Dam Breach Analysis – 1D vs. 2D

PREPARED FOR:

2018 KAMM CONFERENCE

Prepared By: Mario G. Sebastiani

Civil & Environmental Consultants, Inc.

September 19, 2018

Importance of Being Prepared

1D vs. 2D Modeling

Representative Project Overview

1D vs. 2D Results Comparison

Overview

Introduction

-

10,000

20,000

30,000

40,000

50,000

60,000

70,000

High Significant Low Undetermined

15,498 11,882

60,705

2,495

Nu

mb

er

of

Dam

s

Hazard Potential

Dams by Hazard Potential

-

1,000

2,000

3,000

4,000

5,000

Yes No Not Required

4,288 3,902

3,692

Nu

mb

er

of

Dam

s

Emergency Action Plan (EAP)

Number of Significant Hazard Potential Dams with an EAP

Statistics from the National Inventory of Dams

27,380

Introduction

2,565 1,853 1,857 2,188

3,774 4,120

11,902

20,257

13,600

5,521 4,599 4,125

-

5,000

10,000

15,000

20,000

25,000

Before 1900 1900-1909 1910-1919 1920-1929 1930-1939 1940-1949 1950-1959 1960-1969 1970-1979 1980-1989 1990-1999 Since 2000

Nu

mb

er

of

Dam

s

Dams by Completion Date

Statistics from the National Inventory of Dams

Introduction

“By failing to prepare, you are preparing to fail.”

- Benjamin Franklin

2D vs. 1D Modeling

Definitions

▪ 1D Modeling

Solves the fully dynamic St. Venant

equations of conservation of mass and

momentum along a singular dimension.

▪ 2D Modeling

Solves the fully dynamic St. Venant

equations of conservation of mass and

momentum along two dimensions.

What’s the difference?

2D vs. 1D Modeling

Hydraulic Modeling Utilizing HEC-RAS 2D

▪ 1D Advantageso Fewer geometric data are required

o Shorter computational timeo In-bank flows computed more efficiently

o Relatively smaller output files

o Hydraulic structures (2D uses culvert eqns)

▪ 2D Advantageso Flowpaths do not need to be predefined

o Provides realistic depiction of flow throughout a systemo Perform 1D and 2D modeling within the same unsteady flow model allows users to model larger river systems, 1D where appropriate (main river) and 2D

modeling in areas that require a higher level of hydrodynamics

o Flowpaths can change with flow deptho Cross-momentum of flow splits is accounted for (significant for road systems)

o Losses due to 2D effects (i.e. bends, flow separations, etc.) automatically included within computations

o Floodplain storage is implicitly defined

o Inputs and outputs can be defined spatially in GIS-type environments (better data continuity)

o Does not require extraction of cross sections from survey data

o Detailed Flood Mapping and Flood Animations – based on underlying terrain, each cell can be partially wet/dry reflected in the mapping and animations

o Can provide results directly for mapping flood extents and inundation depths, velocities, and safety hazards

2D vs. 1D Modeling

Hydraulic Modeling Utilizing HEC-RAS 2D

▪ When is 1D Okayo Locations where flow isn’t required to spread (uni-directional flow)

o Well-defined channel/overbank systems (defined valleys)

o Simply-connected floodplains where flow in main channel is well connected to flow in the overbank and both are primarily uni-directional

o When elevation data of only limited quality/quantity are available

▪ When is 2D Preferableo Anywhere flow is expected to spread

o Urbanized Areas

o Wide Floodplains

o Downstream of Levee Breaks

o Downstream of Upground Reservoir Breaks

o Wetland Studies

o Lake or Estuary Studies

o Water Quality and Sediment Transport

2D vs. 1D Modeling

Hydraulic Modeling Utilizing HEC-RAS 2D

▪ 1D or 2D?o What is the length-to-width ratio of the

project area? (> or < 3:1?)

o Does the project have features that force

flow to rapidly contract or expand?

o What kind of output animations are

needed to convey the results to the

stakeholders?

Representative Project Overview

Four Eagles Lake – Camden,

Ohio

▪ 25.5-acre reservoir at normal pool

▪ 45-foot high, 620-foot long

embankment

▪ 3.1 sq mi drainage area

▪ High Hazard Dam

Four Eagles Lake Dam

Representative Project Overview

Representative Project Overview

Project Overview Aerial Map

Representative Project Overview

Project Overview Hillshade Map

Project Overview

Dam Breach Assumptions/Parameters

Feature/Parameter Value

Maximum Embankment Height 45 feet

Length of Dam 620 feet

Crest Width 18 feet

Crest Elevation 957.96 feet

Reservoir Area at Top of Dam 42 acres

Storage Capacity at Top of Dam 616 acre-feet

Principal Spillway TypeConcrete weir with baffled

chute

Principal Spillway Crest Elevation 947.46 feet

Storage Capacity at Principal

Spillway Elevation255 acre-feet

Reservoir Area at Principal Spillway 25.5 acres

Emergency Spillway TypeGrass-lined open channel with

3H:1V side slopes

Emergency Spillway Crest Elevation 952.46 feet

Four Eagles Lake Dam Characteristics

Dam Breach Assumptions/Parameters

Summary of Breach Results

Scenario PMF

Pool Elevation at

Breach,

Initial (ft)

957.77

Time Breach Occurs 13:04

Breach Type Piping

Storage Volume at

Breach (ac-ft)608.61

Discharge at Dam, Peak

(cfs)63,063

Probable Maximum Flood

HEC-RAS Geometry Map

1D 2D

2D: 10-foot by 10-foot grid

2D Results – Inundation Boundary

2D Results – Inundation Boundary

2D Results – Inundation Boundary

2D Results – Inundation Boundary

2D Results – Particle Tracing

2D Results - Particle Tracing

2D Results - Particle Tracing

2D Results - Particle Tracing

2D Results - Particle Tracing

2D Results – Time Series

2D Results – Time Series

2D Results – Inundation Boundary

2D Results – Inundation Boundary

2D Results – Inundation Boundary

1D vs 2D Results – Inundation Boundary

1D vs 2D Results – Inundation Boundary

1D vs 2D Results – Inundation Boundary

1D vs 2D Results – Inundation Boundary

1D vs 2D Results – WSEL Profiles

840

850

860

870

880

890

900

910

920

930

940

100 2100 4100 6100 8100 10100

WS

EL

(fe

et)

Station

PMF Breach - Water Surface Elevation Profiles(Entire Downstream Reach)

2D

1D

1D vs 2D Results – WSEL Profiles

840

845

850

855

860

865

870

875

880

885

100 1100 2100 3100 4100 5100

WS

EL

(fe

et)

Station

PMF Breach - Water Surface Elevation Profiles(Downstream End of Study)

2D

1D

1D vs 2D Results – Velocity Profiles

0

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100 200 300 400 500 600

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fps)

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1D vs 2D Velocity

1D

2D

1D vs 2D Results – Velocity Profiles

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1D vs 2D Velocity

1D

2D

1D vs 2D Results – Velocity Profiles

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1200 1400 1600 1800 2000 2200

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1D vs 2D Velocity

1D

2D

1D vs. 2D Modeling

1D or 2D? Which one should we use?

1D vs. 2D Modeling

Hydraulic Modeling Utilizing HEC-RAS 2D

“All models are wrong, but some are useful.”

-George E. P. Box

“For every complex problem there is an answer that is clear, simple, and

wrong.”

-H.L. Mencken

Questions

Thank You

Mario G. Sebastiani

Water Resources Engineer- Civil & Environmental Consultants, Inc.

msebastiani@cecinc.com | P: 513.985.0226