Geoscience Reference
In-Depth Information
Table 7.1
Types of change and the associated impacts in deltas and estuaries.
Nature of change
Impact
Reduced sediment supply
Inability of vegetation surfaces to keep pace with sea-level rise. Loss of sediment
supply to mudflats. Loss of supply to delta front, increasing net marine erosion
Increased sediment supply
Infilling of channels, levee breaching in deltas. Burial and mortality of infauna and
vegetation
Changes in tidal range
Emergence or submergence of vegetated surfaces. Variation in water table.
Changes to status/position of salt wedge. Changes in mixing. Increased saline
penetration upstream
Changes in storminess
Increased erosion and delta/marsh recession (see Case Study 7.1). Increased defence
overtopping
Increasing sea-level
Coastal squeeze, marsh loss, increased water table. Landward movement of salt
wedge. Increased tidal penetration up-river
Decreasing sea-level
Delta/salt marsh advance. Falling water table. Seaward movement of salt wedge.
Down river advance of freshwater habitats
Land claim
Increased tidal constriction producing net increased sea-level (see above). Loss of
flood areas. Coastal squeeze
Dredging
Increased tidal prism with potential net drop in sea-level (see above). Modified
currents and tidal flood/ebb patterns
Increased wave activity
Cutting back of delta front or marsh edge. Increased sediment input
7.3
PROCESSES AND IMPACTS OF NATURAL CHANGE IN
DELTAS AND ESTUARIES
patterns of a salt marsh, or increasing sea-level
may result in changes to water table condi-
tions, and thus the stability of stored pollutants,
or the stability of marsh flora. The increase in
wave activity caused through increased stormi-
ness will result in higher energy conditions at
the delta front or marsh edge. This could lead
to an increase in net erosion and the loss of
delta or marsh sediments over a period of
many years. In the Severn estuary (UK), for
example, the salt marshes show a series of
accretion and erosion events that can be cor-
related with episodes of increased storminess
(see Case Study 7.1).
7.3.1 Erosion of intertidal sediment substrates
Deltas and estuaries are natural systems and,
therefore, undergo change as the processes con-
trolling them change (Table 7.1). Such changes
can be simple, such as increased wave activity,
which may erode more sediment from the
delta front or marsh edge, or changes in the
volume of sediment brought down stream to
supply the delta. More subtly, changes in storm
frequency may alter the erosion and accretion
Case study 7.1 Accretion and erosion cyclicity in the Severn estuary, UK
The Severn estuary is a large, macrotidal estuary system in south-west England. Its maximum
spring tidal range is 14.8 m and extensive freshwater drainage from its catchment means that
it is a well-mixed and extremely high-energy estuary. The intertidal morphology is complex
in that it reflects a series of individual salt marsh units overlying or banked up against other
marshes. The visual effect is that in many locations marsh surfaces descend, step-like towards
the present channel. Allen & Rae (1987) first studied these marshes and categorized them
stratigraphically. The oldest marsh unit, termed the Wentlooge Formation (Case Fig. 7.1a(i)),
comprises a complex series of clays and silts, interspersed with peat layers of varying thickness.
