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Depth and Velocity Grid Methodology EvaluationPowerPoint Presentation

Depth and Velocity Grid Methodology Evaluation

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Depth and Velocity Grid Methodology Evaluation. John Ingargiola, EMA Dr. Shane Parson, URS Don Glondys, URS ASFPM National Conference 2011 May 19, 2011. Introduction.

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### 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:

- Use flood depth and velocity data to better communicate flood risk for the built environment (i.e., increase flood risk awareness)
- 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

Introduction (continued)

Opportunities (continued):

- Increase understanding and awareness of hydrodynamic loads from flooding on foundations and basement walls
- 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)

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

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

Introduction (continued)

Sample of Toolkit CD Contents

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

Overview of Depth and Velocity Grid Datasets

- The development and use of the Risk MAP enhanced datasets and products are an evolving process.

Overview of Depth and Velocity Grid Datasets

Notes:

- The availability and use of non-regulatory datasets, including depth and velocity grid data, will be discussed during the Risk MAP Discovery Meeting

Overview of Depth and Velocity Grid Datasets

Notes (continued):

- It is extremely important that the community or State request inclusion of the enhanced datasets as part of the deliverables.
- The community must also be prepared to explain how it will manage and use the enhanced datasets.

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

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.

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)

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)

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)

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.

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

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 cross-sections

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

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

Regulatory and Non-regulatory Products

Risk MAP products include both regulatory and non-regulatory 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)

Purpose of Flood Depth and Velocity Analysis Grids

- Demonstrate flood inundation as a function of the flood event’s magnitude or severity
- Serve as key input to HAZUS Risk Assessment analyses
- 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)
- Increase flood risk awareness from varied contexts (depth, probability, velocity, etc.)
- Communicate that hazard and risk vary within the mapped floodplain

Key Building Science Objectives

Increase Risk Awareness by:

- Informing local building, permit, and construction inspection officials about hazards beyond flood elevations

Key Building Science Objectives

Increase Risk Awareness by (continued):

- Explaining the benefits of higher construction and development standards, including updated or enhanced building codes, to local officials
- Discussing the usefulness of hazard resistant designs and construction materials with building owners to assist in managing their risk
- Identifying and discussing building elements that are at risk from high velocities, especially foundations
- Identifying and discussing how to use the data within velocity grid dataset and coordinate with local building codes

Key Building Science Objectives

Increase Risk Awareness by (continued):

- 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
- Providing useful technical guidance based on research and expert opinions for developing asset-based mitigation options
- Using the wealth of information generated as part of the hydraulic modeling process for new Risk MAP flood studies

Key Building Science Objectives

Increase Risk Awareness by (continued):

- 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

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

- 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

Velocity and Depth Grid Data Development Issues

Requirements(continued):

- Methods for displaying (tabular, mapping, graphing) depth and velocity data in the DFIRM environment

Velocity and Depth Grid Data Development Issues

Requirements (continued):

- Need to discuss the request for Risk MAP enhanced datasets at the Discovery Meeting
- Agreement for stake holders to use the data after delivery
- Not every community will be able to use the depth and velocity grid datasets

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

Risk MAP Early Demonstration Projects

The Early Demonstration Projects were undertaken to:

- Field test proposed products and fine tune prior to full project use.
- Demonstrate progress toward achieving the Risk MAP goals
- Validate that new/enhanced Risk MAP products either increased value to State, local, and tribal stakeholders, or demonstrate which products require refinement
- Develop Lessons Learned for sharing across Regions and States

Risk MAP Early Demonstration Projects

The Early Demonstration Projects were undertaken to (continued):

- Validate data requirements
- Demonstrate to Regions how products can be applied to ongoing projects
- Support HQ development of guidance

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

Montgomery County (Mohawk River), New York

- 1D Manning’s equation-based calculation: Water Depth

The deepest depth is blue with shallower areas as green, yellow, and brown (shallowest)

Montgomery County (Mohawk River), New York

- 1D Manning’s equation-based calculation: Roughness coefficient
(each land use has a different roughness value)

Montgomery County (Mohawk River), New York

- 1D Manning’s equation-based calculation: Velocity

The slowest velocities are shown in yellow and green, while the faster velocities are shown in blue, pink, and red.

Montgomery County (Mohawk River), New York

- 2D with velocity vectors (modeled with structures)

Building footprint

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

Questions?

Building Science Helpline:

FEMA-BuildingScienceHelp@dhs.gov

(866) 927-2104

http://www.fema.gov/rebuild/buildingscience

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