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Erosion characteristics and horizontal variability for small erosion depths in the Sacramento-San Joaquin River Delta, California, USA
No full textErodibility of cohesive sediment in the Sacramento-San Joaquin River Delta (Delta) was investigated with an erosion microcosm. Erosion depths in the Delta and in the microcosm were estimated to be about one floc diameter over a range of shear stresses and times comparable to half of a typical tidal cycle. Using the conventional assumption of horizontally homogeneous bed sediment, data from 27 of 34 microcosm experiments indicate that the erosion rate coefficient increased as eroded mass increased, contrary to theory. We believe that small erosion depths, erosion rate coefficient deviation from theory, and visual observation of horizontally varying biota and texture at the sediment surface indicate that erosion cannot solely be a function of depth but must also vary horizontally. We test this hypothesis by developing a simple numerical model that includes horizontal heterogeneity, use it to develop an artificial time series of suspended-sediment concentration (SSC) in an erosion microcosm, then analyze that time series assuming horizontal homogeneity. A shear vane was used to estimate that the horizontal standard deviation of critical shear stress was about 30% of the mean value at a site in the Delta. The numerical model of the erosion microcosm included a normal distribution of initial critical shear stress, a linear increase in critical shear stress with eroded mass, an exponential decrease of erosion rate coefficient with eroded mass, and a stepped increase in applied shear stress. The maximum SSC for each step increased gradually, thus confounding identification of a single well-defined critical shear stress as encountered with the empirical data. Analysis of the artificial SSC time series with the assumption of a homogeneous bed reproduced the original profile of critical shear stress, but the erosion rate coefficient increased with eroded mass, similar to the empirical data. Thus, the numerical experiment confirms the small-depth erosion hypothesis. A linear model of critical shear stress and eroded mass is proposed to simulate small-depth erosion, assuming that the applied and critical shear stresses quickly reach equilibrium
Ten years of lessons learned from English levee performance during severe flood events
No full textThe Environment Agency have carried out a structured review of defence performance after each of the five major events from the 2007 Summer floods, up to and including the winter 2015/16 floods. This presentation draws out overarching conclusions about how English flood defences perform during floods based on real life experience, and how this has driven and is driving improvements to asset management and design, supported by research. The five reviews covered all major breaches or near-breaches, but also assessed assets that survived in spite of being loaded beyond their design. Each review typically started with site visits to collate factual information but also to understand the local asset managers’ point of view and collect anecdotal evidence, essential to establish the story of the flood event. This was used to determine the failure modes that occurred, supported by hydraulic/geotechnical modelling as required. Key conclusions were that visual condition can only tell so much about risk of breach, and that local irregularities are often the trigger for failure. This has influenced how the Environment Agency prioritises its asset management investment in relation to Condition Grade, and has initiated focused studies to identify weak points such as transitions and historical channel crossings
Regional-scale probabilistic shoreline evolution modelling for flood-risk assessment
No full textRapid deterministic modelling of shoreline evolution at regional and coastal-scheme scale enables Monte-Carlo simulations by which long-term shoreline statistics can be estimated. This paper describes UnaLinea, a fast, accurate finite difference solver of the one-line sediment continuity equation. The model is verified for the evolution of an initially straight shoreline of a plane beach subject to regular breaking waves at constant angle of incidence in the presence of either a groyne or a continuous single-point feed of sediment. Grid convergence and stability tests are used to obtain accurate, stable results, with satisfactory computational efficiency. Influences of wave input filtering and event-based sediment loading are considered. The rapid deterministic model is applied to Monte-Carlo simulations of the evolution of the west coast of Calabria, Italy for different scenarios including increased sediment load from a river and selected beach nourishment. The potential role of probabilistic shoreline evolution in regional coastal flood-risk assessment is explored through application to an idealised stretch of the Holderness coastline, U.K., where flood depths and expected damage are estimated for a 1000 year return period event
Probabilistic design of riprap bed sills
No full textFlexible bed sills are extensively used in rivers for different purposes, one of them being the protection of buried assets such as pipelines. This paper proposes developments to enable the use of a probabilistic approach in the design of bed sills, covering arguably the most common material: riprap. A probabilistic design of bed sills is expected to lead to safer and more cost-effective designs. A methodology involving a number of stages for estimating the annual probability of bed sill failure is presented along with fragility curves that describe the probability of failure of the structure dependent on the loading or design conditions of the sill. The methodology is applied to a particular example to illustrate the advantages of the approach
Flood forecasting and warning for river basins in Malaysia: non-structural measures for flood mitigation
No full textThe Department of Irrigation and Drainage (JPS) of the Malaysian government has adopted an integrated approach to river basin development and flood management across the country. The major rivers of Peninsular Malaysia have their sources in the band of hills running along the centre of the peninsula. The country has a humid tropical climate; Average Annual Rainfall is 2500 mm, although seasonal distribution of rainfall varies widely from place to place. Many of the river basins have repeatedly suffered prolonged, significant flood events which have caused widespread disruption and impacts to residents, businesses and infrastructure. The impacts of flood events 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, via JPS, is implementing a range of flood management projects, which will provide an integrated approach based on structural and non-structural measures.
