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Prediction and observation of scour around the temporary river works for the Thames Tideway Tunnel
No full textEstimations of scour potential at the Thames Tideway Tunnel temporary works cofferdams were made during the design phase using the Erodibility Index approach of Annandale (1995, 2006), modified by Harris et al. (2010). Intense quarterly monitoring ongoing since 2014 designed to allow an adaptive approach to scour risk management also provides a case study, allowing the approach taken for the pre-construction scour predictions to be verified. The results show excellent agreement between predictions and observations, supporting the use of a method that can take into account soil profile variability when predicting scour on the inter- and sub- tidal foreshore
Development of an agent-based model to improve emergency planning for floods and dam failures
No full textThe Life Safety Model (LSM) is an agent-based model which assists with emergency planning and risk assessments for floods and dam failures by providing estimates of fatalities and evacuation times. The LSM represents the interactions of agents (i.e. people, vehicles, and buildings) with the floodwater. The LSM helps to increase the accuracy of estimates of loss of life and evacuation times for these events by taking into account a number of parameters which are not described in empirical models, such as the people's characteristics (e.g. age and gender), building construction types, and the road network. The LSM has been applied to three historic flood-related disasters: the 1953 coastal floods, in the UK; the 1959 Malpasset Dam failure, in France; the 2019 Brumadinho tailings dam disaster, in Brazil. These illustrate how the LSM has been verified and improvements to evacuation routes, early warnings, and the refuge locations could have reduced the number of fatalities. The value of using the LSM is not to calculate the ‘exact’ number of flood deaths or evacuation times, but to assess if emergency management interventions can significantly reduce them. The LSM can also be used to assess whether the societal risk posed by dams and flood defences is ‘acceptable’
Revision to the exposed jetties manual
No full textThe original “Exposed Jetties Manual” compiled in 2004 by McConnell et al. has provided designers and owners of jetties / piers, and related structures with elevated decks, guidance on wave loads on decks and beams since it was published in 2004. That manual developed general guidance on the design of elevated decks subject to wave loadings from site experience, but also included new prediction methods for wave forces on beams and decks based on results from a dedicated series of physical model tests, discussed in 2002 by Tirindelli et al.
Since that manual, further research information has become available, in particular from re-analysing the original test data to reduce uncertainty in the predictions, narrow the confidence bands, and reduce corruption by dynamic effects for short duration loadings. The suggested changes to formulae / coefficients were described in 2007 by Cuomo et al. Since then, further data have become available from laboratory tests on wave loads on bridge decks, on vertical walls (Cuomo et al. 2010) and on piles (Wienke & Oumeraci, 2005).
This paper introduces the new edition of the “Exposed Jetties Manual”, reminding readers of the 1st
edition, and then describing the principal changes for the 2nd edition
Do cohesive sediment flocs contain sand particles and does it matter?
No full textThe extent to which sand grains can be trapped within the fine sediment and organic matrix of a floc is still a contested scientific topic. Evidence for the interaction of sand and fine sediment was presented in measurements of floc settling velocities from different sand-mud proportions (Manning et al., 2011). These results were used to develop equations for settling velocity for different sand-mud percentages and concentrations and shear. Those equations were then used with numerical modelling to successfully reproduce time series of sand-mud concentration profiles from highly detailed field experiments (Spearman et al., 2011), providing further evidence of the interaction between sand and cohesive sediment flocs. Most recently those equations were also included in the Regional Ocean Modeling System implemented in the Coupled Ocean–Atmosphere–Wave–Sediment Transport Modeling System by Sherwood et al. (2018).
In the last decade the use of X-ray micro-tomography and electron microscopy on floc structure has demonstrated how organic material (extracellular polymeric substances, algae, bacteria, etc.) dominates the cohesion and structure of fine sediment flocs (e.g., Wheatland et al., 2020; Spencer et al., 2021, 2022). These techniques have also shown how clay and organic matrices can include sand particles in muddy sediments (Figure 1), though to date such techniques have not been applied to capture the structure of flocs including sand particles.
