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Numerical Modeling in Flood Risk Assessment: UK Case Study
... To date, 2D modelling of urban floods is performed almost exclusively using digital elevation models (DEMs) and involving large computational power. A study by Moreta and Lopez-Querol (2017) used numerical experiments combining 2D shallow water model with extremely fine-resolution terrain data in the United Kingdom. Ferrari et al., (2019) used a porosity-based numerical scheme for the Shallow Water Equations to model flood inundation in urbanized environments, while (Okaka and Odhiambo, 2019) on the other hand used a probabilistic and modular approach for calculating flooding risk in terms of the mean annual frequency of exceeding a specific limit state for each building within the informal settlement and the expected number of people affected. ...
Flood disasters have increased in frequency and severity over the recent decades causing untold destruction to vulnerable physical infrastructure such as sanitation facilities. Factors including construction quality, design, siting, and users’ behaviour further exacerbate the vulnerability of facilities. Despite this reality, very little has been done to document the extent of flood risk facing such facilities in the pro-poor urban informal settlements in developing countries. This study assessed the flood risks of vulnerable sanitation facilities in the urban informal settlements of Kisumu city, Kenya. The methodology involved assessment of sanitation facilities’ flood vulnerabilities and assessment of flood risk models. Flood risk was assessed by estimating runoff from yearly rainfall totals and also by calculating storm return period and probability of exceedance. Vulnerability assessment for each sanitation facility was done by scoring against flood risk indicators ordered by weighted rank. The study observed that majority sanitation facilities in the urban informal settlements were considered “highly vulnerable” (57%). Flood risk analysis predicted growing vulnerability due to shorter storm return periods, especially under the RCP 8.5 scenario. It was established that over 20% of all rainfall events in the 50-year timeline had a higher than 80% probability of exceedance rainfall, signifying higher storm risks. Additionally, the study showed that between 44% of rainfall received in the study area could translate to runoff, in the near future, further compounding flood risk predictions. With key informal settlements such as Nyalenda and Manyatta facing stronger future flood risks, general public health may be threatened, leading to increased social and economic instability on families and households. The study recommends adherence to improved toilet standards of construction and toilet-raising as methods of improving flood risk resilience and adaptation.
A method for estimating velocity and discharge in compound channels is presented. This is based on solving the equation for the lateral distribution of flow in a channel. Results are given for laboratory and natural river channels.
A holistic perspective on changing rainfall-driven flood risk is provided for the late 20th and early 21st centuries. Economic losses from floods have greatly increased, principally driven by the expanding exposure of assets at risk. It has not been possible to attribute rain-generated peak streamflow trends to anthropogenic climate change over the past several decades. Projected increases in the frequency and intensity of heavy rainfall, based on climate models, should contribute to increases in precipitation-generated local flooding (e.g. flash flooding and urban flooding). This article assesses the literature included in the IPCC SREX report and new literature published since, and includes an assessment of changes in flood risk in seven of the regions considered in the recent IPCC SREX report—Africa, Asia, Central and South America, Europe, North America, Oceania and Polar regions. Also considering newer publications, this article is consistent with the recent IPCC SREX assessment finding that the impacts of climate change on flood characteristics are highly sensitive to the detailed nature of those changes and that presently we have only low confidence 1 in numerical projections of changes in flood magnitude or frequency resulting from climate change.Editor D. KoutsoyiannisCitation Kundzewicz, Z.W., et al., 2013. Flood risk and climate change: global and regional perspectives. Hydrological Sciences Journal, 59 (1), 1–28.
