Base Map Preparation for

           Digital Flood Insurance Rate Maps

                   Near Lago Vista, Texas
                                                         By: Kevin Donnelly

                                                                                  

Table of Contents

        Introduction

        Background

        FIRMs and DFIRMs

        DFIRM Standards and Specifications

        DFIRM Contents

        Data Requirements

        Working with Data

        Acknowledgements

        References

Introduction

In May, 1999, the Lower Colorado River Authority (LCRA) entered into an agreement with the Federal Emergency Management Agency (FEMA) to act as a Cooperating Technical Community (CTC) partner.  The CTC program is an initiative by FEMA to increase local involvement and ownership of the flood mapping process.  This agreement essentially means that the LCRA will act as the map custodian for flood mapping of the Lower Colorado River.  As part of this agreement, the LCRA is to prepare six Digital Flood Insurance Rate Maps (DFIRMs) near Lago Vista, Texas.

DFIRMs are a digital version of FEMA's effective Flood Insurance Rate Maps (FIRMs).  Because this is a new program for both the LCRA and FEMA, an incremental approach is planned for the DFIRM conversion project.  The objective of this six panel test project is to perfect the DFIRM process and to establish the production procedures and processes.  Lessons learned for this project will then be applied to the entire Lower Colorado river basin corridor.

The objectives of my term project were to examine the benefits of DFIRM products compared to FIRM maps, summarize the DFIRM specifications required by FEMA, discuss the techniques used to prepare the final base map products, and prepare the base maps for the six DFIRM panels along Lake Travis near Lago Vista, Texas.

Background

For decades, the national response to flood disasters was the creation of flood-control structures, such as dams and levees, and providing disaster relief to flood victims.  In 1968, the U.S. Congress created the National Flood Insurance Program (NFIP) to combat mounting flood losses and escalating costs of disaster relief to the general taxpayer. The intent was to reduce future damage and to provide protection for property owners from potential losses through an insurance program that allows a premium to be paid by those most in need of the protection.  The National Flood Insurance Program is administered by FEMA.  FEMA produces FIRMs and now DFIRMs that show areas subject to flooding based upon historic, meteorologic, hydrologic, and hydraulic data.

The LCRA is a conservation and reclamation district established by the Texas legislature in 1934.  The district consists of ten counties comprising the watershed of the lower Colorado River.  The ten counties include: Blanco, Burnet, Colorado, Llano, Travis, Bastrop, Wharton, San Saba, and Matagorda.  The LCRA's mission is to prevent or aid in the prevention of damage to person or property caused by the waters of the Colorado River and its tributaries within the boundaries of the district. Recently the LCRA spent a great deal of money on aerial mapping of a zone 2000 feet wider than the 500 year floodplain (1000 feet on each side) along the Colorado River.  This digital topographic data was collected as part of the LCRA Phase 1 Contour mapping project.

           +   = 

FIRMs and DFIRMs

Flood Insurance Rate Maps are developed by FEMA or a FEMA contractor by conducting engineering studies referred to as Flood Insurance Studies in a flood prone community.  Using historic, meteorologic, hydrologic, and hydraulic data, flood elevations are developed for the 100-year and 500-year flood.  A 100-year flood has a 1 percent chance of being equaled or exceeded during any given year (a 500-year flood has a 0.2 percent chance).  This type of flood is referred to as the base flood and is a regulatory standard used by federal agencies and most states to administer floodplain managment programs.  The 100-year flood is also used by the NFIP as the basis for insurance requirements nationwide. Any area within the base flood limits is referred to by FEMA as a Special Flood Hazard Area.

FIRMs are distributed by FEMA to private citizens, insurance agents and brokers, community officials, lending institutions, and federal agencies.  Private citizens and insurance agents use the FIRM to locate properties and buildings in relation to their corresponding flood insurance risk zone.  Community officials use the FIRMs  to administer floodplain management regulations and reduce flood damage.  Lending institutions and federal agencies use the Flood Maps to locate properties and buildings and determine if flood insurance is required when making loans or providing grants for the purchase or construction of buildings.  Flood insurance is required to receive any loans or grants intended for the purchase or construction of any property or building within the 100-year floodplain.

Existing FIRMs are produced using manual cartographic methods and are distributed only through paper copies.  FEMA is now in the process of using new digital technology to convert  the existing FIRMs to new digital versions called Digital Flood Insurance Rate Maps.  DFIRMs will be a vast improvement over their paper counterparts in that they will be much easier to produce, store, and distribute.  Eventually, FEMA intends to allow easy access to DFIRMs through the internet, where anybody could search for the DFIRM panel in their area of interest, view or download the data, and print out a copy for themselves.  DFIRMs are also easier to update when new data becomes available (such as the digital aerial photos recently taken by the LCRA).  Only the portion of the data that is out of date needs to be changed.  Additionally, DFIRMs enable FEMA to store information that is not shown on the actual map product, but which is critical to the development of the floodplains, with the map product.  Examples of this kind of information are elevation data (contours are not displayed on FIRMs or DFIRMs) and digital versions of the text from the Flood Insurance Study report which includes the technical information and calculations used to developed the 100-year and 500-year flood elevations.

