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)

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

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

Introduction

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

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

• 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

Complex Hydraulics in Texas

•

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Integrated 1D/2D Model

1D-2D interface allows flow exchange between 1D/2D engines by balancing energy grade

Well defined channels modeled in 1D

Hydraulic structures modeled in 1D

Overbank areas modeled as 2D grid

Results Highlight Need for 2D

• 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)

Parallel HEC-RAS Model

• 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

xp2D for Complex Hydraulics

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

FIRM Mapping of 2D Model Results

Finite Volume Methods

Example of HEC-RAS 5 FVMCalculation

• 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)

• 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

Finite Volumes Models

• Finite Volume Method Double Mesh

• Flexible Triangular Mesh: Lump Momentum Equations Along

Edges

• Honeycomb Mesh: Lump Mass Balance Equations

ICPR Version 4

Information courtesy of Streamline Technologies, Inc.

• 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

ICPR Version 4

Information courtesy of Streamline Technologies, Inc.

• 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.

ICPR Version 4

Information courtesy of Streamline Technologies, Inc.

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.

XP2DFV

Bespoke and advanced meshing using triangles Easily conforms to complex boundaries using: Feature “fuzzy” simplification Mesh refinements

refined meshingfuzzy meshing

Information courtesy of XP Solutions Inc.

• 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

Finite Volume Methods

• 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

Conclusion