwash, (iii) strong wave-backwash interaction, with a stronger backwash than incoming uprush resulting in a hydraulic jump and an offshore flow. Wave-swash interactions are important with swash periods equal to, or greater than, the incident wave period, i.e. more on dissipative than on reflective beaches. Wave-swash interactions can mobilize and advect sediment and have a strong effect on sand transport processes (Cáceres & Alsina; 2012; Van der Zanden et al, 2019).

Sand transport in the swash zone is transported as sheet-flow and suspended load. The sheet-flow layer is a few cm’s thin layer directly above the non-moving bed with high sediment concentrations (between ~100-1600 kg/m3). Depending mainly on wave conditions and grain-size, one transport mode may prevail during certain parts of the swash cycle (Fig. 2). Both sand suspension and sheet-flow layer dynamics in the swash are not only controlled by vertical processes, but also by cross-shore advection (Alsina et al., 2009; Van der Zanden et al., 2019).

Net (i.e., swash-averaged) transport and bed level changes are the result of an imbalance between typically much larger landward uprush and seaward backwash transport. Net transport rates by different swash events may vary strongly in terms of direction and magnitude, which can partly be attributed to wave-swash interactions (Alsina et al., 2018). In certain cases, the beach morphology is controlled by only a small number of large swash events (Blenkinsopp et al., 2011).

We know that wave-swash interactions, cross-shore advection and individual swash events are important for net sand transport. However, this knowledge still has a strong qualitative nature, and is limited to a relatively small number of wave conditions and beach configurations. As a result of this knowledge gap, we do not fully understand what controls onshore and offshore net transport and the corresponding beach development. This hampers the development of numerical models. This lack of knowledge can largely be attributed to a lack of data from controlled, large-scale laboratory experiments.

Fig. 2. (a) Water depth; (b) ADV measured flow velocities at 0.03 m above the bed (solid lines; mean +/- standard deviations) and sand particle velocity measured by CCM+ at the top of the sheet-flow layer (circles); (c) suspended sand concentration 0.03 m above the bed; (d) sheet-flow layer thickness; (e) depth-integrated suspended (blue solid line) and sheet-flow layer (circles and dashed line) sand transport rates. Figure taken from Van der Zanden et al. (2019).

2.2 Experiments

The swash zone hydrodynamics and sand transport have been studied through experiments both in the field (e.g. Masselink et al., 2005; Lanckriet et al., 2014; Puleo et al., 2014) and in large-scale wave flumes (e.g. Van der Zanden et al., 2015; Alsina et al., 2018; Van der Zanden et al., 2019). Large-scale wave flume experiments enable studying field-scale cross-shore swash zone dynamics under repeatable, controlled conditions in isolation from other processes, such as tide and alongshore variations.

Existing wave flume experiments have gained a lot of valuable data and process-insights. However, the following type of data is currently lacking:

• swash zone dynamics for more dissipative, typically mild-sloping beaches;

• coherent dataset of net sand transport per swash event for a large number of wave conditions and beach configurations, including more dissipative beaches;

• high spatial and temporal coverage to resolve wave-swash interactions and cross-shore advection processes.

2.3 Intra-swash modelling

Intra-swash numerical models resolve the swash motion. Since swash water depths are small (low kh-values, with k the wave number and h the water depth), depth-integrated models that solve the non-linear shallow water equations (NLSWE) have frequently been used (see e.g. Briganti et al., 2016). NLSWE are computational inexpensive compared to depth-resolving models, but the parameterizations for unresolved vertical processes introduce uncertainty. For example, this type of models has difficulty to reproduce accretive swash events (see e.g. Incelli et al., 2016; Jongedijk, 2017).

Most depth-resolving models are based on a Reynolds-Averaged Navier-Stokes (RANS) equations with a turbulence closure. In the swash zone, depth-resolving models have almost exclusively been used to investigate the hydrodynamics (e.g. Torres-Freyermuth et al., 2013; Higuera et al., 2018). Only very recently have Li et al. (2019) used a depth-resolving model to study swash zone morphology. However, depth-dependent processes affecting sand transport have not yet been systematically investigated in the swash zone using a depth-resolving model.

2.4 Swash-averaged, practical modelling

Practical morphodynamic models (e.g. Delft3D, Mike, Telemac) have a wave-averaged framework that does not resolve the swash motion. They include highly-empirical methods to either redistribute erosion from ‘wet beach’ to adjacent ‘dry beach’ cells or to compute the total net swash zone sand transport from the last ‘wet’ point to a predicted run-up level (see e.g. Larson et al., 2004; Van Rijn, 2009). It is generally recognized that these methods are too simplistic as a result of which practical models have great difficulty in predicting shoreline evolution correctly (Van Rijn et al., 2011).

3 Shaping The Beach research project 3.1 Objectives

The Shaping The Beach research project (2018-2023) aims to improve understanding of swash zone sand transport processes and to develop new model formulations. This is reflected in the following objectives:

1. The systematic investigation of net cross-shore sand transport rates across a laboratory swash zone for a wide range of wave and beach conditions.

