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BROAD SCALE TIDAL FLOOD RISK ASSESSMENT FOR LONDON USING MDSF AND FLOODRANGER

Tim Reeder Regional Climate Change Project Manager Environment Agency Bill Donovan ESPACE Development Officer Environment Agency Jon Wicks Associate Director Halcrow Group Ltd

ABSTRACT Understanding current flood risk and changes to flood risk over time periods of decades should be a key component in both flood risk management and spatial planning. As part of the European Union funded ESPACE (European Spatial Planning Adapting to Climate Events) project, a Decision Testing Framework has been developed which makes use of the MDSF and FloodRangerPro software tools to improve assessment, visualisation and stakeholder engagement of flood risk at the broad scale (eg catchment or whole estuary).

This paper describes the application of MDSF and enhanced FloodRanger to London and the Thames Estuary. The application provides an initial broad scale flood risk assessment for use within the Environment Agency’s Thames Estuary 2100 project which is developing a flood risk management plan for London and the Thames Estuary for the next 100 years.

The findings from the Thames Estuary application are generalised to provide advice on the appropriateness of the Decision Testing Framework (including MDSF and FloodRanger) within broad scale flood risk assessment. Key issues described include: interpretation of climate change scenarios, appropriate spatial resolution, broad scale flood risk management measures, need for modelling and relative importance of different types of input data.

Key words: Flood risk assessment, climate change, decision making, Thames Estuary

INTRODUCTION Climate change and its inherent uncertainties for the future presents a totally new challenge to decision makers in the broad field of spatial planning. In view of this the Environment Agency proposed the development of appropriate tools to simulate and test long term decisions in the light of changing future. This proposal was embedded as part of the formation of the ESPACE (European Spatial Planning: Adapting to Climate Events) project. The ESPACE research project is an ambitious four-year European project that aims to promote awareness of the importance of adapting to climate change and to encourage adaptation within spatial planning mechanisms at local, regional, national and European levels. Focussing on North West Europe, ESPACE is looking at how we manage our water resources and plan for a future with

1 Paper 3rd National CIWEM Conference a changing climate. It has been running now for nearly two years and is working with Dutch, Belgian and German partners to develop and compare tools for the future management of these issues.

The Environment Agency is piloting the development of decision making frameworks and tools through its Thames Estuary 2100 project, which is looking at the future of flood risk management for the Thames Estuary over the next 100 years. The initial phase of the research included an assessment of the requirements of the framework and a thorough review of a set of candidate tools. The review concluded that a suitable generic Decision Making Framework is provided by the UKCIP Decision Making Framework(1) as summarised in Figure 1.

Figure 1: UKCIP Decision Making Framework

DECISION TESTING FRAMEWORK AND TOOLS The review did not identify a specific decision-testing tool that is appropriate for all sectors, locations and scales. Instead, it recognised that there is a range of tools that may be beneficial for particular studies. For application to the TE2100 project, the following tools were identified for piloting on the Thames Estuary at the broad and local scale (Figure 2Figure 2) - the tools are described in more detail in subsequent sections of this paper:

 Source-Pathway-Receptor model to help identify the problem and objectives (as used in Foresight Future Flooding(2)).

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 IPCC/UKCIP climate change scenarios to define climate change scenarios and their impact on the sources of flooding (primarily sea level rise and surges)

 TUFLOW and ISIS hydraulic modelling software to convert changes in extreme sea level to water depths at the receptors (primarily properties and people)

 MSDF software to calculate flood risk (consequences x probability) by translating scenario-neutral water depths at receptors into economic flood damage and social impact

 Excel Workbook to post process results and map the scenario-neutral data to specific strategic options

 FloodRanger Professional software as a strategic option exploration and visualisation tool for stakeholders

Tidal Flood Risk Areas

London Thames Estuary Dartford Local Pilot Area

Thames Broad-scale Pilot Area Figure 2: Thames Estuary pilot area showing flood risk areas (‘embayments’)

The UKCIP Decision Making Framework provided clear structured guidance on decision making, highlighting both the sequential stages involved in decision making (stages 1 to 8 in Figure 1) and the need to iterate between stages (particularly stages 3, 4 and 5). Completion of the formal questions posed in the UKCIP Framework provide a very valuable audit trail which will encourage a systematic approach to the decision making and provide documentation suitable for stakeholder scrutiny.