The problem of quantifying the effects of flexible plants on flow resistance and eddy viscosity by vegetated floodplains is first addressed with a one-dimensional (1D) approximation based upon the so-called lateral distribution method. The estimates so obtained are then tested with two-dimensional (2D) numerical simulations based on the full shallow water equations through the use of the computational code Telemac-2D. Data obtained on a physical model of the Be os River (Spain), whose floodplains were covered with plastic ornamental plants to mimic the effect of flexible vegetation, is used for the validation of the numerical results. Additionally, the values of flow resistance estimated numerically with the 1D and 2D simulations are compared with values obtained in a rectangular flume under flow conditions (slope, water depth and artificial lining) similar to those used on the reduced model. It is then established that as more physical mechanisms are included in the mathematical model used to study the problem, the ratio between the floodplain and the main channel flow resistance coefficient increases. The approach demonstrates that whenever enough flow data is available, the lateral distribution method delivers values of flow resistance and eddy viscosity which are highly consistent with 2D numerical modelling. This finding could mean considerable savings in the burdensome task of specifying flow resistance and turbulence dissipation values for 2D modelling of large compound channel systems.
In 2001, the Environment Agency of England and Wales commissioned a Targeted Programme of Research (Evans et al, 2001) into reducing uncertainty in conveyance estimation, with the intention of bridging the gap between the advances in scientific knowledge over the past three decades and the calculation approaches adopted in United Kingdom industry practice. The key output was the Conveyance Estimation System (CES) software, which incorporates a methodology (Mc Gahey & Samuels, 2003) for estimating conveyance in a range of channel types and flow conditions, including straight, skewed and meandering plan form shape; simple, twostage and multi-thread channels; and a variety of vegetation and substrate covers (Defra/EA, 2003; Mc Gahey & Samuels, 2004). In this paper, the CES methodology is applied to fourteen river sites from England Northern Ireland New Zealand Ecuador and Argentina, illustrating reasonable model predictions over a broad application range. Discharge and lateral velocity predictions are compared to observed data, with emphasis on practical application, calibration technique and effects of scale. Consideration is given to the relative magnitude of the equation terms with depth, for example, the role of boundary friction, lateral shear and secondary flows, and how these relate to the observed flow structure and the local site characteristics.
The flow of water in straight open channels with prismatic complex cross-sections is considered. Lateral distributions of depth-mean velocity and boundary shear stress are derived theoretically for channels of any shape, provided that the boundary geometry can be discretized into linear elements. The analytical model includes the effects of bed-generated turbulence, lateral shear turbulence and secondary flows. Experimental data from the Science and Engineering Research Council (SERC) Flood Channel Facility are used to illustrate the relative importance of these three effects on internal shear stresses. New experimental evidence concerning the spatial distribution of Reynolds stresses τyx and τzx is presented for the particular case of compound or two-stage channels. In such channels the vertical distributions of τzx are shown to be highly nonlinear in the regions of strongest lateral shear and the depth-averaged values of τyx are shown to be significantly different from the depth mean apparent shear stresses. The importance of secondary flows in the lateral shear layer region is therefore established. The influence of both Reynolds stresses and secondary flows on eddy viscosity values is quantified. A numerical study is undertaken of the lateral distributions of local friction factor and dimensionless eddy viscosity. The results of this study are then used in the analytical model to reproduce lateral distributions of depth-mean velocity and boundary shear stress in a two stage channel. The work will be of interest to engineers engaged in flood channel hydraulics and overbank flow in particular.
Reducing uncertainty in river flood conveyance, Interim report 2 Review of methods for Estimating Conveyance
- Defra Environment Agency
DEFRA Environment Agency (2003) Reducing uncertainty in river
flood conveyance, Interim report 2 Review of methods for Estimating
Conveyance. Project W5A-057 HR Wallingford UK.
Theory and user manual for SRH-2D
- Y G Lai
Lai YG (2008) Theory and user manual for SRH-2D. Denver, Bureau of
Reclamation Colorado: USA.
Mapping the impacts of natural hazards and technological accidents in Europe an overview of the last decade
EEA (2011) Mapping the impacts of natural hazards and technological
accidents in Europe an overview of the last decade. EEA Technical
Report. European Environment Agency Copenhagen, Denmark.
Estimates of flow resistance and eddy-viscosity coefficients for 2D modelling on vegetated floodplains
- C Vionnet
- P Tassi
- Martínv Vide
Vionnet C, Tassi P, Martínv Vide, JP (2004) Estimates of flow resistance
and eddy-viscosity coefficients for 2D modelling on vegetated
floodplains. Hydrological processes 18(15): 2907-2926.
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