SandyDuck: A Field Study of Sediment and Bathymetric Response to Fluid Forcing William A. Birkemeier (ed)
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.
SummaryThis 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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