The adoption of the Source-Pathway-Receptor (Figure 3) model helped to both identify the problem and objectives and to establish the decision-making criteria. Through the application of expert knowledge, a comprehensive list of risk components were identified and ranked. This process enabled the identification of tidal flood risk as the main ‘source’ of risk in the Thames estuary, and the resultant impact on properties and people as the main ‘receptor’ of this risk. The main ‘drivers’ were identified as climate change (increasing sea levels and surges) and land

3 Paper 3rd National CIWEM Conference development pressures in the Thames Gateway. ‘Responses’ were identified at two levels of detail: at the broad scale responses were represented in terms of generic strategic options (such as maintain existing defence system or maintain existing standard of protection); whereas at the local scale responses included specific defence level and defence realignment options. For the piloting the decision-making criteria were the identification of cost-effective flood risk management strategies for properties and people given future likely climate change scenarios over the next 100 years. (Note that full TE2100 project has a wider remit and will, for example, include environmental benefit in the criteria).

Drivers Processes that change the state of the system

System descriptors Sources Pathways Receptors System Risk rainfall fields, rivers people analysis sea level drains, roads properties economic storms floodplains infrastructure social etc flood defences ecosystems environmental flood storages etc etc etc

Responses Interventions that change the state of the system Figure 3: Drivers and responses can change the sources, pathways and receptors of risk

A distinctive feature of the UKCIP Decision Making Framework is the iterative application of stages 3 to 5 – assessing risk, identifying options and appraising options. This explicitly recognises that different approaches to risk assessment are required according to the level of understanding of the problem, structuring this approach through: risk screening; qualitative and generic quantitative risk assessment; and specific quantitative risk assessment. Within the Thames Estuary study area, the piloting focussed on the first two tiers of these stages at both the broad (Estuary wide) and local (Dartford embayment) scale.

For the risk assessment, a number of climate change scenarios based on IPCC SRES (Special Report on Emissions Scenarios) emissions scenarios were developed to provide a range of possible future tidal water levels to 2100. During piloting, it was recognised that UKCIP02 climate change scenarios(3) provided a good starting point for the development of these scenarios, but did not include consideration of key components of Thames estuary tidal water levels, namely, storm surge and tidal propagation. These two components were therefore added to the sea-level rise climate change estimates.

Generic quantitative risk assessment was undertaken through the application of the selected Decision Testing Tools. The principal tool used during this stage of the piloting was the MDSF (Modelling and Decision Support Framework) (4). This permitted the rapid estimation of direct economic damages associated with the

4 Paper 3rd National CIWEM Conference flooding of residential and commercial properties, and an estimation of the number of people affected by flooding. This tool was supported by the use of the ISIS 1- dimensional and TUFLOW 2-dimensional hydraulic modelling software applied to the study area to provide information on estimated flood extent, depth and rate of flooding. These data were further processed to enable the calculation of risk of loss of life based on a rate of rise in flood water criterion.

Importantly, the application of the MDSF decision testing tool enabled the wide evaluation of strategic options and the identification and appraisal of options that were robust to climate change impacts. This appraisal was undertaken iteratively at a broad-scale to filter strategic options. During this process, a scenario-neutral approach was undertaken to modelling and application of the MDSF decision testing tool. An initial matrix of modelling was undertaken independently of climate change scenario and strategic option. This initial matrix was subsequently mapped across to particular strategic options using an Excel Workbook (the computationally intensive inundation modelling was thus decoupled from the economic damage calculation and strategic option). Such an approach enabled a wide variety of strategic options to be considered without the need for each strategic option to be explicitly modelled (see Figure 4).

Figure 4: Scenario-neutral database of modelling results supports appraisal of strategic options

Once a limited number of strategic options had been identified, a further iteration of the appraisal stage was undertaken at a more detailed spatial resolution for the Dartford ‘local’ pilot area within the wider study area (see Figure 2). This iteration included the explicit modelling of strategic option scenarios, enabling both a comparison of scale and method to be undertaken.

Further stages of the UKCIP Decision Making Framework were not applied during this piloting as a full assessment of preceding stages using specific quantitative risk

5 Paper 3rd National CIWEM Conference assessment were not completed (ie the process stopped at step 6 of Figure 1). However, the development and trialling of ‘FloodRanger Professional’ as a visualisation and strategic option exploration tool was undertaken, both to assist with option appraisal and stakeholder engagement (Figure 5 shows example screen shots). The version of FloodRanger developed through the ESPACE project (called ‘Professional’ to differentiate it from the previous ‘educational game’ version) was able to import the MDSF generated Thames Estuary flood risk data (for current conditions, 2050 and 2100) and interpolate between these time slices to enable estimates of flood risks for 10-year time slices. The software concept is considered a significant innovation as it allows non-modellers to view outputs of potentially complicated modelling and risk assessment calculations in an intuitive and visually appealing software product. Further development of the concept is recommended to provide a simplified fit-for-purpose tool that will enable flood risk managers and other stakeholders to be able to assess, and to communicate to others, the positive and negative impacts of proposed development.

