Water

Abstracts from the 37th Flood and Coastal Management Conference (Sept. 2002)

Please see the main Conference page for details of ordering Conference proceedings or individual papers. Please note that the views expressed in Conference papers are those of the speaker and do not necessarily represent Defra views or Government policy.

Number: 01-1

Title: Design Standards of Protection: a snare and a delusion?
By: Colin Green (Flood Hazard Research Centre, Middlesex University)

Abstract: I shall argue that it is time to stop thinking in terms of design standards of protection. First, because we should be thinking about how to manage all floods and not just some. Thus, the important part of the loss-probability curve is the zone which covers floods more extreme than the design standard. In this zone, the different possible solutions perform very differently. Secondly, it promotes a spurious sense of certainty when the one thing about which the rational individual can be certain is that the future is uncertain. Our estimates of the return period of a flood of a given magnitude are likely to be in error: therefore, we need to explore how we should make decisions when we know that uncertainty is inevitable and also what type of options we should adopt in those circumstances. Thirdly, it is based upon a static view of the world when a dynamic approach (e.g. a consideration of antecedent conditions, climate change) is more appropriate. We need therefore to adopt adaptive management approaches so that adjustments can be made as change occurs. So, thinking in terms of design standards of protection leads us in precisely the wrong direction.

Number: 01-2

Title: A Comparison of Flood Defence and Other Public Sector Investments
By: Robin Smale (OXERA)

Abstract: This is a paper about flood defence and coastal protection and the similarities with other public investments, particularly utility services. There are strong parallels to draw between them, and lessons from the regulation of utility services with an application to flood defence. This is a sufficient topic for a short presentation. There is another topic that could have been presented under the same billing: the comparison of flood defence with services that remain within the public sector, such as health services, the police force, and education. Perhaps someone will take up that challenge at next year’s conference. The similarities with utilities are both in the activities—the maintenance and enhancement of a network of assets—and the drivers, such as increasing load on the system and public demand for improved services. Since privatisation, utilities such as the water industry have experienced a rapid evolution of their management, structure and regulation. In comparison, flood defence activities have changed little, so far, since the creation of the Environment Agency (EA). It may be that some beneficial changes of the utilities could be transferred to the management, structure or regulation of flood defence. It may even be that the utilities’ experiences could be introduced through their direct involvement in the delivery of flood defence services. The second reason is that there is a debate, in the Spending Review, over the proposed increase in the rate of investment in flood defence. On the one hand, the Environment Agency, Defra and the Association of British Insurers have put together cases for increased investment; on the other hand, HM Treasury will apply tests of value for money and efficiency of spending relative to competing calls on public funds. The case for increased flood defence expenditure has been made in a public document called the ‘National Appraisal of Assets at Risk’. This allows comparisons to be drawn between the business cases prepared by utilities, checked by appointed reporters, and challenged by regulators, and the flood defence document. Not least, this conference provides the opportunity to explore some issues that were not discussed in the first Financial Management and Performance Review of the Environment Agency. This paper covers the following issues, in the order listed below: Public-sector investments and why they are made; types of comparison available; lessons from utilities; detailed comments on the case for investment in flood defences.

Number: 01-3

Title: PFI for Flood Defence: Broadland flood alleviation project
By: Bernard Ayling BSc (Hons) CEng MICE (Environment Agency), James Rowntree BEng (Hons) CEng MICE (Halcrow Group Ltd), Bob Lancastle BEng (Hons) CEng MICE (Edmund Nuttall Ltd)

Abstract: The flood defences of the Norfolk Broads extend for some 240km and protect approximately 21,000 hectares of low-lying land, including 1,700 properties, from predominantly tidal flooding. Broadland is an area of unique environmental value with National Park status and is recognised as a wetland of international importance for nature conservation. In 1997 the Environment Agency identified the future maintenance and improvement of these flood defences as a candidate for the Private Finance Initiative. In 2001, Broadland Environmental Services Ltd (BESL), a joint venture between Edmund Nuttall Ltd and Halcrow Group Ltd, was awarded the 20 year commission to deliver a comprehensive service for flood defence. This includes maintenance, emergency response, design & improvements.

Number: 01-4

Title: Thorngumbald: the route to approval
lBy: Philip Winn (Environment Agency), John Pygott (Environment Agency) Abstract: The Environment Agency has been working on the development of a long-term flood defence strategy for the Humber estuary since 1997.

