Boundaries of 1D 2D modelling. Suzanne Callaway Senior Hydraulic Modeller

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1 Boundaries of 1D 2D modelling Suzanne Callaway Senior Hydraulic Modeller

2 Introduction Why is it important to define 1D 2D boundaries carefully? Defining boundaries between 1D and 2D models (Flood Modeller 1D and TUFLOW) Common issues related to 1D-2D linking identified during model reviews Model stability potential solutions 2

3 Why is it important to define the boundaries between 1D and 2D carefully? A good 1D-2D boundary setup is required to achieve: accuracy in the model s representation of flooding mechanisms e.g. timing and location of bank overtopping, flow rates leaving/entering channel accuracy in results reduce uncertainty model stability 3

Defining boundaries between 1D and 2D model domain Considerations when linking 1D and 2D domains: Decide if and where a 2D model domain can add value Definition of the river channel and floodplain in

4 Defining boundaries between 1D and 2D model domain Considerations when linking 1D and 2D domains: Decide if and where a 2D model domain can add value Definition of the river channel and floodplain in 1D and 2D respectively Definition of bank elevations Location and orientation of 2D domain Grid size Type of 1D 2D boundary Location of 1D 2D boundary Flow over structures, 1D or 2D? Range of depths anticipated 4

Flood Mapping, Hazard Mapping, Flood Alleviation Scheme options appraisal

5 Defining boundaries between 1D and 2D model domain Decide whether linking Flood Modeller 1D to a 2D model domain can add value: Study objectives e.g. Flood Mapping, Hazard Mapping, Flood Alleviation Scheme options appraisal Study area characteristics e.g. Urban areas Flooding mechanisms Data availability 5

6 Defining boundaries between 1D and 2D model domain Definition of the river channel and floodplain in 1D and 2D respectively 6

Null area width in TUFLOW approximately 12 m.

7 Common Issues inconsistency between 1D and 2D Width of cross sections in Flood Modeller approximately 6 to 7 m. Null area width in TUFLOW approximately 12 m. Approximately 5 m of flow width adjacent to the channel not represented in 1D or 2D. 7

8 Defining boundaries between 1D and 2D model domain Definition of the 1D 2D boundary elevations in TUFLOW: HX line defines grid cells that form the boundary between 1D and 2D model ZP points can be used to enforce elevations along the boundary 8

9 Defining boundaries between 1D and 2D model domain Definition of the bank elevations Survey Cross sections Bank crest spot heights Defence crest elevations LiDAR Provides elevations along the river banks (including areas between survey) Less accurate than survey 9

10 Defining boundaries between 1D and 2D model domain Survey Accurate at location surveyed Is it representative of river banks upstream and downstream? 10

11 Defining boundaries between 1D and 2D model domain LiDAR Have buildings/vegetation been over/under filtered? 11

12 Common Issues 1D 2D boundary elevation Interpolation of bank elevations applied from survey at structures Variations in bank elevation not represented 12

13 Defining boundaries between 1D and 2D model domain What is the best way to define bank elevations? Survey LiDAR More accurate than LiDAR (caution - raised structure abutments at reach ends) Interpolating bank elevations between two surveyed sections may not be appropriate Location of HX (boundary) line needs to be accurate and grid size appropriate to pick up top of bank elevations Combined approach ZP points based on survey, with additional points based on LiDAR if required Automated process are helpful but need careful checking 13

14 Defining boundaries between 1D and 2D model domain Example of combined approach - Left bank elevations between two surveyed river sections 14

15 Defining boundaries between 1D and 2D model domain Exporting bank elevations from Flood Modeller 15

16 Defining boundaries between 1D and 2D model domain Left Bank elevations defined by LiDAR and surveyed cross sections Left Bank elevations interpolated between elevations at surveyed sections 16

Level Grid shows out of bank flooding, starting at approximately

17 Defining boundaries between 1D and 2D model domain Bank elevations defined by LiDAR and surveyed cross sections Water Level Grid shows out of bank flooding, starting at approximately 4 hours on the left bank. Maximum 100 year flood extent Flooding at 4.2 hours 17

18 Defining boundaries between 1D and 2D model domain Left Bank elevations interpolated between two surveyed sections. Water Level Grid shows no out of bank flooding along the left bank between the two surveyed sections. Maximum 100 year flood extent 18

19 Model Stability Checks and potential solutions: Check schematisation Check definition of 1D channel and 2D null width Check elevations along HX line (boundary cells) Improve calibration and stability in general Improve uniformity of conveyance in 1D channel Increase width of channel/floodplain represented in 1D if appropriate Avoid slithers of floodplain in 2D 19

20 Model Stability Checks and potential solutions: Improve elevations along HX lines Add interpolates to 1D Tighter tolerances (e.g. Htol), especially if short time steps are used Increase loss coefficients on HX ( a value) For H boundaries/links that experience oscillations or instabilities, using a BOUNDARY VISCOSITY FACTOR == of one to five may provide improved performance with no measurable or adverse effect on model results, but can significantly improve the flow patterns at the boundary/link (TUFLOW Forum, May 2015) Roughness increases (boundary and floodplain) due to shallow depths 20

21 Model Stability across 1D 2D boundary Checks: Use 1D and 2D results to identify areas of instability Use 2D flow output to check stability of flow across 1D 2D boundary 21

22 Common Issues Model Stability across 1D 2D boundary Location and elevation of HX line Bank heights based on LiDAR Bank heights approximately 2.4 m lower along the 1D 2D boundary than in the Flood Modeller cross section Before: After: 22

23 Varying Loss Coefficient on HX line Run 031 uses an 'a' value of 0.3 Run 053 uses an 'a' value of 1 (NB - Draft results shown) 23

24 Flow, m 3 /s Varying Loss Coefficient on HX line 0 Flow between 1D and 2D along left bank at node LW5_ MWEY_DM_0050YR_031 MWEY_DM_0050YR_ Time, Hours 24

25 Flow, m 3 /s Varying Loss Coefficient on HX line 9 Flow between 1D and 2D along left bank at node LW5_9.024_L MWEY_DM_0050YR_031 MWEY_DM_0050YR_ Time, Hours 25

26 Summary 1. Careful definition of the 1D 2D boundary is required to accurately represent flooding mechanisms. 2. Bank elevations between surveyed sections are an important considerations when defining 1D 2D boundaries. 3. There are several areas to investigate when improving model stability, including applying a loss coefficient to HX lines (since some physical processes are not reflected in the software). 26

27 What will you do differently? 27

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