DFIRM Standards and Specifications

A major part of my project involved collecting and familiarizing myself with the standards and specifications developed for the creation of DFIRMs.  This was a much more challenging task than first anticipated.  One of the main problems is that FEMA is currently in the process of developing standards that address new DFIRM development using digital images as the base map, which is exactly what I am using.  It seems that the current standards were mainly developed to specify how paper FIRMs are to be converted into DFIRMs using digitizing techniques.  The LCRA and FEMA have agreed to use the following set of standards for the initial six panel pilot project:

1.Base Map Specifications for New Digital Flood Insurance Rate Map Product (FEMA, May 26, 1999) - This supersedes Section 6.6.1 of Guidelines

     and Specifications for Flood Map Production Coordination Contractors - http://www.fema.gov/mit/tsd/DL_bsmp.htm
 
 

2.Content Standards for Digital Geospatial Metadata (Federal Geographic Data Committee, June, 1998)

http://www.fgdc.gov/standards/documents/standards/metadata/v2_0698.pdf

3.Flood Insurance Study Guidelines and Specifications for Study Contractors (FEMA) - http://www.fema.gov/mit/tsd/FRM_SCg.zip

4.Guide for Preparing Technical Support Data Notebook (FEMA, May 1989, Revised January 1990) - mailto:mapmod@fema.gov

5.Guidelines and Specifications for Flood Map Production Coordination Contractors (FEMA, Draft February 17, 1999) http://www.fema.gov/mit/tsd/DL_FMPCC.htm

6.Specifications for Preparing Maps and Graphics (FEMA, Appendix B of Guidelines and Specifications for Flood Map Production Coordination Contractors, 1999) -  mailto:mapmod@fema.gov

7.Standards for Digital Orthophotos (U.S. Geological Survey, National Mapping Program, December 1996)http://rmmcweb.cr.usgs.gov/public/nmpstds/doqstds.html

Additional requirements and guidance were obtained through communication with Jane Huzil and Jim Murphy at Michael Baker, Jr., Inc. who is currently acting as Flood Map Production Coordination Contractor (FMPCC) for FEMA Region VI.

DFIRM Contents

The base maps for DFIRMs are to be either community supplied data or United States Geological Survey (USGS) Digital Orthophoto Quarter Quadrangles (DOQQs).  Community supplied data may be in either raster (image) or vector (line, point, ...) format.  Locally produced digital orthophotos may be at larger scales and higher resolution than USGS DOQQs, but must meet USGSDOQQ standards at a minimum. Aerial images that are not ortho-rectified are not acceptable. Vector files may be photogrammetrically compiled or digitized from orthophotos.

 The files must contain all transportation features (roads, railroads, and airports) for the community. If digital orthophotos are supplied, thesefeatures must be clearly visible. If vector files are supplied they must also contain transportation features. In vector format, roads may be portrayed as road centerlines, edge of pavement, or right-of-ways.

FEMA also desires to augment the USGS DOQQs or community supplied data with the following vector data:

Optional Contents – FEMA also desires the following features, if available: FEMA requires thematic separation of data by level, layer,attribute, or file (theme in ArcView). In other words, the roads should be separated from the streams or corporate limits by one of the methods listed in Appendix 7 of Flood Insurance Study Guidelines and Specifications for Study Contractors.  I will be using a modification of Method 3 described on page A7-28, which is the simplest method of separating and attributing the data.  Other methods require specific colors, lineweights, and layer names for each feature.  Method 3 simply separates the data by feature (county line, USGS quad neatline, Flood Zone A, ...) and has standard names for the data files.  The method described in the guidelines also has coded attribute numbers that go along with the data.  I will not be using these attributes as directed by the FMPCC.

Data Requirements

When data other than DOQQs are used, the following minimum standards must be met:

* A quad sheet paneling scheme is a way of dividing a standard 7.5' by 7.5' USGS Quad Sheet into either quarters or sixteenths.

Working with Data
 
 

Where is Lago Vista, Texas?


