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A step towards risk-based flood forecasting - A pilot application in Dumfries, Scotland
No full textThe Scottish Environment Protection Agency (SEPA), Deltares and HR Wallingford have recently collaborated to develop new tools and techniques to extend existing flood forecasting capabilities. The focus was on developing timely ‘impact’ information to support flood response activities. Within this research, the River Nith to Dumfries was used as a pilot catchment.
SEPA operates a Delft-FEWS based flood forecasting and warning system which consists of multiple models that use numerical weather predictions to estimate future river levels and flows. Over time, SEPA’s flood forecasting system has continually evolved to incorporate the latest science and technology. This study focussed on the following aspects:
1. Live flood maps: forecasting flood inundation;
2. Live impact information: number of properties affected and damages;
3. Ensemble runs: incorporating meteorological uncertainty.
These elements were connected to the existing forecasting system in a framework titled FEWS-Risk.
Flood inundation mapping was performed using HR Wallingford’s Rapid Flood Spreading Model (RFSM-EDA). The RFSM-EDA is a 2D inundation model that is used for rapid flood inundation prediction in practical run times, appropriate for operational use. The RFSM-EDA was setup using Lidar digital elevation data on a 5 metre grid. For the Dumfries catchment the RFSM-EDA embedded in FEWS-Risk framework takes 5 minutes to run a 2 day simulation.
Part of the FEWS-Risk framework, the potential damages were assessed by linking the flood depths with receptor impact information using Delft-FIAT (Flood Impact Assessment Tool).
The possible benefits of including uncertainty in the forecasts using ensemble forecasting were also explored. To achieve this, 5 ensemble members were randomly generated based on Storm Frank (December 2015) data and used as boundary conditions for the RFSM-EDA model. The simulations were used to calculate the probability of exceedance of a given water level threshold.
The results were presented and discussed at a workshop hosted at the Scottish National Centre for Resilience, with delegates attending from SEPA, Scottish Fire and Rescue Service, Police Scotland, Scottish Government and Dumfries & Galloway Council.
Feedback indicated that the strength of such risk-based operational modelling is greatest for events and regions where impacts are unknown. Communication of risk is a challenge, but guidance with long lead times and high uncertainty can still be useful (e.g. to trigger preparation). Information regarding the onset of floods is especially useful for flood preparedness. Overall, probabilistic forecasting can give good guidance in advance of a flood event but needs to link closely with operational procedures to maximise its effectiveness. Results from this pilot study indicate that the FEWS-Risk framework may have great potential in helping to minimize flood impacts
Power and Scour: Laboratory simulations of tsunami-induced scour
Get PDFThe world’s coastal regions are becoming increasingly urbanised and densely populated. Recent major tsunami
events in regions such as Samoa (2007), Indonesia (2004, 2006, 2010), and Japan (2011) have starkly highlighted
this effect, resulting in catastrophic loss of both life and property, with much of the damage to buildings being
reported in EEFIT mission reports following each of these events.
The URBANWAVES project, led by UCL in collaboration with HR Wallingford, brings the power of the
tsunami to the laboratory for the first time. The Pneumatic Tsunami Simulator is capable of tsimulating both
idealised and real-world tsunami traces at a scale of 1:50.
Experiments undertaken in the Fast Flow Facility at HR Wallingford using square and rectangular buildings placed
on a sediment bed have allow us to measure, for the first time under laboratory conditions, the variations in the
flow field around buildings produced by tsunami waves as a result of the scour process.
The results of these tests are presented, providing insight into the process of scour development under different
types of tsunami, giving a glimpse into the power of tsunamis that have already occurred, and helping us to
inform the designs of future buildings so that we can be better prepared to analyse and design against these failure
modes in the future
Physical and numerical modelling of trench infill
Get PDFPhysical modelling of trench infill, for trenches perpendicular to the flow direction, was undertaken in the Fast Flow Facility at HR Wallingford. Parameters including trench width, flow velocity (including reversing tidal flow), and the presence of berms along the sides of the trench were varied. Following the experiments, a numerical model was constructed with the T0 bathymetry from the physical model used as the starting numerical model bathymetry, allowing direct comparison between models. Numerical model parameters were tuned to represent the physical modelling results, with the calibrated model then used to predict infill rates for other scenarios modelled in the Fast Flow Facility
Flood forecasting and warning for Muar River: non-structural measures for flood mitigation
Get PDFThe Muar River catchment has repeatedly suffered prolonged, significant flood events which have caused widespread disruption and impacts to residents, businesses and infrastructure; the impacts have been exacerbated by considerable rapid development over the past decade, which has modified the flow regimes and flooding mechanisms. To help prepare for, and mitigate, the effects of future floods, the Malaysian government is implementing a range of flood management projects, which will provide an integrated approach based on structural and non-structural measures.
