David KnipeEngineering Section Manager

Automated Zone A Floodplain Mapping

FEMA floodplain designations

Zone AE: has elevation and

floodway mapped – from detailed

hydraulic model

Zone A: has only 1% chance annual

flood mapped – could be from any

source

Why Map Zone A’s?

• New FEMA guidance – all new zones must be model based

• Still 10,000+ stream miles in Indiana that are not model based

• Cost prohibitive to provide detailed zones in all areas

• Need quick, easy and inexpensive way to map quality floodplains

The Zone A project

• IDNR funded by OCRA / CDGB (not FEMA) to map 3,200 miles of Zone A floodplains

• Modeling exclusively using new LiDAR DEM’s

• Full “RiskMAP” modeling, including 5 profiles, Flood Elevation Points, FBS Points, SFHA boundaries and depth grids

Zone A tools

• Tools are a set of Python scripts inside of an Arc Toolbox – other tools (such as HEC-GeoRAS) are not needed

• Only stream centerline, cross sections and overbank flow paths need to be created in Arc.

• HEC-RAS model will be considered an approximate model (no bridges, no floodway).

Stream Selection

Use the CNMS database to determine

stream reaches that were not studied during

the Map Modernization

processClay

Parke

Putnam

Owen

Montgomery

Boone

Hendricks

Vigo

Morgan

Fountain

Monroe

Sullivan

Hydrology: Streamstats

hydrology for a stream reach will be completed by using the Purdue

regression equations as

found in the USGS application, Streamstats

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Hydrology: Results

• Results from Streamstats service returned as XML file and imported into Excel, plotted on log-log plot

• Final discharges based on best fit line, not actual values (standard Division practice)

• discharges adjusted by defining a split point to improve the fit of the line

• the user to choose either the regression results (50%) or the upper 68% or 90% confidence interval for use in the model

Hydrology: Results

Hydraulic pre processing

three hydraulic data features will need to be

created; stream

centerline, cross sections and overbank

flow paths

Hydraulic processing

• the initial script checks the geometry of the input data for anomalies• the second script pulls elevation data from the DEM and creates a HEC-RAS import file

HEC-RAS import

• File created from Hydraulic script imported into HEC-RAS

HEC-RAS Cross Sections

• Sections as imported represent LiDAR data for each line

Mannings’s N Values

• Automatically derived using relationships based on Anderson Land Use classification and NLCD data (2006)

HEC-RAS model development

• Cross section points must be filtered (> 500 points)

• Data evaluated for reasonableness and ineffective flow areas

• Discharges added to model from Excel spreadsheet

• Bridges not modeled, but adjusted for using ineffective flow, where necessary

Bridge “modeling”

Hydraulic Post Processing

• Run a series of scripts that generate– 1% annual chance floodplain

(S_FLD_HAZ_AR)– Depth grids and WSEL TIN’s for all 5

profiles– Floodplain Boundary Standard check

point– Flood Elevation Points (for INFIP)– Format largely meets DCS standards

1% annual chance floodplain

• Derived from subtraction of WSEL tin and DEM, with cleaning of edges and elimination of extraneous small shapes

Depth Grids

• New requirement from “RiskMAP” used in HAZUS and for visualization

Flood Elevation Points

• Used in INFIP for calculation of RFE at a point

• Created along stream centerline at 50 foot spacing, tagged with 1% flood elevation

Floodplain Boundary Standard check

• Ensures that floodplain delineations are matching topography

• Standard FEMA evaluation technique

• Points spaced along floodplain boundary at 100 foot interval