Abstract: It was clear from the beginning of the strategy development process however that some defence improvement and repair work could not wait for the long-term plan, and so an ‘urgent works’ programme was also initiated. Thorngumbald on the north bank of the Humber near Hull and Pyewipe (Urgent Works 15 to 17) on the south bank near Immingham were identified early on as sites needing such attention. Defence improvement at Thorngumbald offered great opportunities, but there were also serious constraints. Setting back the defences would create much new habitat, countering past habitat losses. Potentially the scheme would provide compensation for other projects where foreshore and habitat loss was unavoidable. Managing the land purchase and Habitats Regulations issues with the site itself within the SPA were the chief constraints. After a lengthy development process, the scheme was approved last year, and construction has begun. The old banks on the estuary shoreline will be breached in 2003. The paper describes the approach that was adopted and charts the decisions and actions taken through to approval for this scheme and for a linked scheme at Pyewipe on the south bank of the estuary. Thorngumbald now shows the way for the likely future estuary-wide Humber setback programme, and the lessons from it will assist setback projects elsewhere in the country.

Number: 01-5

Title: National Flood and Coastal Defence Database
By: Trevor Linford, Yolande Jones, Julia Barrett and Grant Butler (Environment Agency)

Abstract: The paper will set out the background to the development of the National Flood and Coastal Defence Database (NFCDD), the objectives of the project and how those objectives have been realised. The paper will also set out the information that the database is designed to hold and the uses that can be made of that data utilising the functionality of the system. A key element of the project is that the database holds data from all the operating authorities and not just the Environment Agency, the paper will therefore cover the involvement of the other operating authorities and Defra in developing the system. It will also deal with how web browser technology is being used to make the system available to all those operating authorities facilitating both data input and access to data stored on the system. Finally the paper will deal with potential opportunities for further enhancements and developments. The paper will contribute to the aims of the conference by showing how NFCDD has been developed to become a national repository of core data relating to flood defence that can be added to and accessed with ease by all operating, however large or small. This data will be a national resource that will assist in the future management of flood and coastal defence.

Number: 02-1

Title: Environmental Change Indicators (ECIs) to Assist Flood and Coastal Defence
By: Frank M. Law (independent hydrologist), Frank A.K.Farquharson (Centre for Hydrology and Ecology Wallingford)

Abstract: The Central England Temperature index is well known for tracking one element of climate change. However there are many environmental measures of changing catchment and coastal conditions that could assist flood defence managers more directly. A current Defra project [Defra Ref. SLD 1753, under the Risk Evaluation and Understanding Uncertainty Theme FD 2311] is exploring possibilities more fully with stakeholders. Direct measures of possible change are preferable, and in 2001 the National Flood Warning Centre was recommended by its consultant to track POT3 i.e. the flood flow threshold that is exceeded by an average of three flood peaks per year at gauged river sites. At a February 2002 inception workshop on this project the value of a wide range of candidate ECIs was debated, and a short list accepted for initial trials. This paper defines some of these parameters, covering measures of both inland and coastal threats. The importance of historic back calculation is discussed, as is the need for public domain presentation of such indicator series.

Number: 02-2

Title: UKCIP02 Climate Change Scenarios: Implementation for flood and coastal defence
By: Dr Peter Hawkes (HR Wallingford Ltd), Dr Steven Wade (WS Atkins), Dr Nick Reynard (Environment Agency)

Abstract: The UK Climate Impacts Programme released new future climate scenarios in April 2002, including temperature, soil moisture, rainfall, runoff, sea level rise, wind speed and wind direction, based on global and 50km grid regional climate modelling at the Hadley Centre. These data provide a consistent source of information for use in UK climate change impact studies. HR Wallingford and WS Atkins were commissioned through the Defra / Environment Agency joint R&D programme, under the Risk and Uncertainty theme, to determine how UKCIP02 could be applied effectively and consistently in UK flood and coastal defence studies. The main strands were: consultation with users to assess current practice and requirements of climate change scenarios; comparison of the content of UKCIP02 with the requirements; best practice guidance for consistent and appropriate use of the scenarios by the flood and coastal defence community; recommendations for further research to extend scope for use within the flood and coastal defence community (e.g. waves and river flows) and facilitate take-up. The paper summarises some of the results from UKCIP02 and introduces the implementation for flood and coastal defence.