 
 
 
 


 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
                Texas w/ Travis County Highlighted                                                              Travis County showing location of

                                                                                                                        Austin, Lago Vista and Lakeway
 
 


 
 

    Here are the locations of the six DFIRM panels for which I will be creating base maps.  The panels are located along Lake Travis and contain most of Lago Vista and parts of Lakeway and Jonestown.  The panels to be produced include:
 
 

                          48453C0350E        48453C0315E

                          48453C0355E        48453C0320E

                          48453C0360E        48453C0325E
 
 
 
 

I am using the LCRA's aerial photos as the main part of the base map.  However, as you can see below the imagery only follows about 1000' on either side of the 500-year floodplain.  Therefore, I must use the USGS DOQQs to supplement the LCRA's data.  Conveniently, the DOQQs happen to be the same size and in the same paneling scheme as the existing FIRM panels.

                  22 Aerial Photo Image Files                                                                 Six DOQQs

The aerial photo image files are each one quarter the size of a DOQQ.  There are 22 files instead of 24 because the two aerial photo panels in the Northeast corner are more than 1000' from the 500-year floodplain. As you can see the DOQQs have various shades of red in them, while the aerial photos are in black and white.  The red color of the DOQQ can be changed to display as black and white.  Some adjustment in the brightness and contrast of the new black and white DOQQ is necessary to form a close match between the two data sets.

                                          1 Red DOQQ                                                    1 B/W DOQQ
 
 

Now I am able to combine the two data sets together to form one.  FEMA requires that there is no data overlapping, so the part of the DOQQ where there is aerial photo data available must be clipped off before "quilting" it to the aerial photo data.  Due to time constraints, I have not yet tried to clip the DOQQs but have found some possible methods (all using ArcInfo).

One method uses the RECTIFY command, specifying a clip box or clip cover (both of which are rectangular).  Another method requires a TIN license along with ArcInfo.  This involves converting the image to a grid with IMAGEGRID, then clipping the grid with the LATTICECLIP command.  LATTICECLIP can use an irregular polygon to clip a grid.  The output will still be rectangular with cells outside of the clipping polygon having values of NODATA.  The grid can then be converted back to an image with the GRIDIMAGE command.  The third method is very similar to the second except that it requires a GRID license and uses the GRIDCLIP command to clip the grid with a box or a polygon.

So pretend that the DOQQ below only has data were the aerial photo on the left is white.  I can then splice these two together to form a complete image base map file.


 
 
 
 
 
 
 
 


 
 
 
 


 
 


 
 

     Here is a representative picture of what a DFIRM base map panel will look like.  The red and yellow lines represent city streets and county roads, respectively.  The green polygons are building footprints.  The streets near the floodplain will have to be annotated.  This image is one quarter the size of the DOQQ shown above (lower right quadrant) and is the size of the new DFIRM panel.  The base map will be shown at a scale of 1" = 500' and will therefore have dimensions of about 20" wide and 22.5" tall.  Also, the image will be clipped along the black lines (DFIRM neat lines) and a standard FEMA DFIRM border, legend and title block will have to be added.

    When the final base map is completed the floodplains will have to be laid on top and annotated.  The floodplains will be created by intersecting the constant elevation surface of Lake Travis during the 100-year and 500-year floods with the elevation data acquired from the LCRA's aerial photos (shown below).  The flood studies were done by the LCRA during the 1980's using a DWOPER flood model.  Lake Travis was found to have a constant surface elevation even during the flood events.  The 100-year and the 500-year flood levels were found to be 716 feet and 728.5 feet above mean sea level, respectively.  The spillway elevation at Mansfield Dam is 714 feet above mean sea level.

   After determining the floodplains by intersecting the surface water elevation and the contour data, the floodplains will have to be adjusted so they match what is currently shown on the paper FIRM.  This is done because the scope of this project is to update the base map using the new image data and convert the FIRM into a DFIRM, but does not include the adjustment of the actual floodplains using the new contour data.
 
 


 
 
 
 
 
 
 
 
 
 

Contour data developed from the aerial photos taken along the Colorado River will also be included with the DFIRM submittal, however, this data is not displayed on the actual DFRIM panel.  Here is one DOQQ sized sample of the contour data.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Acknowledgments

Many thanks to the following people for helping me along the way!

References

Along with those listed in the DFIRM Standards section, information was taken from the following sources:

    Federal Emergency Management Agency, FEMA 258 - How to Use a Flood Map to Determine Flood Risk for a Property, May 1995.
    Lower Colorado River Authority, Task Agreement TA-01 - The DFIRM Pilot Project, June 1999.

    Lower Colorado River Authority WaterCo.,  High Water - A Guide to the Colorado River/Highland Lakes Floodplains From Lampasas to Bastrop Counties, August 1997.

    Maidment, David R., Olivera, F., Project Proposal: Flood Mapping in the Lower Colorado River, July 14, 1999.

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Questions, comments, suggestions?

Email me at kevin@mail.utexas.edu