Geoscience Reference
In-Depth Information
Fig. 4.3
Model of the effect of
tsunami upon a sandy barrier
coastline. Based on Minoura and
Nakaya (
1991
), and Andrade
(
1992
)
Lagoon
Ocean
Relict tsunami backwash channel
Coalesced tsunami overwash deposits
Secondary tidal channels
Estuarine lagoon tsunami deposits
Chenier ridges
Modern barrier
Modern tidal inlet
may account for pools that are tens of meters deep and
morphologically stable under present tidal flow regimes in
coastal estuaries along the New South Wales coast.
Water piled up behind barriers by tsunami overwash
tends to drain seaward through existing channels. However,
if the tsunami wave crest impinges at an angle to the coast,
channels can be opened up, or widened, at the downdrift end
of barriers. In some cases, on low barriers, water may simply
drain back into the ocean as a sheet along the full length of
the barrier. Because barriers are breached at so many loca-
tions, the resulting tidal inlets that form compete for the
available tidal prism rushing into the lagoon under normal
tides (Minoura and Nakaya
1991
; Andrade
1992
). As these
inlets become less efficient in flushing out sediment, they
lose their integrity and rapidly close. Hence, tsunami-swept
barriers may show evidence of numerous relict tidal inlets
without any obvious outlet to the sea. Reorganization of tidal
flow in the lagoon because of these openings may lead to the
formation of a secondary, shallow, bifurcating distributary
channel network. In contrast, under storm waves, new tidal
inlets are usually located opposite, or close to, contemporary
estuary mouths (Bryant and McCann
1973
).
silt or sand as a landward-tapering unit ranging from a few
centimeters to over a meter in thickness. This feature is the
most commonly identified signature of tsunami as described
10 km or more inland. In extreme cases, where sand is
abundant, a swash bar or chevron ridge may be deposited at
the landward limit of penetration.
Very little attention has been paid to the resulting back-
wash, which according to hydrological principles must
become concentrated into a network of interconnected
channels that increase in size, but decrease in number sea-
ward. Only one description of such a network has appeared
in the literature to date (Young et al.
1996b
), and this is for
the Shoalhaven Delta on the New South Wales south coast
unit up to 10 km from the coast. The sand contains open
marine shells, such as Polinices didymus, Austrocochlea
constricta, and Bankivia fasciata that are 4730-5050 years
old. A network of meandering backflow channels drains off
the delta to the southeast (Fig.
4.4
). Significantly, these
smaller channels are elevated above the regional landscape
and are bordered by broad swamps. These channels are
distinct from the main channel of the Shoalhaven River in
that they have developed within Holocene sediment,
whereas the river is entrenched into a Pleistocene surface
that is buried about 4 m below the surface of the delta. The
backwash channels are not only an order of magnitude
smaller than the main river, but they also have a much lower
carrying capacity. The channels increase in width from 40 m
about 10 km upstream to 100 m at the mouth of the
Crookhaven River, which exits to the sea at the sheltered
4.4.2
Deltas and Alluvial Plains
The pattern on deltas and alluvial plains is different. If these
low-lying areas are cleared of vegetation, the low frictional
coefficient permits the tsunami wave to penetrate far inland
before its energy is dissipated. The limit of penetration is
defined by Eq. (
2.14
). In this instance, the wave can deposit
