Future impacts on sewer systems in England and Wales

1. Future impacts on sewer systems in England and Wales Overview of a project for Ofwat Presented to the Water Statistics Group Andrew Heather and Adam Grove29th November 2011

2. Objective • Ofwat commissioned the study to provide evidence from the water and sewerage industry on: • the possible long-term impacts on sewer flooding of population growth, climate change, and urban creep. • The study was commissioned to provide evidence on the potential scale of any change, not to propose solutions at this stage. • Future studies may investigate the costs and benefits of different approaches to mitigating any increases that arise – using either traditional solutions or ‘softer’, potentially more sustainable solutions. • Our report was published by Ofwat on its internet site:http://www.ofwat.gov.uk/sustainability/climatechange/rpt_com201106mottmacsewer.pdf

3. Introduction 2010 2035 Flow Current demand Populationgrowth Urbancreep Climatechange Futuredemand [Illustrative] How will population growth, urban creep, and climate change influence future demand on the wastewater network?

4. Introduction 2010 2035 Flow Current demand Populationgrowth Urbancreep Climatechange Futuredemand [Illustrative] How certain are the forecasts?

5. Introduction Flow 2035 2010 [Illustrative] How much certainty is there in the forecast? Could demand decrease?

6. Modelling simulations • A workshop was held with all ten water and sewerage companies and Ofwat, to agree modelling scenarios for the study: • Companies nominated 15 catchments, from which MM selected 10 • 10 catchments each, selected over full range of populations • 1-3 rainfall scenarios: low (where time), most likely, high (where time) based on UKCP09 • 1 population growth scenario based on Local Plans and Office of National Statistics figures • 1 urban creep scenario based on UKWIR report • 1-3 combined scenarios as above : low (where time), most likely, high (where time)

7. Output for Models • 100 models, up to 6 runs per scenario • For each scenario, only analysed: • Worst change in total flood volume • Worst change in number of flooded manholes • Plotted % change in rainfall/population/property creep v % change in flood volume/nr of flooded manholes

8. Decreasing frequency Intensity Duration Rainfall characteristics Illustration of the typical relationship between rainfall intensity, duration and return period (event frequency). Under some climate change scenarios, the return period of events may shorten, so that high-volume storms occur more often.

9. Scenario set-up We hoped companies would cover all the scenario combinations but there were insufficient data to allow some scenarios to be modelled. For example there was no distribution of the projections for population or urban creep. Is this a topic for future research or method development? The industry appears to lack a probabilistic approach to population and urban creep forecasting – both of which are highly uncertain and so would benefit from the technique

10. Models run • Planning horizon: 2040 or nearest available year in change scenario forecast • Growth - one population growth scenario based on Local Plans and Office of National Statistics figures • Urban Creep – one urban creep scenario based on UKWIR report • Climate Change – medium emissions scenario was adopted. Scenarios were run for 10th, 50th and 90th percentile storms. • Summer and winter scenarios were run for each of the three factors and for climate change, for each of the three percentiles. • The percentage change in rainfall/population/property creep was plotted against the change in flood volume or number of flooded manholes for each scenario.

11. Coverage of models run • Catchment sizes ranged from 150 – 2,500,000 • Total population covered 8.87 million – 16.2% population England and Wales • Development assumptions increase the population to 10.87M by 2040 (+16.2%) • In the models, “flooding” included water ponding above ground, and water escaping from the system to flood above ground.

12. Results: Combined scenario 50%ile Winter Figure i: Results of the combined scenario with 50th percentile winter rainfall, ranked in order of the increase in predicted flooding

13. Results: current summer rainfall with growth 100 Change in flood volume % 0 50 0 % growth

14. Results: current winter rainfall with creep 100 Change in flood volume % 0 20 0 Creep %

15. Results: 50th percentile climate changewinter rainfall 100 Change in flood volume % 0 14 0 Uplift %

16. Results: 50th percentile climate changewinter rainfall • In virtually all cases the model runs show that significant increases in predicted flood volumes and flood locations occur. • The top two lines of the table show that in 58 catchments (i.e. 41+17), the climate change scenario at 50th percentile has the greatest effect on flooding, after the combined scenario. • Climate change had the smallest predicted impact on flooding in six of the 80 catchments included in the ranking.

17. Cumulative effects • The chart shows results for the catchments where All>50%ileCC>Creep>Growth • The combined effects are more than the sum of the individual effects • This is because each effect reduces capacity in the network • Once the network is full, flooding increases in proportion to additional water input 100 Change in flood volume % 0

18. Results – summary of all results Summer and winter • These tables summarise the results of all the model runs. • Taking the year as a whole, some catchments showed no change in flooding under the 10th percentile scenario. • But looking at winter scenarios alone, none showed no increase. • Therefore any potential reduction in summer rainfall may be offset by increases in winter rainfall and its associated flooding Winter only

19. Challenges • Timescale • Addressed by company use of contractors to run the models • The timescale limited the study to models already in good condition. Potential for bias to ‘troublesome’ catchments? • Some scenarios, such as developing a probabilistic approach to population growth and urban creep, were not possible in the timescale of this project • Consistency of approach • Addressed through the method statement and company visits • Forecasting number of properties flooded • Cannot forecast accurately the increase in DG5 properties because flooding is very sensitive to local topography • Analytical approach • On balance it would be unwise to extrapolate to the rest of England and Wales • Catchments are unique and so it is not appropriate to assume that results for one catchment would apply in the same way to another • But the overall conclusions are likely to stand in most catchments

20. Conclusions • The combined scenario has a greater effect than the sum of individual changes • For the individual changes, the order of influence, on average, appears to be: • Climate Change • Urban Creep • Growth • The extent of change varies by company, but with no clear geographic trend. • There is a wide range of uncertainty but the results are logical: more water in = more water out!

21. Andrew.Heather@mottmac.com Charles.Wilson@mottmac.com www.mottmac.com