GRIMSBY TIDAL DEFENCES: WAVE OVERTOPPING AND COASTAL FLOODING

Transcription

1 14 May 2013 GRIMSBY TIDAL DEFENCES: WAVE OVERTOPPING AND COASTAL FLOODING EMAY TOHA B&V WATER

2 INTRODUCTION, CONTENTS AND WHY GRIMSBY? 2

3 I N T R O D U C T I O N S & C O N T E N T S INTRODUCTIONS Senior Hydraulics Modeller in Redhill, Black & Veatch, UK More than 20 years experience of mathematical modelling for water resources and flood control studies and designs in the UK and overseas. 3

4 I N T R O D U C T I O N S & C O N T E N T S CONTENTS Project Background Flooding in Grimsby TUFLOW Modelling Conclusions 4

5 I N T R O D U C T I O N S & C O N T E N T S WHY GRIMSBY? BBC NEWS Humberside (7 February 2013): A 14.5m flood improvement scheme in Grimsby will reduce the risk of flooding to 14,000 properties, according to the Environment Agency. ( Overtopping of the Cleethorpes Seawall 5

6 PROJECT BACKGROUND 6

7 P R O J E C T B A C K G R O U N D PROJECT BACKGROUND Promoting authority Environment Agency, Anglian Region Consultants Halcrow: Design Black & Veatch: Modelling, Benefit appraisal and Environment appraisal Key stakeholders Associated British Ports (ABP) North East Lincolnshire Council 7

8 P R O J E C T B A C K G R O U N D Study Area 8

9 P R O J E C T B A C K G R O U N D THE PROBLEM The conurbations of Grimsby and Cleethorpes are low-lying A significant number of residential and commercial properties located below the level of predicted current and future extreme tidal storm water levels The current defences provide inconsistent levels of protection and are susceptible to breaching failure In some areas the defences have deteriorated 9

10 FLOODING IN GRIMSBY 10

11 F L O O D I N G I N G R I M S B Y HISTORY OF FLOODING Tidal flood events occurring in Grimsby and Cleethorpes in 1919, 1920, 1938, 1953, 1973, 1976 and The worst recorded flooding was from the major East Coast storm event of A storm event on 3 January 1976 overtopped the Cleethorpes seawall In January 1978 the Cleethorpes frontage area was hit by a further surge 11

12 F L O O D I N G I N G R I M S B Y FLOOD RISK WITH EXISTING DEFENCES Current tidal flood risk: 1 in 50 year event from the Fish Dock 1 in 100 year event from both the Fish Dock and Royal Dock (Assuming no breaching of defences and present day defences configuration/crest levels ) 12

13 F L O O D I N G I N G R I M S B Y SHANDONG Principal flooding routes into Grimsby over existing defences 13

14 F L O O D I N G I N G R I M S B Y Crown Copyright All rights reserved. Ordnance Survey Licence No Note: Areas shaded red are above 4.0 maod Low areas around docks 14

15 TUFLOW MODELLING 15

16 MODEL BACKGROUND Modelling for benefit appraisal Project Appraisal Report Based on the EA Tuflow model of the Northern Area Tidal Modelling Hazard Mapping Study constructed by Mott MacDonald in 2010: LiDAR data TUFLOW Build AA 8 x 8m spatial grid resolution 16

17 MODEL BACKGROUND Crown Copyright All rights reserved. Ordnance Survey Licence No Model Extent 17

18 MODEL DETAILS Breach model for assessing the Do Nothing and Do Minimum scenarios Overtopping model for assessing Do Something scenarios TUFLOW Build AA-iSP-w32 with the following amendments to the original model: Inclusion of the Freshney Pumping Station weir Adjustment to the cill level at the Royal Dock entrance Modification to the initial water level in the docks and inclusion of dock gates Inclusion of sea (in the overtopping model) to allow water to flow over the defences and also flow back to the sea if the topography of the ground and the defences permitted this to happen 18

19 MODEL DETAILS Description Royal Dock Fish Dock Level (maod) Cill Dock Bottom Lock Gate crest Flood Gate crest Max. water level Min. water level Model initial water level Area (km 2 ) Crown Copyright All rights reserved. Ordnance Survey Licence No