2. A detailed examination of sand transport processes across the inner surf and swash zone in relation to hydrodynamic parameters.

3. To further develop two detailed numerical models (the 2DV OpenFoam model and the 1D XBeach model) describing the intra-swash cross-shore sand transport and morphodynamics, by implementing new formulations for key hydrodynamic and sand transport processes in the swash zone based on the experimental observations.

4. To develop a new parameterization for net total load sand transport in the swash zone for application in practical morphological modelling systems.

These objectives will be met by a combination of 1) field-scale wave flume experiments, 2) detailed numerical modelling, and 3) practical swash zone sand transport modelling.

3.2 Barcelona wave flume experiments

Objectives 1 and 2 will be realized through mobile-bed experiments conducted in the large-scale CIEM wave flume at the Technical University of Catalunya (Barcelona), which is in terms of size and swash zone track record arguably the most well-suited wave flume in Europe for the experiments foreseen. The total campaign will comprise 9 weeks of experiments, whilst another 8 weeks are scheduled for preparing the bed profiles and the instrumental set-up.

The bed conditions will comprise one grain size (medium sand, 0.25 mm median diameter) and two bed slopes (1:15 and 1:25). The purpose of the two bed slopes is to widen the range of swash

conditions, covering bore-driven reflective swash events up to infra-gravity-driven dissipative events. Two types of experiments are foreseen: ‘TRANS’ experiments and the ‘PROC’ experiments. Fig. 3 shows the preliminary experimental set-up.

Fig. 3. Preliminary experimental set-up for the 1:15 Shaping The Beach wave flume experiments.

The TRANS experiments focus on quantifying the net (i.e. swash-averaged) total transport rates across the swash zone for a high number and wide range of swash events. Exposed bed levels in the swash zone will be measured with high spatial coverage using LiDAR (Blenkinsopp et al., 2011; Almeida et al., 2015), with additional local bed level measurements by acoustic wave gauges (AWGs) for calibration/validation purposes. At multiple locations around the shoreline and in the inner surf zone, the exposed and submerged bed level will be measured continuously using CCPs (Lanckriet et al., 2013) and two CCM+ tanks (Van der Zanden et al., 2015). For each swash event, the net total transport rates will be inferred from the bed level measurements by solving the sediment mass balance.

For selected conditions, hydrodynamic and sediment transport processes will be investigated in great detail (PROC experiments). Bichromatic waves will be used, which are similar to natural random waves in terms of cross-shore distributions of time-averaged wave, and velocity and sand transport statistics, and in terms of the occurrence of wave-swash interactions. At the same time, bichromatic waves allow ensemble-averaging over many wave repeats in order to improve data convergence. Hydro- and sand dynamics will be measured with dense spatial coverage and sophisticated instrumentation. Water levels will be measured by LiDAR, pressure transducers, and acoustic wave gauges; velocities by ADVs, and inferred from LiDAR through the volumetric fluid conservation balance. Sand concentrations will be measured with OBSs in the suspension layer, and with CCPs and CCM+ in the sheet-flow layer; bed levels by CCPs and CCM+ (intra-swash), LiDAR and AWGs (between events), and a mechanic wheel profiler (between runs).

3.3 Detailed numerical modelling

Two detailed numerical models that resolve the low- and high-frequency intra-swash fluid motion are used: the depth-resolving (2DV) OpenFoam model and the depth-averaged (1D) XBeach model. Both models will be improved for the simulation of swash zone sand transport and morphodynamics by implementing novel formulations for turbulence, bed shear stress, sand pick-up, sand mixing, and bedload transport on the basis of new and existing data.

The OpenFoam model solves the 2DV non-hydrostatic RANS equations with a turbulence closure and a volume of fluid approach to capture the free surface. It includes the advection-diffusion model for suspended sand, an empirical bedload transport formula, and the Exner equation to compute morphological change (Jacobsen & Fredsøe, 2014). The depth-resolving character of OpenFoam makes it especially suitable to study wave bottom boundary layer processes, vertical turbulence and sand mixing, and effects of vertical pressure gradients at an intra-swash time scale.

The 1D XBeach model computes the flow by solving the NLSWE (accounting for non-hydrostatic pressure effects), sand transport with an advection-diffusion solver and empirical bedload formula, and morphological change by solving the Exner equation (Reniers et al., 2013; McCall et al., 2015). The 1D character makes this model suitable to study cross-shore swash hydrodynamics, sand dynamics, and morphodynamics on the time scales of days (e.g. simulations of dune erosion under storm conditions).

3.4 Practical swash modelling

The final research activity in the Shaping The Beach project is the development of a new parameterization for the net total load sand transport from the lower swash region to the wave run-up level, to be applied in engineering morphological models such as Delft3D, Mike and Telemac. This parameterization will use existing formulations for sand transport and wave run-up as a starting point (e.g. Larson et al., 2004), which can be extended and improved by adding important hydrodynamic and sand transport processes in a parameterized way. Specifically, the parameterization will include a two-way coupling between the inner surf and the swash zone to account for cross-shore sediment advection between these zones. Moreover, effects of wave-swash interactions on sand transport will be accounted for. Parameterizations for these processes will be based on the new insights from the new wave flume experiments and detailed numerical modelling, and on existing knowledge on swash zone sand transport.