Flood forecasting and warning systems are currently being developed for key river basins. These systems are characterised by a need for flexibility in their handling of sources of incoming data, combining spatial and gauged observations, along with Numerical Weather Prediction (NWP) forecasts, as well as techniques for providing robustness to missing or poor quality inputs. The fully automated end-to-end systems are designed to ingest data, run simulations, and post-process results to provide flood managers and the general public with relevant information in a timely manner. Flexible dissemination routes via dedicated interfaces for operational staff, and websites, SMS messaging and smart phone applications for the general public, are designed to ensure that warnings reach stakeholders in time for action to be taken, so that the widely used government slogan ‘Rakyat didahulukan, pencapaian diutamakan’ (‘People first, performance now’) is key to resilient flood management
Tsunami inundation forces on coastal buildings – effects of building layout
No full textA simplified experimental method using sliding buildings has been coupled with a unique pneumatic Tsunami Simulator (TS) to investigate the influence of blockage ratio and building position on tsunami loading on sets of idealised buildings. The TS generates the full duration of the 1:50 scale tsunami waves, allowing for the first time the transient effects of the full tsunami wave to be investigated. The simplified sliding building technique allows the resultant horizontal force acting on a building, or array of buildings to be assessed quickly, identifying general trends and patterns before more complicated and time-consuming measurement techniques are used. This paper presents the influence of blockage ratio, the interaction between the first and second row of buildings in relation to the resultant horizontal force acting on the building and the influence of multiple rows of buildings on the first and last rows
Potential sources and magnitude of errors associated with the measurement of suspended sediment concentration
No full textDredging 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
Three-Dimensional Quantification of Pore Space in Flocculated Sediments
No full textFlocculated sediment structure plays a vital role in determining sediment dynamics within the water column in fresh and saline water bodies. The porosity of flocs contributes to their specific density and therefore their settling characteristics, and can also affect settling characteristics via through-flow. The process of settling and resuspension of flocculated material causes the formation of larger and more complex individual flocs, about which little is known quantitatively of the internal micro-structure and therefore porosity. Hydrological and sedimentological modelling software currently uses estimations of porosity, because it is difficult to capture and analyse flocs. To combat this, we use a novel microscopy method usually performed on biological material to scan the flocs, the output of which can be used to quantify the dimensions and arrangement of pores. This involves capturing flocculated sediment, staining the sample with heavy metal elements to highlight organic content in the Scanning Electron Microscope later, and finally setting the sample in resin.
The overall research aim is to quantitatively characterise the dimensions and distribution of pore space in flocs in three dimensions. In order to gather data, Scanning Electron Microscopy and micro-Computed Tomography have been utilised to produce the necessary images to identify and quantify the pore space. The first objective is to determine the dimensional limits of pores in the structure (i.e. what area do they encapsulate? Are they interconnected or discreet?). This requires a repeatable definition to be established, so that all floc pore spaces can be quantified using the same parameters. The LabSFLOC settling column and dyes will be used as one possible method of determining the outer limits of the discreet pore space. LabSFLOC is a sediment settling column that uses a camera to record the flocs, enabling analysis of settling characteristics. The second objective is to develop a reliable method for quantifying the dimensions of the pores. The dimensions to be quantified are the long- and short-axis lengths, measured using ImageJ. The third objective will be to quantify the distribution of the pore space within the structure, utilising point-to-point measurements and distance from centre of the floc, again utilising software capable of providing accurate measurements between the centres of each pore within the structure.
