Field Research Facility
Coastal & Hydraulics Laboratory

SandyDuck: A Field Study of Sediment and Bathymetric Response to Fluid Forcing    William A. Birkemeier (ed)
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Editor's Note:
This early 1992 document provided the original motivation for the DUCK94/SandyDuck series of experiments. Many members of the nearshore science and engineering community contributed to its content and authorship.

Introduction

The nearshore research community met at St. Petersburg, Florida in April of 1989 and outlined research priorities for the next years. Specific areas requiring special emphasis were identified, most notably sediment transport. At a subsequent community meeting in December 1991, researchers identified field studies focused on nearshore sediment transport that could be executed within the next several years. This document summarizes possible scientific objectives of such a field experiment (tentatively known as SandyDuck).

Objectives

A long term goal of nearshore processes research is to predict the evolution of the bathymetry of a natural beach given the initial bathymetry, sediment characteristics, and the temporal variation of the wind, tide, and incident wave field. Such predictions are not presently possible because we do not understand the complex and interacting fluid and sediment processes, particularly small scale boundary layer processes and the three-dimensional circulation on complex bathymetry. Recent field experiments focused on measurements of wave-induced flows in mid-water column over simple topography (either no sand bar or a predominantly linear bar). Models of sediment response remain primitive and empirical. SandyDuck will expand our knowledge of the nearshore by properly sampling processes on more complex, three-dimensional bathymetries. More specifically, the objectives of the experiment would be to study:
  • The dynamics of the bottom boundary layer and associated sediment transport. Both are poorly understood, partly owing to severely limited field data. Without field-tested sediment transport models, our capabilities for predicting nearshore evolution will remain poor.
  • The feedback between complex topography, waves and the three-dimensional nearshore circulation. Spatial gradients in fluid flows are caused by depth variations on many scales, from ripples that affect bottom roughness to large scale bar morphology. The bathymetry in turn evolves in response to the flow field.
SandyDuck will have two main parts: development of a conceptual model (or a system of models) appropriate to a realistic nearshore regime; and execution of an experiment to test the hypotheses of this model. There exists now no complete quantitative description of the nearshore in the sense that it is a region where several equally important (order one) processes occur and interact. The conceptual model must be developed to provide a framework for the various parts of the experiment, to ensure that hypotheses are posed properly and to enhance the interaction among investigators representing different subspecialty fields of research.

The experimental approach will be to measure sediment flux, bathymetry, and fluid flow at spatial scales ranging from a few cm to 100 m and temporal scales ranging from seconds to weeks. The model will relate changes in large-scale bathymetry to spatial gradients in time-averaged sediment flux. Topographic processes which may influence waves and currents span a range of scales including the formation and migration of ripples, megaripples, beach cusps, bars, channels and other morphologic features. Observations will be designed to test existing large and small-scale sediment transport models and to provide information about processes (e.g., effects of megaripples on bottom stress and sand transport) for which there are as yet no models.

Background

SandyDuck will be another step in a continuing sequence of major nearshore field experiments in the US. These started with the Nearshore Sediment Transport Study (NSTS) conducted between 1977-1982. The NSTS experiments emphasized nearshore sediment transport combined with hydrodynamic monitoring. However, because of limitations of sediment measurement instruments at the time, the greatest impact of NSTS was on hydrodynamic knowledge. The DUCK series of experiments began with DUCK82 in 1982, followed by DUCK85, SUPERDUCK in 1986, and DELILAH in 1990. Although the first three experiments included some element of sediment transport, the focus of these experiments was on nearshore hydrodynamics. Hydrodynamic process data were collected, along with complete surveys of the large scale morphology. Each DUCK experiment improved on the previous one. For example, the DELILAH current meter array was designed using knowledge and results gained during SUPERDUCK.

While the DUCK experiments were significant and have contributed to fundamental knowledge of surf zone processes, the data from these experiments have some significant limitations. Chief among these has been the lack of synoptic measurements of the vertical distribution of horizontal flow. Limited data collected with a mobile, instrumented sled along with theoretical work has pointed out the importance of these measurements. Similarly, there have been only limited measurements of real-time bottom changes (DUCK85) and no measurements of sediment flux at any scale under moderate to high wave conditions. Successful small-scale experiments have been conducted by individual investigators, but they did not quantify the interaction of the large scale fluid and bathymetric environments with the small-scale processes. Finally, although morphologic changes and nearshore flow are affected by bottom characteristics including texture, ripples, and bed forms, these features have not been investigated. Their importance was emphasized during DELILAH when the presence of megaripples at Duck was documented for the first time.

