the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Water level fluctuations drive bank instability in a hypertidal estuary
Andrea Gasparotto
Stephen E. Darby
Julian Leyland
Paul A. Carling
Abstract. Hypertidal estuaries are very dynamic environments characterised by high tidal ranges (>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 ~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 (~3x10-10 m s-1), such that variations in positive pore water pressures within the saturated bank are smaller, ranging between about 10 kPa (at low tide) to ~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 on a longer timeframe. In the spring to neaps 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.
Andrea Gasparotto et al.
Status: final response (author comments only)
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RC1: 'Comment on esurf-2022-44', Anonymous Referee #1, 21 Jan 2023
The paper concentrates on bank erosion processes in hypertidal estuary environment. The research has been well justified, and the results and discussion are very valuable to scientific community within the field of geomorphology, coastal processes and related fields. The paper has been written clearly and structure is also very good. I have only minor comments.
- Introduction: the last paragraph is heavy and I suggest that it would be split in couple of paragraphs. For example lines 51-55 could be a separate paragraph.
- Line 53: It could be good to mention related to the field observations, how many years and when the measurements took place?
- Lines 115-116: Did the drone also have accurate (RTK-GPS) location information, or was the georeferencing solely based on the GCPs?
- Figure 2: Where does the P-3 picture locate on the left hand side’s map? I cannot find it from the map, only P-1 and P-2 are shown. In photos P-1, P-2 and P-3 the direction of the view is different to each other. Could it be possible to show for example the north direction in these photos, so that the reader could easier interpret the figure and which part of the bank is seen in each photo.
- Line 196: In which laboratory the triaxial shear tests were performed? Could you please explain a little bit more about the triaxial shear test procedure. How large was the grain size, and what kind of approach/equipment were used for the tests?
- Line 200: “preliminary tests” -> what tests are meant with this, does it refer to triaxial shear tests or model tests? This sentence could be clarified.
- Table 3: How was the friction angle and water contents derived? It seems that these were not described in the methods section yet. Could you please clarify, and add shortly about these calculations?
- Line 231: How it was decided that the resolution will be 0.5 m? Or do “sensitivity tests” on lines 251-252 refer to these test of grid size impact on results? Please, could you add in the methods section more clearly how the resolution was selected to be 0.5 m. These parts of methodology section need clarification.
Citation: https://doi.org/10.5194/esurf-2022-44-RC1 -
AC1: 'Reply on RC1', Andrea Gasparotto, 26 Jan 2023
Comment 1) Introduction: the last paragraph is heavy and I suggest that it would be split in couple of paragraphs. For example lines 51-55 could be a separate paragraph.
Thank you for the suggestion. We have simplified the text, shortening it a little, while also breaking the original paragraph down into two more accessible ones in the revised manuscript. Here below the revised introduction:
Rising sea level and increased storminess, driven by climate change, pose significant risks to coastal communities due to their 25 increased exposure to flooding and erosive events. For example, it has been estimated that 17% of coastlines in the British Isles, and almost 20% in Ireland, are being affected by the combination of sea level rise and the increased frequency of severe storms (MCCIP, 2020). In total, the UK coastline is 17,381 km long, 17.3% of which is experiencing erosive trends (EUROSION, 2004) with the yearly costs of coastal erosion in the UK rising to a possible £126 million per year by 2080 (Foresight, 2004). The evolution of fine-grained shorelines within estuaries is closely connected to bank retreat processes (Zhang et al., 2004, 2021; Guo et al., 2021; Zhao et al., 2022). While beach retreat (Jolivet et al., 2019; Bain et al., 2016; Hird et al., 2021; Carvalho and Woodroffe, 2021; Masselink et al., 2016) and cliff erosion (Brooks et al., 2012; Leyland and Darby, 2008; del Río and Gracia, 2009; Young et al., 2014; Hackney et al., 2013) have been well researched, sensitive estuarine environments have received less attention despite their societal importance. About 60 % of the world’s population is concentrated along coasts, and 22 of the largest cities on Earth are located adjacent to estuaries (Harris et al., 2016). Furthermore, estuarine environments such as salt marshes are essential in the mitigation of coastal flooding, attenuate wave activity (Möller et al., 2014; Fairchild et al., 2021; Leonardi and Fagherazzi, 2015), and aid carbon sequestration (Li et al., 2022; Pendleton et al., 2012).
Although some studies (Bendoni et al., 2014; Mel et al., 2022; Carniello et al., 2009; Marani et al., 2011; D’Alpaos et al., 2007) have explored marsh retreat behaviours in microtidal settings (e.g. Venice Lagoon, Italy), and others (Shimozono et al., 2019; Roy et al., 2021) have investigated erosion in large tidal-dominated estuaries , studies that consider the problem of bank collapse geomechanically, and with a particular focus on hypertidal environments, are lacking. Given the centrality of estuaries as transitional zones between the sea and land, a more complete understanding of the sources, mechanics and rates of bank erosion is of substantial importance. Yet, bank failure processes in tidal settings have to date been poorly studied and quantified (Gong et al., 2018; Zhao et al., 2022, 2019), especially when compared with the large literature on bank erosion in non-tidal (fluvial) environments (e.g. Rinaldi and Nardi, 2013; Nardi et al., 2012; Patsinghasanee et al., 2018; Julian and Torres, 2006; Darby and Thorne, 1996b; Darby et al., 2000; Majumdar and Mandal, 2022; Zhang et al., 2021; Thorne and Abt, 1993; Darby et al., 2010; Duong Thi and do Minh, 2019). Given the additional complexity of the process mechanics involved in tidal settings, arising mainly from the presence of bidirectional flows, process insights gained from studies of fluvial bank erosion may not necessarily be transferable to estuarine contexts. The present study seeks to address this gap through an investigation in which a combination of field observations and geotechnical modelling is employed to elucidate the bank failure processes operating in a hypertidal environment (the Severn estuary, UK).
