Depth and Velocity Grid
Methodology Evaluation
John Ingargiola, EMA
Dr. Shane Parson, URS
Don Glondys, URS
ASFPM National Conference 2011
May 19, 2011
Introduction
With the availability of new, non-regulatory Risk MAP
datasets such as velocity and depth grids, the FEMA
Building Science Branch has an opportunity to:
1.
Use flood depth and velocity data to better communicate
flood risk for the built environment (i.e., increase flood risk
awareness)
2.
Provide additional data and tools to local officials so they
can improve the permitting process and ensure that new
or retrofit construction is not only sited properly but
designed and built for specific hazard levels
2
Introduction (continued)
Opportunities (continued):
3.
Increase understanding and awareness of hydrodynamic
loads from flooding on foundations and basement walls
4.
Demonstrate that hazard and risk vary in the mapped
floodplain based on:

floodwater depth

location with respect to the channel

location of the floodway (velocities and depths may be
smaller outside the floodway)
3
Introduction (continued)
Opportunities (continued):
5.
Show that risks extend beyond flood depths
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Introduction (continued)
Purpose
 The purpose of this presentation is to show the results of
some FEMA early demonstration projects
 The intent is not to discuss the pros and cons of any of the
methodologies used to develop the datasets, but rather to
focus on the end products and why Building Science
believes that this type of data will be useful for permit
officials, design professionals, contractors and building
owners
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Introduction (continued)
Additional FEMA Building Science Branch outreach efforts:
 New Building Science Toolkit CD of resources (FEMA and
other publications) for new and retrofit construction,
including best practices for velocity considerations for
community outreach during the Risk MAP Resilience
meeting
• List of current Building Science publications by hazard:
riverine, coastal, seismic, and wind
• Links to online publications, from pamphlets to design guides
for hazard-resistant construction and retrofits
• Useful for local officials, builders/contractors, design
professionals, and residents
6
Introduction (continued)
Sample of Toolkit CD Contents
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Introduction (continued)
Additional FEMA Building Science Branch outreach efforts
(continued):
 Attendance by Regional Building Science POCs at Risk
MAP Resilience meetings
 Active integration of Building Science resources with the
Risk MAP Outreach program at the HQ level
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Overview of Depth and Velocity
Grid Datasets
 The development and
use of the Risk MAP
enhanced datasets and
products are an evolving
process.
9
Overview of Depth and Velocity
Grid Datasets
Notes:
1. The availability and
use of non-regulatory
datasets, including
depth and velocity
grid data, will be
discussed during the
Risk MAP Discovery
Meeting
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Overview of Depth and Velocity
Grid Datasets
Notes (continued):
2. It is extremely important that the community or State
request inclusion of the enhanced datasets as part of
the deliverables.
3. The community must also be prepared to explain how
it will manage and use the enhanced datasets.
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Overview of Depth and Velocity
Grid Datasets
 Depth and velocity distributions are based on:
• Hydraulic model analysis
• One dimensional (1D), quasi-two dimensional, or two
dimensional (2D) hydraulic models
• Flood depth
• Channel vs. overbank flood areas
 In 1D and 2D hydraulic models, flow velocity is
expressed as a depth-averaged flow velocity
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Overview of Depth and Velocity
Grid Datasets
 Velocity Distribution Plot from HEC-RAS
The dark blue
shows the
fastest velocity
while the green
is the slowest.
13
Overview of Depth and Velocity
Grid Datasets
Example of tabular
flow distribution
output for a single
cross-section from
HEC-RAS
HEC-RAS flow distribution option (USACE 2008)
14
Overview of Depth and Velocity
Grid Datasets
 In the table on the previous slide, the velocities listed are
the average velocities between two sets of points in the
cross-sections
 The velocities in the main channel (Chan.) are greater than
velocities in the left overbank (LOB)
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Overview of Depth and Velocity
Grid Datasets
Sample plot of velocity vs.
depth
Velocity increases as depth
decreases from the channel
bottom to the water surface
Open channel flow velocity profile and average
velocity (Leopold, 1994)
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Overview of Depth and Velocity
Grid Datasets
Hypothetical
example of a
DFIRM depth
– velocity grid
Note that the cross
hairs for the text
box should be
located over the
stream centerline.
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Overview of Depth and Velocity
Grid Datasets
Velocity grid datasets
 1D hydraulic models
• Show average velocity across the channel cross-section
• HEC-RAS provides velocity distribution along the cross-
section at specified intervals
• Require users to interpolate velocity distribution between
consecutive cross-sections
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Overview of Depth and Velocity
Grid Datasets
 Quasi-2D hydraulic models
• Based on 1D model results plus HEC-RAS flow distribution
calculations
• Combination of channel velocity and bank velocity data
• Still require users to interpolate velocity between crosssections
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Overview of Depth and Velocity
Grid Datasets
 2D hydraulic models
• Grid or mesh format
• Usually based on finite element or finite difference methods
• Spacing and size of grid or mesh can be changed to show
more detail in areas of interest (near structures, bridges,
confluences)
• Velocity can be calculated and shown as a “vector” that
includes both magnitude and direction
• Can also account for tidal effects
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Overview of Depth and Velocity
Grid Datasets
Sample output
from a 2D model
The dark blue
coloring
indicates slow
or no velocity,
while higher
velocities are
shown in green,
yellow, and red.
From FLO-2D software
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Regulatory and Non-regulatory
Products
Risk MAP products include both regulatory and nonregulatory data
 Regulatory – FIRMs and FIS reports (BFE, floodplain, and
floodway), required to be adopted by the community