SandyDuck will differ significantly from the earlier experiments by addressing these limitations and focusing on small- and mid-scale sediment transport processes within the framework of the driving large-scale hydrodynamics and bathymetric evolution.

Models

The coupling between nearshore bathymetry and fluid flow is too complex to study by observation alone, without the benefit of a theoretical framework. Modeling must therefore be an integral part of all phases of SandyDuck, including experiment design. Existing models for three-dimensional flows and bathymetric evolution in the nearshore are relatively crude, and field observations to verify them are sparse. Although well-designed field measurements can help understand model weaknesses and capabilities, the process of using field data to test and improve models is not straightforward. For example, most hydrodynamic models require more detailed specification of boundary and initial conditions than can be obtained in the field.

The SandyDuck experiments will be an initial step towards model-experiment integration and will provide the basis for the design and execution of future experiments and for the development of improved models. SandyDuck data will lead to improved models of wave transformation, three-dimensional circulation, boundary layer processes, infragravity wave effects, and, most importantly, interactions of these processes with the evolving and complex bathymetry.

Field Experiments

The proposed experiment consists of intensive and monitoring phases. The several-week-long intensive phase, encompassing one or two storms, will include direct measurements of sediment flux, boundary layer processes, and detailed studies of beach-face and bar-bathymetry interaction with the local flow field. This phase will employ a large array of fluid and sediment monitoring instruments. The duration of the intensive phase is limited by the difficulty of maintaining instruments in the surf zone.

The monitoring phase, of perhaps a year, will couple measurements of bathymetric evolution with detailed measurements of the incident wave field. A smaller array of instruments will be deployed in the surf zone. The longer time span will allow sampling of bathymetric changes caused by many storms. Fewer instruments will be needed by virtue of relationships between incident waves and the surf zone flow field established during the intensive phase.

The field site for this experiment must offer a variable wave climate and a beach that is known to have substantial bathymetric changes in response to fluid forcing. While many beaches satisfy these requirements, the US Army Corps of Engineers Field Research Facility (FRF) at Duck, NC, is an ideal location because it satisfies the environmental constraints and also provides unparalleled logistical support (eg., pier, LARC and CRAB vehicles, high-resolution directional wave array). Moreover, it is an extremely well studied site with over 10 years of wave and large-scale morphologic observations.

Sediment Transport Experiments

Excepting aeolian processes, all nearshore morphologic change evolves from the divergence of fluxes of sediments that reside in the water body. Relating these divergences to governing hydrodynamic processes necessitates measurements of local sediment flux, a global question being how local sediment transport rates are coupled to morphology and hydrodynamics at all relevant scales. Important research issues identified in the St. Petersburg report (and elsewhere) include the:
  • dynamics of the time-varying, turbulent boundary layer
  • threshold for sediment motion under time-varying bottom stress
  • role of sediment-induced stratification in controlling the bottom stress field sediment flux under coupled mean and oscillatory flows

Progress over the last decade in the development of fast response acoustic and optical sensors for measurement of suspended sediments, as well as advances in velocity measurement technology for measurements of turbulent flows, now make it possible to begin addressing these problems. SandyDuck represents an opportunity to make, for the first time, local measurements of sediment transport nested within a large-scale, three-dimensional study of nearshore hydrodynamics. Consequently, a set of target objectives is to measure, directly or indirectly,

  • local suspended sediment transport over a broad frequency band
  • changes in local bathymetry including temporal evolution of bedforms over
  • horizontal scales of order cm to 10 m
  • bedload transport
  • bottom stress
  • surface roughness

None of the five objectives is trivial, but success in any one of them represents a significant advance in our understanding of the overall process. Implicit to meeting these objectives is to determine the vertical structure of suspended sediment and velocity fields. These observations provide a basic test of analytic sediment transport models.

Fixed instrumentation will be deployed in four main morphological regions: outside the surf zone, on the offshore bar, in the nearshore trough and in the swash zone. Because dynamically important regions change with tides, surf zone width and morphology, further observations will be obtained with portable instrumentation using sleds, tripods or the CRAB. The temporal and spatial variation of bedforms that strongly influence local sediment fluxes (i.e., ripples, megaripples) will be monitored with point and scanning acoustic altimeters, and side scan sonar.

