Flood Defence Schemes Research Articles

Net-zero carbon dioxide emissions in flood defence schemes

This paper compares the carbon dioxide emissions of constructing flood protection schemes with the emissions saved by providing flood defences. There is considerable information available on the carbon dioxide ‘cost’ of construction, but less on repairing flooded properties. Drawing on existing research into flooding, this paper identifies a carbon dioxide cost of repairs of 6 tonnes carbon dioxide equivalent per property flooded and uses this to provide a method to determine emission benefit for constructing flood protection schemes. While emission reduction should be the first priority, flood defences defend against future emissions. Using Stonehaven in the UK as a case study, this paper demonstrates that the carbon dioxide cost of constructing a flood protection scheme can be less than the carbon dioxide cost of undertaking repairs following flooding over the life of a scheme, with a benefit ratio of 2 identified for the case study scheme. The paper concludes with a review of the UK government appraisal process and cautions that the carbon value many not differentiate a flood protection scheme option when compared with capital cost. It is considered that an independent assessment should be undertaken to demonstrate that a net-zero-emission option is selected.

Estuarine Infauna Within Incidentally Retained Sediment in Artificial Rockpools

Artificial coastal structures (ACSs) are primarily designed to provide services for human use, such as flood defence or shipping, and are generally poor for marine biodiversity. Consequently, there has been significant research effort to enhance these hard structures to increase biodiversity and habitat availability via eco-engineering. On seawalls and breakwaters, this has included the creation of habitats for benthic species found on natural rocky shores, including the provision of cracks, crevices and water retaining features, such as artificial rockpools. When sediment retention in these features has occurred, it has often been deemed detrimental to the overarching aim of the intervention. Yet, it is soft sediment habitat that is impacted the most through coastal construction. As ecological enhancement of a flood defence scheme, nine concrete retrofit rockpools were installed at three different tidal elevations between mean high water neap tide and mean tide level on steel sheet piling on the Arun Estuary in Littlehampton Harbour, United Kingdom, which naturally filled with mud 1 year after installation. To explore how analogous the faunal assemblages and sediment profile of rockpool mud were to two local mudflats, core samples were taken and analysed for species richness, abundance, biomass, assemblage structure, median grain size, and organic matter content. More benthic species were observed in the artificial rockpool than in the local mudflats. Although the rockpools were placed at higher tidal levels than the lower shore mudflat, their assemblage structure and species richness were more similar to the lower shore mudflat at the base of the sheet piling than the upper shore mudflat. This study demonstrates that retained sediment within eco-engineered features on hard ACSs can create habitat for benthic assemblages. Providing sediment-retentive features on ACSs has the potential to provide a novel eco-engineering option that may be appropriate for some heavily modified waterbodies on sheltered, depositional coasts.

Reconstructing the peak flow of historical flood events using a hydraulic model: The city of Bath, United Kingdom

AbstractHistorical flood events on the River Avon in the city of Bath were reconstructed using a 1D hydraulic model. The model represents the river prior to the completion of the Bath flood defence scheme and was developed based on archived data on archival materials including historical cross sections of channel geometry, technical drawings of hydraulic structures, descriptions of floodplain extent, epigraphic markings and photographs. The model was calibrated on two historical floods for which peak flow values were available from hydrological data (March 1947 and December 1960) and the calibration was based on adjustment of Manning's n as well as the introduction of floodplains identified from historical maps and photos. Subsequently, the peak flow was assessed for a total of 16 events for which flood marks existed in the centre of the city. A composite annual maximum series of peak flow was created upstream of Bath city centre and presented a flow series consisting of data from 1866 to present day. Finally, flood frequency analysis was undertaken on the dataset, showing that the inclusion of historical floods increased the 1 in 100 year design flood by up to 30%, that is, a substantial influence.

