A morphological investigation of marine transgression in estuaries

Abstract

The landscape setting for an estuary varies widely and is an important aspect of determining how they evolve. This paper focusses on alluvial estuaries in river valleys and how they respond to sea level rise. We examine the implications of marine transgression, as a response to sea level rise, where the estuary moves upwards and landwards to maintain its position in the tidal frame (so-called “stratigraphic rollover”). Here we encapsulate such kinematic movement of the estuary morphology using a ‘morphokinematic’ model, to assess the potential response to sea level rise and sediment supply. The model of the estuary form includes a single convergent channel, intertidal and surrounding flood plains (the valley) and allows the relative importance of the space available for deposition of sediments, the accommodation space, to be investigated as a function of rates of sea level rise and sediment supply. The transgression of the system is determined using a sediment mass balance, taking account of any supply from the river and marine environment. Model results confirm that the transgression distance, measured as the distance the entity moves landward, varies in proportion to the change in accommodation space, which mainly depends on the flood plain area. As the size of the flood plain reduces, the transgression distance is less and the system becomes much more sensitive to changes in the rate of sea level rise, or changes in sediment supply. The greater demand for sediment when a flood plain is present results in greater cannibalisation of the estuary form (i.e. greater landward movement) to meet the sediment demand. When the flood plain is disconnected from the estuary, the synergistic relationship is lost and the accommodation space increases. The capacity for restoration will depend on the availability of sediment and the prevailing rate of sea level rise.

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ORCID for Ian Townend: orcid.org/0000-0003-2101-3858

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