The integrated Flood Forecasting and River Monitoring system (iFFRM) for the river Muar is a key non-structural measure that has been recently implemented. The government’s Department of Irrigation and Drainage (DID) is responsible for providing a flood forecasting and warning service to the public; the iFFRM is a tool designed to enable effective decision support by DID.
The iFFRM is a fully automated system that is driven by a combination of live, telemetered gauged data from DID’s own InfoBanjir database, spatial rainfall radar data, and Numerical Weather Prediction (NWP) rainfall forecasts from the Malaysian Meteorological Department. Hourly simulations are carried out automatically, to forecast water levels and flows in the river channels, and to map the flood inundation process within the flood plains. Simulation results are used to warn DID staff so that immediate action can be taken to provide an effective and proactive emergency response. Results are also passed to the project website, and dedicated smartphone application, enabling forecasts to be disseminated more widely. A parallel analytical modelling network can take over the forecasting role should the primary iFFRM system fail.
Ongoing structural measures for flood mitigation are captured through a flexible modelling approach that can incorporate model updates to reflect real changes in the catchment, complementing the structural measures being implemented by DID and ensuring a sustainable solution
Does organic matter have an unacknowledged influence on the dynamics and deposits of fine-grained submarine sediment gravity flows?
No full textFine-grained sediment gravity flows are common in the deep marine environment and an important process by which many desirable hydrocarbon source rocks and seals have and can be formed. Our understanding of the flow dynamics and depositional processes of these flows is limited. Clay particles can alter the flow rheology because of their ability to aggregate, forming floccules and more pervasive cohesive structures, called gels. These flocs and gels can enhance or dampen turbulent forces in sediment gravity flows, such that increasing the cohesive sediment content causes a transition from turbulent, Newtonian flow, via transient-turbulent flow, to laminar, non-Newtonian debris flow. Because the flow rheology controls the depositional style, a thorough understanding of how flow composition relates to flow rheology is essential for our interpretation of the architecture and the palaeo-environmental setting of deep-marine deposits. In the natural environment, clay-rich sediments are commonly associated with the presence of organic matter. There is a reasonable understanding of the effect that organic matter, in particular ‘sticky’ extracellular polymeric substances (EPS), has on clay flocculation and the stability of sedimentary deposits in shallow-marine environment, but it's impact on deep-marine sediment gravity flows has not been explored yet. Here, the influence of similar interactions for suspended clay and EPS within sediment gravity flows is presented. The above research gaps were addressed by means of flume experiments that recorded changes in dynamic behaviour and deposit run out distances of sediment gravity flows with variable amounts of biologically cohesive xanthan gum (a commonly used proxy for natural EPS) and physically cohesive kaolin clay (one of the most common clay minerals on Earth) and non-cohesive silica flour. Results indicate that very small quantities of EPS – three orders of magnitude smaller than the quantity of clay – are sufficient to enhance flocculation and alter the size distribution of clay flocs compared to a flow that lacks EPS. Ultrasonic Doppler Velocity data demonstrate changes in the internal turbulence behaviour of transitional flows as a result of adding EPS. These findings have the potential to change our understanding of sediment gravity flows in the natural environment, in particular those that result in the organic rich, fine-grained deposits regarded as potentially favourable source rocks
CFD Modelling coupled with Floating Structures and Mooring Dynamics for Offshore Renewable Energy Devices using the Proteus Simulation Toolkit
No full textWith many countries showing a growing interest in offshore renewables, the development of floating support structures able to withstand extreme environmental loads is key to winning the race in offshore renewable energy deployment.
Numerical modelling of these structures allows relatively inexpensive testing and selection of suitable designs. The research presented here focuses on the numerical analysis of the behaviour of different floating structure designs and the associated mooring dynamics under various wave conditions. Simulations are performed with Proteus, a relatively new and open-source computational fluid dynamics (CFD) software actively developed by the ERDC and HR Wallingford, using the Finite Element Method (FEM) to model two phase flows.
In order to allow the simulation of moving bodies in Proteus, a mesh motion module has been developed. The mesh nodes in the fluid domain are moved using the equations of linear elastostatics while displacement of the nodes placed on the surface of the moving structure is imposed through boundary conditions (see Figure 1 for an illustrative example of the mesh motion for an oscillating floating body). The mesh motion is taken into account directly in the Navier-Stokes equation that are solved in the Eulerian frame.