Number: 02-3

Title: Risk, Performance and Uncertainty in Flood and Coastal Defence - a defining review
By: Ian Meadowcroft AMICE (Environment Agency), Paul Sayers CEng MICE (HR Wallingford), Jim Hall PhD CEng MICE (University of Bristol)

Abstract: The language is changing - from flood and coastal defence, to flood and coastal risk management. This reflects a real change in the way policy is applied. Risk management shifts the focus to outcomes: decisions are made based on assessment of the impacts of flooding and erosion, together with the likelihoods of those outcomes. This unlocks the potential for a more integrated approach to risk management in which a whole range of co-ordinated activities is planned and implemented. For widespread and consistent uptake of risk methods, it is essential to have in place information giving clear and well-founded advice on the concepts, definitions and tools which are common for all applications. A defining review has been carried out (R&D Project FD 2302) which describes the language of risk, performance and uncertainty, and the tools available to decision-makers (Defra / Environment Agency, 2002). The review looks forward to ways in which risk and performance assessment can support more integrated decision-making. For risk methods to be taken up and used more widely the benefits must clearly outweigh the costs. We believe the main challenge now is to develop practical guidance and tools which genuinely support a wide range of decision-makers, and information needs, within a coherent framework for risk, uncertainty and performance.

Number: 02-4

Title: Performance in the Design and Management of Flood and Coastal Defence Systems
By: Jonathan Simm (HR Wallingford), Jim Hall (University Of Bristol), Paul Sayers (HR Wallingford), Andrew Bradbury (New Forest District Council)

Abstract: Performance evaluation is important for Flood and Coastal Defence, in order to: audit investment efficiency and demonstrate achievement of Best Value; provide insights for effective future monitoring and management; identify lessons for future practice on similar projects elsewhere. Performance is influenced by both the human system which organises and delivers flood and coastal defence and the natural system of loadings and responses. The concept of performance is hierarchical and is applicable at all levels - from policies, plans and strategies to schemes and operations. Defra will be publishing FCD PAG6 , which will establish principles and promote best practice in performance evaluation. This paper presents concepts and definitions upon which FCD PAG6 will be based and addressed, with the aim of promoting discussion, some of the issues that will be tackled in FCD PAG6.

Number: 02-5

Title: A New Conveyance Estimation System
By: Dr Paul G Samuels (HR Wallingford Ltd), Dr Mervyn E Bramley (Environment Agency), Edward P Evans (Independent Consultant)

Abstract: The estimation of conveyance is a core component of flood management, water level prediction and flood defence design. All river modelling software includes one or more methods for conveyance estimation, usually based upon methods dating from research completed more than 50 years ago with little or no account taken of recent advances in knowledge and understanding. In 2001 the Environment Agency commissioned a scoping study on reducing uncertainty in conveyance estimation. The paper describes some of the conclusions of that scoping study including the needs of different users, the diversity of current knowledge and provides the outline for a targeted programme of research to make a step forward in the management of river capacity. This Targeted Programme is now underway involving a partnership between academic researchers, experts and users. Particular issues of concern are the effects of riverine vegetation, the influence of natural shaped (and re-naturalised) channels and the interaction between river channels and flood plain flows. The output from the programme will be a new Conveyance Estimation System (CES) as open code and enabled for use within modelling packages. The CES includes a roughness advisor to provide access to the wide body of knowledge on the estimation of river resistance.

Number: 03-1

Title: Regulated Tidal Exchange: intertidal habitat without the tears
By: John Sharpe (RSPB), Graham Hirons (RSPB), Jeff Kew (RSPB), Rob Pilcher (RSPB), Wendy Brooks (Environment Agency), Curt Lamberth (Haycock Associates)

Abstract: Coastal habitats are under threat from erosion, which will accelerate due to sea level rise and climate change, leading to losses of ‘intertidal’ habitats and their flood defence and environmental functions. Regulated Tidal Exchange (RTE) presents an opportunity to offset some of these losses. The paper explains the technique, identifies where RTE has been used and the benefits that have resulted and suggests further applications. RTE uses pipes or sluice gates to allow regulated tidal flushing to create saline/brackish habitats behind sea walls. It is a potentially valuable tool in two particular scenarios: where coastal defences are likely to remain in place for the foreseeable future; and as the first phase of a longer term realignment strategy. A small number of projects have been developed in the UK but there are examples of larger projects overseas in the Netherlands, Germany and the US. To assess the potential value of RTE, we have identified potential areas for intertidal habitat creation around Britain, considered the potential ecological benefits, identified practical considerations of design, operation and effectiveness and incorporated lessons leant elsewhere, including by visiting sites in Delaware and Connecticut. Our work concludes that it is possible to create viable saline habitats using RTE in the UK and that practical demonstration sites are now needed.

