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SuDS design for catchment flood protection – are current criteria appropriate?
No full textCurrent criteria on the design of runoff storage for developments can result in surface water attenuation storage volumes in excess of 1000m3 or more per hectare which is of the order of 20m3 to 40m3 per property for large developments. The use of up to 40% increase in rainfall intensity, based on the latest guidance from the Environment Agency, for addressing expected future climate conditions, results in volumes being twice what they would be if designed for the present day. The cost of the attenuation storage is therefore a very significant component of development construction costs. Considering the expected investment in housing stock is of the order of 250,000 dwellings per year for many years ahead, this is a massive investment.
So it is therefore reasonable to ask: Is there evidence that this investment is effective in protecting catchments from flooding? Are the design rules suitable for addressing flooding problems in the specific catchments in which the developments are located? Are methods of assessing runoff from small development sites accurate? If there is any doubt that current methods are not cost-effective in achieving the intended goal of effective catchment flood protection, then appropriate research should be carried out and a new approach should be developed to provide the appropriate control of site runoff which minimises this investment in storage, is targeted at catchment specific issues and uses suitable technology.
This presentation will explain how drainage design criteria have evolved in the last 30 years, discuss the limitations these have and explore possible options for a new approach
An examination of the rheology of flocculated clay suspensions
No full textA dense cohesive sediment suspension, sometimes referred to as fluid mud, is a thixotropic fluid with a true yield stress. Current rheological formulations struggle to reconcile the structural dynamics of cohesive sediment suspensions with the equilibrium behaviour of these suspensions across the range of concentrations and shear. This paper is concerned with establishing a rheological framework for the range of sediment concentrations from the yield point to Newtonian flow. The shear stress equation is based on floc fractal theory, put forward by Mills and Snabre (1988). This results in a Casson-like rheology equation. Additional structural dynamics is then added, using a theory on the self-similarity of clay suspensions proposed by Coussot (1995), giving an equation which has the ability to match the equilibrium and time-dependent viscous rheology of a wide range of suspensions of different concentration and mineralogy
Tsunami simulators in physical modelling – concept to practical solutions
Get PDFWhilst many researchers have conducted simple ’tsunami impact’ studies, few engineering tools are available to
assess the onshore impacts of tsunami, with no agreed methods available to predict loadings on coastal defences,
buildings or related infrastructure. Most previous impact studies have relied upon unrealistic waveforms (solitary or
dam-break waves and bores) rather than full-duration tsunami waves, or have used simplified models of nearshore
and over-land flows.
Over the last 10+ years, pneumatic Tsunami Simulators for the hydraulic laboratory have been developed into an
exciting and versatile technology, allowing the forces of real-world tsunami to be reproduced and measured in a
laboratory environment for the first time. These devices have been used to model generic elevated and N-wave
tsunamis up to and over simple shorelines, and at example coastal defences and infrastructure. They have also
reproduced full-duration tsunamis including Mercator 2004 and Tohoku 2011, both at 1:50 scale.
Engineering scale models of these tsunamis have measured wave run-up on simple slopes, forces on idealised sea
defences, pressures / forces on buildings, and scour at idealised buildings. This presentation will describe how these
Tsunami Simulators work, demonstrate how they have generated tsunami waves longer than the facilities within
which they operate, and will present research results from three generations of Tsunami Simulators. Highlights of
direct importance to natural hazard modellers and coastal engineers include measurements of wave run-up levels,
forces on single and multiple buildings and comparison with previous theoretical predictions.
Multiple buildings have two malign effects. The density of buildings to flow area (blockage ratio) increases water
depths and flow velocities in the ‘streets’. But the increased building densities themselves also increase the cost
of flow per unit area (both personal and monetary). The most recent study with the Tsunami Simulators therefore
focussed on the influence of multiple buildings (up to 4 rows) which showed (for instance) that the greatest forces
can act on the landward (not seaward) rows of buildings.