One of the questions governing the interaction of sand particles with muddy flocs is whether, and how, this interaction changes the deposition flux of mud and sand. Most sediment transport modelling codes in use today assume segregation - i.e., there is little direct interaction between mud and sand in the water column, and the only interaction occurring at, and within, the sediment bed (e.g. as postulated by Van Ledden, 2003). The evidence from the earlier work by Manning et al. (2011) and Spearman et al. (2011) suggests this may not be true. However, the generally accepted assumption of independent settling of sand particles and mud flocs with the latter settling at a speed in the region of 1 mm/s, is normally a practical and successful approach to sediment modelling, and this could imply that any interaction between mud flocs and sand particles in the water column is not significant.
We examine this issue through the careful analysis of suspended settling data of sediment flocs resulting from the suspension of varying proportions of mud and sand, measured as part of the TKI - MUSA research project (https://publicwiki.deltares.nl/display/TKIP/DEL112+-+MUSA). The measurements of floc size and settling speed were undertaken using the high resolution LabsFLOC2 video system (e.g., Manning et al., 2006, 2017) using sediment samples collected both from the seabed and from the water column. These were used, together with the information about the mud and sand concentrations of the samples used, to infer how mud and sand particles are distributed within the floc population. A 1DV model was then used to examine whether the distribution of mud and sand within the floc population aligns with the observed field information. The results of the study were used to evaluate whether the interaction of sand and mud particles is important in a practical sense for sediment transport modelling
A simplified approach to modelling all types of obstacles in TELEMAC-2D and 3D
No full textA simplified approach has been implemented in the openTELEMAC system to represent most types of obstacles, or hydraulic structures, including weirs, culverts, bridges, turbines, pumps, sluices, porous or solid barriers, storage ponds, etc., each including several variations of their defining hydraulic characteristics taken from empirical formulations. For instance, weirs can now either be sharp- or broad-crested, v-shaped or tilted, submerged or not, reversable or linked to a storage pond.
The objective of the work was to simplify user inputs and completely generalise hydraulics structures based on an abstraction of “obstacle”. For instance, the user should not have to enter node numbers or duplicate information while providing georeferenced definition of geometries.
To achieve this, the geometry of the obstacle is defined through a set of georeferenced lines, within a single file for all geometries, whether open polylines or closed polygons. The defining parameters are keyword-based similar to those of the steering files of the openTELEMAC system, again, stored in a single file for all obstacles. Other files can then be used to control the operation of the obstacle, whether driven by time sequences or water level variations. Finally, as it is customary with the openTELEMAC system, user subroutines can also help manually control gates, sluices, pumps, turbines, etc
Emergency Bulletins – Forecasting flood inundation and exposure to recent tropical cyclones in East Africa
No full textTropical cyclones can be especially devastating due to their multi-hazard nature and potential to generate exceptional flood magnitudes. In Africa, a combination of high vulnerability to flooding and limited pre-emptive action can exacerbate a disaster. Accurate and timely forecasts of tropical cyclone flooding can potentially aid humanitarian response and save lives and livelihoods. However, impact forecasts always include significant uncertainty that complicates their use by decision makers. We present a summary of efforts to forecast population exposure to tropical cyclone flooding for the UK Foreign and Commonwealth Development Office and various humanitarian partners via the production of Emergency Bulletins. Bulletins were produced for eight tropical cyclones (Idai, Kenneth, Eloise, Ana, Batsirai, Emnati, Gombe and Freddie), with bulletins often issued or updated over multiple days.
