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Increasing the Resilience and Improving the Environmental Performance of Earthen Flood Defense Structures with High Performance Turf Reinforcement Mat Reinforced Vegetation
Get PDFAdding resilience to earthen flood defense structures, such as dams and levees, is critical to future risk mitigation as building higher and stronger structures to prevent overtopping waves, storm surge, and flood waters becomes more prohibitive. To add resilience riprap, articulated concrete blocks, concrete slope paving, and other traditional hard armor solutions are often used typically at a great cost to the owner and the environment. The United States Army Corps of Engineers (USACE) sought to mitigate these costs when selecting an armoring system for the earthen levees in the 214 km (133 miles) of the Hurricane & Storm Damage Risk Reduction System for southeast Louisiana. The USACE armoring focus turned to High Performance Turf Reinforcement Mats (HPTRMs) after a levee armored with vegetation reinforced by this synthetic mat in Lafitte, Louisiana survived the storm surge and wave overtopping produced during Hurricane Ike in 2008. This success encouraged the USACE to begin a 10 year intensive research program to determine the hydraulic performance threshold, cost effectiveness, and long-term durability of vegetation reinforced by a HPTRM for adding resilience to the re-built levee system destroyed by Hurricane Katrina. Research at the Hydraulics Laboratory of Colorado State University has established the HPTRM reinforced vegetation performance in both outdoor flume testing and in the world’s largest, full scale Wave Overtopping Simulator. As a result of this research, this paper will focus on using hydraulic data to reposition HPTRMs as a suitable alternative to traditional hard erosion control solutions and explain the importance of key material properties when comparing different HPTRMs. This paper will show that HPTRMs with a more closed structure and a smaller percent vegetation establishment perform better than the more open HPTRMs with a higher percentage of vegetation establishment. More research is required to determine specific design guidelines for correlating percent HPTRM openness to vegetation densities in different soil types as it relates to hydraulic performance. Even with low vegetation densities, HPTRM reinforced vegetation provides improved environmental outcomes and lower carbon emissions when compared to traditional hard armor solutions. Countries around the world may benefit greatly from investing in HPTRM reinforced vegetation to provide resilience on earthen flood defenses as an alternate to traditional hard armor systems that are more expensive, less aesthetically pleasing, and more detrimental to the environment
Modelling wave overtopping for flood defense reliability
Get PDFFailure of dike covers due to wave overtopping erosion may initiate dike breach. Surface transitions in the dike cover, such as cure points, height difference, roughness difference and objects are often weak spots, but the effects of such transitions on the wave overtopping discharge and associated location and evolution of dike cover erosion are highly uncertain. Dike cover erosion is dominated by the turbulence-dominated shear stress at the jet front. Therefore, a detailed numerical Finite Element model was developed. In this paper the preliminary results of the model of Aguilar-Lopez (2016) and Bomers et al. (2018) are briefly presented. At the University of Twente, two PhD students recently started on the challenge of quantifying the effect of transitions in grass covered dikes on dike erosion. In this paper, we present their research plans
How can policy makers in sub-Saharan Africa make early warning systems more effective? The case of Uganda
No full textSub-Saharan Africa is affected by three main weather-related hazards: floods, droughts, and tropical cyclones. Effective early warning systems (EWSs) can reduce the risks posed by these hazards. There have been numerous EWSs set up throughout Africa; however, work out to assess their effectiveness has been limited. This paper covers sub-Saharan Africa using Uganda as a case study. Recently in Uganda there has been a proliferation of EWSs targeted at various beneficiaries. Engagement with a range of stakeholders via an internet-based survey and interviews in Uganda and the rest of sub-Saharan Africa found that EWSs often do not communicate hazards well to vulnerable communities or reduce their impacts at a local level. There are numerous barriers to providing effective warnings. In many sub-Saharan African countries EWSs are not sufficiently budgeted for by the government despite the benefits that can be realised. In Uganda EWSs and the hydro-meteorological networks on which they often rely are underfunded, a situation that is mirrored throughout Africa. This can lead to an absence of confidence in EWSs, meaning that stakeholders do not use formal thresholds to provide triggers for early action. For EWSs in Uganda and the rest of Africa to be effective they need to have not only a strong scientific and technical basis, but also a strong focus on the people at risk and governments need to set aside sufficient funding to ensure that as a minimum their recurrent costs are covered, which currently is not generally the case
Soil/structure transitions in flood defence structures
No full textTransitions in flood defence structures are locations where a change occurs in the defence, whether in terms of internal or external geometry, construction or foundation materials or between flood defence segments. Such transitions can be viewed as falling into two major categories: Transitions consisting of spatial changes in the cross section of either the cover layer or the internal construction of a definable segment of a flood defence; Longitudinal transitions between flood defence segments of different types (e.g. between flood embankments and flood walls) or of different foundation conditions.