Boston tidal barrier, UK: adapting to climate change and delivering social outcomes

Climate-adaptation projects such as flood defence schemes must deliver wider societal benefits in the communities they protect to ensure long-term resilience and regeneration. This paper presents a case study of the £100 million Boston tidal barrier across the River Witham in Lincolnshire, UK, that now better protects over 13 000 homes from tidal flooding. The United Nations sustainable development goals provided a framework for monitoring and evaluating the wider benefits of the project and enabled its full societal benefits to be understood and communicated by all key project stakeholders. The primary barrier, completed in 2020, has delivered benefits against all 17 goals and is an example of how sustainability can be embedded into all aspects of a civil engineering project.

Combining hydraulic modelling with partnership working: towards practical natural flood management

The UK government’s Future Flooding Inquiry called for more integrative methods to respond to flood risk management challenges. The 25-year plan for the environment, A Green Future (published by the UK Department for Environment, Food and Rural Affairs in 2018), has reiterated the requirement for integrated catchment management. There is growing acceptance that natural flood management (NFM) can complement traditional urban flood defence schemes. This paper examines the outcomes of a knowledge transfer partnership (KTP) between Waterco Consultants and the University of Liverpool which explores some of the challenges of implementing what appear to be relative simple NFM measures. Through a multidisciplinary partnership, the KTP project explored multiple delivery challenges. Using case study evidence from north-west England, the paper demonstrates the need for combining partnership working with more traditional hydraulic modelling approaches that can predict the potential flood risk reduction benefits of multiple NFM features, combined with the need to design structurally resilient interventions, so that appropriate permits can be approved. One of the key findings is that while NFM can contribute to flood risk alleviation, with multiple socio-environmental benefits, NFM can only be part of a more holistic approach. Primary evidence for hard and soft engineering measures, combined with use of automated attenuation management, could provide opportunities for more significant integrated flood risk benefits.

MODELING WAVE ATTENUATION DUE TO SALTMARSH VEGETATION USING A MODIFIED SWAN MODEL

Vegetated shorelines have been increasingly recognized for their contribution to natural coastal protection due to their ability to dissipate wave energy. Within the UK, salt marshes are beginning to be included in flood defence schemes. Predicting wave dissipation over vegetation requires accurate representation of salt marsh canopies and the feedback relationship between vegetation and wave conditions. We present a modification to the SWAN vegetation model, which includes a variable drag coefficient and a spatially varying vegetation height. Its application is demonstrated by modelling wave propagation over UK salt marshes. The third generation wave model, SWAN includes a vegetation module for calculation of wave attenuation over vegetation. Wave dissipation is determined based on the vegetation properties and a drag coefficient. This drag coefficient, C_D, is used to calibrate the model, and a fixed value is used per model run. Empirically the drag coefficient has been found to vary with ambient wave conditions. Typically the drag coefficients are defined empirically as a function of either the stem Reynolds number, Rev, or the Keulegan-Carpenter number, KC. The parameter values have been shown to vary with vegetation type. In this paper, we modify the SWAN vegetation module to include a temporally varying CD. This allows the drag coefficient to vary with ambient wave parameters, which gives an improved prediction under time varying wave conditions (e.g. passage of a storm) and includes the change in wave conditions as they travel through the vegetation. We also incorporate spatially varying vegetation height into the model to further improve the representation of the complexity of vegetated shorelines. Using the new formulation we find improved prediction of wave dissipation over both idealized laboratory and field salt marsh vegetation.

The Micro-Politics of Water Management in Early Modern England: Regulation and Representation in Commissions of Sewers

Abstract Early modern water management was as much a social and political endeavour as an environmental one. This paper explores this assertion by analysing the different forms of knowledge used by English Commissions of Sewers in the governance of flood defence and drainage in the sixteenth and seventeenth centuries. Using examples drawn primarily from Gloucestershire and Lincolnshire, in south-west and eastern England respectively, this paper traces the rise and decline of popular influence over water management. Where Commissions of Sewers operated harmoniously, they were staffed by significant numbers of local people, who valued their right to participate in water management. With the involvement of large numbers of 'ordinary' people, Commissions of Sewers, and particularly the minutes of Courts of Sewers, became repositories of oral and customary knowledge about the functioning of local drainage networks and flood defence schemes. However, the paper argues that, over time, as customary knowledge that was divulged and negotiated orally became codified, and decisions about water management became founded on textual precedent, those who lacked access to textual resources lost influence over water management decision making.

