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Flood Velocity Data –Benefits, Examples, and 2D Model Considerations
Andy Bonner, P.E., CFM
Brian Cahill, P.E., CFM
ASFPM Annual ConferenceJune 21, 2018
Which scenario has the highest hazard/risk?
Informed Decision-Making… What Data Do We Have?
Flood Depth:3 ft
Flood Depth:1 ft
Scenario 1 Scenario 2
Fast-moving water Slow-moving water
Is the hazard/risk for the car the same as for the people or for the home?
Applications and Benefits of Velocity Data
Modeling Data (1D vs. 2D Differences)
Other 2D Modeling Velocity Outputs and Examples
Floodway Delineation and Other Uses
Agenda
From weather.gov (“Turn Around, Don’t Drown”)
• ______ percent of all flood-related drownings are
due to vehicles being driven into hazardous flood
waters
• It only takes ______ inches of rushing water to
carry away a small car
• ______ inches of rushing water can carry away
most vehicles
• ______ inches of fast-moving flood water can
knock over an adult
Other facts
• A moderately-sized person begins to lose stability
in ______ feet of water flowing at 2 ft/sec.
• Water depths greater than ______ feet generally
cannot be negotiated by those who can’t swim
The Dangers of Rushing Waters
50%
12
24
6
3
3
Velocity Applications
Vehicles and
Road Crossings
Credits: Accuweather.com
1-2 Feet of Swiftly Moving Water
Velocity Applications
Human
Maneuverability
Credits: The Daily Universe, Brigham Young University
Human Maneuverability:Ellicott City, MD – July 30, 2016
Human Maneuverability:Ellicott City, MD – July 30, 2016
NFHL
Human Maneuverability:Ellicott City, MD – July 30, 2016
“6 inches of fast-moving flood water can knock over an adult”
Ellicott City, MD – Quick & Dirty 2D Model
Flood Depth
Depth:1-2 ft
Ellicott City, MD – Quick & Dirty 2D Model
Flood Velocity
Velocity:> 10 fps
Velocity Applications
Erosion
Credits: Rollrecovery.com
In stream channels, erosion is most
likely to occur on the outside of the
meanders, where velocities are
highest
Actual erosion potential is based on
more than just velocity (channel
geometry, soil type, etc.), but erosion
generally doesn’t occur without higher
velocities
Erosion Basics
Velocity at Stream Meander
10% Annual Chance 1% Annual Chance
Velocity Velocity
Potential Erosion “Hot Spot”?
Velocity Applications
Buildings/Homes
Credits: Rollrecovery.comCredits: Yahoo News
Publications
• ASCE 24, Flood Resistant Design and
Construction
• FEMA P-259, Engineering Principles
and Practices for Retrofitting Flood-
Prone Residential Structures
Hazards in high velocity areas include:
• Debris impact forces
• Scour/erosion around foundation
• Hydrodynamic pressure on walls
5 feet per second is often cited as a
key design threshold for when open
foundations without enclosures are
recommended
Hazards to Buildings
Velocity Data
How Do We Get
and Use It?
Water Surface Elevation
(WSEL) Rasters (Grids)
Depth Rasters
Velocity Rasters
Model Outputs
Depth:3 ft
Velocity:2 ft/sec
WSEL:542.6 ft
100-yr Velocity Raster should not be confused with the 100-yr Velociraptor…
Velocity Rasters
…or the 100-yr Velociraster
Velocity Rasters
…or the 10-yr Velociraster
Velocity Rasters
1D Velocity Data• Confined to one direction• Velocity is calculated at every cross-
section• Up to 40 velocity distributions per
section o divided between the channel, left and right
overbanks Average Estimate of 1D Results
• Velocity data is interpolated between sections – Use with caution: Data is assumed to have the same direction of flow
1D vs. 2D Velocity Data
2D Velocity Data
• Flow of water is multi-directional
• Velocity data is calculated normal to
the cell face
• Spatially interpolated in between
• Can be displayed at terrain cell level
• Velocity vectors show direction and
magnitude of flow
• Accuracy dependent on quality of
terrain
4200 4400 4600 4800 5000 5200 5400
1120
1140
1160
1180
1200
Big Sioux River Detailed Study Plan: 100-YR 1D/2D DxV Floodway T rial 8/30/2017 Big Sioux River 438.0
Station (ft)
Ele
vation
(ft)
Legend
WS Max WS
0.5 ft/s
1.0 ft/s
1.5 ft/s
2.0 ft/s
2.5 ft/s
3.0 ft/s
3.5 ft/s
Ground
Bank Sta
.045 .15 .045 .033 .15
1D vs. 2D Velocity Data
2D Model Results
1D Model Results
Compare
Compare
Used extensively internationally to communicate flood hazard
Queensland Reconstruction Authority (Australia):
What About Depth x Velocity (DxV)?
FEMA G&S Flood Depth & Analysis Grids Guidance
Flood Severity Category Depth * Velocity Range
(ft2/sec) Depth * Velocity Range
(m2/sec)
Low < 2.2 < 0.2
Medium 2.2 – 5.4 0.2 – 0.5
High 5.4 – 16.1 0.5 – 1.5
Very High 16.1 – 26.9 1.5 – 2.5
Extreme > 26.9 > 2.5
Example Area (Kingman, KS)1
2
3
Flood Depths
DRAFT – not Official
Depth:1.5 ft
Depth:3.5 ft
Depth:3.5 ft
1% Annual Chance
Flood Velocities
DRAFT – not Official
Vel:4.5 fps
Vel:4.5 fps
Vel:3.5 fps
1% Annual Chance
Depth x Velocity (DxV)
DRAFT – not Official
DxV:High
DxV:Very High
DxV:High
1% Annual Chance
Depth * Velocity Data
Potential for
Floodway Use
2D Floodway Challenges
2D modeling considers overbank storage – remove it and flows will increase; like 1D
unsteady-flow floodways, 2D floodways will generally be wider than a 1D steady-flow
floodway
2D floodway must be “correct” everywhere – not just at XS
Effective Floodway (1D Steady)
New Floodway (1D Unsteady)
From www.fema.gov/floodway - “Managing Floodplain Development Through the
NFIP” publication:
“The floodway is the stream channel and that portion of the adjacent floodplain that must
remain open to permit passage of the base flood. Floodwaters generally are deepest and
swiftest in the floodway, and anything in this area is in the greatest danger during a flood.” (pg.
3-19)
Summary & Recommendations from a 2016 PTS Innovation Project for FEMA
HQ
• Long-term: Redefine the floodway concept altogether
• Short-term: Delineate 2D floodways that are initially based on Depth x Velocity
(DxV) – will be an iterative process until surcharges are within acceptable limits
Compass PTS Innovation Project for FEMA HQ:2D Modeling – Floodways
Comparison of Existing Floodways vs. 2D Depth x Velocity Grid
DxV EffectiveFloodway
More AppropriateFloodway??
Comparison of Existing Floodways vs. 2D Depth x Velocity Grid
DxV
EffectiveFloodway
More AppropriateFloodway??
Comparison of Existing Floodways vs. Depth x Velocity Grid
EffectiveFloodway
More AppropriateFloodway??
DxV
Big Sioux River Example (South Dakota)
Floodway: Iterative process using the
Depth x Velocity approach as guide
Big Sioux River Example (South Dakota)