Representing Detail in Large Hydraulic Models: Lower Thames and Humber Estuary. Neil Hunter, Kevin Haseldine and Matthew Scott

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1 Representing Detail in Large Hydraulic Models: Lower Thames and Humber Estuary Neil Hunter, Kevin Haseldine and Matthew Scott

2 Overview What do we mean by detail? Lower Thames Humber Estuary How is software helping to represent detail? Final thoughts

3 What do we mean by detail? Model grid resolution? Hydrology Rainfall, runoff, upstream/lateral catchments, infrastructure operations/rules Channel and floodplain flow processes SWEs vs. simpler alternatives, 1D vs. 2D Topographic detail Grid resolution / cross-section spacing Representation of flood defences Defended / undefended vs. RASP Interactions with other flood sources Number of RPs / joint probability considerations

4 How can this be represented? Sliding scale from effective/proxy treatments (e.g. runoff coefficients) to explicit, physics-based models (pipe network models) Single optimised parameter sets vs. multiple plausible combinations De-coupled vs. dynamically-linked models Only where it matters a risk-based approach to model schematisation Compromises are likely to be required in some areas to allow more detailed representation in others Inevitably (and unsurprisingly) linked to study objectives, time and budget

5 LOWER THAMES

6 The challenge Combine existing Thames modelling into single model in order to allow better understanding of the complex mechanisms and processes in the system Integrate all significant tributaries to develop model capable of meeting the requirements of current and future projects Key aims: (1) Qualitative: Improved understanding of flood risk in order to provide updated floodplain mapping able to reflect complex overland flow routes (2) Quantitative: Provide tool for analysis and development of River Thames Strategy (RTS)

7 Conceptualisation Pre 2013 Modelling: Mixture of individual 1D only and 1D-2D models of varying resolution and quality no single model of Thames and key tributaries update Single domain 1D model: 20+ existing and newly built models merged into single 1D Model domain: 190km of watercourse 7000 model nodes Speedier model calibration Multi-domain 1D-2D model: 10 separate 2D domains: Variable domain resolution (TUFLOW SCENARIO function) Active 2D area 200km ESTRY floodplain structures

8 2D domains Marlow Maidenhead Hammersmith Bray Slough Windsor Staines Walton Addlestone Chertsey

2D domains Bray example Domain includes multiple

River Hunter Coombe Lane Stream BRAY_LEFT All

Cut Roundmoor Ditch Jubilee River Chalvey Ditches Model

resolution depending on requirements of model run

9 2D domains Bray example Domain includes multiple watercourses: Chalvey Ditches The Cut Thames Jubilee River Hunter Coombe Lane Stream BRAY_LEFT All watercourses represent important sources of flooding The Cut Roundmoor Ditch Jubilee River Chalvey Ditches Model able to represent domain in higher (4m) or lower (20m) resolution depending on requirements of model run BRAY_RIGHT BRAY_CENTRAL Cress Brook Boveney Ditch Colemorton Brook ec26 24h.4 24h.6U 24.0

10 Floodplain structures Represent significant conveyance routes for floodplain flow Key flooding mechanisms in all major Thames flood events Project has undertaken a detailed review and update of floodplain structures Current model includes 104 ESTRY floodplain structures representing flow through: Railway embankments Major roadways (M4 etc.)

11 Model calibration Run time constraints meant that 2-stage approach was adopted: (1) Focus on in-channel hydraulics by analysing in-channel events (2007, 2009 and 2012) undertaken using 1D model (2) Analysis of significant out of bank events (2003 and 2014) undertaken using 1D-2D model Flow and level comparisons undertaken at 6 locations Level comparisons undertaken at a further 18 locations including all lock structures Operation of key structures including Taplow Sluices and Thames Barrier represented using time series data from observed records

12 Model development Key challenges (1D): (1) Develop consistency between 1D models prior to merging into single model: Tributary models initially developed in groups (Eton, Chertsey etc.) before merging with main Thames (2) Survey data: Thames channel data verified against bathymetry data collected in last 5 years Key challenges (1D-2D): (1) Run times - 3 weeks to run 350 hour hydrograph Maximise processing power of PCs through virtualisation All lakes represented in 1D model

13 Lessons learned Optimise use of 1D modelling: Calibration of in-channel hydraulics using 1D only model Out-of-bank events can then calibrated more easily Lakes and other significant water bodies retained in 1D model Where possible retain non-critical floodplain elements in 1D Representation of control structures very important in calibration (Thames Barrier, Taplow sluices) Linked model development: Survey of boundaries between domains critical, particularly on smaller tributaries Variable resolution domains very useful especially in model development phase Engagement with previous modelling teams: Build on their experience and avoid repetition of mistakes

14 HUMBER ESTUARY

15 Humber Estuary study domain

16 The challenge Construct a detailed hydraulic model representing both tidal & fluvial flood risk to the Humber Estuary area Calibrate to the late 2013 tidal surge Single biggest challenge was size of the floodplain (450km 2 ) Solution utilised four individual models: 1. Coarse resolution joint probability model 2. Fine resolution design model 3. Fine resolution TUFLOW only breach models 4. Very coarse resolution undefended model

17 Joint probability model Joint probability model developed to identify what combination of flood events would result in the greatest flood risk to a given area Tidal and inflow conditions on each river were defined based on new Chi values Required level of detail: Grid resolution Defence network Floodplain culverts Parameters

18 Detailed design model Joint probability results were used to determine which combination of flood events needed to be simulated for each flood cell, therefore reducing the required simulations Each non-critical flood cell for a given event was set to a 50m grid resolution. Each critical cell flood was set to 8m (only one critical cell per model run to reduce run times). TUFLOW scenarios Defence networks (updates required) Floodplain culverts Parameters

19 Humber breach scenarios TUFLOW only Source Receptors Breach level

20 Undefended model approach TUFLOW only (with ESTRY culverts) Defence networks removed Floodplain embankments maintained Coarse grid resolution Parameters

21 Lessons learned Model build: Start at a relatively coarse resolution for quick initial model development and slowly add detail Careful model schematisation can significantly decrease model run times Use TUFLOW scenario functionality In flat systems such as the Humber great care has to be taken to the manner in which defences are represented; a slight error in crest elevation in one or two boundary cells have huge implications to the area of inundation Agree which floodplain structures should be included General: Consider a conceptual modelling approach prior to starting development Does the Client and Consultant understanding of level of detail align with one another?

22 How is software helping to represent detail? Improved functionality for representing hydrological inputs and dynamic interactions between different flood sources Enhanced options for channel and floodplain linking i.e. grid-scale dependent HX vs. SX/lateral spill connections Wide range of options for floodplain modelling, many of which can be combined in a single model Extended sections, storage cell, simplified FAST-type, 2D diffusion wave, 2D SWE Efficient configuration of domains with different grid resolution / orientation Switching out of different domains using Boolean logic / Scenario commands Map and time-series outputs that can vary across the model Different hardware options (e.g. GPUs) Different parallelisation options (e.g. multi-threading) Batching / sampling of uncertain inputs

23 Final thoughts Rapid progress in recent years but we can always do better, so Let s keep talking Share good practice Challenge the software developers

The TUFLOW Link. Past, Present and Future. Stephanie Dufour

The TUFLOW Link. Past, Present and Future. Stephanie Dufour The TUFLOW Link Past, Present and Future Stephanie Dufour stephanie.dufour@bmtwbm.co.uk Contents PAST Software background Thames Embayments Inundation Study PRESENT Flood Modeller -TUFLOW link Flood Modeller