Water level fluctuations drive bank instability in a hypertidal estuary
<p>Hypertidal estuaries are very dynamic environments characterized by high tidal ranges (<span class="inline-formula"><i>></i></span> 6 m) that can experience rapid rates of bank retreat. Whilst a large body of work on the processes, rates, patterns, a...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2023-05-01
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| Series: | Earth Surface Dynamics |
| Online Access: | https://esurf.copernicus.org/articles/11/343/2023/esurf-11-343-2023.pdf |
| Summary: | <p>Hypertidal estuaries are very dynamic environments
characterized by high tidal ranges (<span class="inline-formula"><i>></i></span> 6 m) that can experience
rapid rates of bank retreat. Whilst a large body of work on the processes,
rates, patterns, and factors driving bank erosion has been undertaken in
fluvial environments, the process mechanics affecting the stability of the
banks with respect to mass failure in hypertidal settings are not well-documented. In this study, the processes and trends leading to bank failure
and consequent retreat in hypertidal estuaries are treated within the
context of the Severn Estuary (UK) by employing a combination of numerical
models and field-based observations. Our results highlight that the periodic
fluctuations in water level associated with the hypertidal environment drive
regular fluctuations in the hydrostatic pressure exerted on the incipient
failure surfaces that range from a confinement pressure of 0 kPa (at low
tide) to <span class="inline-formula">∼</span> 100 kPa (at high tide). However, the relatively low
transmissivity of the fine-grained banks (that are typical of estuarine
environments) results in low seepage inflow/outflow velocities
(<span class="inline-formula">∼</span> 3 <span class="inline-formula">×</span> 10<span class="inline-formula"><sup>−10</sup></span> m s<span class="inline-formula"><sup>−1</sup></span>), such that variations in
positive pore water pressures within the saturated bank are smaller, ranging
between about 10 kPa (at low tide) and <span class="inline-formula">∼</span> 43 kPa (at high
tides). This imbalance in the resisting (hydrostatic confinement) versus
driving (positive pore water pressures) forces thereby drives a frequent
oscillation of bank stability between stable (at high tide) and unstable
states (at low tide). This transition between stability and instability is
found not only on a semidiurnal basis but also within a longer time frame. In
the spring-to-neap transitional period, banks experience the coexistence of
high degrees of saturation due to the high spring tides and decreasing
confinement pressures favoured by the still moderately high channel water
levels. This transitional period creates conditions when failures are more
likely to occur.</p> |
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| ISSN: | 2196-6311 2196-632X |