The impact of Surface Data Accuracy on Floodplain Mapping

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The impact of Surface Data Accuracy on
Floodplain Mapping

• CE 394K.3 GIS in Water Resources
• Term Project Presentation
• Fall 2007
• Prepared By Chelsea Cohen
• Tuesday, December 4, 2007

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Scope of Project

• Compare the plotting accuracy of HEC-RAS water
  surface elevations (WSE) when plotted on
• 10ft contours
• 2ft contours
• TIN
• Analyze the effect of floodplain
• plotting accuracy when in a
• Rural area
• Urban area
• Analyze the effect of varying peak flow and
  mannings roughness on water surface elevation
  using HEC-RAS

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Study Location

• Data taken from internship at Freese and Nichols,
  Inc. in Dallas, TX
• Study area is in Cleburne, TX
• Analyzed 2miles along Stone Lake Stream
• Stone Lake is a tributary that enters into East
  Buffalo Creek

DALLAS
50 miles
CLEBURNE
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TIN Terrain Model

• The TIN data was collected by the City of
  Cleburne in 2005
• CLIPPED TIN to study area
• Created 2ft and 10ft contours from the TIN
  surface using 3D Analyst
• Used Topo to Raster feature in Spatial Analysis
  to create raster surface from contour lines
• Convert Rasters to TINs using 3D Analyst

TIN Surface
2ft Surface
10ft Surface
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Assumptions

• Several factors that affect WSE
• Assumed that using accurate hydrology and
  hydraulic data
• Want to prove that

Poor Hydrology Poor Hydraulics Good Terrain
Inaccurate Floodplain
Good Hydrology Good Hydraulics Poor Terrain
Inaccurate Floodplain
Good Hydrology Good Hydraulics Good Terrain
Accurate Floodplain
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Hydrology and Hydraulic Data

• Used hydrology and peak flows developed at Freese
  and Nichols, Inc.
• Created a hydraulics model using HEC-GeoRAS
  Geometry and HEC-RAS
• 100YR Peak Flows
• No Structures
• No ineffective flow or levees
• Plotted floodplains using HEC-GeoRAS Mapping

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Benefits of GeoRAS Mapping

• Reduce time and effort of manually digitizing
  floodplain boundaries
• Tedious to interpolate WSE between cross-sections
  using 2ft contours
• GeoRAS automates plotting floodplains on TIN or
  raster surfaces, which are more accurate than
  contours
• GeoRAS can digitized floodplains in minutes,
  which used to take hours and days

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Drawbacks of GeoRAS Mapping

• The larger the raster cell size, the quicker that
  GeoRAS can process the floodplain delineation and
  less accurate
• The size of TIN controls the raster cell size for
  floodplain delineation
• For example, my tin was approximately 3.5 sq.
  miles
• I had originally wanted to plot it based on at
  least 1ft to 0.5ft raster cell size
• The smallest raster cell size I could use was 2ft
  because

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Drawbacks (cont.)

• Lesson Learned
• Size of the TIN area controls the raster cell
  size of the floodplain
• Highly accurate TINs dont always produce highly
  accurate floodplains

2ft
2ft
Legend TIN Surface 2ft Surface 10ft Surface
2ft
10ft
2ft Raster Cell Size for TIN and 2ft Surface
Floodplain Mapping
10ft Raster Cell Size for 10ft Surface
Floodplain Mapping
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Results 10ft Surface

• FEMA checks floodplain delineation to make sure
  that WSE at cross-sections follow contours
• 10ft Contour Floodplain
• USGS Quad Maps come with 10ft contours
• Extremely Inaccurate Method
• Creates floodplain that is smoothed out

Legend TIN Surface 10ft Surface FEMA 100YR
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Results 2ft Surface

• 2ft Contour Floodplain
• More accurate than 10ft contours
• Frequently crossed contours between
  cross-sections

Legend TIN Surface 2ft Surface
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Results TIN Surface

• Tin-based Floodplain
• More consistently followed 2ft contour lines

Legend TIN Surface 2ft Surface
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What happened?

• Need to double-check RAS generated floodplains
  for accuracy

Legend TIN Surface 2ft Surface
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Rural Areas

• In rural areas, crops can be ruined where
  flooding occurs
• Shows how much of the overbank will be flooded

Legend TIN Surface 2ft Surface
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Urban Areas

• Impact of detailed topography felt greater in
  urban areas that have structures and streets
  close to the floodplain
• Shows houses that are in and out of the
  floodplain
• Shows how water floods streets and ditches

Legend TIN Surface
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Conclusions

• Embrace technology, using HEC-GeoRAS to map
  floodplains can reduce mapping effort and
  increase efficiency and accuracy
• Need to increase the capability of HEC-GeoRAS to
  map low raster cell sizes for large TIN terrain
  surfaces
• Good Hydrology Good Hydraulics Poor Terrain
  Inaccurate Floodplain
• Good Hydrology Good Hydraulics Good Terrain
  Accurate Floodplain

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Project Work Left

• Analyze the effect of varying peak flow and
  mannings roughness on water surface elevation
  using HEC-RAS
• Compare different accuracy floodplain
  delineations for entire study area

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Future Work

• Effect of ineffective flow areas vs. levees on
  GIS Floodplain mapping
• Effect of structures on floodplain mapping
• Flood prediction model linking detailed
  floodplain mapping and weather data to warning
  systems for evacuation during storms

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Picture References

• http//www.ci.cleburne.tx.us/community/index.htm
• http//news.bbc.co.uk/1/hi/in_pictures/6948090.stm
  
• http//www.semp.us/publications/biot_reader.php?Bi
  otID215
• http//www.technotuesday.com/wp-content/uploads/20
  07/04/hug1.jpg

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Questions?