TECHNICAL NOTES
Beach-Profile Evolution under Spilling and Plunging
Breakers
Ping Wang1; Bruce A. Ebersole2; and Ernest R. Smith3
Abstract: Beach-profile evolution, along with measurements of waves, currents, and sediment concentration, under spilling and plung-
ing breakers of similar height were studied in the three-dimensional Large-Scale Sediment Transport Facility at the U.S. Army Engineer
Research and Development Center. Unidirectional irregular waves were generated over a fine-sand beach. Beach-profile shape reached
equilibrium after 1,330 and 280 min of spilling and plunging wave actions, respectively. Near the main breaker line, the profile evolved
differently under plunging and spilling breakers. Across most of the midsurf zone dominated by surf bores, the equilibrium profile shapes
were similar. Uniform energy dissipation per unit volume at equilibrium, as assumed in the Dean 1977 model and often used in
cross-shore sediment-transport modeling, was measured for both cases across most of the surf zone except at the main breaker line, where
a much greater rate of dissipation occurred. The bar/trough formation and maintenance were closely related to the local patterns of
sediment suspension and bed scour at the plunging point.
DOI: 10.1061/ ASCE 0733-950X 2003 129:1 41
C
E Database keywords: Profiles; Sediment transport; Coastal morphology; Breaking waves.
Introduction
Realizing that the forcing mechanisms landward and seaward
af the breaker line are significantly different, Inman et al. 1993
o
The equilibrium concept is often embedded in modeling of beach-
nd Larson et al. 1999 divided a beach profile into two indepen-
profile evolution. A commonly used assumption is that cross-
dent portions separated at the breaker point. Power functions were
shore sediment transport rate and therefore profile evolution de-
used to model each section of the profile. The two-segment mod-
pend on the deviation from an equilibrium state e.g., Kriebel and
els are still monotonic and cannot reproduce the landward slope
Dean 1985; Kraus and Larson 1988 . One of the most commonly
of the bar or bar crest. Wang and Davis 1998 added a third
used beach-profile models, which was developed by Dean 1977 ,
segment to represent the landward slope of the bar. The three-
is
segment model improved the representation of barred profiles as
h A x 2/3
(1)
indicated by a case study along the west-central Florida coast.
where h still-water depth, x distance from the shoreline, and
Bars/troughs are common dynamic nearshore features. Two
A dimensional parameter determined by sediment grain size.
general mechanisms have been developed to explain the forma-
tions of bars and troughs. One mechanism involves a convergence
Various mechanisms have been suggested to explain beach-
at the bar between offshore-direct sand transport carried by the
profile equilibrium in the surf zone. Two of the mechanisms that
lead to Eq. 1 are examined here. The beach profile adjusts so
undertow and an onshore transport due to wave asymmetry
Thornton et al. 1996 . A second mechanism involves current pat-
that 1 the rate of wave-energy dissipation per unit water volume
is uniform Dean 1977 , and 2 a nonlocal balance between on-
terns under standing or partially standing infragravity waves
Carter et al. 1973 . Detailed patterns/gradients of sediment sus-
shore sediment transport and offshore transport is reached Larson
et al. 1999 .
pension at the breaker line and equilibrium of bar/trough features
were not examined in these models.
In this study, beach-profile evolution was examined at the
1
Assistant Professor, Dept. of Geology, Univ. of South Florida, 4202
Large-Scale Sediment Transport Facility LSTF . The objectives
E. Fowler Ave., Tampa, FL 33620. E-mail: pwang@chuma1.cas.usf.edu
were to 1 investigate the beach-profile evolution under spilling
2
Research Engineer, U.S. Army Engineer Research and Development
and plunging breakers; 2 examine mechanisms that produce
Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road,
shoreface equilibrium; and 3 examine the equilibrium of bar/
Vicksburg, MS 39180.
3
Research Engineer, U.S. Army Engineer Research and Development
trough features.
Center, Coastal and Hydraulics Laboratory, 3909 Halls Ferry Road,
Vicksburg, MS 39180.
Note. Discussion open until June 1, 2003. Separate discussions must
Methodology
be submitted for individual papers. To extend the closing date by one
month, a written request must be filed with the ASCE Managing Editor.
The capabilities of the LSTF and procedures of establishing long-
The manuscript for this technical note was submitted for review and
shore uniformity are discussed in Hamilton and Ebersole 2001 .
possible publication on September 4, 2001; approved on July 10, 2002.
The beach is composed of approximately 150 m3 of very well-
This technical note is part of the Journal of Waterway, Port, Coastal,
sorted fine quartz sand with a median grain size of 0.15 mm. The
and Ocean Engineering, Vol. 129, No. 1, January 1, 2003. ASCE,
sand beach was approximately 25 cm thick over a planar concrete
ISSN 0733-950X/2003/1-41 46/.00.
JOURNAL OF WATERWAY, PORT, COASTAL AND OCEAN ENGINEERING / JANUARY/FEBRUARY 2003 / 41
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