Number: 03-2

Title: Environmental Enhancement in Flood Defence: a case study in historic Chepstow
By: Richard G Samphier CEng MICE and Nicholas J Rowson MLI MIHort (WS Atkins Consultants Ltd)

Abstract: The historic town of Chepstow is situated on the River Wye. It has been subject to frequent tidal flooding with event records dating back to 1883. The 3 hectare flood risk area includes 80 houses and small businesses. Whilst the economic case for flood defence was clear, the environmental value of the valley raised challenges in implementing an appropriate scheme:

  • The river corridor is designated for its landscape and ecological value.
  • A public park and riverside walk offer amenities which would have been compromised using a traditional approach.
  • The fabric of an historic bridge could have been affected.
  • Striking views across the river could have been interrupted.
  • Parking and traffic flow was an important local issue.

An integrated team of engineers and landscape architects was tasked with developing a combined technical and environmental solution. This team challenged traditionally accepted constraints, for example whether roads could be closed to improve the defence alignment. Ultimately, innovative materials such as laminated glass were used to overcome aesthetic problems. The resulting scheme has fully met the technical requirements for flood defence. In addition, it has provided an enhanced and sustainable traffic-free riverside park area. The valley’s aesthetic value and the historic bridge have been preserved in full. The public perception of the scheme is one of an environmental enhancement rather than an engineering necessity. Approaches developed for this project and the lessons learned can be applied to sensitive sites across the country. Such practise can deliver unobtrusive flood defence where previously this had not been thought possible.

Number: 03-3

Title: Vegetation in Channels: good or bad?
By: David Noble, CEng FICE FICWEM FIAgrE (Chief Executive of the Association of Drainage Authorities)

Abstract: I have an obsession with trees, I never fail to be impressed by the visual quality of a tree lined river which embodies the very best of the English landscape pursued by artists, photographers and the casual observer. In contrast the obsession becomes one of dislike when trees are seen within the profile of river and drainage channels, the presence of which inevitably increases water levels during higher flow periods and will, in some situations, unnecessarily increase or put at risk adjacent and upstream property and land.

Number: 03-4

Title: Achieving Biodiversity Gain from Sustainable Flood Management
By: David Withrington (English Nature)

Abstract: The thesis of this paper is that flood management needs to work with nature to be sustainable. The challenge is to find technical solutions, strategic catchment approaches and an effective policy framework to deliver benefits to both biodiversity and flood defence. The paper examines recent policy developments in this area, gives examples of good practice on the ground and sets out the challenges for the future.

Number: 03-5

Title: The Wise Use of Floodplains
By: Richard Horrocks (Environment Agency), David Collins (Defra)

Abstract: The Wise Use of Floodplains Project was a trans-national partnership based in six case-study areas throughout England, Scotland, Ireland and France. It was supported by the European Union LIFE programme and involves fifteen partner organisations. The object was to demonstrate how floodplain wetlands can contribute to the sustainable management of water resources within river basins. Floodplain management and restoration are potential measures under the Water Framework Directive (WFD). This aims to implement the integrated management of river catchments throughout Europe to achieve ‘good status` or better for aquatic environments and sustainable water use. The case studies focused on public participation to develop local action plans and analyse potential barriers to change at local, national and European levels. The project produced a series of technical reports covering public participation, hydrological modelling, sustainability appraisal and economic issues. The experience that has been gained, from the case studies and from examination of practice across Europe, contributes to the development of best practice for the management of our rivers and is timely given the introduction of Catchment Flood Management Plans and the WFD.

Number: 04-1

Title: Local Flood Protection - First and Foremost
By: Pam Bowker (HR Wallingford), Mervyn Bramley (Environment Agency)

Abstract: The objective of the paper is to present an overview of the development of an integrated approach to local flood protection brought about by the collaboration of all stakeholders following the Autumn 2000 floods. The paper discusses current practices of all stakeholders who are involved in the promotion, development, implementation and use of local protection techniques and products and explains how these stakeholders have come together in a research project to raise the standards of flood protection and promote best practice within the industry.

