Comparing 2D Approaches for Complex FEMA Studies

Transcription

1 Comparing 2D Approaches for Complex FEMA Studies Presented by: Ted Cassidy, P.E., PH (STARR2-Dewberry) Sam Crampton, P.E., CFM (STARR2-Dewberry) Mike DePue, P.E., CFM (STARR2-Atkins)

2 Agenda Introduction and Overview 2D Case Studies for FEMA Risk MAP Projects Rush Creek, Arlington TX, XP- 2D Study Low relief modeling and flood zone determination Finite Volume Methods General approach and advantages Thoughts on upcoming 2- D models ICPR 4 for 2D modeling and coupled groundwater modeling RAS 5 XP2DFV

3 Introduction 2-D Modeling for FEMA increasing But still a relatively small % of current studies <1,000 miles nationwide out of 1,200,000 total mapped (excluding coastal 2-D) Quasi-2D and Unsteady 1-D add another 10,000 miles Standards for FEMA 2-D use are limited ASFPM identified this in March 2014 White Paper Quality Assurance Parameters Unique to 2-D Maintenance and Updates Floodways

4 Introduction Typical 2-D Modeling Types for FEMA Studies Coastal Modeling (Not covered here) Alluvial Fans Flow Breakouts Shallow Flooding Lake and Complex Flow

5 Introduction New 2-D Modeling Type for FEMA Studies First Order Approximation (FOA), used to: Compare against old floodplains to test validity of effective data Estimate what new floodplains might look like As a basis for communication 2-D FOA Advantages Depth, Velocity Guidance on 2-D FOA is very basic at this time 2-D FOA

6 Complex Hydraulics in Texas Highly altered flooding source containing: Excavated flood relief channel parallel to and below a perched main channel Converging flooding sources Multiple openings through major road embankments Major grading activities have occurred influencing flow direction Integrated 1D-2D model provided a solution for modeling complex conditions Developed 2D informed 1D HEC-RAS model for FEMA purposes

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8 Integrated 1D/2D Model Overbank areas modeled as 2D grid 1D-2D interface allows flow exchange between 1D/2D engines by balancing energy grade Hydraulic structures modeled in 1D Well defined channels modeled in 1D

9 Results Highlight Need for 2D

10 Parallel HEC-RAS Model Traditional 1D models are generally more readily accepted for FEMA studies xp2d model enabled a 2D informed HEC-RAS geometry setup to be confidently determined xp2d grid elevations could be contoured to determine RAS cross section alignments Areas of negligible energy slope can be modeled in RAS as storage nodes Calibration and comparison of results HEC-RAS generally gave a little higher results Hard to model in just 1D where elevations suddenly change laterally (break outs)

11 xp2d for Complex Hydraulics Benefits of the Integrated xp1d/2d model Straight forward to integrate 1D features and refine 2D model, much easier than many other 2D models SWMM Link-Node method is very versatile, enabling complex hydraulic structures to be effectively modeled Good stability, relatively low effort required to stabilize model compared to other dynamic models on the market Excellent continuity (generally easier than just 1D model) Recommendations Develop 1D model incrementally to help diagnose stability issues Choose suitable mesh resolution to optimize run times

12 Low Relief 1D/2D Modeling XP-SWMM/XP2D Defined channels with low capacity Drainage boundaries difficult to define Culvert embankments control elevation Flow can be shared based on timing Flow paths do not support a Profile Base Line

13 FIRM Mapping of 2D Model Results

14 Finite Volume Methods Example of HEC-RAS 5 FVM Calculation Solid black lines represent the mesh Black nodes are cell computational centers Each cell face (yellow line) represents a detailed cross section based on underlying terrain Red arrows denotes flow through cell face (longer representing more flow potential)

15 Finite Volumes Models Several models not yet accepted by FEMA HEC-RAS 5.0 Finite volume Still in beta phase. Release date expected soon ICPR 4 Finite volume, coupled 1D/2D and groundwater Released and pending FEMA approval XP2D Finite volume method being developed Available as prototype in testing phase SRH-2D Finite volume method including sediment transport Available through BOR and AquaVeo SMS

16 ICPR Version 4 Finite Volume Method Double Mesh Flexible Triangular Mesh: Lump Momentum Equations Along Edges Honeycomb Mesh: Lump Mass Balance Equations Information courtesy of Streamline Technologies, Inc.

17 ICPR Version 4 Groundwater Finite Element Method 6-Point Quadratic Triangular Element Heads are Calculated at Nodes Includes: Saturated Horizontal Flow Leakage Through Confining Layer Seepage at Ground Surface Irrigation/Water Supply Wells Information courtesy of Streamline Technologies, Inc.

18 ICPR Version 4 Levee Breach Example Coupled 2D ground and 2D overland model 1 foot increase in ambient GWT resulted in nearly 3-4 inch increase in flood elevation. Information courtesy of Streamline Technologies, Inc.

19 XP Solutions - XP2DFV XP2DFV New 2D inundation engine available as prototype In the future, it will be available in new flood modelling software suite Why has XP Solutions created this new flow engine? To design a flow engine with the right capabilities for modern inundation modelling To have direct control of the development path Information courtesy of XP Solutions Inc.

20 XP2DFV Bespoke and advanced meshing using triangles Easily conforms to complex boundaries using: Feature fuzzy simplification Mesh refinements fuzzy meshing refined meshing Information courtesy of XP Solutions Inc.

21 Finite Volume Methods Benefits of approach Each cell considers full range of elevation-volume Not averaged over entire cell Each cell considers full range of irregular conveyance along cell face Not based on average elevation Approach eliminates elevation averaged bias due to channels, embankments etc. Reduced need for 1D channel and embankment features due to full elevation range of volume and conveyance Generally supports larger cell sizes

22 Conclusion 2D modeling is gaining recognition and acceptance 2D can be a better solution for complex flow Can minimize engineering judgment in a positive way Technology is evolving fast New technologies including finite volume models have huge potential for FEMA applications Challenges to fit traditional FEMA standards and mapping approaches, especially floodways Need for new and updated guidance for FEMA studies