Studies in the 70m long, 4m wide main channel of the Fast Flow Facility on tsunami defence structures have also
measured forces on buildings in the lee of a failed defence wall and tsunami induced scour. Supporting presentations
at this conference: McGovern et al on tsunami induced scour at coastal structures and Foster et al on building
loads
A 3D numerical study of solitary wave interaction with vertical cylinders using a parallelised Particle-In-Cell solver
No full textThis paper aims to provide a better understanding of the interaction between solitary waves and vertical circular cylinders. This is achieved via process based numerical modelling using the parallel particle-in-cell based incompressible flow solver PICIN. The numerical model solves the Navier-Stokes equations for free-surface flows and incorporates a Cartesian cut cell method for fluid-structure interaction. Solitary waves are generated using a piston-type wave paddle. The PICIN model is first validated using a test case that involves solitary wave scattering by a single vertical cylinder. Comparisons between the present results and experimental data show good agreement for the free surface elevations around the cylinder and the horizontal wave force on the cylinder. The model is then employed to investigate solitary wave interaction with a group of eleven vertical cylinders. The wave run-up and wave forces on the cylinders are discussed
Visualising flood risk: a view from the street
No full textCommunicating flood risk presents a significant challenge to flood managers. Members of the public often have difficulty relating to hydraulic model results and there is sometimes a degree of scepticism regarding their accuracy.
To make flood information to the public and other stakeholders more accessible, an application has been developed to superimpose flood model outputs onto a photographic representation of the affected area. This can be viewed on site using a mobile device or on a desktop computer.
The application enables the location of the user to be provided either by mobile device sensors or through map-based navigation. The user can then explore different flood water levels on request to represent “what if” scenarios and observed flood events. This approach enables stakeholders to see what a flood will mean for individual streets and properties, and will facilitate comparison of flood information from modelling with their own experiences.
The presentation will describe the technique, and demonstrate a number of potential applications. For example, the impacts of climate change and flood alleviation schemes on floods of different magnitudes can be shown based on flood levels predicted by hydraulic models.
The technique has been tested on the floodplains of the River Thames, where major flood schemes are planned that will use flood relief channels to lower flood levels. In this case the impact of the schemes on flood levels for different flood events can be clearly demonstrated
A stakeholder consultation into hydro-meteorological e-science environments
No full textThe purpose of this paper is to report on and analyse an international consultation into hydro-meteorological e-Science environments with the objective of identifying key functions and features together with exploring show-stopping issues and organisational structure. Transatlantic experiences were compared and contrasted. Including strong participation from both Europe and the USA with high quality responses from experienced practitioners, the consultation was undertaken as part of a joint initiative and took the form of an online questionnaire supported by a set of stakeholder interviews and other discussions. Topics included were functions and features such as provision of numerical models and data, usability, and ease of access; show-stopping issues such as flexibility, reliability and longevity; centralised and distributed structures, and funding models. The results demonstrated a broadly similar set of experiences and implied a future as an evolution of that which exists today. The consultation exercise ran alongside the development of the DRIHM e-Infrastructure which had, in itself, already benefitted from the prior DRIHMS consultation. Results were fed in to the development process at appropriate intervals allowing the consultation to shape the resultant services
A dredging strategy for flood management on the River Parrett and the River Tone in Somerset
No full textFollowing the severe floods of winter 2013/14, an emergency programme of dredging was carried out on the tidal reaches of the River Parrett and the River Tone to reduce flood risk to the moors and the communities that were affected by the flooding.
The Somerset Rivers Authority developed a flood action plan for the Somerset Levels and Moors that covered all aspects of flood management including engineering works and non-structural measures. The plan includes a programme of dredging to reduce flood risk. In order to inform the dredging programme, a dredging strategy was prepared to identify the possibilities of dredging for flood risk reduction.
The first stage of the strategy covered the tidal reaches of the River Parrett and the River Tone, and the River Brue. The presentation will cover the River Parrett and the River Tone.
Particular challenges on these tidal rivers include the high tidal range and the very high sediment loads from the Severn estuary. There was a need to develop dredging designs which balanced the hydraulic benefit (in terms of reduction in flood levels) with the amount of maintenance dredging that will be needed in the future.
The strategy includes some new dredging and a programme of maintenance dredging. The works will be guided by regular monitoring of the river channels to determine the requirements for each maintenance dredge.
