Utilizing a Basinwide Approach for the North Carolina Flood Mapping

ProgramJohn Dorman, Program Director,NC CTS Flood Mapping Program

Gib Jones, P.E., Dewberry & Davis LLC

Background

Why North Carolina Is Undertaking This Project

NC vulnerability to hurricanes and flooding 14 federally declared disasters since 1989 Hurricane Floyd damages = $3.5 billion 4,117 uninsured/under-insured homes

destroyed as result of Hurricane Floyd Accurate, up-to-date flood hazard

information crucial to protect NC livesand properties

Why North Carolina Is Undertaking This Project

(Continued) Hurricane Floyd revealed flood hazard

data and map limitations Age of North Carolina FIRMS

55% at least 10 years old 75% at least 5 years old

FEMA’s mapping budget is finite NC receives only one updated flood

studyfor one county per year

Many counties and communities lack resources to take on this responsibility

North Carolina CTS Flood Mapping Program

Cooperating Technical State (CTS) partnership with Federal Emergency ManagementAgency (FEMA)

State ownership and responsibility for Flood Insurance Rate Maps (FIRMs)

Mapping Products High-resolution topographic data

and accurate Digital ElevationModels (DEMs)

Engineering studies Countywide digital FIRMs State-of-the-art, dynamic

Information Technology (IT) infrastructure

Real-time flood forecasting and inundation mapping capability

Basinwide Approach to Floodplain Mapping

Three Phases to Mapping Project

Phase I—bluePhase II—gold

Phase III—green

Phase I — Eastern Six River Basins

Addressing eastern six basins first Basins most affected by

Hurricane Floyd Comprise approximately half

of State

Phase I — Eastern Six River Basins

Affect 48 counties and 334 incorporated municipalities (in whole or in part)

Encompass approximately 21,200 linear miles of streams and rivers

Flood hazard mapping for remaining 11 basins to be discussed with General Assembly in 2001

Project “Scoping”

Meetings held with every community in 6 eastern basins

Their needs synthesized into basinwide objectives

Basinwide work plans developed

Flood Map Production Process

Topographic Data Collection and DEM Creation

Hydrologic & Hydraulic (H&H) Analyses

DFIRM Production Community Review and Due

Process

Two Study Contractors

Each to perform Topographic data collection

and DEM production H&H Analyses DFIRM production

Work broken out by basin

Topographic Data Collectionand DEM Creation

Includes LIDAR surveys Field surveys of structures and

channels Automated and manual post-processing DEM creation

Conducted on a basinwide approach Efficiency of scale realized in surveying

large geographic areas

Topographic Data Collectionand DEM Creation

Example: Status of Bare Earth DEM

Creation

Engineering Analyses

Hydrology and stream hydraulicsdo not follow corporate boundaries

Set up H&H models using basinwide approach Allows large areas to be study

contiguously Minimizes discontinuities and

mismatches

Digital FIRM Production

“Carve” up data into countywide maps Countywide maps more user friendly Corresponds to community-based

approach to floodplain management Merge data for counties in more

than one basin

Community Review andDue Process

Community meetings held oncounty or basinwide basis

Due Process 90-day appeal period Evaluate and resolve community

comments and concerns Update community ordinances

Printing and distribution of maps

Multi-Basin Counties

Many counties are in more than one basin

Basins will be completed at different times

Insert figure with schedule for different basins

Multi-Basin Counties

Neuse Basin

Tar-Pamlico Basin

Processing Multi-Basin Counties

Process different portions of county separately Produce separate preliminary maps Separate due process

Merge different portions after all portions have gone through due process

Prioritize multi-basin counties

Preliminary Map for Multi-Basin County

BLADEN COUNTYAND INCORPORATED AREAS

LumberRiver Basin

Cape Fear River Basin

“Preliminary revised flood hazard information not yet available for Cape Fear Basin. Please

refer to effective FIRM.”FIRM panel neatlines

Timeline for Multi-Basin County

FinalMeeting(Lumber)

End ofAppeal Period

(Lumber)

FinalMeeting

(Cape Fear)

End ofAppeal Period

(Cape Fear)

FinalElevationDetermin.

6 month compliance period

90-day appeal

period (Lumber)

90-day appeal

period (Cape Fear)

1stPublicNotice

2ndPublicNotice

1stPublicNotice

2ndPublicNotice

9-28-01 10-5-01 10-12-01 10-26-01 1-10-02 8-28-02 9-6-02 4-25-032-28-0312-26-0212-12-029-27-029-13-02

Eff

ecti

ve

Cou

nty

wid

e F

IRM

PreliminaryIssued

(Lumber)

Preliminary Issued

(Cape Fear)

Utilizing a Basinwide Approach for the North Carolina Flood Mapping

Program

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