Using input forces and moments from the CFD solver, floating body and mooring dynamics are solved using the open source C++ library Project Chrono. This library allows a fully coupled simulation of rigid and flexible bodies with cable dynamics using FEM, where collision detection of the cables with structures is enabled using node clouds for seabed and other obstacles.
Verification and validation of the coupled two-phase flow and body/moorings dynamics is conducted in this paper with the help of experimental data of floating bodies with varying constrains and degrees of freedom. Figure 2 shows results of one of the validation tests where the Response Amplitude Operator (RAO) was obtained with Proteus for the roll motion of a floating body under different wave loads and compared to experimental data [1] and to other numerical models [2].
Validating the model for these cases allows us to simulate with confidence more representational scenarios for offshore renewable energy devices using, for example, a selection of the following floating wind support structures: deep draught platforms (SPAR), tension-leg platforms (taut mooring), and semi-submersible (buoyancy stabilized)
Towards better design of riprap bed sills - an experimental study
No full textFlexible bed sills are extensively used in rivers in several applications: to protect pipeline crossings, for slope control and to enhance environmental aspects. Use of riprap provides the flexibility required to adjust to small changes in river bed level and makes this material one of the most commonly used. Shear failure of riprap has been extensively studied, but little knowledge is available on edge failure of flexible bed sills and on the design of the required extent of sills to protect assets. Experimental work in a large test facility was carried out to gain improved understanding of flat sills and sills protruding above the bed. Significantly lower depth and length of the scour hole were observed at the downstream edge of flat riprap sills than at rigid sills. Existing equations for rigid bed sills were found not to be appropriate for estimating scour at the flexible bed sills tested. As expected, protrusion enhances scour potential significantly: it was found that scour depths were of the order of 100% or greater than those associated with flat sills. The research has provided useful evidence for avoiding placement of pipelines straight on to river beds and instead to adopt trenches whenever feasible
The use of existing reservoirs for flood alleviation
No full textThe move towards catchment flood management includes the retention of water in rivers catchments in order to reduce flood risks downstream. There are many existing dams and reservoirs in the UK that were designed for non-flood uses including water supply and hydropower. There is an opportunity to use some of these reservoirs for flood attenuation, particularly those that are no longer required for their original purpose.
In Scotland, some reservoirs have already been handed over to local authorities and form part of flood management schemes. Following the severe floods of December 2015, Scottish Water (SW) is now exploring further opportunities for using their reservoirs for flood attenuation.
In order to select reservoirs that may be suitable for flood attenuation, the reservoirs are screened using criteria that include their location relative to flood risk areas, reservoir volume, existing use and the scope for creating an effective volume for flood storage. Having identified reservoirs that may be suitable for flood attenuation, detailed studies are then carried out to investigate the feasibility for each reservoir.
In some cases a flood storage volume can be created with minimal changes to the existing reservoirs. In other cases, more significant changes are needed, such as the construction of gated spillways. The detailed studies include utilising HR Wallingford’s OPTIONISE approach, which is used to develop operating rules that balance the different requirements of reservoir system performance including dam safety, water supply reliability, hydropower generation (where plants exist) and flood attenuation
Sensitivity framework to assess the resilience of a conjunctive use system to drought
Get PDFTraditionally water resource systems in the UK are assessed
based on their performance during the worst historic drought
Current guidance suggests UK water companies need to plan system resilience to foreseeable future events and historic events. The uncertainty surrounding the predictions of groundwater supply during periods of low groundwater levels poses a challenge to water resource managers
Development and application of a framework to understand the available capacity in the UK’s drainage systems
No full textThe vision of the 21st Century Drainage Programme is to enable the UK water industry, in partnership with the UK’s governments and regulators, to take action now that will ensure the resilience and sustainability of our drainage infrastructure in the future.
As part of this Programme, Workstream 2 has been focused on understanding the available capacity in the UK’s foul and combined drainage systems to accommodate present-day flows and flows expected in the future. The first phase of this work resulted in The 21st Century Drainage Programme Capacity Assessment Framework - a consistent, transparent and high-level approach to assessing available capacity and investment needed in the long term.
This paper describes the approach adopted by the Framework (completed in April 2017) for assessing performance and investment (present day, future and future with intervention), the metrics used by the assessment and how this information is to be visualised.
This paper also introduces the second phase of work, which is currently underway, supporting all 12 of the UK’s sewerage undertakers as they embed the Framework into their organisations. This includes the development of a first draft National Picture of present day drainage capacity, due for publication in late November 2017