Our approach uses a combination of ensemble river discharge forecasts from the GloFAS system and 30 m resolution pluvial, fluvial and coastal flood inundation mapping provided by Fathom. Given these data, a bulletin will summarise and map the likelihood and location of flooding and the number of people directly affected. The performance of previous ensemble forecasts was evaluated using inundation extents from satellite data and user feedback. We find that the inundation mapping could identify the most exposed districts once uncertainty in the track and landfall location was sufficiently low, often around a day before landfall. Longer 2-5 day forecasts could be highly uncertain but were not disinformative. Key components of our bulletin production process include: 1) expert interpretation of the discharge forecasts and inundation model outputs, 2) comparative analysis with previous events, and 3) post event appraisal aimed at understanding how the bulletin was used. We believe that each of these add significant value over automated modelling outputs available online and that the value of fully automated bulletin and flood forecast services for decision makers should be considered carefully
Scour prediction in cohesive marine soils: a hybrid approach
No full textRegardless of the advancement in scour research in the last three decades, scour prediction at offshore foundations in cohesive and non-uniform soils still involves great uncertainty and remains a challenge for designing structurally efficient and effective foundations offshore. One approach to scour prediction in these marine soils is the application of the Erodibility Index method, allowing for the physical properties of the soil to be considered. The method does not directly take into account the chemical properties of the material, although the mass strength number, MS, represents the relative influence of chemical bonding properties of the soil through the unconfined compressive strength. This paper presents a case study for a planned offshore wind farm located in the Baltic Sea. The soils in the upper seabed have extremely low to low undrained shear strengths (0 - 15 kPa). The study combines the Erodibility Index method with site-specific geotechnical information and targeted soil sample erosion testing to provide a means of calibrating the method for site-wide use, utilizing the cone penetrometer test data undertaken at every proposed monopile location to enable the Erodibility Index method to be calibrated at each foundation
Time evolution of scour: the importance of event duration
No full textThe assessment of scour risk at offshore foundations is dependent on a number of different factors including metocean conditions, water depth, soil conditions and structure dimensions and layout. At foundations where the soils are dominated by non-cohesive soils the scour risk is potentially greater than for clay dominated sites. However, even in those locations where sand overlies less erodible soils, it is important to understand the possible time-scale for the loss of this material. Further, as the location of foundation structures moves further offshore into deeper water, not only does the scour process change, the scour development is likely to become event duration limited. However, this latter effect may not only be confined to deeper water sites, but to sites where the metocean conditions are constrained (e.g. weak tidal currents, limited fetch lengths etc). This paper explores the importance of this effect through the use of a time-evolution model of scour
Insights on flow-structure-seabeed interaction gained from laboratory testing
No full text[Keynote] Offshore marine renewable energy projects require an understanding of sediment mobility as part of an agile assessment of foundation scour. Furthermore, the interaction of hydraulics and geotechnics is key to developing representative scour profiles for design as well as suitable scour mitigation measures where they are required.
At each project location seabed mobility and scour affect the performance of foundations and affect the security of high voltage subsea cables. Where the effects are known they can be addressed in design, or mitigated for during Operations & Maintenance, through monitoring and intervention as part of a Scour Management Plan.
Physical model testing is used to provide underpinning research knowledge for the process based understanding of scour, support for the development of predictive methods, and validation of designs.
The talk will present some insights gained from physical model testing at HR Wallingford’s laboratory and, with reference to published data from other laboratories and field cases, cover a range of topics including: Scour development and sediment mobility; Erosion testing and its application to scour modelling; Sediment transport and bedform migration; Scour protection and foundation performance; UXO burial and migration.
The talk will be rounded off with a summary of conclusions based on experimental evidence, and highlight some key gaps in research knowledge
Recommendations to improve the interpretation of global flood forecasts to support international humanitarian operations for tropical cyclones
No full textInternational humanitarian organisations increasingly turn to forecast teams to support the coordination of efforts to respond to disasters caused by hazards such as tropical cyclones and large-scale fluvial floods. Such disasters often occur where there is limited local capacity or information available to support decision making and so global forecasting capacity is utilised to provide impact-based flood forecast bulletins. A multidisciplinary team joined together to provide forecast bulletins and expertise for such events through the UK Foreign and Commonwealth Development Office (FCDO). This paper captures the successes and challenges from two cyclones: Hurricane Iota in Central America (November 2020) and Cyclone Eloise in Mozambique (January 2021). Recommendations to improve global forecasting systems are made which will benefit the international community of researchers and practitioners involved in disaster prediction, anticipatory action and response. These include the need for additional data and expertise to support the interpretation of global models, clear documentation to support decision makers faced with multiple sources of information, and the development of user relevant metrics to assess the skill of global models. We discuss the value of effective partnerships and improving synergies between global models and local contexts, highlighting how global forecasting can help build local forecasting capability