Michael Cranston, Claire Harley, Lauren McLean, Amy Tavendale, Darroch Kaye, and Lisa Naysmith (Flood Forecasting and Warning section)

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1 Firths of Forth and Tay Flood Warning System and the Storms of Keming Hu Acknowledgements Michael Cranston, Claire Harley, Lauren McLean, Amy Tavendale, Darroch Kaye, and Lisa Naysmith (Flood Forecasting and Warning section) Paul Hart, Chris Whitlow and Clive Powis (Royal HaskoningDHV project team) 1

2 Aims of the talk Present science developments for coastal flood forecasting Firths of Forth and Tay Flood Warning System Review the recent storms and the forecasts and warnings provided The science objective To develop a flood forecasting system that delivers reliable and timely flood warnings Enhance our flood forecasting through better understand coastal flooding processes Improve approaches to wind and wave forecasting in the coastal environment 2

3 Early Warning Systems Coastal flooding process (Tides, Surge and Waves) Flood forecasting system design Flood warning communication TRITON System TRITON is a real-time coastal flood forecasting and warning system developed by Royal HaskoningDHV and Plan B 6 3

4 Coastal Flood Warning its evolution Water level at key (or A-Class) stations (STFS after 1953 event) for example, Leith in Forth and Tay area transformed to local water levels (e.g. Humber Database) Offshore waves and winds + water levels (Critical Condition Tables) Inshore waves and winds + water levels + Rh/Hs ratio Wave overtopping Inundation depth TRITON System - history TRITON s history reflects the science development of coastal flood forecasting. Starting from Humber Database for providing flood warning in Yorkshire Coast in early 1990 s National Coastal Flood Warning Trial Study (1998) North West Tidal Flood Forecasting (2002) Dublin Bay Coastal Flood Forecasting (2005) EA s National Flood Forecast System (NFFS from 2006) SEPA s Forth and Tay ( ) 4

5 TRITON System what does it do? Transfer Met Office forecasts on waves, tides and surges to the warning site For waves, from offshore to inshore For tides + surges, from A-Class gauges (Leith for Forth and Tay) to warning sites TRITON is a data display platform TRITON is a data processor TRITON is a data manipulator (e.g. what-if scenario) TRITON offers freedom in choosing warning triggers (e.g. offers wave overtopping as a trigger as well as water level) Forth and Tay - Flood Warning Sites 5

6 Wave Transformation Models Wave Transformation Models Hindcast model Offshore and wind hindcast model comes from the UK Met Office forecasts SWAN (Simulating WAves Nearshore) SWAN transforms offshore wave forecasts to inshore locations 6

7 Wave Transformation Matrices Wave transformation matrices (look-up tables) Matrix Dimensions Steps of Dimension Dimensions Wave Height (m) 0, 2, 4, 6, 8, 10 6 Wave Period (s) 4, 6, 8, 10, 12, 16 6 Wave Direction ( N) 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, , 330 Sea Level (m CD) 0, 3.5, 7 3 Wind Speed (m/s) 0, 20, 40 3 The matrices were obtained by pre-running SWAN models. In total, the number of SWAN model runs are: 5 x 6 x 12 x 3 x 3 x 5 = 16,200 Tidal Model Extents 7

8 One-Dimensional Model Extents Hydrological Routing Model Extension The one-dimensional model represents the River Tay watercourse between Ballathie and Newport-on-Tay, and the River Earn between Kinkell Bridge and the confluence with the River Tay. Modelled Tide Level Relationships 8

9 Wave Overtopping Hedges Reis Formulation Main advantages: It considers zero overtopping cases that were often omitted from other overtopping formulae; It concentrates on the low overtopping range and gives more realistic values for flood warning and flood risk assessment The forecasting system Transforms tidal surge forecasts from Leith to 28 flood warning sites along the Firth of Forth and Tay Transforms offshore wave forecasts to inshore locations Transforms inshore wave forecasts to mean wave overtopping forecasts at selected sites Use pre-run matrices and level relationships so that system is computationally efficient and real-time reliable. 9

10 Flood Warning Trigger Wave Overtopping Rate or Volume Level 1: Flood Alert: Overtopping rate exceeds 0.1 l/m/s. In the fluvial Forth and Tay without waves, this means bank full conditions. Level 2: Flood Warning: 0.1m inundation depth above Level 1 for urban areas and 0.3m inundation depth in rural areas assuming after 2 hours continuous overtopping. Level 3: Severe Flood Warning: 0.1m inundation depth for national transport (roads and railways), 0.3m inundation depth above Level 1 in urban areas and 0.5m inundation depth in rural areas assuming after 2 hours continuous overtopping. 19 Flood Early Warning System 10

11 The Storm of December 2012 Press and Journal, 12 th January

12 Coastal flooding threat (Thursday 13 th ) 36 to 48 hours lead time Offshore waves forecast 3m to 4.5m Coastal impacts possible for East Coast Flood Alert issued Coastal flooding threat (Friday 14 th ) 12 to 24 hours lead time Offshore waves forecast 5m to 8m Flooding impacts likely due to wave impacts Flood Warnings 12

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14 Islands: Road damage and closures Wick and Brora: Damage to harbour, properties flooded and A9 closed due to flooding. Highland Council reporting 2m damages Moray coastal villages: Sea defences and harbours damaged Peterhead and Stonehaven: Properties and infrastructure flooded, some evacuations Fife coast: Sea defences and harbours damaged, risk to properties The Storm of December 2012 Coastal Forecasts Near shore waves of 1 to 2 metres forecast Significant wave overtopping predicted in Kirkcaldy Warnings issued Emergency Response Road closures, especially along Kirkcaldy promenade Damage to two 20 metre sections of sea defences 14

15 Culross, Longannet and Kincardine Flood warning threshold 3.94 maod Alloa (not a flood warning location) Top level of Quay 3.55 maod Grangemouth Flood warning threshold 3.94 maod Culross, Longannet and Kincardine 6 Dec 4.32 predicted 4 Jan 4.23 predicted Alloa (survey levels) 5 th /6 th Dec 4.26 maod 4 th Jan 4.11 maod Grangemouth 6 Dec 4.37 predicted (4.25 observed) 4 Jan 4.28 predicted (4.18 observed) 15

16 December 5/6 Event Analysis Grangemouth Observed 5 th Dec 3.7 maod 6 th Dec 4.25 maod Forecast 5 th Dec 4.04 maod 6 th Dec 4.37 maod Photo: Looking upstream from A904 in Grangemouth on the peak of the tide/surge on the 5 th December (Source: JBA Consulting) Photo: Musselburgh at peak tide/surge (source: JBA Consulting) 16

17 In summary Major new flood warning scheme launched for Scotland s critical communities in December 2012 Based on latest science developments for forecasting tide, surge and wave impacts Delft FEWS system used to integrate Met Office offshore coastal data with near shore assessment Enabled better flooding response during the storm of December 2012 and December 2013 / January 2014 Increased awareness of coastal flooding issue The wave transformation and overtopping calculations do lead to some false warnings needs to be reviewed and improved THANK YOU Q & A 34 17