Enhancing the Communication of Flood Risk Through Inundation Mapping

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Enhancing the Communication of Flood Risk Through
Inundation Mapping
National Hydrologic Program Managers
Conference July 11, 2007

• Doug Marcy
• OCWWS HSD
• NOS Coastal Services Center

Dan Miller Assistant City Engineer Overland Park,
KS
Frank Richards OCWWS HSD
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Workshop Agenda

• Overview and Logistics Doug Marcy
• NWS Flood Severity Inundation Mapping Doug
  Marcy
• - Presentation
• - AHPS web demo
• Pilot Project Indian Creek, KS (Dan Miller
  City of Overland Park, KS)
• Internal Project Website and Draft Content
  Discussion Frank Richards
• Feedback and Discussion Audience Input

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Brief Overview

• Background
• Pilot projects
• Methodologies/Standards
• Current efforts
• Dissemination
• Future plans
• Some questions to think about

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Background

• The NWS utilizes a three-tier, impact-based flood
  severity scale with the categories minor,
  moderate, and major flooding
• For each NWS river forecast location, flood stage
  and the stage associated with each of the NWS
  flood severity categories are established in
  cooperation with local public officials
• NWS river forecasts and flood categories are
  presented via Advanced Hydrologic Prediction
  Services (AHPS)

NWS Manual 10-950, Definitions and
General Terminology (NWSPD 10-9).
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NFIP vs. NWS Categories
NWS
NFIP

• Based on potential damages to property and/or
  public threat
• Subjective analysis based on historical impacts
  and coordination with local emergency managers
• One forecast value given at flood forecast point
  only
• Traditionally not mapped
• Used in flood forecasting and warning
• No regulatory/legal implications
• Used for evacuation and road/bridge closure
  decisions

• Based on historical stream flow statistics
• Based on hydrologic and hydraulic analysis
• Based on steady state conditions
• Objective analysis
• Uses GIS techniques for mapping
• Relies on accurate elevation data
• Values given at multiple cross sections along
  river reach
• Has regulatory/legal implications

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Need For Collaboration

• Two Agencies conveying flood risk
• One for regulatory/legal insurance purposes to
  reduce damage costs
• One for protection of life and property
• Both have different methods of showing flood risk
• Both need to work together to limit overlap and
  potential confusion to users of flood risk
  information

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Confusion?
HURREVAC Inland Flood Module

• At some NWS forecast points major flooding
  occurs before the 100 year (1 chance flood)
• NWS Definition extensive inundation of
  structures and roads
• Both products show elevation of water surface,
  so they can be compared.
• How does 100 year (1 chance flood) relate to
  the NWS categories?

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Advanced Hydrologic Prediction Services (AHPS)
Objectives

• More precise forecasts at many time-scales
• Information to make risk-based decisions
• Easy product access
• Visually oriented products

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NWS Flood Severity and Local Impacts
Text Based Format on AHPS Web pages (From NWS
Form E-19)
Graphical Format (Gage Height vs. Elevation)
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Pilot Projects

• North Carolina Tar River Basin as part of
  enhanced flood warning system
• - Graphical E-19 maps for 9 sites based on flat
  WS
• Albany, GA maps of NWS flood categories based on
  Georgia Power dam break model data WFO
  Tallahassee, FL
• Charleston, WV WFO mapping using flat WS
• St. Johns River attempt at graphical e-19s NOAA
  Coastal Storms
• NWRFC prototype maps illustrating graphical e-19
  data for Portland, OR NOAA Coastal Storms
• Indian Creek at Overland Park, KS as part of FEMA
  flood insurance study enhancement with Region 7,
  City of Overland Park, and Johnson County
• - Modeled and mapped NWS flood categories
• - Changed flood stage and major flooding
• - Pleasant Hill WFO
• Goldsboro North Carolina pilot and methods and
  standards
• - Watershed Concepts
• - NC Floodplain Mapping Program and USGS
  datasets
• Mapping 35 points in 5 Gulf of Mexico States
• - Hurricane Supplemental
• - Topographic and Engineering Inventory
• 17 sites in NC will come on-line in October based
  on NC floodplain mapping work with watershed
  concepts and USGS

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Pilot project with FEMA Region 7

• Johnson County, KS (City of Overland Park)
• Current flood study was underway for the entire
  county
• County is an example of a CTP
• Flooding Source Indian Creek
• Overland Park NWS forecast point (OPDK1)
• Redefine NWS Flood Categories
• Adjust flood stage and major flooding 2 feet
  based on mapping and reanalysis
• Inform EMs and all affected agencies of change
• Perform Inundation Mapping using NWS flood
  categories
• Flood Stage, Moderate, and Major Flooding
• Combine with NFIP HH Analysis to produce map
  products for NFIP and NWS Flood Severity

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Benefits Of Combining Efforts

