2D Modeling for Approximate Areas. Monica S. Urisko, P.E. CFM

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1 2D Modeling for Approximate Areas Monica S. Urisko, P.E. CFM Ferrin P. Affleck, P.E. CFM

2 Outline Types of Modeling Available Steady vs Unsteady 1D vs 2D Software available Basics of 2D modeling Inputs Outputs Things to consider Case Study

3 Types of Modeling Available Steady vs Unsteady 1-dimensional vs 2-dimensional Software

4 Steady vs Unsteady Steady Flow is constant Only need peak Storage not a factor Unsteady Flow changes with time Hydrograph Storage is a factor

5 Unsteady - Hydrograph Steady State

6 Unsteady - Storage

7 1D vs 2D 1-dimensional Node & Link Based Cross sections Junctions Storage Ponds Diversions 2-dimensional Grid or Spatially Based Flow in multiple directions Storage accounted for in model

8 1D Node & Link Based Flow Follows your network Controlled by downstream feature

9 Steady 1D Models Clearly defined channels Structures Flow direction is clear Floodways

10 Unsteady 1D Models Storage significant factor Diversions occur Structures Swamp/wetlands

11 2D Grid Based Flow follows terrain Flow is determined by neighboring water surface elevation

12 fa18 2D Models Storage significant, flat, slow moving Flow paths are unclear Unconfined flooding - structures minimal Alluvial Fans Swamps Size can be a limitation

13 Slide 12 fa18 you could add a slide after this one, "if you really need to use a 1D model, sometimes a 2D model can help you understhow to layout the model." Ferrin Affleck, 03/24/2014

14 How Do I Decide? One type is not necessarily better than another Let your circumstances dictate which model you use

15 Some Products Available Model Steady Unsteady 1D 2D Gage/Regression X HEC-HMS X X HEC-RAS X X X ICPR X X EPA SWMM X X XP SWMM X X X X FLO-2D X X X TUFLOW X X X

16 Basics of 2D Modeling Flo-2D

17 Simple model 2D Inflow (Point or Rain) Topography Infiltration (CN) Manning s roughness Outflow Extras: structures, levees, 1D channels

18 Outputs Water surface elevation grid Depth grid Velocity grid Flow hydrographs at cross sections

19 Things to Consider Be willing to invest some effort into the setup Choose grid cell size Start simple then add detail

20 Case Study Coastal Georgia PMRs Approximate Area Effingham County

21 GUYTONN Bulloch Co. Effective Zone A GA-17 Study Area Effingham Co. Ogeechee River Effective Zone AE Little Ogeechee River Bryan Co.

22 Ogeechee Detailed Study 1D Modeling Work Begins

23 Ogeechee Detailed Study Ogeechee Approximate Study

24 Ogeechee Approximate Study Ogeechee Run Study Ogeechee Detailed Study Little Ogeechee Study

25 What do we do with this area?

27 EASY!!!! 1D Approximate Models

28 Let s take a closer look!!!

29 Model split flow? What s happening here?

30 What s happening here?

31 What s happening here?

32 Let s look at this area now

34 Model split flow?

35 Model split flow?

36 Let s look at this area now

40 Q ~ 350 cfs

41 Let s think about flow attenuation Too many assumptions???

45 Let s try a 2D model for our area Flo2D

46 Flow is unpredictable Split flow 33 sq. mi 100 ft cell size

47 Inputs Inflow - Rain Topo 100 ft cell size from LiDAR Infiltration CN Manning s Land Use and Roads Outflow encompassing watershed

48 Terrain High 112 ft Low 30 ft

49 CN High 98 Low 30

50 Manning s n High 0.2 Low 0.03

51 Case Study Results We have a simple model Good for an approximate area (Zone A) Easier and faster than modeling split flow and flow attenuation

52 Water Surface Elevation

53 Depth over LiDAR Greater than 0.5 ft

54 Floodplains

55 fa6 Floodplains

56 Slide 54 fa6 not sure I like the floodplains hollow, hard to tell what is in/out. can you do a hatch or a transparet fill? Ferrin Affleck, 03/24/2014

57 Floodplains

61 Was the 2D model worth it? 1D Automated tools No additional training Level of Effort: 2-3 days Results: Not very defendable A lot of questionable areas Populated areas needed a better understanding of their flood risk

62 Was the 2D model worth it? 2D 1-day training course (simple 2D model) 1-2 days to create and run an approximate model 1 day for processing results Results: Comparable to 1D results But, captured all the flow split locations and storage areas Still an approximate model (no structures were modeled) Overall, it provides a better picture of the area

63 Was the 2D model worth it for this approximate area? YES!

64 If you really need to use a 1D model, sometimes a 2D model can help you understand how to layout the model.

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