The practicality of being able to meet all the above objectives with the instrumentation currently available or under development needs to be assessed. Newly developed acoustic instruments are capable of providing measurements of microtopography and sediment concentration profiles with sufficient resolution outside the surf zone. Optical backscatter sensors (OBS) can obtain point measurements of suspended load across the nearshore zone. Coupled OBS and current meters can be used to estimate suspended sediment transport. These instruments are currently available and, by themselves, will provide a wealth of information about nearshore sediment transport processes. Instruments currently under development, including bedload sensors along with high-frequency-response current meters and suspended sediment sensors are highly desirable and may be available for SandyDuck.

Bathymetry Experiments

Instrument arrays in past experiments have been designed around fairly simple bathymetries because hydrodynamic models were available for these cases and available instrumentation was severely limited. In the NSTS experiments, featureless bathymetry was selected and longshore homogeneity was assumed. At the next level of complexity, the Duck experiment arrays were designed for the case of a linear bar. However, the Duck gages had no capability for resolving synoptically the observed three-dimensional flows on what was frequently complex morphology. While these experiments have provided knowledge and suggested important driving forces, our understanding of flows over an evolving, complex bathymetry remains poor.

At the largest bathymetric scale (a few hundred meters) of interest, erosion and deposition are believed to be driven by nonlinearly shoaling waves and mean currents driven by waves and wind. In SandyDuck, measurements of the velocity field will be made at enough horizontal locations to sample variations in these flows and to test theories for bulk changes in bathymetry (measured with sonic altimeters and CRAB surveys). Although the importance of near-bottom steady flows is accepted, detailed field verification of models for undertow (offshore directed mean flow) and its effect on sediment transport is lacking on all but the simplest bathymetries. In addition, models of nonlinearly shoaling waves have been field verified only for nonbreaking, nearly normally incident waves. Further development of all models for fluid driven bathymetric changes is hindered by a lack of high quality field data. SandyDuck data will be used to test and develop these models, particularly emphasizing sediment transport/bathymetric evolution and cross-shore evolution of mean flows and wave orbital velocities.

Shoreline change is, by definition, a swash process, yet the beach face-swash region is one of the least-understood nearshore areas. Observations show that low-frequency motions sometimes dominate the swash because higher-frequency incident band motions are dissipated by breaking. The dynamics of these infragravity motions continues to be an important research problem, particularly their interaction with complex topography. In addition, swash dynamics can be highly nonlinear at all frequencies. The consequences to local sediment transport as well as the roles of changing ground water levels and variable beach porosity are critical for swash modeling. Measurements which will aid in understanding these processes are a component of the proposed experiment.

In addition to studying the behavior of natural bathymetry, experiments in which bathymetry is changed artificially may be instructive. The focus will be on experiments which test specific theoretical assumptions or model hypotheses concerning the effect of initial bathymetry on subsequent sediment transport, bathymetry and flow. Bathymetry manipulation experiments will be most easily performed in the swash zone.

Sediment Texture and Stratigraphic Experiments

Geologic studies of the nearshore zone are often descriptive, lacking measurements of the hydrodynamics that modify the bed. Little detailed information concerning relationships between bedforms and flow fields exists. However, the internal structure and properties of the nearshore bed represent a highly convolved temporal record of profile shape, bedform development, and sediment transport. Sedimentology can provide important insights about processes affecting the bulk of the shoreface prism on a historically important basis. For example, if the helical depositional structures found in Lake Michigan longshore bars is found at Duck, it is likely that sediment is transported in patterns much different than conceptualized in standard surf zone transport models.

The FRF provides an ideal location for sedimentological studies because of the varied nature of the sediments, availability of logistic support required by a coring and sampling operation, and the available background information. Much of the coring activity will be conducted during the monitoring phase with additional surface samples and short cores being collected during the intensive phase. Through accurate surveys and instrument measurements, it will be possible for the first time to relate the stratigraphic record to historic profile configurations. The results of this investigation are of interest not only to other SandyDuck investigators, but also to geologists who often encounter these structures in the rock record and seek to understand them.

Summary

This document describes scientific investigations that could form the core of the SandyDuck experiment. This plan originated from community discussion and will continue to evolve. SandyDuck will be a major effort, bringing together a wide spectrum of nearshore field experimenters and modelers to examine fundamental processes affecting sediment transport and bedform evolution in the nearshore zone. Integrating modeling efforts into all aspects of experiment planning and execution will yield experiment results that lead directly to improved modeling skill. Conceptually, SandyDuck continues the sequence of increasingly insightful Duck experiments by including small- and mid-scale sediment, bedform, and bed core monitoring, along with measurements of the governing three-dimensional flow structure.
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