1. IMPACT 3-5th November 20044th IMPACT Project Workshop Zaragoza 1 Investigation of extreme flood Processes and uncertainty IMPACT Investigation of Extreme Flood Processes And Uncertainty

2. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 2 Some key points (Wed / Thu) Firstly, Discussion covers a fraction of the issues Lot of work done; lot of results presented; lot of comparisons made and conclusions started......but we do need to push to complete the final step of drawing conclusions from this work

3. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 3 Some key points (Wed / Thu) Mesh size –This is not a simple problem! Not a simple solution - depends upon a number of factors : numerics, flow path, topography, urban areas, etc –Different solution for different models –Depends upon ‘end user’ application –Detailed analysis of different model schemes required to give answer

4. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 4 Some key points (Wed / Thu) Roughness selection –Tous: 2 m variation in wl in town for n 0.050 / 0.100 –How is n chosen? –Estimation of n not same for extreme flows as ‘normal’ river modelling depth here 8-10 m; orange trees submerged effective n varies with flow depth... –Guidance on selection for extreme events required –Effect > mesh size

5. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 5 Some key points (Wed / Thu) Influence of inflow hydrograph shape –Lower peak, longer duration --> flow volume gives higher flood levels than short duration higher peak discharge –Implications for uncertainty analysis approach (upper / mid / lower - upper not necessarily highest flood level) –Implications for typical approach of using Qp from hydrograph or Qp equations

6. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 6 Some key points (Wed / Thu) Influence of inflow hydrograph shape –? Accuracy of given Tous hydrograph compared to upper / mid / lower (u/m/l) hydrographs –u/m/l shape gave better fit to some field observed data… –What really happened? –How does interaction of structure with embankment breach affect hydrograph shape significant if hydrograph shape important to determine worst case water levels

7. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 7 Some key points (Wed / Thu) Time of arrival –Seems to be notable differences between models – up to 50 min - what’s missing? Sediment?

8. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 8 Some key points (Wed / Thu) Topography uncertainty –Notable difference in model predictions for different topography (1982 & 1998) –Which is correct? Neither? What sediment movement really took place? Difference > 8m in places –If these are indicative of magnitude of changes, then this notably affects predictions –Effect > mesh size

9. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 9 Some key points (Wed / Thu) Compare like with like –Not all modelling results are comparing same data different mesh --> different points of comparison model differences not immediately obvious –To compare models : water levels –For end users : compare water depths

10. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 10 Some key points (Wed / Thu) End Users: there are many different end users, each with different perspectives (Politician to Commercial Client) –One solution does not fit all –Approach will vary from user to user –Type I and Type II scenarios (a wrong decision that means money was spent and subsequently found to be unnecessary is unfortunate; a wrong decision where money was not spent and subsequently found to be needing is unacceptable) –How does an end user evaluate an expert / model when they are not calibrated?

11. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 11 Some key points (Wed / Thu) End Users: emergency planning –Current approach is to adopt a conservative value where uncertain of the modelling results

12. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 12 Some key points (Wed / Thu) Urban flow simulation: –Best approach depends upon flow conditions –Slow flood - all approaches give reasonable results –If computing power there, exact definition of buildings and urban features –Are models reproducing rapid flow changes correctly?

13. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 13 Some key points (Wed / Thu) Future development of propagation models –Models are sufficiently complex; focus on increasing density of calculation rather than complexity of maths to see if different model solutions converge results from detailed lab modelling good; need to apply same resolution to real case (grid <1m) 2nd order models seem to give little improvement on 1st order (for large-scale real cases) –Focus on roughness modelling –How much further can the basic equations be taken?

14. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 14 Some key points (Wed / Thu) Future development of propagation models (cont) –Should focus now be on helping the manager? –No point in increasing model resolution unless topo data resolution (and cost) to match

15. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 15 Some key points (Wed / Thu) Modelling approach: –1D first, define need for 2D areas, mesh refinement etc –Who will pay for such an approach? –Cost versus detail –If end users does not understand or know, then cannot make a judgement on paying...

16. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 16 State of the art Geophysics –Promising approach, needs pilot application Flood propagation –Models provide good prediction for water level –Arrival time to be improved –Roughness modelling

17. IMPACT Investigation of extreme flood Processes and uncertainty 3-5th November 20044th IMPACT Project Workshop Zaragoza 17 State of the art Sediments –No general agreement about physics / equations –Important improvement of models in IMPACT –Still at research level Breaching –Situation between Flood prop and Sediments –Important and unique data set –Current models can be improved