Geology Reference
In-Depth Information
11
Tidal Channels on Tidal Flats
and Marshes
Zoe J. Hughes
Abstract
In shallow coastal settings channels provide a pathway for the tide to propagate and
are, thus, a primary control on the sedimentation and ecology of these environments.
Being shaped by bidirectional fl ows, tidal channels exhibit morphologies, which,
despite apparent similarities, bear signifi cant and fundamental differences to fl uvial
channels, specifi cally their scaling with size. This chapter considers the classifi ca-
tion of tidal channels and the networks they form. We examine the hydrodynamics
of shallow tidal channels, including asymmetry in period or velocity between the
ebb and fl ood tides, and the hysteresis seen in stage-velocity curves in regions with
large intertidal areas. Channel initiation may occur either through incision or by
variations in rates of deposition. Tidal channels evolve over time and a number of
relationships are presented that have been derived to describe the geometry of tidal
channels. Mutually-evasive pathways of fl ood and ebb fl ows may produce cuspate
meanders; a morphology unique to tidal channels. Of particular importance, in terms
of preservation potential, is the development of meanders in channels and the result-
ing pointbars. Pointbars in tidal environments are often fully or partially detached
from the bank by a channel formed by the subordinate tidal current, however their
exact morphology varies being dependent on channel sinuosity and tidal asymmetry.
Channels are preserved through infi lling (as tidal prism is reduced) and through lat-
eral accretion, particularly at meanders. Pointbars in tidal regions are generally
heavily bioturbated in the upper tidal range, and mid-tidal zones will exhibit inclined
stratigraphy, often with intercalated beds of muddier and sandier deposits.
11.1
Introduction
sediment between the outer and inner regions of a
coastal water body. The nature of the channel net-
work will infl uence local tidal conditions, specifi -
cally tidal range, and tidal fl ow velocity. Within tidal
fl ats and marshes, which are in the intertidal zone,
this translates to the period and depth of inundation
and potential for erosion and deposition. These con-
ditions in turn determine the fl ux of sediment, nutrients
and biota across an environment, ultimately impacting
the long-term morphological evolution of the region.
Within tidally dominated coastal landscapes, chan-
nels provide the conduit through which the tidal
wave propagates, driving the exchange of water and