Preliminary data demonstrating pore dimensional limits and quantification will be presented. This will establish a definition of pore space based on limits of interaction between pore water and the water column, including experimental data from LabSFLOC, and visual representations of pore outer limits. Further to this, I will include some investigational data from ImageJ relating to the dimensions being measured for sub-aim 2. This information is vital in providing accurate and reliable information for hydrological and sedimentological model input, ultimately increasing the value of the outputs
Are there lessons that can be learnt from Bangladesh and Cuba that can increase American coastal communities’ resilience to flooding?
No full textOver the past decade, the US Gulf Coast has been hit by a series of major hurricanes, including Ivan, Katrina, Rita, Ike and Sandy which have led to coastal flooding and fatalities, as well as significant economic losses. In the USA over the past 60 years the disparity between the rich and poor has increased, with the poorest people becoming increasing physically and socially isolated. These isolated households often have limited social networks to call upon in times of emergency reducing their resilience to natural hazards such as coastal floods. In many coastal states, such as Florida and Georgia, it is expected that coastal populations will have increased by up to 20% by 2020. This increase, which will mainly comprise people of retirement age, coupled with increasing sea-levels, will make American coastal communities increasingly vulnerable to coastal flood events. Some states, such as Georgia, have not experienced an extreme coastal surge for more than 100 years. A recent survey found that less than one quarter of Georgians living on the coast were very concerned about hurricanes and many people living in low lying areas did not believe their homes could flood.
Despite their relative lack of resources the resilience of communities to coastal floods in Cuba and Bangladesh has increased via a range of measures including: improving the comprehensibility and effectiveness of warnings; building social capital; increasing the levels of trust in organisations responsible for issuing warnings and coordinating the emergency response; and methods of providing education in disaster risk reduction at primary through to tertiary level institutions. Despite financial challenges, both Cuba and Bangladesh have made significant progress in reducing deaths, as well as ensuring that people are able to maintain their livelihoods following tropical cyclones and coastal flood events.
This paper looks at how the resilience of disadvantaged and isolated American coastal communities to floods could be increased based on the success of strategies and measures employed in Bangladesh and Cuba, where governments face significant financial constraints in providing infrastructure to protect populations from severe storms. Between 2003 and 2011, there were almost 20 times fewer deaths per million people at risk in in Cuba than in the USA as a result of hurricanes and coastal floods and in Bangladesh the number of fatalities from cyclones has reduced by two orders of magnitude over the past 25 years. This paper will investigate if the lessons learnt in Bangladesh and Cuba can be applied in the USA. It will also discuss why there needs to be a recognition of the importance of social capital and community self-sufficiency and how these can be encouraged and measures can be designed to leverage it in the context of future coastal flood events
On the hindered settling of sand-mud suspensions
No full textHindered settling, the process by which the settling of sediment particles becomes impeded due to the proximity of other sediment particles, can be an important process for the coastal modeller, especially in highly muddy environments. It is also a significant process in other disciplines such as chemical engineering, the modelling of debris flow, the study of turbidites, piping of slurries and the understanding of processes occurring within a dredger hopper. This study first examines the hindered settling behaviour of monodisperse suspensions in order to create a framework for polydisperse hindered settling that works for both non-cohesive and cohesive suspensions. The Richardson–Zaki equation is adapted to make it compatible with the changes with viscosity that occur near the point at which suspensions become solid. The modified monodisperse settling equation is then compared to data for hindered settling of cohesive suspensions and shown to be consistent with the transition between hindered settling and the initial permeability phase of consolidation. Based on the monodisperse framework developed initially, this paper proposes a hindered settling model for sand/mud mixtures which is based on a modification of the Masliyah (1979) and Lockett and Bassoon (1979) hindered settling equation. The model is shown to reproduce the hindered settling of a variety of different sediment mixtures whilst reducing the extent of empiricism often associated with the modelling of polydisperse hindered settling of mud/sand mixtures