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Estuary translation (rollover)

Method indicator
Bottom-Up Hybrid Top-Down
    YES

Summary of key issues:

Description Stratigraphic rollover describes one possible outcome of the morphological and sedimentological response of an estuary to sea level rise. The rollover model is the tendency for erosion of the upper intertidal in the outer estuary and transport of derived sediment to the estuary head where its intertidal deposition results in headward transgression.
Temporal applicability Long-term. This model can be applied to geological timescales (millennial to centuries).
Spatial applicability Whole estuary.
Links with other tools This method is more of an observation of a natural tendency in some estuaries and requires other tools to evaluate how the process of rollover might occur.
Data sources As it is a general concept no data is required. To find evidence of this process for a specific estuary requires considerable bathymetric and archaeological evidence.
Key issues None except those normally associated with EGA.
Necessary software tools / skills Prediction of the long-term effects of sea level rise.
Typical analyses The tool is a general concept that does not apply to every estuary and does not provide a means of evaluating how this might occur in practice.
Limitations Estuarine response to sea level rise may take the form of:
  • Drowning; or
  • Vertical sedimentation in pace with sea level rise; or
  • Vertical sedimentation and horizontal translation of the estuarine morphology (rollover).
The type of morphological response depends on sediment availability and rate of sea level rise. Intertidal drowning results when sediment availability is low or sea level rise rates are high. Availability of marine-sourced sediment may allow intertidal warping with no increase in tidal prism and no spatial translation. Restricted external sediment sources results in internal redistribution of sediment. Increased wave propagation in the outer estuary results in intertidal erosion. Increased water depths increases flood dominance and thus landward sediment transport resulting in headward transgression.

Introduction

The rollover model is a concept regarding a general tendency of estuary response to sea level rise which can be then quantified by applying other top-down approaches such as regime theory. The basis of the rollover model can be attributed mainly to Allen (1990) and Pethick (2000).

“Rollback” of the system has been observed in the Severn Estuary and the Blackwater, Essex (Pethick, 2000). As yet however, there is no theoretical model to represent such translation from a top-down perspective (i.e. looking at a parameterisation of the gross changes rather than some form of detailed process modelling to derive a bottom-up prediction of the system response to sea level rise).

In some cases, a further consequence of such landward movement is that, at any particular location on the estuary, the magnitude of sea level rise may be masked because the tidal wave also translates landwards (Figure 1). Depending on the degree of the tidal wave’s amplification as it propagates up the estuary, it is possible for levels to increase more or less than any rise in the mean sea level along the length of the estuary.

Figure 1 Schematic diagram to show transgression of tidal wave in an estuary due to sea level rise

Schematic diagram to show transgression of tidal wave in an estuary due to sea level rise

Read the full PDF document for more information on this methodPDF version

Analysis and modelling

Last Modified on: 19 June 2011
Printed from the Estuary Guide on 15/09/2014 05:01:19