Silt curtains - a review of their role in dredging projects

As environmental mitigation associated with dredging and marine construction activities becomes ever more a focus of attention, the use of silt curtains to contain fine material locally to the dredging operation has increased. A silt curtain is typically simple in design, comprising of a geo-textile sheet attached to floats that are weighed down to the sea floor and anchored in place. The expected result is to prevent the transport of sediment out of the work site and into the surrounding environment. However, few documents exist that consolidate the current understanding of the use of curtains and the controlling factors associated with the deployment of them, including consideration of their purpose, design, development and limitations. A cynic’s view is that silt curtains are often installed to comply with regulatory constraints and may offer limited practical value, especially when they are left unmaintained. Poor understanding of silt curtain design and limitations often means they are installed in unsuitable environments, resulting in unsatisfactory performance. Critically, once a curtain is installed; without good management and regular maintenance, the geo-textile will often perish leading to a significant reduction in its effectiveness. Different installation methods exist along with multiple configurations; consideration of the environmental conditions should be made prior to the design and installation of a silt curtain. This paper will consider a range of parameters that affect the effectiveness of silt curtains

Comparison between methods for creating DEMs of physical models

Within physical modelling, it is often necessary to create DEMs (digital elevation models) when testing the stability of rock structures or the filter layers and scour protection around foundations and other marine structures. These DEMs are used to detect changes in the position of the structure or surrounding protective material. Several methods are available to create these models, yet no one technique has been selected as an industry standard. A comparison between three widely used methods – terrestrial laser scanner (TLS), combined laser scanner (CLS) and structure from motion (SfM) – are presented within this paper. The CLS in underwater mode gave low measurement errors and can be deployed without having to drain the facility but requires a traverser system. An area of approximately 7 m by 4 m can be measured in half an hour. The TLS can survey a much larger area in the same time, but requires the facility to be drained. SfM is cheapest method, but struggles to create a full shape and more care must be taken. The CLS in underwater mode has been chosen for use in scour studies in the Fast Flow Facility, with high volumes of water but a relatively limited area

Distribution of and hydrographic controls on ferromanganese crusts: Tropic Seamount, Atlantic

Hydrogenetic ferromanganese crusts are likely to be exploited as resources for critical metals in the near future, yet the processes controlling where and how they grow are poorly understood. Using detailed mapping of seafloor outcrop and well constrained hydrographic modelling alongside scanning electron microscope imagery of samples from the Tropic Seamount, a star-shaped guyot located in the Tropical East Atlantic, we investigate the relationship between currents, ferromanganese crustal texture and the locations and intensity of crustal erosion. Here, we report the distribution of FeMn crusts and explore factors controlling their growth and erosion. We find that just over 35% of the summit plateau of the guyot exposes some form of ferromanganese crust mineralisation, with the rest variably covered by plains of mobile sediment and slim cliff exposures of carbonate. The steep flanks of the guyot largely expose ferromanganese crust both in situ and as debris flows. The strongest currents are located on the upper flanks of the guyot, the central part of its eastern limb, and across the summit plateau. Three categories of surface morphologies are identified; from pristine botryoidal surfaces to flat areas that have been completely polished by the erosive action of currents and sediment. The relationship between the outcrop of crusts, their erosional states and the hydrographic current regime to which they are exposed is complicated. There is a general correlation between the degree of erosion and location across the seamount, with the least eroded being found on the flanks below 2000 m water depth and the most heavily eroded crusts largely restricted to the summit area. Furthermore, the pristine samples all reside in areas that rarely experience current magnitudes over 0.2 m/s, suggesting that above this the currents have the ability to erode ferromanganese crust. However, there is a strong overlap between the measured current magnitudes at the locations of partially and completely eroded crusts, as well as partial overlap with the current magnitudes measured at pristine crust locations. This complexity is likely due to the presence of cliffs and plateaus increasing current magnitudes and turbidity at a scale smaller than the model resolution

Women's leadership in the Asian Century: does expansion mean inclusion?

