GRIMSBY TIDAL DEFENCES: WAVE OVERTOPPING AND COASTAL FLOODING

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

INTRODUCTION, CONTENTS AND WHY GRIMSBY? 2

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

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

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. (http://www.bbc.co.uk/news/uk-england-humber-21367758) Overtopping of the Cleethorpes Seawall 5

PROJECT BACKGROUND 6

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

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

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

FLOODING IN GRIMSBY 10

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 1978. The worst recorded flooding was from the major East Coast storm event of 1953. 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

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

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

F L O O D I N G I N G R I M S B Y Crown Copyright 2012. All rights reserved. Ordnance Survey Licence No. 100026380 Note: Areas shaded red are above 4.0 maod Low areas around docks 14

TUFLOW MODELLING 15

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 2009-07-AA 8 x 8m spatial grid resolution 16

MODEL BACKGROUND Crown Copyright 2012. All rights reserved. Ordnance Survey Licence No. 100026380 Model Extent 17

MODEL DETAILS Breach model for assessing the Do Nothing and Do Minimum scenarios Overtopping model for assessing Do Something scenarios TUFLOW Build 2012-05-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

MODEL DETAILS Description Royal Dock Fish Dock Level (maod) Cill -5.10-5.10 Dock Bottom -2.80-2.80 Lock Gate crest 3.41 4.90 Flood Gate crest 4.49 4.90 Max. water level 3.41 3.12 Min. water level 1.11 1.22 Model initial water level 3.40 3.10 Area (km 2 ) 0.34 0.24 Crown Copyright 2012. All rights reserved. Ordnance Survey Licence No. 100026380 19

DOCK GATES Fish Dock Gates Royal Dock Gates 20

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 2012. All rights reserved. Ordnance Survey Licence No. 100026380 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

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

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

MODEL BOUNDARIES Return period (years) Design Sea Levels (maod) 2008 2012 2018 2032 2055 2091 1 4.00 4.02 4.05 4.13 4.29 4.57 2 4.10 4.12 4.15 4.23 4.39 4.67 5 4.24 4.26 4.29 4.37 4.53 4.81 10 4.35 4.37 4.40 4.48 4.64 4.92 20 4.46 4.48 4.51 4.59 4.75 5.03 25 4.50 4.52 4.55 4.63 4.79 5.07 50 4.61 4.63 4.66 4.74 4.90 5.18 75 4.67 4.69 4.72 4.80 4.96 5.24 100 4.71 4.73 4.76 4.84 5.00 5.28 150 4.79 4.81 4.84 4.92 5.08 5.36 200 4.84 4.86 4.89 4.97 5.13 5.41 250 4.87 4.89 4.92 5.00 5.16 5.44 300 4.91 4.93 4.96 5.04 5.20 5.48 500 5.00 5.02 5.05 5.13 5.29 5.57 1000 5.12 5.14 5.17 5.25 5.41 5.69 10000 5.55 5.57 5.60 5.68 5.84 6.12 Design Sea levels 24

Level (maod) MODEL BOUNDARIES EA report SC060064/TR2&4: Coastal flood boundary conditions for UK mainland and islands 6.00 5.00 4.00 3.00 2.00 1.00 0.00-1.00-2.00-3.00-4.00 0 24 48 72 96 120 144 168 192 216 240 264 Time (hours) Base tide+slr Surge shape Net design tide Design Sea levels: 1 in 200 year (2055) 25

Flow (m 3 /s) MODEL BOUNDARIES Reach Length (m) Proposed crest level (maod) 1 in 200yr overtopping rate in 2055 (l/s/m) 18.0 16.0 14.0 R01a 1a 426 5.48 8.4 12.0 R01 R02 1 604 6.25 2.5 2 83 5.62 10.7 10.0 R03 R04 R05 3 23 4.49 o/t 4 137 5.48 14.1 5 187 5.95 0.3 6 48 5.11 o/t 8.0 6.0 4.0 R06 R07 R08a R08b R08c R09 7 17 4.9 o/t 8a = 8c 610/632 6.92 27.8 2.0 R10 8b 404 7.03 23.7 0.0 0 3 6 9 12 9 = 10 113 5.8 0 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

MODEL RUNS Do Nothing scenario: all breaches H6 to H10 took place at once Do Minimum scenario: individual breach Crown Copyright 2012. All rights reserved. Ordnance Survey Licence No. 100026380 Breach runs 27

MODEL RUNS Existing defences Proposed defences Crown Copyright 2012. All rights reserved. Ordnance Survey License No. 100026380 Overtopping runs Crown Copyright 2012. All rights reserved. Ordnance Survey Licence No. 100026380 28

MODEL RUNS Reach Length (m) Existing crest level (maod) Proposed crest level (maod) Overtopping rate (l/s/m) 1a 426 5.64-6.04 5.48 8.4 1 604 5.64-6.50 6.25 2.5 2 83 5.62 5.62 10.7 3 (RDG) 23 4.49 4.49 o/t 4 137 4.76 5.48 14.1 5 187 6.50-6.72 5.95 0.3 6 48 5.11 5.11 o/t 7 (FDG) 17 4.90 4.90 o/t 8a = 8c 610/362 6.82 6.92 27.8 8b 404 6.84 7.03 23.7 9 = 10 113/465 [4.96] 5.80 0.0 [] figure is ground level Bold figures assume no change to the exiting defence o/t means defence overtopped in the design event Crown Copyright 2012. All rights reserved. Ordnance Survey Licence No. 100026380 Design overtopping rate: 1 in 200yr in 2005 29

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

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

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

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

FDEM OUTPUT 34

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

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

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

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

CONCLUSIONS 39

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