 Non-regulatory – Flood Risk Database, Flood Risk Map, and
Flood Risk Report – used for local planning efforts, public
outreach and to communicate risks
The non-regulatory data include:
 Flood Risk Datasets (includes depth grid data)
 Enhanced Dataset Options (includes velocity grid data)
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Purpose of Flood Depth and Velocity
Analysis Grids
1. Demonstrate flood inundation as a function of the flood
event’s magnitude or severity
2. Serve as key input to HAZUS Risk Assessment analyses
3. Used as pre-screening criteria for mitigation project
potential (e.g., BCA ≥ 1.0 with first floor elevations in the
range of 10-year depths)
4. Increase flood risk awareness from varied contexts
(depth, probability, velocity, etc.)
5. Communicate that hazard and risk vary within the
mapped floodplain
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Key Building Science Objectives
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Key Building Science Objectives
Increase Risk Awareness by:
1.
Informing local building, permit, and construction
inspection officials about hazards beyond flood elevations
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Key Building Science Objectives
Increase Risk Awareness by (continued):
2.
Explaining the benefits of higher construction and
development standards, including updated or enhanced
building codes, to local officials
3.
Discussing the usefulness of hazard resistant designs and
construction materials with building owners to assist in
managing their risk
4.
Identifying and discussing building elements that are at risk
from high velocities, especially foundations
5.
Identifying and discussing how to use the data within velocity
grid dataset and coordinate with local building codes
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Key Building Science Objectives
Increase Risk Awareness by (continued):
6.
Demonstrating through success stories that hazard-resistant
designs or retrofits can reduce or eliminate future damage
from flood heights, velocity, and hydrostatic forces on
basement walls and other foundation types
7.
Providing useful technical guidance based on research and
expert opinions for developing asset-based mitigation options
8.
Using the wealth of information generated as part of the
hydraulic modeling process for new Risk MAP flood studies
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Key Building Science Objectives
Increase Risk Awareness by
(continued):
9.
Providing a comprehensive
view of resources and
assistance available to
communities and identifying
potential mitigation actions
that communities can initiate
to develop, design, and
construct stronger structures
to reduce flood losses
through distribution of the
Toolkit CD
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Velocity and Depth Grid Data
Development Issues
Understanding the requirements for each modeling
methodology will allow FEMA to provide guidance on the
best use of depth and velocity grid data
1.
•
•
•
•
•
Modeling
Complexity – models besides HEC-RAS may require some
simplifying assumptions
Coastal data is more complicated than riverine data
• Surge-dominated vs. runup-dominated
• May require 3D modeling
Data requirements
Accuracy
Availability of depth and velocity data within the model
output
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Velocity and Depth Grid Data
Development Issues
Requirements (continued):
2.
Methods for displaying (tabular, mapping, graphing) depth and
velocity data in the DFIRM environment
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Velocity and Depth Grid Data
Development Issues
Requirements (continued):
3.
Need to discuss the request for Risk MAP enhanced datasets
at the Discovery Meeting
4.
Agreement for stake holders to use the data after delivery
5.
Not every community will be able to use the depth and velocity
grid datasets
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Modeling Options
Varies based on funds and available data
 1D – use of Manning’s equation (steady flow) and UNET model
(unsteady flow)
 Quasi-2D – use of 1D model results plus HEC-RAS flow
distribution calculations or RAS Mapper to produce smoother
extrapolated values
 2D – use of finite element or finite difference methods with
equations for conservation of flow and energy and boundary
conditions
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Risk MAP
Early Demonstration Projects
The Early Demonstration Projects were undertaken to:
1.
Field test proposed products and fine tune prior to full
project use.
2.
Demonstrate progress toward achieving the Risk MAP goals
3.
Validate that new/enhanced Risk MAP products either
increased value to State, local, and tribal stakeholders, or
demonstrate which products require refinement
4.
Develop Lessons Learned for sharing across Regions and
States
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Risk MAP
Early Demonstration Projects
The Early Demonstration Projects were undertaken to
(continued):
5.
Validate data requirements
6.
Demonstrate to Regions how products can be applied to
ongoing projects
7.
Support HQ development of guidance
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Montgomery County
(Mohawk River), New York
 1D hydraulic model
• XS average velocity
• HEC-RAS: velocity distribution along cross section (XS)
• Mike 11: No velocity distribution along XS
• Need to estimate velocity distribution in between XSs
• Some methods: GIS techniques to interpolate in B/W XSs
• Manning’s equation
 2D hydraulic model
• Grid or mesh
• Computed as a result
• Velocity directionality
35
Montgomery County
(Mohawk River), New York
 1D Manning’s
equationbased
calculation:
Water Depth
The deepest
depth is blue with
shallower areas
as green, yellow,
and brown
(shallowest)
36
Montgomery County
(Mohawk River), New York
 1D
Manning’s
equationbased
calculation:
Roughness
coefficient
(each land use
has a different
roughness value)
37
Montgomery County
(Mohawk River), New York
 1D Manning’s
equationbased
calculation:
Velocity
The slowest
velocities are
shown in yellow
and green, while
the faster
velocities are
shown in blue,
pink, and red.
38
Montgomery County
(Mohawk River), New York
 2D with
velocity vectors
(modeled with
structures)
Building footprint
39
Quasi-2D: Georgia
 Based on 1D model results plus HEC-RAS flow
distribution calculations
• Combination of channel velocity and bank velocity data
• Still requires user to estimate velocity between cross-sections
1D Model
Quasi 2D Model
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Questions?
Building Science Helpline:
[email protected]
(866) 927-2104
http://www.fema.gov/rebuild/buildingscience
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