Number: 04-2

Title: Ensuring Safe and Effective Use of Temporary and Demountable Flood Protection Systems
By: Fola Ogunyoye, BSc MSc CEng (Posford Haskoning Limited)

Abstract: The use of non-permanent forms of flood protection can provide much needed flexibility and increased opportunities for effective flood management within an increasingly complex environment. This realisation has created the present scenario where an increasing number of temporary and demountable flood protection products are available or are being developed, each with their own innovative features and to differing standards and quality. As with All flood protection systems, the consequences of failure can be significant. There is therefore a need for guidance to support developers and potential users in its appropriate development and use. This paper describes a risk-based process for choosing appropriate systems for various scenarios and considerations to ensure their effective and safe use.

Number: 04-3

Title: Shrewsbury Flood Alleviation Scheme: working together
By: Simon Lewin CEng MICE MCIWEM, Scott Nield CEng MICE and Stuart Ryder DipLA MLI (Binnie Black & Veatch / Ferguson & McIlveen); Tim Ive Ceng MICE (Environment Agency)

Abstract: The aim of this paper is to discuss the challenges met on implementing a flood alleviation scheme in a tight urban setting, within a short timeframe. The paper also aims to pass on knowledge to other professionals, giving examples of lessons learned, which may be of benefit to those currently working on flood alleviation schemes, or to those who may be embarking on future schemes.

Number: 04-4

Title: Risk Assesment for Flood and Coastal Defence Strategic Planning (RASP)
By: Paul Sayers CEng MICE (HR Wallingford), Jim Hall PhD CEng MICE (University of Bristol), Richard Dawson (University of Bristol), Corina Rosu PhD (HR Wallingford), John Chatterton PhD, (John Chatterton and Associates), Rob Deakin BSc (Halcrow Group Ltd)

Abstract: It has long been recognised that flood risk cannot be eliminated completely - but understanding the risks is key to improving risk management. In particular, this means deciding on risk management actions such as:

  • Construction of new defences where they are most needed.
  • Maintaining and operating defences and defence systems to minimise risk.
  • Flood forecasting and warning to minimise the consequences in the event of flooding.
  • Restricting development in flood and erosion-prone areas to control the impacts.

Risk assessment is rapidly becoming the basis for decision-making in all of these areas. It is also being used to support policy development to address strategic or overarching issues such as:

  • What is the appropriate level of spending on flood and coastal defence to ensure risk is reduced, including the possible effects of climate change?
  • What combination of risk management measures provides the best value?
  • What is the 'residual risk' remaining after all risk management measures, and is this acceptable?

To better understand flood risks and improve the performance of flood defences it is necessary to consider the performance of systems of defences rather then merely considering single defences in isolation. If, for example, a town is protected by several different defences then it is necessary to consider how this flood defence system functions as a whole in order to assess and manage the flood risk to the inhabitants and assets in the town. With moves towards more integrated flood management, it is essential that risk managers have recourse to sound and practical tools and techniques for assessing the performance of whole systems in order to develop balanced, integrated risk management strategies.

This paper describes the progress of an R&D project titled Risk Assessment for flood and coastal defence for Strategic Planning (RASP) funded through the Risk Evaluation Understanding of Uncertainty Theme of the joint EA/Defra research programme. The RASP Project aims to develop and demonstrate methods for supporting Integrated Flood Risk Management through the development and demonstration of methods for assessing the performance and risks associated with systems of linear flood defences (see Figure 1). The focus of this paper is a high level methodology, which is designed for application on a national basis. More detailed methodologies will follow over the next two years.

Number: 04-5

Title: Managing Coastal Risk: Developing Shared Responsibility
By: Rhoda Ballinger (Cardiff University), Jane Taussik And Jonathan Potts (National Maritime Museum)

Abstract: This paper presents key findings of a recent study conducted for the Local Government Association’s (LGA) Coastal Issues Special Interest Group. The study, focusing on the interface between shoreline management and the statutory planning system in England and Wales, identified ways in which integration could be fostered within the existing legislative and policy framework. Informed by a workshop and interviews with a range of stakeholders, the study identified critical issues currently impeding integration. These include institutional, policy and organisational shortcomings as well as issues relating to knowledge, awareness, training, information, research, resources and wider planning matters. An overview of these issues is provided along with a series of recommendations to improve liaison and co-operation between planners and engineers, particularly at local levels.