The presentation will described the dredging strategy including the proposed techniques for both the new dredging and the maintenance dredging
Scour development around large-diameter monopiles in cohesive soils: evidence from the field
Get PDFDespite the progress in scour research over the last three decades, seabed scour development in cohesive and nonuniform soils is still an area of great uncertainty and remains a challenge for designing structurally efficient and effective foundations in the offshore marine environment. The uncertainty is made greater by the timescale required for scouring and effects, such as sediment abrasion, pile installation impacts, and operationally and environmentally induced dynamic motions. The rapid growth in offshore wind, particularly in European waters, has led to a requirement for estimates for scour development in such soils. This becomes very pertinent for large-volume serial installation of foundations, such as those required for offshore wind farm developments, given that there is a limit to the amount of detailed geotechnical information that can be collected as part of a project, and soil erosion testing is not standard. There is a reliance in geotechnical data, such as undrained shear strength, derived from cone penetration tests, supplemented with borehole data collected at a limited number of locations across the wind farm site combined with laboratory analysis of soil samples. This paper reviews the present evidence from both field and laboratory measurements of scour potential and looks at possible approaches for determining scour magnitude in cohesive soils including hydraulic and mechanical effects
Transient controls on estuarine SPM fluxes: case study in the Dee Estuary, UK
No full textEstuaries are a critical interface between land and coastal ocean across which freshwater, suspended particulate
matter (SPM), and consequently terrestrial carbon, nutrients and anthropogenic contaminants are exchanged.
Suspended particulate matter is closely linked to estuarine turbidity; it affects water quality and estuarine ecology;
and it contributes to overall estuarine sediment budgets. However, predicting the response of estuarine ecosystems
to climate change and human interventions remains difficult partly due to a lack of comprehensive understanding
of SPM concentrations and fluxes across time scales from intratidal to seasonal and interannual variability.
We investigate the dynamics of suspended sediment and suspended particulate matter in a hypertidal estuary
with a maximum tidal range in excess of 10 m and tidal currents reaching over 1 m/s: the Dee Estuary. This
estuary is located in northwest England and outflows in Liverpool Bay, itself in the eastern Irish Sea. The Dee
Estuary is a funnel-shaped, coastal plain estuary, which is about 30 km long with a maximum width of 8.5 km at the
mouth, and consists of mixed sediments.We focus on field observations, collected during several campaigns in the
channels of the Dee Estuary from 2004 to 2009 using acoustic and optical instrumentation, which provide intratidal
measurements of flow velocity and suspended sediment, and thus sediment fluxes, over approximately a month.
Measurements in February-March 2008 highlight three distinct hydrodynamic regimes: a current dominant
regime at neap tides (14-21 February); a combined wave-current regime at spring tides (21-29 February); and a
wave dominant regime at neap tide (1-4 March). While analysis of tidal distortion and dominance predicts weak
ebb dominant channels, the observations yield flood dominant sediment transport. The net sediment flux exhibits a
two-layer structure – import near the bed, export near the surface – that is consistent with the residual circulations
in the estuary. Wavelet analysis provides clear evidence that such influx of sediment in the estuary is alternatively
the result of periodic stratification at neap tides and of tidal asymmetry in suspended sediment concentration at
spring tides. Such transient processes will be important to determine and predict estuarine responses to short-lived
perturbations. Further analysis of other field campaigns will enable to determine the persistence of these processes
over seasonal and annual timescales
The Conundrum of Specifying very low Wave Overtopping Discharges
No full textThe design of certain seawalls / breakwaters has often been required to achieve very low target overtopping discharges when these structures protect vulnerable infrastructure or activities. The balance between economically viable protection and performance requirements is often difficult to achieve without good knowledge on low overtopping. The paucity of data in this space and the high uncertainties associated with existing methods, increase the challenge. The occurrence of a low number of overtopping waves has the consequence that any test results are substantially more affected by the inherent variation of random waves, therefore more uncertain. The physical model test results presented hereafter were successful in obtaining low to very low overtopping discharge data. For low / very low overtopping, these test data present considerable scatter relative to the latest empirical prediction. A number of repetitions was performed for conditions giving very low overtopping discharges, which illustrated the inherent uncertainty associated with low overtopping