The role of local stakeholder participation in flood defence decisions in the United Kingdom and Germany

An important aspect of integrated flood risk management (FRM) around the world is accepted as being the involvement of a range of stakeholders in flood‐related decision‐making processes. Achieving local stakeholder participation in ways that lead to the expected benefits is burdened by challenges and difficulties. By drawing on examples of practices of local stakeholder participation in FRM in two European countries, the United Kingdom and Germany, this paper aims to understand the extent to which local stakeholders are able to influence FRM. Empirically, the paper focuses on flood defence planning and implementation‐related decisions as they still remain the dominant approach of managing flood risks in those locations. The findings from the two case studies show that involvement of local stakeholders in decisions related to flood defence schemes is limited and likely to lead to conflict and frustration as well as, potentially, a strengthening of inequalities. These lessons have implications for the United Kingdom and Germany as well as for other locations around the world.

Method for Incorporating Morphological Sensitivity into Flood Inundation Modeling

AbstractTypically, the analysis and design of fluvial flood defence schemes is based on a single N year extreme flow event using a single survey of the river channel and flood plains. Adopting this approach assumes that the channel capacity is identical for all subsequent N year events. If one assumes that the typical design life for a flood defence scheme is of the order of 50 years, then such an approach is flawed because river channel morphology, and hence flood conveyance, may change considerably over this time scale. Therefore, to provide a more robust estimate of future flood inundation, a sensitivity analysis of these changes should be undertaken. This paper proposes a modeling methodology that combines a stochastic model, for estimating streamflow throughout the design period, and a 1D sediment transport model (HEC-RAS), to enable this sensitivity to be included in flood inundation modeling and defence scheme design. The methodology is demonstrated through conceptual implementation to evaluate the...

Local controversies of flood risk reduction measures in Germany. An explorative overview and recent insights

AbstractIn light of the flood event in June 2013, local disputes of flood risk reduction measures became a public concern in Germany, as it was argued that these controversies delayed the implementation of flood defence schemes and thus aggravated the flood impacts. However, actual knowledge about such disputes is generally quite limited. Therefore, this paper uses different empirical approaches to present first an explorative overview of such ongoing controversies with a focus on the measures under dispute and their geographical distribution. Second, current insights in the disputes are delineated, and the following four central conflict lines are expounded: (1) the desire for safety, (2) arguments of nature and landscape protection, (3) questions regarding economic development, and (4) participation issues. The results are discussed comprehensively, and conclusions are drawn regarding further research as well as planning practice in the field of risk reduction measures.

Public attitudes towards flooding and property-level flood protection measures

The number of residential properties at risk from flooding is predicted to rise in the future, and it is clear that large-scale flood defence schemes are not always feasible. There is thus an increasing onus on the public to protect their own properties. This paper reports the results of a stakeholder consultation investigating public attitudes towards flooding and property-level flood protection (PLFP) in general, and peoples’ “willingness to pay” for PFLP specifically. The findings show that flooded households have suffered significant financial and social impacts. Despite some continued uncertainty surrounding flood risk responsibility, the majority of the public surveyed were willing to pay for PLFP, with a mean contribution of approximately £800. Whilst this paper broadly confirms some of the findings of earlier studies, it also indicates that public education and promotion campaigns have been effective in raising awareness and uptake of PLFP and that people are willing to pay more to protect their properties. The findings also support the notion that an increased awareness of PFLP, and an increased willingness to pay for PLFP, is linked to the scale of flooding and impacts, rather than just the frequency, as well as financial subsidies. The results of the study are particularly relevant to institutional stakeholders, as they can help guide the development of strategies to increase the uptake of such measures. Whilst the project focuses on the situation in Scotland, the findings will have resonance in similar countries throughout Europe and beyond.