20 DOCK GATES Fish Dock Gates Royal Dock Gates 20

21 ROYAL DOCK GATES Flood Gate ("normal weir"). Crest level = 4.49mOD. Sea Null code Lock Gate ("uni-directional weir"). Crest level = 3.41mOD. Royal Dock Crown Copyright All rights reserved. Ordnance Survey Licence No During low tides and when water levels in the dock > 3.41 maod, water will spill over the lock gate crest During high tides, the flood gate prevents the sea water entering the dock, but the gate will be overtopped if the sea level rises above the crest of the gate 21

22 FISH DOCK GATES The flood gate crest level is the same as the lock gate crest level (i.e. at 4.9 maod) The flood gate is closed for tides > 3.12 maod 22

23 MODEL BOUNDARIES Tidal boundaries based on the extreme sea level data from the EA Base year 2008 and surge shape 7 (Immingham) The UKCP09 User Interface for the 95%ile medium emission scenario of UKCP09 Overtopping boundary flows based on wave overtopping calculations in accordance with the EurOtop Manual (2007) Water level in the River Freshney upstream of the pumping station 23

24 MODEL BOUNDARIES Return period (years) Design Sea Levels (maod) Design Sea levels 24

25 Level (maod) MODEL BOUNDARIES EA report SC060064/TR2&4: Coastal flood boundary conditions for UK mainland and islands Time (hours) Base tide+slr Surge shape Net design tide Design Sea levels: 1 in 200 year (2055) 25

26 Flow (m 3 /s) MODEL BOUNDARIES Reach Length (m) Proposed crest level (maod) 1 in 200yr overtopping rate in 2055 (l/s/m) R01a 1a R01 R R03 R04 R o/t o/t R06 R07 R08a R08b R08c R o/t 8a = 8c 610/ R10 8b = Time (hr) Due to the nature of overtopping calculations, an approximate and simplified figure for the overtopping amounts has been used Normal distribution shape flow hydrographs were derived from the overtopping rates based on the expected duration of overtopping Overtopping rates: 1 in 200 year (2055) 26

27 MODEL RUNS Do Nothing scenario: all breaches H6 to H10 took place at once Do Minimum scenario: individual breach Crown Copyright All rights reserved. Ordnance Survey Licence No Breach runs 27

28 MODEL RUNS Existing defences Proposed defences Crown Copyright All rights reserved. Ordnance Survey License No Overtopping runs Crown Copyright All rights reserved. Ordnance Survey Licence No

29 MODEL RUNS Reach Length (m) Existing crest level (maod) Proposed crest level (maod) Overtopping rate (l/s/m) 1a (RDG) o/t o/t 7 (FDG) o/t 8a = 8c 610/ b = /465 [4.96] [] figure is ground level Bold figures assume no change to the exiting defence o/t means defence overtopped in the design event Crown Copyright All rights reserved. Ordnance Survey Licence No Design overtopping rate: 1 in 200yr in

30 MODEL RUNS Breach runs Do Nothing scenario: 10 return periods/4 epochs = 40 runs. Do Minimum scenario: 5 breaches/9 return periods/4 epochs = 180 runs. Overtopping runs Existing scenario: 8 runs. Do Something scenario: 26 runs Final model runs 30

31 MODEL RUNS *save up to 80% of disk space and reduce file clutter Run management: Using batch (.bat) files; 4 simultaneous runs 31

32 POST PROCESSING Import into FDEM* Quality control Flood damage assessment TUFLOW h max grids *Flood Damage Economic Method 32

33 FLOOD DAMAGE ECONOMIC METHOD (FDEM)* GIS based flood economic assessment Quality control: Compare flood depths Compare flood outlines (on development) Automated calculation and population of flood depth for a particular flood scenario Visualisation of results: allows to focus on the highest contributors to benefits * Developed by Black & Veatch GIS Team 33

34 FDEM OUTPUT 34

35 MODEL OUTPUT Do Nothing (all breaches): 1 in 200yr (2055) 35

36 MODEL OUTPUT Breach H10 Breach H08 Do Minimum (Single breach): 1 in 200yr (2055) 36

37 MODEL OUTPUT Do Something (proposed defences): 1 in 200yr (2055) 37

38 MODEL OUTPUT Initial dock level: Low Initial dock level: High Do Something Sensitivity Test: 1 in 500yr (2055) 38

39 CONCLUSIONS 39

40 C O N C L U S I O N S KEY POINTS The model run management resulted in significant savings in modelling time FDEM offers faster benefit appraisal Integration of hydraulic modelling, GIS and economic assessment Quality check The modelling effort on Grimsby has enabled refinement of the design of the proposed defences and resulted in significant savings in construction cost 40

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