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2D Hydraulic Models For Complex Floodplain Analysis: Case Study – FM 1442

Golam Mohiuddin, P.E., CFMProject Engineer (H&H)

Mike McGovern, P.E., CFMVice President, Director Water Resources

Presentation Team

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Woolpert at a glance. 

1911Founded in

Dayton, Ohio

1,200+Global

employees

40+Offices

Worldwide

Woolpert at a Glance

1911Founded in

Dayton, Ohio

1200+Global

Employees

40+Offices Worldwide

ARCHITECTURE ENGINEERING GEOSPATIAL

Areas of Expertise

LiDAR Derived Digital Surface Model ‐ Niger

Areas of Specialization

ARCHITECTURE ENGINEERING GEOSPATIAL

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Woolpert Office Locations

The map data provided in this presentation provided for informational and communication purposes only.

Arlington, VAAtlanta, GAAustin, TXCalgary, Alberta CACharleston, SCCharlotte, NC

Chesapeake, VAChicago, ILCincinnati, OHCleveland, OHColumbia, SCColumbus, OH

Cypress, TXDayton, OHDenver, COEgg Harbor, NJFairview Heights, ILFort Worth, TX

Glendale, COGreenville, SCHouston, TXIndianapolis, INJackson Hole, WYJefferson City, MO

Johannesburg, RSAKansas City, MOLexington, KYMason, OHMiami, FLOrlando, FL

Pensacola, FLPittsburgh, PAPortland, ORProvo, UT Richmond, VASalt Lake City, UT

San Antonio, TXSan Jose, CASpringfield, OHStennis, MSSt. George, UTSt. Louis, MO

Tampa, FLToledo, OHVirginia Beach, VA

Outline

What is 2D Modeling 2D vs 1D Model Case Study Why 2D Modeling Hydrology Development 2D Model Development 2D Model Validation Model Results Summary

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What is 2D Modeling

What is 2D ModelingHydraulic property changes in both X and Y 

direction

Discretization of floodplain into grid cells/ meshes

Utilize high resolution terrain data even with coarse cell size

Solve either 2D Shallow Water Equation or 2D Diffusion Wave Equation

Implicit finite volume algorithm 

Stream

X

Y

Z

X

Y Z

X

Larger time step

More stability

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2D vs 1D Model

1D Model

Unidirectional stream with defined banks (canal, mountain stream)

Unavailability of good terrain data –Easy to fix terrain anomalies in 1D model

2D Model

Wider floodplain

Shallow overland flow

Low flow condition

Split flow

Narrow crossing with significantcontraction / expansion

“The MODEL is as good as the INPUT (DATA)”

2D vs 1D

Modeler’s judgement

Project Specific

2D Modeling Case Study: FM 1442 Project

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Project Background TxDOT Project – Beaumont District

Project located in Orange County, Texas

FM 1442 between FM 105 South and FM 408

4.91 miles of proposed roadway improvements

FM 105 South

FM 408

Project Background- H&H

Located within Cows Bayou and Neches River watershed

Flat coastal terrain

Tidal impact

Project Start

Project End

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Project Background- H&H Mostly within FEMA zone AE

14 drainage crossings

11 non‐bridge class

1 bridge class (CV5)

2 equalizers

Why 2D Hydraulic Modeling?

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Hydraulic Modeling – Why 2D

Flow interaction between drainage crossings

Complexity in the upstream floodplain

Multiple crossings draining to the same outfall

Variability in flow direction 

Good terrain data source (3.28’X3.28’)

Recent development in HEC‐RAS 2D

CV1

CV2

CV3

CV4

CV5

Bessie Heights Outfall

Hydrology Development

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Hydrologic Analysis

Validate the Peak Rate Factor (PRF) 

Three gauges along Cows Bayou

Analysis was performed using HEC‐SSP 2.2

Bulletin 17B procedure to perform Log Pearson III Distribution

HMS Model with delineated area for the gauges

Gauges were calibrated with PRF 350 

All subsequent hydrology based on same PRF

Gauge 1

Gauge 3

Gauge 2

2D Model Development

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2D Model Development Floodplain: 41.5 sq.miles

2D cell size: 100’ X 100’

No of cell: 113k (approx.) 

Landuse: NLCD Landcover (USGS)

Inflow hydrographs: At the upstream of each crossing

MSL data: NOAA Local Tides and Currents

2D Model Development

Driveway (DW) culverts were added 

Cell face adjustment where DW culvert info is not available

Terrain modification to incorporate survey flowline EL

Roadway defined using 2D‐2D connection weir

Connection length to match with DA 

Weir coeff = 1.50 (for 1 ~3 ft above natural ground)

Terrain modification

DW Culvert

Cell face adjustment

FM 1442 Roadway

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2D Model Validation

Model Validation

Backyard Flooding

Parking lot flooding

Rainfall Event: May 14th,2020

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Model Validation NOAA’s Next Generation Weather 

Radar (NEXRAD)

NOAA ATLAS 14 Estimates

Model Validation

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Model Results

Model Results- Design Storm Event

CV2

CV3

CV4

CV5

Rising Storm  Peak Storm  Storm  Recession

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Rising Storm  Peak Storm  Storm Recession 

CV10

EQ1

CV11

Model Results- Design Storm Event

Model Results- HY8 vs RAS 2D

Storm Event

Peak Discharge

(cfs)

HY8 Result RAS 2D Result

Culvert Discharge

(cfs)

Roadway Discharge

(cfs)

Culvert Discharge

(cfs)

Roadway Discharge

(cfs)

25 Year 1067 536.42 529.75 373.84 NA100 Year 1474 532.10 941.76 396.80 12.6

Storm Event

HY8 Result RAS 2D Result

Roadway Elevation

HW Elevation

Overtopping depth (ft)

HW Elevation

Overtopping depth (ft)

25 Year 13.3 14.21 0.91 12.97 NA100 Year 14.64 1.34 13.46 0.16

100Y Result‐ HY8

100Y Result‐ RAS2D

CV2 : (1)‐ 8’X5’ RCB

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Summary

2D model is an effective tool to model complex floodplain

Physics based approach, less assumptions

Less model instability than traditional 1D unsteady or 1D‐2D couple models

Hydraulic modeling option for alternative analysis on an accelerated schedule

Simulation time could be adjusted (based on computational time step, cell size, solution equations, etc)  

THANK YOU

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