• Clear relationship between NFIP flood zones and
  NWS flood categories
• Graphical displays of flood severity
• Easier to understand
• Communicates flood risk more effectively
• Re-evaluate existing flood categories based on
  new data or development

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Goldsboro Pilot and Methods and Standards

• Develop Methods Standards for Graphical Flood
  Severity Inundation Mapping
• Develop prototype mapping for sites in the Neuse
  River basin

Prototype flood severity inundation map that
meets FEMA DFIRM standards (scale, cartography)
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Methods and Standards for NWS Flood Severity
Inundation Maps

• Created for NWS by Watershed Concepts via
  EarthData International
• Minimum standards for mapping topographic and
  base data
• Vertical Datums
• Hydraulic modeling methods
• Inundation map data standards
• Mapping methods
• Map scales and layout
• Consistent with FEMA DFIRM standards
• Pilot mapping study (Goldsboro, NC)

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Methodology

• Data Requirements and Sources (Datums) NAD83 and
  NAVD88
• Topography and Engineering Data Inventory
• Data Request
• Hydraulic Modeling
• GIS Methods
• Final Products (Dissemination)

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Data Requirements and Sources

• Stage Impact Data (Form E-19)
• Vertical Datum Reference Data
• Topographic Data (DEM)
• LIDAR Elevation Data
• NED USGS Elevation Data
• Local Elevation Data
• Cross Section Data
• Engineering Data (FIS)
• Derived From DEMs
• USGS Rating Curves (stage/flow relationships)

Flood Categories 19.0 Feet MINOR FLOODING
(FLOOD STAGE) 24.0 Feet MODERATE
FLOODING 32.0 Feet MAJOR FLOODING
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Data Requirements and Sources

• National Flood Insurance Program (NFIP) Flood
  Insurance Studies (FIS)
• HEC Model Data (Channel Geometry)
• WS Profiles
• Elevation Data Sources
• 1/3 Arc Second (10m) DEMs from USGS
• Local LIDAR data or stereo contours
• NWS Form E-19 Data from WFOs and RFCs
• Flood Definitions
• Impact Information
• Gage Datums and Rating Curves from USGS
• Stage vs. Flow Relationships
• Vertical Datum of Gage
• Datum Transformation From NGS (VERTCON)
• NGVD29 To NAVD88 Conversion

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Which Vertical Datum?
NGVD 29
We have to switch!!
NAVD 88
The official vertical datum according to the
Federal Register is NAVD 88 and is summarized as
follows This notice announces a decision by the
Federal Geodetic Control Subcommittee (FGCS) to
affirm the NAVD 88 as the official civilian
vertical datum for surveying and mapping
activities in the United States performed or
financed by the Federal Government, require that
all Federal Agencies using or producing vertical
height information undertake an orderly
transition to NAVD 88. Dated June 21, 1993
http//www.ngs.noaa.gov/PUBS_LIB/NAVD88/Navd88-F
edReg.pdf
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Example Louisburg, NC
USGS Datum NGVD 29 - 176.71 FT Lat 3605'34",
Long 7817'48"
VERTCON - Conversion Latitude 036 05 34
Longitude 078 17 48 NGVD 29 height 176.71 FT
Datum shift (NAVD 88 minus NGVD 29) -0.965 feet
Converted to NAVD 88 height 175.745 feet
http//www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.p
rl
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Current Efforts (ongoing)

• Develop a topographic and engineering data
  inventory for future mapping at 526 points in 5
  Gulf Coast States (AL, FL, MS, LA, TX)
• Develop flood severity inundation mapping at 35
  NWS forecast points in 5 Gulf Coast States
• Flood inundation map libraries will be delivered
  via the AHPS Web pages
• Supported by Hurricane supplemental funds

MAP LIBRARIES, not just categories Dont have to
worry about category changes Incremental stages
action/flood stage to major (FOR)
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Topographic and Engineering Data Inventory

• Develop a geospatial database for the NWS river
  forecast sites in study area
• Topographic data inventory developed by phone
  interviews with local State officials
• Engineering data inventory developed by combined
  review of available Q3 data and existing FIS
  reports, along with data request to FEMA archives
• Data entered through GIS interface database can
  be queried and displayed spatially

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Topographic and Engineering Data Inventory
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Criteria for Site Selection

• Available topographic data of sufficient
  resolution (roughly 2 contour interval or 5
  meter DEM)
• NFIP detailed study for stream reach at NWS
  forecast site (study completed since mid 90s)
• Prioritization by NWS local service hydrologist
  based on impacts and severity of flooding at sites

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Flood Severity Inundation Mapping at 35 NWS
forecast points in 5 Gulf Coast States

• 50 potential sites have been identified
• Request for engineering data from FEMA archives
  for selected sites is being processed
• Terrain data has been received for 5 sites
  terrain data requests and processing for
  remaining selected sites is ongoing

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Potential NWS forecast points for Inundation
Mapping
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Hydraulic Modeling Details

