Geoscience Reference
In-Depth Information
Despite the great thicknesses of sediment deposited, the period over which the Rumney
Formation accreted was relatively short. By the end of the nineteenth century, the Rumney
marshes were being cut back by renewed erosion (Case Fig. 7.1a(v)), releasing further quantities
of sediment into the estuary but, importantly, sediments which were now contaminated with
pollutants. This period of reworking, however, lasted just a few decades as by the early twentieth
century the estuary had switched to being accretional again, with the deposition of the Awre
Formation. These sediments again show an increase in coarseness over their predecessors, with
sandy clays to clayey sands dominating these deposits (Case Fig. 7.1a(vi)). Importantly, how-
ever, the deposition of these sediments incorporated pollution levels present in the water body
from continued industrialization as well as those reworked from the Rumney Formation. This
marsh unit, however has only minimal development in the estuary because by the 1930s there
was further erosion (Case Fig. 7.1a(vii)) before accretion began again in the 1960s. At this time,
the last recognizable marsh unit present in the estuary, the Northwick Formation, began to
form (Case Fig. 7.1a(viii)) with the deposition of silty clays and silty sandy clays. At the present
time, the estuary is eroding again, cutting back into the Northwick deposits.
The causes of this clear erosion and accretion cyclicity are not fully understood, but they do
correspond to periods of increased storminess in the estuary, when larger waves from the pre-
vailing westerly wind direction enter the estuary and cause erosion to dominate. Interestingly, the
period of time between erosion and accretion cycles appears to be shortening, possibly a response
to deepening water caused by a combination of sea-level rise and channel restriction due to land
claim. The appearance of the estuary today is of a series of marshes stepping down towards the
channel, but often terminated in a marsh cliff, several metres in height (Case Fig. 7.1c).
Although this apparent storm-driven erosion and accretion cyclicity is particularly evident in
the Severn, the last decade of the nineteenth century also led to considerable erosion and defence
breaching in many other UK estuaries, such as those of the Essex coast, and the Medway
(French 1999). This storm-driven erosion and accretion has important repercussions for human
use of estuaries and deltas. Once human activity has claimed part of an estuary or delta for
some use, it is considered sacrosanct. Hence, the erosion of these areas as a result of natural
cyclicity inevitably leads to demands for protection and intervention in these natural processes.
Relevant reading
Allen, J.R.L. & Rae, J.E. (1987) Late Flandrian shoreline oscillations in the Severn Estuary: a geomorphological
and stratigraphical reconnaissance.
Philosophical Transactions of the Royal Society of London, Series B
315
,
185-30.
French, P.W. (1999) Managed retreat: a natural analogue from the Medway estuary, UK.
Ocean and Coastal
Management
42
, 49 - 62.
Erosion of vegetated surfaces does not just
include uniform retreat following wave attack.
Stripping of the vegetation surface and the expo-
sure of underlying sediment to erosion is another
way in which marsh loss can occur (Fig. 7.10). It
is well accepted that vegetation roots are effec-
tive in increasing sediment resistance to erosion.
These roots, however, penetrate only to a certain
depth, for example, up to 1 m for
Spartina
and
up to
c
. 20 cm for
Salicornia
, and if the vegeta-
tion is largely the same across the marsh surface,
then root depth will be uniform. Figure 7.10
shows the effect of this at Silverdale marsh
in Morecambe Bay, north-west England. Storm