Figure 5: FloodRanger Professional visualises MDSF results for the Thames Estuary

SENSITIVITY ANALYSIS Sensitivity analysis was undertaken to assess the sensitivity of results to variations in input data and calculation method. The analysis found that significant ‘short cuts’ in the analysis are possible by identifying and focussing on the major contributors to overall risk. For example, for the local pilot study, less than 20% of all properties contributed over 90% of the risk (measured in terms of annual average economic flood damage). Thus, results are likely to be insensitive to sensible changes in data quality for most of the property data set and any effort to improve data quality should only address the major contributors to overall risk.

There are likely to be important ‘thresholds’ in the analysis beyond which there are changes in the relative importance

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of variability in input data. For example, the estuary wide annual average flood damage values are relatively insensitive to the sea level rise prediction changing from 7mm/year to 8.9mm/year for the strategic option to maintain a 1:1000 year standard of protection (damages increase by only 5%). However, the same variation in sea level rise for the ‘maintenance only – declining standards’ strategic option results in a 350% increase in flood damage.

Similarly, there are also likely to be important calculation method ‘thresholds’. An example of this is the potentially time saving assumption of expressing flood damage per embayment as only a function of relative water level (expressed as local river water level minus flood defence crest level). Sensitivity testing showed that while this relationship was valid for some river and defence levels, it became inappropriate for the most important river levels (and thus flood damage had to be related to both river level and defence level in the scenario-neutral database).

CONCLUSIONS The following conclusions can be drawn from the piloting of the Decision Making Framework and Tools on the Thames Estuary:

1. The application of the UKCIP ‘Risk, Uncertainty and Decision Making’ Framework provides excellent generic guidance and a set of procedures appropriate for assessing the impact of climate change on spatial planning. Despite its ‘UK’ title, it is appropriate for use throughout the ESPACE partner countries and outside flood risk management (eg for scarcity of water resources, threat to biodiversity, threat to water quality).

2. The Framework proposes an iterative and tiered approach to the assessment of risk, identification of options and appraisal of options. This enables a level of analysis that is appropriate to both the level of decision and the level of understanding of the risk problems and objectives.

3. The tiered approach is consistent with the development of the scenario-neutral approach to strategic option appraisal (as used in the broad scale piloting) which provides rapid quantitative estimates of risk. This approach enables the identification of sets of robust strategic options that can be further assessed using more detailed, scenario-specific quantitative methods (and the early screening out of ‘non-sensible’ options).

4. No single Decision Testing Tool will be appropriate for all studies. However it is likely that tools (ie structured methodologies and/or software products) will be required to:  Help identify the problem and objectives (eg Source-Pathway-Receptor)  Define appropriate climate change scenarios (eg IPCC/UKCIP)

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 Assess the impact of drivers and responses on risk using an appropriate level of scientific rigour (TUFLOW, ISIS, MDSF and Excel were used in the piloting)  Help communicate the consequences of action and lack of action to stakeholders (FloodRanger Professional was used in the piloting)

ACKNOWLEDGEMENTS The broad scale risk assessment was undertaken for the Environment Agency TE2100 team. Their help and encouragement is gratefully acknowledged although the views and opinions expressed in this paper do not necessarily reflect those of the Agency. The research in this paper was part funded by the European Commission Interreg 3c ESPACE (European Spatial Planning: Adapting to Climate Events) project. The authors acknowledge the contribution to the work presented in this paper by Kevin Morris (Discovery Software Ltd – developer of FloodRangerPro), Matt Scott and Rodolfo Aradas (Halcrow) and members of the TE2100 team.

REFERENCES (1) Climate adaptation: Risk, uncertainty and decision-making. UKCIP Technical Report. May 2003.

(2) Evans, E, Ashley, R, Hall, J, Penning-Rowsell, E, Sayers, P, Thorne, C and Watkinson, A (2004) Foresight. Future Flooding. Scientific Summary: Volumes I and II. Office of Science and Technology, London. 2004

(3) Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. DEFRA, Tyndall Centre and Hadley Centre. April 2002.

(4) Penning-Rowsell, EC, Evans, EP, Ramsbottom, DM, Wicks, JM, Packman, JC. Catchment Flood Management Plans and the Modelling and Decision Support Framework. ICE Proceedings, Civil Engineering, Vol 150, Special Issue 1, May 2002, 43-48.

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