2D Modelling Series. Modelling Structures in Floodplains

Transcription

1 2D Modelling Series Modelling Structures in Floodplains

2 Jessica Jefferys Products Manager XP Solutions

3 XP-LIVE Webinars XP-LIVE educational program Webinars have been recorded and are available at Certificates available password at end of webinar Question/Answer

4 Bill Syme Associate xp2d Engine Author BMT WBM

5 How do 1D and Fully 2D schemes handle energy losses Bridges/Culverts/Embankments Buildings & Fences Model Demonstration Q&A Modelling Structures in Floodplains

6 Form (Energy) Losses Energy dissipated where water changes its speed and direction (in addition to energy dissipation from bed friction) Pronounced at Bends Flow constrictions (eg. hydraulic structure) Energy lost is a proportion of the kinetic energy (V 2 /2g) V = 1m/s; Dynamic Head = 0.05m V = 4m/s; Dynamic Head = 0.82m

7 Simulating Form Losses 1D St Venant vs Full 2D Equations 4 # Water Surface Profiles (V = 2m/s) Coarse 2D Model 1D Model 1D Model 1 2 # # # A Water Level (m) A Distance (m)

8 In 1D we use entrance and exit losses (fraction of V 2 /2g) to approximate the losses Entrance and exit loss values vary depending on how much the water speeds up and slows down changes direction User and/or software must estimate these loss values Loss values can vary significantly with the height of the water 1D Approach Entrance/Exit Losses

9 Contraction/expansion (due to embankment) Piers Eccentricity Skew Dual bridges Deck surcharging 1D Approach for Bridges Estimate Loss Values

10 Full 2D equations (not 1D equations over a 2D mesh) accurately simulate speeding up, slowing down, eddy formations and change in direction dissipate kinetic energy Implies no need for entrance/exit losses? However, not all losses accounted for may require additional losses fine-scale losses (eg. piers, vena contracta) losses in vertical (eg. bridge deck) Full 2D Equations No Entrance/Exit Losses

11 Simple Example 2.86 m 2.42 m 2.88 m 2.30 m m # # # # 0.7 m/s 2.9 m/s 0.8 m/s Desktop calculations Entrance loss coeff = 0.38 Exit loss coeff = 0.55 Head drop ~0.42m Water Level (m) Upstream of Culvert Water Surface Profiles - Outlet Controlled Downstream of Culvert 1D Model Distance (m)

12 Simple Example Modify 2D Cells to represent culvert Full 2D Afflux too low (fine-scale losses not accounted for) Calibrate/adjust by adding additional losses (in this case 0.2) Water Level (m) Water Surface Water Profiles Surface - Outlet Profiles Controlled - Outlet - Adjusted ControlledForm Losses D Model 2.8 2D Model (Culvert as 2D Cells) Upstream Downstream of of Culvert of of Culvert Distance (m) Distance (m)

13 Flow Constrictions Bridge and Floating Deck Options Cell Obvert Partially block cell sides Deck FLC Form Loss Coefficient

14 Flow Constrictions Layered Option Up to 3 layers varying Energy Loss Blockage A layer can spatially vary in depth (eg. arched bridge deck) Top layer always unblocked Blockage = 0% FLC = 0 Blockage = 50% FLC = 0.5 Blockage = 100% FLC = 0.8 Blockage = 5% Form Loss Coeff = 0.1

15 Guidance Literature Hydraulics of Bridge and Waterways (Bradley, 1978) AustRoads Calibration Data (if available!) Other software (which could be wrong!) Careful entering energy losses for 2D flow constrictions Line objects full value Polygons loss per metre in direction of flow (ie. divide by flow length) This is so that values are cell size independent

16 Bridge Example 1D/2D 2D

17 Bridge Example 1D/2D 2D

18 Make sure width of WET 2D cells EXCEEDS width of 1D flow at all elevations Use an interface line to select more than one cell for large structures Check ground elevations are not above culvert invert! 1D/2D Structure Linking Culvert Through an Embankment

19 Modelling Buildings Mark Pearson/REX

20 Modelling Buildings Block 2D Cells Out 100% 0.137m 0m/s 1.9m/s

21 Building Walls on 3 Sides 100% 0.145m 0m/s 1.93m/s

22 Building as High Roughness 79% 0.122m 0.38m/s 1.41m/s

23 Buildings as Inactive Cell Example 2m 5m

24 Buildings as 3 Sides Blocked 2m 5m

25 Buildings as High Roughness 2m 5m

26 Pros and Cons Building Approaches Inactive buildings Do not include storage effects Closed on three sides Storage effects represented If slab on ground construction raise ground elevations using fill areas feature If open on upstream side water level in building is higher than if open on downstream side High Manning s n only option for coarse grids

27 Inactive cells approach don t receive rainfall therefore don t use or adjust rainfall Direct Rainfall Implications High Manning s n approach can cause water mounds on buildings, therefore Vary n with depth for building landuse Low (0.01) Manning s n for very shallow depths to model runoff off roof High (>0.1) Manning s n for depths greater than say 0.1m to model as obstruction to flow

28 Elevate cell sides (not whole cells) Can wet and dry Layer parameters eg. vary blockage and losses with height Can collapse sides Handles u/s and d/s controlled flow regimes Modelling Fences in 2D

29 Collapsing Fences/Walls

30 This type of handrail is recommended as it does not collect debris! Some agencies require that all rails are blocked Can use Layered Flow Constrictions if in 2D Blockages!

31 Working on this one

32 Provides general guidance on many of the topics discussed today Section on structures Section for buildings, fences, etc And much more! AR&R Project 15

33 Live Demonstration

34 Webinar recording Future webinars in 2D Modelling Series Certificates Questions

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37 Questions? Comments? Thank you for joining this presentation Modelling with Different Cell Sizes By Bill Syme & Jessica Jefferys Contact XP Solutions Asia Pacific: ausales@xpsolutions.com