4 Preliminary research results 4.1 Numerical modelling

O’Donoghue et al. (2010) measured swash hydrodynamics in detail for a dambreak experiment with a single bore collapse on a steep (1:10) beach. These data were used as first test case for the XBeach and OpenFoam numerical models.

Fig. 4 shows two snapshots of OpenFoam results. The first (t = 2 s), shows the bore arriving at the beach. The second (t = 5 s) shows the early stage of backwash. At the early uprush, the localized patches of high flow velocity and turbulence induced by the previously broken wave are clearly visible. The velocity peaks reach 2 m/s and are localized near the swash tip. It is evident that the velocity varies strongly in the vertical direction. The turbulent kinetic energy is also concentrated around the swash tip and the velocity peaks. Around the transition to backwash, the localized high velocity and turbulent kinetic energy patches have dispersed into the water column. At the lower parts of the beach this transition happens earlier than higher up the beach. Details on this can be found in the Coastal Structures paper by Kranenborg et al. (2019).

Fig. 4. Results from OpenFoam on the cross-shore velocity and turbulent kinetic energy density for the dambreak case of O’Donoghue et al. (2010). The upper two panels show the turbulent bore reaching the beach after two seconds. The lower two panels show the early backwash stage. In all four plots only the results for the water phase are shown.

4.2 LiDAR test experiments at MARIN

Laboratory experiments were carried out to test the ability of the LiDAR instrument to measure bed level and free-surface elevation at MARIN concept basin. This basin has a length of 220 m, a width of 4 m, and a depth of 3.64 m and is equipped with a single flap wave maker on one end and a parabolic wave absorbing beach at the other end. The fixed beach induces wave breaking and swash events allowing the LiDAR to measure the free-surface elevation during uprush and backwash.

A series of irregular and regular waves with wave heights ranging from 0.05 m to 0.35 m and wave periods in the range of 1 to 4.5 s were generated with a duration of 5 minutes. The experimental work was undertaken using a LMS511-20100 PRO 2D laser measurement sensor manufactured by SICK (2012). The instrument was configured to scan at a sample rate of 25 Hz with an angular resolution of 0.1667° within a 190° field view. The instrument was positioned at two different horizontal positions, (i) directly above the dry part of the beach and (ii) above water (~3.6 m more offshore), both 4 m above still water level with a scanning range of 7 and 10 m along the length of the flume, respectively. For validation, 3 resistive and 3 acoustic wave gauges were deployed at 0.50 m of from the flume wall and with at 0.70 m intervals in cross-shore direction to measure time-varying free-surface elevations at fixed points at a sample rate of 100 Hz.

Preliminary results (Fig. 5) show the ability of the LiDAR to detect both the water surface and the bed level. The bed level is detected when the water surface is tranquil, while the water surface is detected as soon as the water surface roughens due to the breaking waves. The angle of incidence of the laser beam into the water was found to influence the ability to detect water levels. The more offshore location with a smaller angle of incidence resulted in an additional ~2 m horizontal range at which water levels could be measured.

Fig. 5. LiDAR measurement of (black) fixed bed level during still water and (grey) water level of a breaking wave.

5 Conclusion

This paper presented an overview of the Shaping The Beach research project (2018-2023) on cross-shore sand transport in the swash zone. The projects aims to develop a new parameterization for sand transport in the swash zone, through a combination of detailed laboratory experiments and advanced numerical modeling.

The large-scale laboratory wave flume experiments involve high-resolution measurements of hydrodynamics, sand transport processes and bed level changes for a wide range of wave conditions, using sophisticated instrumentation including LiDAR, CCP and CCM+. These experiments will give new insights in detailed bedload and suspended load sand transport processes in the swash, and also in net sand transport rates for a wide range of swash conditions. The new experimental data will be used to progress two intra-swash process-based numerical models for swash zone sand transport and morphodynamics (XBeach, OpenFoam). With these models we will gain profound understanding of the detailed sand transport processes that drive beach erosion or accretion. The new insights of the experimental and numerical activities will then be used to develop a new practical parameterization for swash zone sand transport, accounting for the fundamental sand transport processes identified during the detailed experimental and numerical investigations. Such a new parameterization is necessary to advance wave-averaged morphodynamic models (Delft3D, Mike, Telemac), that are applied by coastal engineers worldwide.

As a final step, the new parameterization will be implemented in Delft3D and validated against field cases. This is expected to result in a widely applicable practical morphological model that can reliably predict shoreline evolution.

Acknowledgements

This work is part of the research program Shaping The Beach with project number 16130, which is financed by the Netherlands Organisation for Scientific Research (NWO), with in-kind support by Deltares. The authors would like to thank MARIN for the concept basin time for instrument testing.

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