Geoscience Reference
In-Depth Information
system functions. Around 7000 years ago, the
channel pattern changed from meandering to
anastomosing. This led to the formation of sedi-
ment bars within channels which, as a result,
frequently became blocked and changed course
(Törnqvist 1993). Consequently, the method of
delivery of sediment to the delta front became
less reliable. Larger, more permanent meander-
ing channels deposited sediment at fixed points
around the delta front, but a frequently changing
system of delivery in an anastomosing channel
network meant that the point of sediment delivery
became less certain and dependable. Around 4000
years ago, the system reverted to meandering.
The extent to which sea-level rise will cause
an impact will depend on the ability of the sys-
tem to respond. Bruun (1962) produced a model
by which to predict shoreline behaviour under
sea-level rise (see Chapter 8 for a discussion of
its validity). In essence, a shore profile, such as a
mudflat to salt marsh sequence, will erode and
reform so that it remains at the same position
relative to the tidal frame. Such movement is
seen on the ground as an upward and land-
ward relocation of vegetation zones. In many
situations, however, this landward movement is
restricted by coastal defence structures, mean-
ing that although sea-level rise is forcing the
seaward marsh edge landwards, the landward
edge cannot move into the hinterland (Fig. 7.12).
This process is known as coastal squeeze.
As salt marsh vegetation communities develop
at an elevation controlled by the depth, period
and frequency of tidal inundation (Adam 1990),
different plant species can tolerate different
periods and frequencies of inundation. Those
that can tolerate the greatest inundation occur
on the lowest parts of the marsh, whereas those
Fig. 7.12
Restricted salt marsh development along the southern banks of the Humber estuary, UK. The man-made embankment will
prevent landward migration of salt marshes as sea-levels rise leading to loss of marsh area. (Photograph by Chris Perry.)