Development of Louth flood alleviation scheme outline design

The market town of Louth (in Lincolnshire), on the River Lud, has suffered from flooding on many occasions, most recently in 2007. Atkins was commissioned to conduct a feasibility study into the potential for a flood defence scheme. This paper begins once the preferred option of two online flood storage areas had been identified; the brief from this point was to develop their outline design. Atkins worked closely with a contractor to develop a cost estimate for the works and to ensure buildability was considered. The two dams are designed as homogenous earth fill embankments, with heights of circa 6 m, and retaining a combined volume of 213 000 m3 (during a flood event); both storage areas fall under the requirements of the Reservoirs Act 1975. As in many cases, these reservoirs present particular challenges; designed to be empty for the majority of their life, they are required to meet the needs of a variety of stakeholders. The design was further complicated by the risk of unexploded ordnance and burrowing animals. This paper presents the outline design under three sections: the location, embankments and control structures. For each section the paper discusses the alternatives considered, the selection process and the adopted design.

Putting policy initiatives into practice

Purpose – Amidst the current economic climate, which places many constraints on expensive flood defence schemes, the policy makers tend to favour schemes that are sympathetic to the needs of small and medium-sized enterprises (SMEs) and which promote empowering local communities based on their individual local contexts. Research has shown that although several initiatives are in place to create behavioural change among SMEs in undertaking adaptation approaches against flooding, they often tend to delay their responses by means of a “wait and see” attitude. The paper aims to discuss these issues. Design/methodology/approach – This paper argues that unless there are conscious efforts in the policy-making community to undertake explicit measures to engage with SMEs in a collaborative way, the uptake of adaptation measures will not be achieved as intended. With the use of the “honest broker” approach the paper provides a conceptual way forward of how a sense of collaboration can be instigated in an engagement process between the policy makers and SMEs, so that the scientific knowledge is translated in an appropriately rational way, which best meets the expectations of the SMEs. Findings – The paper proposes a conceptual model for engaging SMEs that will potentially increase the uptake of flood adaptation measures by SMEs. This could be a useful model with which to kick start a collaborative engagement process that could escalate to wider participation in other areas to improve impact of policy initiatives. Originality/value – The paper lays the conceptual foundation for a new theoretical base in the area, which will encourage more empirical investigations that will potentially enhance the practicality of some of the existing policies.

Gainsborough flood defence scheme: improving project delivery by reusing existing assets

This paper shows how an integrated team enabled the UK Environment Agency to deliver a more cost-effective and sustainable solution to improving protection for over 2600 properties in Gainsborough, Lincolnshire, from flooding by the tidal River Trent. The team identified several opportunities to cut costs, mitigate risks and reduce carbon dioxide emissions. At the outset, a detailed condition assessment and numerical analysis of existing flood defence structures enabled many of them to be reused, even though most were originally thought to be at the end of their design life. This and other initiatives resulted in cost savings £10 million, a shorter construction period and significantly lower environmental impact.

Comparison of an ANN approach with 1-D and 2-D methods for estimating discharge capacity of straight compound channels

Most natural streams or rivers exhibit a compound or two-stage geometry consisting of a main channel and one or two floodplains. The discharge capacity of compound channels has an importance in flood defence schemes and in the economic development of floodplain areas for agriculture and parks. Therefore, the comprehensive stage–discharge model studies performed and different one or two-dimensional methods have been developed. In this study, the single-channel method (SCM), the divided-channel method (DCM), the coherence method (COHM), the exchange discharge method (EDM) and the Shiono–Knight method (SKM) have been compared with a multilayer perception neural network (MLP) with Levenberg–Marquardt algorithm. The results of the comparisons reveal that the artificial neural network (ANN) model gives slightly better statistical results than those of the COHM, EDM and these three give more accurate results than those of the SCM, DCM, and SKM.