In addition, if buried in flood embankments, objects such as conduits, culverts and pipes can form transitions of either type.
The paper describes the content of a UK research project which has just commenced which is designed to: consider the presence of transitions during flood defence condition assessment; quantify the effects of transitions on defence performance (via fragility curves) and flood risk (via systems analysis); and manage the risk of transitions with improved design and retrofitted solutions for existing defences
Fuse plug spillways
No full textThere are fuse plug spillways at many large dams around the world. These may be designed to trigger in floods with return periods of, say, 10,000 years but may also trigger in much smaller floods if these are combined with failure of one or more of the main spillway gates to open. The annual probability of the latter combination may be much higher than that of experiencing a flood with a return period exceeding 10,000 years.
Rubber dams are examples of fuse plugs as are dams of conventional appearance sitting on non-erodible foundations ( whether rock or artificial ). Such dams typically have sloping clay cores resting against erodible downstream shoulders.
Whilst both of the above types of fuse plugs appear simple they require very careful design and, in the case of rubber dams, a high standard of maintenance.
The authors describe some of the fuse plug dams with which they have been involved including a very small one in UK which was built when the landowner downstream refused permission for the reservoir undertaker to build a conventional auxiliary spillway.
There are various alternatives such as tipping gates ( Fusegates ), Flowgates etc. which are well covered in the literature and which are not further discussed in this paper
Dams for small hydropower in Scotland
No full textOver recent years government incentives have led to the development of numerous small hydropower schemes in the Scottish Highlands of which five are described in this paper. The schemes have Francis or Pelton turbines with installed capacities between 0.7 and 2.0 MW with gross heads of up to 220 m. The dams are between 4.7and 13.6 m high and of various types.
The dams are all “smart dams” in that they have sophisticated control systems and are operated remotely. The paper describes the control systems and the choices made to optimise the schemes. It reviews construction problems encountered, including those attributable to working in a harsh environment, and experience gained from operating the completed schemes
Long-term modelling of reservoir sedimentation with turbid underflows
Get PDFPurpose - This paper presents the development of a turbid density current module (TDCM) for a one-dimensional quasi-steady reservoir sedimentation model suitable for long-term simulations and an example of its application.
Materials and methods - The module determines the plunging point of a density current based on the criterion of the densimetric Froude number. Following plunging, simulation continues in a supercritical regime as long as the energy of density flow and bed slope are sufficient to support transport in this mode, or until the dam is reached. A muddy pond is then formed. An adapted version of the theory of Toniolo et al. (2007) is used to compute water and sediment balance in a muddy pond. If the level of muddy pond exceeds the lowest level outlet, sediment is vented from the reservoir.
Results and discussion - The model was applied to one of the largest reservoirs in the world, the Nurek reservoir in Tajikistan. Historical data on bed levels and sediment inflow as well as a recent survey from 2015 were used to calibrate and validate the model. The model showed good results both in terms of modelled bed levels and composition of deposited sediment. For comparison purpose, the model was also calibrated without the TDCM module. However, the discrepancy between the observed and modelled data was significantly higher, confirming that the good match in the case of the TDCM module was not achieved solely by calibration, but rather by a more appropriate approach to modelling.
Conclusions - These results support the choice of using the model with TDCM for modelling a general pattern of long-term deposition in the Nurek reservoir, in which turbid density currents play an important role
Drought resilience and climate change analysis: Moving from a top-down to bottom-up approach in the UK
No full textWater resources across the globe face challenges from population growth and climate change. Concerns regarding the potential implications of extreme, previously unseen, droughts are also increasing. Recent research in the UK has developed and demonstrated new, practical ‘bottom-up’ approaches for assessing and communicating the resilience of water supply systems to droughts beyond the historic record and assessing the potential impacts of climate change.
Traditionally water supply planning in the UK has been based around the historic record to identify the worst droughts which in turn are used to test the performance of contemporary water supply systems. Typically, data records extend back to the early twentieth century, although with increasing data uncertainty further back in time, generally providing around five or six events around which to design and plan a water supply system.
Drought events are rare and their characteristics are both spatially and temporally variable. Planning solely to events in the historic record does not reflect that droughts in the future could exhibit very different characteristics. Recent work has demonstrated the benefits of using stochastically generated and synthetic droughts applied as part of a ‘bottom-up’ approach to provide a richer understanding of system resilience. The outputs are presented consistently across the industry in the form of drought response surfaces with pragmatic methods and supporting tools developed to support industry application.
For climate change, impacts have been previously assessed using ‘top-down’ modelling approaches that, particularly for large, complex conjunctive-use systems, require significant effort regardless of the systems’ potential vulnerability and which may not adequately consider and communicate how adaptation to climate change relates to resilience to droughts. Consequently, further work with the UK water resource industry has extended the “bottom-up” approach to droughts, alongside exploiting previous climate change analysis, to ensure an effective, proportionate approach to climate change.
