Pierce County Flood Mapping Project:

1
Pierce County Flood Mapping Project
A Methodology for Digital Delineation of
Regulatory Flood Boundaries
Jared Erickson
Randy Brake
Gael Serviss
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Introduction
This study examined the feasibility of
generating 100 year flood plains from digital
terrain models using GIS.
1. Reason and Benefits of the study. 2. Pilot
Project. 3. Data and Methodology. 4. Project
Expansion 5. QC and Final Editing 6.
Conclusions Problems and Successes
3
Reason for Mapping Project

• County has obtained higher accuracy digital
  topographic mapping (from a DTM) with
  ortho-photos.
• Recent major flooding events have found that
  current FEMA mapping is not accurate to actual
  flooding conditions at many locations.
• Pierce County has the needed GIS resources and
  technical expertise to accomplish such mapping
  projects.
• Pierce County responded with interest and
  commitment to FEMAs invitation to participate as
  a COOPERATING TECHNICAL COMMUNITY (CTC).

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Mapping Project Scope

• Re-mapping focuses on numbered A-Zones only for
  both stream and river systems.
• Floodway delineation also not a part of the
  scope.
• All other other flood zones and floodways remain
  unchanged unless future detailed studies are
  completed.
• Some areas lacking adequate DTM and Ortho-photo
  coverage therefore, no change from existing FEMA
  mapping.

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Project Benefits

• Provide more accurate floodplain mapping than
  what currently exists.
• Aid in floodplain regulation and permitting for
  new development (i.e. elevations certificates,
  finish floor elevations, Deep and/or Fast Flowing
  Floodway, flood insurance, etc).
• Floodwarning and Flood response activities.
• Digital data much more useful in administering
  floodplain management programs.
• Aid many Federal, State and Local agencies and
  officials.

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FEMA CTC Program

• FEMA has very limited funding to complete mapping
  updates nationwide.
• Basically a new plan and strategy developed by
  FEMA to increase local involvement in, and
  ownership of, the flood mapping program.
• Many states, regional agencies and local
  jurisdictions has acquired the needed technical
  resources in flood hazard identification.
• Therefore, program is designed to use local
  resources, knowledge and expertise in developing
  new updated and more accurate mapping and more
  efficiently.
• Program aims to interject a tailored, local focus
  into a national program where unique conditions
  may exist the necessitate special approaches to
  flood hazard identification.

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Project Data
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Stage 1 Data Creation
1. Create hydro, topo, breakline, dtm_point,
and road coverages for the stream under study
from the DXFs.
2. Create centerline coverage and route of
stream using orthophotos and hydrography coverage.
3. Place monitoring stations from FEMAs
database on centerline using the route and
repositioning if necessary.
4. Create a cross section coverage using the
FIS Maps.
Arc/Info
AML
ArcView/Avenue
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Step 1A Getting DXF Tiles.
1. Select DXF Tiles within 1 mile of the stream
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Breaklines
Step 1B Process DXFs.
Roads
DXF Tile
Dtm Points
Hydro
Topo
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Step 1C Append to form stream coverages.
Individual DXF Tiles
Stream wide Coverage
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Step 2 Create Centerline.
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Step 3 Position stations using route.
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Step 4A Create Cross-Sections.
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Step 4B Extend Cross-Sections 1 mile.
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Stage 2 DTM Creation/Modeling
1. Generate DTM (Tin and Grid) of land surface.
2. Order and Intersect extended cross-sections.
Transform modified cross sections into PolygonZs.
3. Generate DTM (Tin and Grid) of 100 year flood
surface.
4. Generate Grids and polygons of 100 year
floodplain.
5. Adjust modeled floodplain with expert
knowledge.
ArcView
Avenue
Spatial Analyst
3D-Analyst
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Avenue Dialog/Scripts
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Hard Breaklines
Step 1 DTM Model Creation
Mass Points
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Step 2 Construct 100 Year Flood Surface
Re-order and Intersect Arcs
Make PolygonZ's
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Step 2 Model 100 year flood surface.
TIN
PolygonZs
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Floodplain Model
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Modeling Products
Floating Point Grid
Integer Grid
Polygon

• gt 1 Foot
• Polygon creation

• Bathymetric Grid
• Depth of Flood

• Floodzone boundary

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FloodZone Models
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3D Visualization
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Pilot Project Estimates
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Project Expansion
30 Streams and 7 Lakes
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Project Expansion
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The Model in Retrospect Sources of Error
New contours, old hydrology.
Stream channel evolution erosion, deposition,
and meandering.
Placement of stations and cross-sections.
Projection of station elevation from NGVD 27.
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FEMA Data Problems

• Problem
• 100-year flood surface from the Flood Insurance
  Study (FIS) is below the DTM surface Elevation
  from the DXFs.
• Basically, the new Ortho-DXF DTM is not
  matching up with the old hydraulic model.
• Result
• Fragmented floodplains with long stretches of
  stream with no 100-year floodplain.

No Floodplain
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FEMA Data Problems

• 100-year flood surface should be
• above the DTM Elevation.
• DTM Elevation from DXF-Orthos
• dips is above the 100-year flood
• surface.

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Quality Control and Final Editing
AutoCAD Process
1. Download GIS data as a .dxf
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Quality Control and Final Editing
AutoCAD Process
1. Download GIS data as a .dxf
2. Create a .dwg from the .dxf
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Creating the Drawing

• Insert roads, hydrology, FEMA floodplain, 100
  year floodplain, centerline, cross sections and
  spot elevations dxfs.
• Clean up scaling, point style, layers and color.
• Create a layout with a legend.
• Plot

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Quality Control and Final Editing
AutoCAD Process
1. Download GIS data as a .dxf
2. Create a .dwg from the .dxf
3. Print the drawing for markups
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Drawing Ready for Markups
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Quality Control and Final Editing
AutoCAD Process
1. Download GIS data as a .dxf
2. Create a .dwg from the .dxf
3. Print the drawing for markups
4. Draw Polygons
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Drawing the Polygons

• Create a new layer.
• Create a polygon by using polylines, making sure
  that they are joined and closed when fininshed
  with the shape.
• Draw on the top of the 100 year flood plain based
  on redlines.
• As you go smooth the line.

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New Polyline on the 100 Year Floodplain
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Quality Control and Final Editing
AutoCAD Process
1. Download GIS data as a .dxf
2. Create a .dwg from the .dxf
3. Print the drawing for markups
4. Draw Polygons
5. Print drawings for QC
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Ready for Quality Control
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Quality Control and Final Editing
AutoCAD Process
1. Download GIS data as a .dxf
2. Create a .dwg from the .dxf
3. Print the drawing for markups
4. Draw Polygons
5. Print drawings for QC
6. Wblock out polygon as a .dxf.
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Isolated Polygon for .dxf
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Quality Control and Final Editing
AutoCAD Process
1. Download GIS data as a .dxf
2. Create a .dwg from the .dxf
3. Print the drawing for markups
4. Draw Polygons
5. Print drawings for QC
6. Wblock out polygon as a .dxf.
7. Send to GIS.
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Conclusion
It is possible to model the 100-Year Floodplain
using GIS and DTMs.

• Faster and Less Expensive
• Reproducible Method

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Contacts
Gael Serviss Engineering Technician gservis_at_co.pie
rce.wa.us [phone]
Randy Brake Civil Engineer 2 rbrake_at_co.pierce.wa.u
s [phone]
Jared Erickson Information Technology Specialist
1 jericks_at_co.pierce.wa.us [phone]