Effective flood alleviation design and construction

Innovation and best practice designs can be encouraged through an effective design process. The award-winning Harbertonford Flood Defence Scheme was described as ‘the future of flood defence schemes’ by Sir John Harman, Chairman of the Environment Agency, and delivered a combination of in-village river channel improvement and upstream flood attenuation to alleviate regular flooding and the misery it caused. This paper focuses on how the design was developed and implemented to demonstrate the benefits that can be delivered through good teamwork and effective management of the civil engineering process.

Transport of mecoprop from agricultural soils to an adjacent salt marsh

Salt marshes are important ecological areas and play a significant role in coastal flood defence schemes. In many areas of the UK they are adjacent to agricultural areas utilised for the growth of cereal crops, for which mecoprop is used as a selective herbicide in the control of broad-leafed weeds. This study measured concentrations of mecoprop in soils, drainage ditch waters and sediments and salt marsh sediments over a period of 138 days following spring application. Soil concentrations of up to 1827 μg/g were recorded after application, which demonstrated a half life for mecoprop of from 9 to 12 days, with first order kinetics. However, a major rainfall event 9 days after application resulted in significant transport of herbicide to the salt marsh via subsurface field drains, drainage ditches and discharge sluice. Mecoprop concentrations of up to 386 μg/l observed in water samples were above UK guidelines.

Environmental impact assessments for flood defence schemes

Flood defence schemes, like most civil engineering infrastructure projects, can have a significant impact on the environment despite their intended benefits. Under the UK Environment Act 1995 and Land Drainage Act 1994, the Environment Agency, internal drainage boards and local authorities are required to contribute to the conservation of nature and heritage when carrying out their flood defence functions. Assessment of environmental implications is thus fundamental to design. Furthermore, environmental appraisal must also be carried out for all flood and coastal defence schemes undertaken with government grant-aid. This paper reviews the legislative framework and the processes for ensuring that flood defence schemes are sympathetic to their surrounding environment.

Environmental impact assessments for flood defence schemes

Flood defence schemes, like most civil engineering infrastructure projects, can have a significant impact on the environment despite their intended benefits. Under the UK Environment Act 1995 and Land Drainage Act 1994, the Environment Agency, internal drainage boards and local authorities are required to contribute to the conservation of nature and heritage when carrying out their flood defence functions. Assessment of environmental implications is thus fundamental to design. Furthermore, environmental appraisal must also be carried out for all flood and coastal defence schemes undertaken with government grant-aid. This paper reviews the legislative framework and the processes for ensuring that flood defence schemes are sympathetic to their surrounding environment.

An investigation of marginal habitat and macrophyte community enhancement on the River Torne, UK

During floodbank raising work as part of a major capital flood defence scheme on the River Torne between 1985 and 1990, selected reaches of the main trapezoidal channel were enhanced. By winning spoil from the channel margins and from borrow pits in the floodplain, a more varied marginal zone was created which maximised the potential habitat for wetland plant communities and their associated fauna. Enhancement comprised bank re-profiling to create narrow wetland shelves (berms), shallow bays, channel margins of varying shape and depth and linear still ponds from the borrow pits. The 1990 planting programme comprised 11 macrophyte species and a total of 7740 individual plants. This paper reports on an initial study to evaluate the marginal habitat enhancements on the River Torne 5 years after completion of the project. Lack of pre-scheme data necessitated a space-time substitution; enhanced river margins were compared with neighbouring reaches that had undergone conventional floodbank repair and remained as trapezoidal channel sections planted with a standard, low maintenance seed mix. Marginal vegetation was surveyed and supported by measurements of the physical habitat at 10 enhanced and 10 conventionally-engineered reaches. The macrophyte surveys and the results from the cluster analysis and polar ordination indicate that enhanced and conventionally-engineered reaches are floristically distinct and that the enhanced reaches have a more varied macrophyte community. The results from the Mann–Whitney U-tests show that enhanced reaches have significantly higher values of wetland species diversity and equitability, percentage of wetland species, bank width and soil moisture and significantly lower bank angles. However, the correlation and linear regression analyses did not show any strong associations between the physical habitat and plant parameters. Copyright © 2000 John Wiley & Sons, Ltd.