• 1.) Topography and Engineering data inventory
• 2.) Filter based on methods and standards
  criteria (2ft contours or 5m DEM data and
  available model data)
• 3.) Data request Region IV and VI RMCs
• 4.) Develop new model from FIS model (truncate
  reach to 1-2 miles up and down stream)
• 5.) Existing geometry left alone
• 6.) Start with 10, 50, 100, and 500 year WS
  profiles
• 7.) Choose suitable mapping interval based on
  topo and areal extent of flooding
• 8.) Input flows at upstream boundary to match
  USGS rating at x-section nearest gage (use
  HEC-RAS rating to ensure match)
• 9.) Use normal depth solution for downstream
  boundary based on WS profile slope from the 4 FIS
  profiles.
• 10.) Calibrate model and export results to GIS
  for mapping

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Hydraulics

• Adapt and revise existing Hydraulic Model
• Determine stage interval for inundation map
  development, based on local conditions and
  terrain resolution
• Develop determine stage-discharge relation
  (rating curve) for range of inundation mapping

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Hydraulics

• Compute 2-D Water Surface profiles for each stage
  interval to be mapped (HEC-RAS)

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Flood Surface DEM Flood Boundary
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GIS Mapping

• Convert 2-D profile to 3-D Water Surface

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GIS Mapping

• Convert to Raster

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GIS Mapping

• Build attributed symbolized polygon shapefiles
  in ArcMap

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• Inundation Maps served at http//www.weather.gov/
  ahps/
• Web Site DEMO

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Project Schedule

• First site came on-line via AHPS web pages for
  Goldsboro, NC on June 1, 2007
• 17 Total Sites in North Carolina coming on-line
  by October 2007 (Tar, Neuse, Lumber, Cape Fear,
  Lumber)
• Indian Creek, KS under review by CRH may come
  on-line soon
• Topographic and Engineering data inventory
  available for 526 forecast points in 5 Gulf Coast
  States as ESRI Geodatabase
• 35 sites in 5 Gulf Coast States will be
  delivered in 4 stages and may start coming online
  in Q1 of FY08 (final delivery expected in January
  2008)

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Future Plans

• Briefings to FEMA Regions by NWS regional staff
  (Region II and III and maybe IV and VI pilots)
• Prioritize forecast locations by FEMA region
  based on discussions
• HPMs attend FIS scoping meetings with either
  CTPs or other communities FEMA will map bring EM
  contacts too
• Obtain other funding sources to bring to the
  table (states, counties, municipalities)
• Provide any hydrologic data to FEMA to assist
  with HH process
• Work with FEMA to show benefits of NWS map
  libraries for HAZUS analysis
• Work with FEMA CRS program to show benefits of
  communities having map libraries to offset
  insurance costs
• Develop internal NWS HSD website for feedback,
  discussion, materials, etc....
• Formalize the methods and standards document
  and co-author with USGS to get approved at ACWI
  subcommittee on hydrology and ultimately by FEMA
  as an appendix to guidelines and specs for
  mapping partners document

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NRC Study Objective 1Map Integration

• Leverage resources to mitigate flood impacts
  through the integration of NOAA Inundation Map
  Library Guidelines into FEMA FIS Guidelines
• Understand the accuracy limitations and benefits
  of this effort.
• Should additional NFIP CRS credits be given to
  communities who produce map libraries?

National Research Council Committee on Floodplain
Mapping Accuracy
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Modeling Approaches?

• Steady flow vs. Unsteady flow
• Single value rating curve vs. looped rating curve
• Same stage can result in different inundation
  patterns
• Is timing of the flood important (peak and
  duration)?
• Site dependent
• Tributary effects on main stem and vice versa
• Tidal and storm surge effects
• One dimensional vs. Two dimensional
• Is flow across floodplain important?
• Flat areas with sinuous/braided pattern
• WS elevations only accurate at point of
  intersection with x-section and channel center
• Friction values are more accurate with more
  physically based model
• Mapping considerations
• Computational constraints

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Mapping Uncertainty?

• Three main sources of uncertainty (error)
• Topographic data for hydraulic modeling and
  inundation mapping
• Effective friction coefficients (Mannings n)
• Model validation data (streamflow, high water
  marks, remote sensing data)
• Models applied in real-time also require forecast
  flows from precipitation data and rainfall runoff
  models
• We are currently mapping a single inundation
  boundary
• Should we be mapping a zone of uncertainty based
  on a /- percent error?

VS.
/- 20
(Bales and others, 2007)
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Verification?

• Streamflow measurements
• (/-) 20 error for peak flow conditions
• Ungaged streams
• High water measurements
• Reliability
• Scarcity
• No timing
• Remote sensing
• SPOT and RADARSAT
• LANDSAT
• AVHRR
• Cooperative observers
• Web cams
• Post event studies and calibration
• Changing channel conditions over time
• Result in map maintenance necessity

(Bales and others, 2007)