This paper draws on British Council commissioned research in response to concerns about women's absence from senior leadership positions in higher education in South Asia. The study sought existing knowledge from literature, policies, and available statistics and collected original interview data from 30 academics in Afghanistan, Bangladesh, India, Nepal, Pakistan, and Sri Lanka. A central finding was that gender is not a category of analysis in higher education policy, research or statistical data in the region. Our interview data suggest that leadership was frequently not an object of desire for women. Being associated with particular types of masculinities, leadership often carried a heavy affective load for those women who transgressed patriarchal socio-cultural norms and disrupted the symbolic order of women being led by men. Leadership was frequently perceived and experienced by women in terms of navigating a range of ugly feelings and toxicities that depleted aspirations, well-being and opportunities

Representation as politics: asserting a feminist ethic in ethnographic research

As ethnographers we are familiar with methodological debates problematizing ethnography’s inherited and inherent connections to ideas of authenticity commonly mobilised to legitimate modes of representation. In this paper, we engage with the post-structural philosophies of Jacques Rancière and Judith Butler, to argue that methodological tools of representation are always ‘political’ and as such shape the limitations of what can be known. In order to trace the overlapping methodological foundations which inform our ethnographic representations, we introduce three paradigmatic constructions of ethnography. By paying attention to the ways in which our ethnographic representations mark the perceptibility of educational practices and purposes, we assert a feminist ethic through the representation of the ‘livable life’ as a productive methodological provocation

The ARC-Boat: a remote controlled rapid response scour monitoring platform

The assessment of scour at hydraulic structures through inspection and monitoring is a key part of scour risk management. Bathymetric data from monitoring or rapid response surveys post storm provide up-to-date information on the channel geometry and maximum scour at structures. The opportunity presents itself for the use of USVs. The ARC-Boat is a 2 m long, remotely operated survey vessel used to collect hydrometric and bathymetric data. It can be deployed close to site without the need for lengthy equipment mobilisation. Operating onshore from safety the surveyors can optimise the data in real-time. The ARC-Boat’s shallow draft and high manoeuvrability enable it to get close to structures which could be hazardous for manned survey vessels. A swath bathymetry system was integrated to accurately measure the bathymetry and morphology around a bridge. The results present significant opportunities for increased definition of erosional and depositional processes that occur around key assets

Principles into practice: monitoring of dredging projects

There are no simple answers to the question “what should the monitoring of my dredging project comprise of?” Instead, there exists a set of principles which can be followed to guide experienced professionals in the design of monitoring (see for example CEDA, 2015 and Laboyrie et al., 2018). Principles (theory) and practice (reality) are often different however. In this paper we seek to provide a useful (quick reference) summary of the key principles (and process) to apply to monitoring design. We provide an explanation of the reality arising from applying these principles in the case of the physical monitoring of a 30Mm3 dredge in a temperate estuarine system

Potential sources and magnitude of errors associated with the measurement of suspended sediment concentration

Dredging activities cause the mobilisation of sediments from the bed of water bodies, often resulting in the suspension of sediment into the water column in the vicinity of the activity. Sediment release to the aquatic environment and the effects of this release are often the prime environmental concern associated with dredging. As a consequence the accurate monitoring of suspended particulate matter (SPM) concentrations is of considerable importance to the industry. The three techniques most widely used for the measurement of SPM associated with dredging are: - Water sampling; - Use of optical (e.g. optical backscatter) sensors (OBSs); and - Use of Acoustic Doppler Current Profilers (ADCPs). The commonly used approach of water sampling combined with optical sensor measurements is particularly important, as: - The techniques are well established; - Widely used; - Relatively simple and inexpensive; and - Provide a means of calibration for other techniques (e.g. ADCP measurements). Although versatile and in widespread use there is often little consideration of the accuracy (or potential lack of accuracy) in the application of OBS and water sampling methods. This is important information for the design and implementation of very many dredging monitoring studies, the costs of which can be exceptionally high. This paper describes the potential error sources and magnitudes associated with water sampling and the use of OBSs for the measurement of suspended sediment concentrations

Measurements of suspended sediment concentration in the deep sea – challenges based on a previous measurement campaign and how those challenges might be addressed

Deep-sea mining can involve the extraction of poly-metallic nodules, ferromanganese crusts and sulphide deposits from the ocean floor and transportation of the mined material to the water surface. The mining of these resources (and potential discharge of unwanted material also excavated from the seabed) will potentially result in adverse ecological effects arising from the formation of sediment plumes that could deposit fine sediment on sensitive species. Identifying the behaviour of such sediment plumes is therefore a critical part of designing deep-sea mining operations that minimise impacts on ecology. Monitoring of the plumes has particular importance given the very low background levels of suspended sediment concentration that the sensitive receptors often exist in. In coastal waters sediment plume characterisation is traditionally carried out using optical (optical backscatter sensor (OBS)) and acoustic (ADCP) technology. Here we present some of the challenges of performing sediment plume monitoring in the deep-sea and how those challenges may be addressed. The information presented is based on experience gained during a previous sediment plume monitoring campaign undertaken on the Tropic Seamount, 550km SSW of the Canary Islands. The monitoring undertaken included the deployment of an ADCP and OBSs in water depths of >1000m