Application of these methods has demonstrated contrasting levels of resilience to different types of droughts and vulnerability to climate change across the UK. The outputs are being used to inform both national policy and underpin local investment planning in demand-side and supply-side measures
Forecasting coastal overtopping: What’s the worst that can happen?
Get PDFMuch is known about what is possible around the coast; Astronomical Tides follow fairly predictable rules, Physics limits the effects of weather on top of that. In recent years we have invested significantly in better forecasting for coastal extremes, attempting to determine whether the apparently random conjunction of tide and weather will overtop our sea walls and beaches, and if it does what the hazard or inundation extent, depth, speed and impact will be. Internationally, these factors may not be the sole drivers for flood risk management, but by recognising short and long term limits of these variables we can better describe the coastal risk and a “reasonable” worst case of possible impact.
We are able to forecast coastal storm possibilities out to 5 days and as flooding approaches we are able to narrow the range of that forecast, which may or may not exclude the worst extremes. How do we use that as a heads-up to prepare for the possibilities without scaring people? By sharing extreme possibilities with responders who understand the low probability, but who may have to mobilise, we can prepare for that worst case. This process may escalate with the proximity of the event and the likely scenarios narrowing towards that expected. Conversely, as the worse events become less likely, we can scale down the response needed.
The biggest variable around our coast is the tide. As it is driven by the pull of the moon and sun predominately, it can be predicted indefinitely ahead, with reasonable accuracy. Around 6 times a year, these work together to generate threatening sea levels. Second to the tide, storm surge is a major contributor, adding up to 3m onto the water level. On top of the sea level, wind driven wave action may give overtopping of defences well above that combined height. We will look at the way these factors combine and we forecast their impact.
As we know from flooding history, these events where the factors combine to their worst effect are rare. The way they add together varies around our coast and more so around the world; that same history means that exposed locations will have been affected by similar events in the past and the risks should be well known. We have defended much of the coast where people and property are at risk, so we are dealing with the remaining vulnerability. We can’t build an un-surmountable wall round our coast and we wouldn’t want to, so we defend against events proportionately and prepare to respond to extreme events.
Our defences are affected by wave action in a storm. Beaches are a major part of our defence as waves will break before they get close, but that wave action tends to mobilise sand or shingle and move it offshore. Hard defences may be damaged by heavily plunging waves or be undermined by overtopping waves. Either of these will affect the way the event is managed and need repairs planning if possible. These and the extreme events need a plan to deal with the failure of that defence.
Major Incident Plans are maintained for events where serious or widespread overtopping or defence failure occurs. In some areas, demountable defences may be applicable, others may require evacuation. Knowing the worst that can happen is vital to planning this response. The uncertainty in our forecast at 5 days out means we shouldn’t mobilise at that point, acting according to our best-most likely- forecast, be prepared for the reasonable worst case and monitor for any unprecedented escalation to give as much warning as possible
Applying Gaussian process emulators for coastal wave modelling
No full textIn an ever increasing need to minimise costs, most coastal engineering work requires reliable information on the environmental forces and governing physical processes that need to be taken into account when designing or assessing the performance of coastal structures. This often involves the application of complex physical process based numerical models (simulators) that can be computationally expensive to run. In general, the more complex the process representation, the more computationally expensive the model simulations will be. Coastal process simulations, flood risk analysis and flood forecasting can all require substantial computational resources. The situation is exacerbated when repeated simulation runs are required in a sensitivity analysis or uncertainty analysis, for example.
When the number of model simulations are excessive, it is possible to make a compromise on time and space resolution or processes represented or apply alternative methods typically with a reduction in accuracy. One such relatively simply approach is populate or train a look-up table (LUT) using only a subset of the full set of runs and then use the LUT to predict the output for the relevant input values typically using linear interpolation. A limitation with such approaches is the number of training simulations required to maintain accuracy by adequately representing the input parameter space increases significantly with increasing dimension. In addition, the relationship between the inputs and outputs can often be non-linear and hence linear interpolation is not necessarily an appropriate interpolation method to apply.
This paper focusses on the use of the Gaussian process emulator (GPE) meta-modelling approach as an alternative approach to traditional LUTs. Using the specific example of wave transformation with the Simulating Waves Nearshore (SWAN) wave model, a GPE has been compared with a traditional LUT approach. In addition, the method of selecting the design points used to train the GPE has been explored and a refined algorithm, that takes account prior knowledge of the boundary conditions, has been introduced.
It is shown that the GPE approach requires significantly fewer model runs to obtain similar or higher accuracy, enabling a substantial reduction in overall computation time when compared to a traditional LUT approach. The refined algorithm also shows significant computational efficiencies, meaning that potential compromises on model resolution or the physical processes can be limited