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)