Geology Reference
In-Depth Information
platform and sand bodies substantially overlap the
inlet shoreline many times the width of the inlet throat
due to wave processes and fl ood-tidal currents (e.g.
Northern New England, Southern New Jersey, Southern
South Carolina, German Friesian Islands). Asymmetries
in ebb delta confi guration commonly result from the
main ebb channel being defl ected along the downdrift
barrier shoreline due to the dominant longshore trans-
port direction or the pattern of the dominant back-
barrier channels.
0.8 m/s, megaripples become superimposed on the
backs of sandwaves. Bedforms are commonly absent
at the inlet throat due to the lack of sand or presence
of a lag deposit, such as gravel or shell armor, that
retards bedform formation. Flood-oriented megarip-
ples (0.6 m < L < 6 m; 2-D small dunes in the scheme
of Ashley
1990
) occur on the low-intertidal ebb shield
areas and in marginal fl ood channels on the ebb delta.
Ebb-oriented megaripples are found in the ebb-spill-
over lobes of fl ood and ebb deltas. The swash bars and
bar complexes comprising intertidal portions of the
ebb delta are commonly covered by fl ood-oriented
megaripples (FitzGerald
1976
) .
As demonstrated in Fig.
12.3
, the size and orienta-
tion of subtidal bedforms at New Inlet, Massachusetts
follow closely the patterns reported by Boothroyd and
Hubbard (
1975
) with the exception that mutually eva-
sive channels in the backbarrier exhibit opposing tidal
dominance and opposite trending sandwaves
(FitzGerald and Montello
1993
). A similar pattern of
opposing bedform orientations has been documented
in the seaward portion of Texel Inlet, (Sha
1989
) .
Nummedal and Penland (
1981
) describe a system of
alternating fl ood- and ebb- dominant channels on the
ebb delta at Friesian tidal inlets having attendant fl ood-
and ebb-oriented bedforms, respectively. The reader is
directed to Boothroyd (
1985
) for additional treatment
of bedforms at tidal inlets, including their genesis,
migration trends, and sedimentary structures.
12.3
Bedform Distribution
The strength of the tidal currents and abundance of
sand at tidal inlets produce a wide distribution of
bedforms in tidal channels and on intertidal shoals.
Boothroyd and Hubbard (
1975
) performed exten-
sive fi eld studies of bedforms at inlets in northern
Massachusetts using direct underwater measurements
and fathometer transects. Since that time, many other
investigators have gathered additional information at
inlets throughout the world showing similar patterns
and migrational behavior of bedforms, including the
German Friesian Seegats (Nummedal and Penland
1981
), Rangaunu Harbor Entrance, New Zealand
(Pickrill
1986
), Texel Inlet, The Netherlands (Sha
1989
), Martens Plate, German North Sea (Davis
and Flemming
1991
) , New Inlet, Massachusetts
(FitzGerald and Montello
1993
) , Piedras Estuary,
Spain (Morales et al.
2001
), and the Algarve (Williams
et al. 2003). Boothroyd and Hubbard (
1975
) have
demonstrated that tidal inlet channels and shoals have
a systematic pattern of bedforms that is governed by
three aspects of the current regime: (1) maximum cur-
rent velocity, (2) velocity asymmetry between the
maximum ebb and fl ood currents, and (3) duration of
the velocity initiating bedform formation. Their stud-
ies have shown that fl ood- oriented sandwaves
(L > 6 m; 2-D medium dunes in the scheme of Ashley
1990
) occur in the intertidal and shallow subtidal por-
tions of the fl ood delta, coinciding with the fl ood ramp
and fl ood channels where there is a pronounced
velocity asymmetry and maximum velocities greater
than 0.8 m/s. Similar conditions produce ebb-oriented
sandwaves fl ooring the main ebb channel. Where
currents exhibit little velocity asymmetry, sandwaves
tend to be symmetrical. Boothroyd and Hubbard (
1975
)
also showed that under fl ow conditions exceeding
12.4
Tidal Inlet Relationships
Tidal inlets throughout the world exhibit several
consistent relationships that have allowed coastal engi-
neers and sedimentologists to formulate predictive
models: (1) Inlet throat cross-sectional area is closely
related to tidal prism, and (2) Ebb-tidal delta volume is
a function of the tidal prism.
12.4.1 Inlet Throat Area - Tidal Prism
Relationship
The cross section of tidal inlets (A) correlates closely to
tidal prism (P) (Eq.
12.1
all units in metric; O'Brien
1931,
1969
) and is secondarily affected by the delivery
of sand to the inlet channel by wave energy. For exam-
ple, at jettied inlets, tidal currents can more effectively