Comment 2) Line 53: It could be good to mention related to the field observations, how many years and when the measurements took place?
The present study seeks to address this gap through an investigation in which a combination of field observations (made during the period 2018 to 2020; for details the reader is referred to Table 1) and geotechnical modelling is employed to elucidate the bank failure processes operating in a hypertidal environment (the Severn estuary, UK).
Comment 3) Lines 115-116: Did the drone also have accurate (RTK-GPS) location information, or was the georeferencing solely based on the GCPs?
We clarify that the drone does not have its own georeferencing system; georeferencing was accomplished by using Ground Control Points (GCPs) linked to an accurate grid of topographic surveys.
We don’t think we need to modify the manuscript because the application of the GCPs in the georeferencing process is already discussed in the manuscript (section 2.2.1 Aerial surveys).
Comment 4) Figure 2: Where does the P-3 picture locate on the left hand side’s map? I cannot find it from the map, only P-1 and P-2 are shown. In photos P-1, P-2 and P-3 the direction of the view is different to each other. Could it be possible to show for example the north direction in these photos, so that the reader could easier interpret the figure and which part of the bank is seen in each photo.
Thank you for the comment. We are pleased to clarify that P-3 is located roughly at the centre of the image on the left-hand side of the figure. In the original manuscript the boxes representing the snapshot locations are slightly too dark and therefore in the revised paper we have revised them to better highlight the locations and we have also added north directions as per the reviewer’s helpful suggestion. The updated figure (Figure 2) has been separately uploaded as PDF document.
Comment 5) Line 196: In which laboratory the triaxial shear tests were performed? Could you please explain a little bit more about the triaxial shear test procedure. How large was the grain size, and what kind of approach/equipment were used for the tests?
Thank you for the useful suggestion. We are happy to add some additional clarification about the suggested points. Here below the additional paragraph we propose for the manuscript:
The triaxial tests were performed in the Geomechanics Laboratory at the Civil, Maritime and Environmental Engineering department of the University of Southampton, following the British Standard methods (British Standard Institution, 1990). All tests were conducted using an electro-mechanical TRITECH machine with a maximum compression capacity of 10 kN. The grain size distribution of the two analysed layers is reported in Table 3.
Comment 6) Line 200: “preliminary tests” -> what tests are meant with this, does it refer to triaxial shear tests or model tests? This sentence could be clarified.
We have revised the paper to clarify that this refers to additional tests (not triaxial tests) to acquire the additional parameters used in the model setup. We have adjusted the text to read: “Prior to undertaking triaxial tests, a portion (c. 40 mm diameter and 80 mm length) of the soil samples retrieved from the field site was removed by cutting the sample in order to estimate the moisture content and unit weight of the material. Moisture content was obtained by using the oven-drying method as stated by the British Standard for geotechnical laboratories. First, the wet mass of the sample was measured before placing it into an oven for 24 hours at 105°C. After 24 hours the dry sample was weighed again and the moisture content (%) calculated. The unit weight (Table 3) of each sample was then calculated by dividing the sample mass by its volume.
Comment 7) Table 3: How was the friction angle and water contents derived? It seems that these were not described in the methods section yet. Could you please clarify, and add shortly about these calculations?
We have revised the text to clarify these points. Specifically, we now include the following text: “After the triaxial testing, the saturated water content (θr in m3 m-3) was calculated for the different samples. At the end of the triaxial testing procedure the samples were fully saturated, so that the saturated water content was estimated using:
θ = (m2-m3) / (m3-m1) 100
where (m1) is the mass of the sample container, m2 is the is the wet soil mass plus container, and m3 is the dry soil mass plus container. Dry soil mass was determined before the triaxial tests on undisturbed samples collected from the bank blocks.
Comment 8) Line 231: How it was decided that the resolution will be 0.5 m? Or do “sensitivity tests” on lines 251-252 refer to these test of grid size impact on results? Please, could you add in the methods section more clearly how the resolution was selected to be 0.5 m. These parts of methodology section need clarification.
We are pleased to confirm that the reviewer is correct: the sensitivity tests allowed the definition of the most appropriate grid resolution (0.5 m). Specifically, comparisons between a coarser and a more refined mesh indicated that model results are insensitive to the selected grid design. Thus, a grid resolution of 0.5 m is appropriate to resolve the key processes of concern here.
We propose to better clarify this in the main text as follow: (Line 231 – text in Italic in parentheses) “Following this process, the investigated bank was discretised into a c. 0.5 m resolution irregular triangular grid (sensitivity tests were used to adopt the selected resolution) composed of 1792 elements subdivided into two bank material layers (Upper and Lower bank deposits) (Fig. 6c).”
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RC2: 'Comment on esurf-2022-44', Anonymous Referee #2, 25 Mar 2023
This study addresses the bank failure problem at hyper-tidal estuary , which is well written and faily presented.
I have only a minor concern and comment: the tidal level at the study site can be predicted rather accurate by various models, it should have been better if tidal level at the site was used to drive the model. Particularly for the long term simulation, the impact may be accumulated over time.
Citation: https://doi.org/10.5194/esurf-2022-44-RC2
Andrea Gasparotto et al.
Andrea Gasparotto et al.
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