Number: 06-1

Title: "Yet More Plans?" The Benefits for Catchment Flood Management Plans
By: Alison Baptiste, MSc CEng MICE MCIWEM (Environment Agency), Richard Harpin PhD CEng MICE MCIWEM (Halcrow Group Ltd), Trevor Linford BSc CEng MICE (Environment Agency), David Richardson MSc CEng MICE (Defra, Flood Management), Chris Wotherspoon MEng CEng MICE MCIWEM (Babtie Group)

Abstract: Catchment Flood Management Plans will be the foundation stones of all future fluvial flood management work. Not only will they give a strategic view of how catchments behave but they are enabling a more co-operative way of working by bringing together all the relevant operating authorities to produce the plans and then to work to them! The CFMPs are seeking to take a long-term view (50 years) of how flood risk can be mitigated taking into account factors such as climate change, land use and social pressures. They will identify "hot spots" for more detailed studies and investigate possible flood management measures such as attenuation through upstream storage, maintenance enhancements, changes in land use etc. The Agency has commissioned five pilot CFMPs across the country to determine best practice. The aim of this paper will be to draw on the experience of the pilots and give a brief summary of the findings and lessons learned for future CFMPs.

Number: 06-2

Title: Catchment Flood Management Plans in Action: the Medway pilot study
By: Andrew Burton BEng CEng MICE MCIWEM MIEnvSc (Babtie Brown & Root JV)

Abstract: Catchment Flood Management Plans aim to provide a high level, holistic understanding of flooding and flood processes at a catchment level. Flood risk management measures and policies are investigated and possible climate and land use changes are accounted for. A pilot study programme, consisting of plans for the Yorkshire Derwent, Irwell, Parrett, Severn and Medway catchments commenced in early 2001. The Medway is the largest river basin within the Southern Region of the Agency, and contains a wide range of geologies and terrains. It is predominantly rural but has significant development, ecological and social pressures. Maidstone and Tonbridge are major urban areas at risk of flooding. A multidisciplinary approach has been adopted to undertake the Medway study by establishing a team of engineers, modellers, and environment and planning specialists. Significant consultation has been undertaken and innovative methods have been established and tested in key areas, including hydraulic modelling, geomorphology and multi-criteria matrix assessments. The Medway study has assessed flood management policies and measures, and has identified the focus of future Agency strategy plans. A commentary on the success of the methods and approaches adopted is emphasised, together with key lessons learnt from the pilot study approach and recommendations for future CFMPs.

Number: 06-3

Title: The Modelling and Decision Support Framework for Catchment Flood Management Plans
By: David Ramsbottom (HR Wallingford Ltd), Peter Von Lany (Halcrow), Edmund Penning-Rowsell (Flood Hazard Research Centre)

Abstract: A programme of Catchment Flood Management Plans (CFMPs) has been launched for England and Wales. The purpose of CFMPs is to identify a preferred policy for managing floods in each catchment together with possible measures (for example, flood defences, etc) to achieve the policy. In order to facilitate the production of CFMPs, and provide a consistent approach, a Modelling and Decision Support Framework (MDSF) has been developed. This paper contains an outline of the MDSF and provides some insight into the approaches adopted for policy evaluation and uncertainty estimation.

Number: 06-4

Title: Catchment Flood Management : the challenge for modellers
By: Rob Cheetham BEng (HR Wallingford Ltd), Dr Roger Bettess BSc PhD DipMgmt MCIWEM (HR Wallingford Ltd), John Packman BSc MSc (Centre for Ecology and Hydrology), Dr Paul Samuels BSc PhD MICE MCIWEM (HR Wallingford Ltd)

Abstract: It is recommended in the MDSF Procedures that in general rainfall runoff flood routing models provide the best compromise between the difficulty of catchment scale modelling and provision of the necessary detail of output. The present generation of routing models, however, do suffer from a number of short-comings and these are discussed in this paper. For a catchment it is not possible to define a single set of inputs that will provide a T-year event throughout a catchment. As a result the MDSF Procedures suggests the use of different design events for different parts of the catchment. An alternative approach is to use continuous simulation in which long time-series are used in the hydraulic model and the results analysed to determine the T-year peak discharge at any location. Though this method is conceptually attractive there are still many issues that need to be resolved and these are discussed in the paper.


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