Boulder County Flooding:

Estimating Population at Risk and Proposing Evacuation Points

1) Overview

For this project I would like you to take the role of a consultant hired by Boulder County to:

1. Estimate the number of county residents that would be threatened by a 100-year flash flood along Boulder Creek and other tributaries that flow into the Boulder Creek floodplain.

2. Propose aid and evacuation points near threatened areas of the city where aid could be coordinated. These would be areas out of the flood plain with easy access to major roads.

However, in order to get the low bid on this contract you submitted a budget without any money set aside to create new GIS data layers. The challenge is to prepare your analysis with only existing data and digital maps.

For a bit of background on Boulder's flood risk see the following website which includes an interactive floodplain map:

City of Boulder, Flood Management Program,,

A flood safety video is linked from:

2) What Data is Available Online and in KESDA?

A wide range of data is available online from:

1) Boulder County, Geographic Information Services, Data Downloads (you can use Esri Shapefiles or Layer Package files):

The most useful files here are those of floodplains.  These can be downloaded as a ZIP file.

2) City of Boulder, Planning and Development Services, GIS Data-Shape Files for Downloading,

The most useful files are:  City 100-year Floodplain, City Limits, and Main Roads & Street Centerlines.

3) Z:\Geog Files\Foote\boulder-county-census. This folder contains Boulder County census block-group boundaries and Boulder County census block centroids. These are subsets of the full census boundary files developed by the U.S. Bureau of Census and provided free with ArcMap software by ESRI.

4) Z:\Geog Files\Foote\BoulderCountyDOQQs. This folder contains a series of subfolders for each USGS 24,000-scale quadrangle that overlap Boulder country.  Within each subfolder are four orthophotos covering the NW, NE, SW, and SE quarters of each quadrangle. 

I have loaded the files for the Lyons, Hygiene, Boulder, Niwot, Eldorado Springs and Louisville quadrangles.   If  you want any of the others, please let  Ken Foote know and he will load them on the server.

The following map shows how the USGS's 24,000-scale DOQQ files overlay Boulder County.  The boxes with red labels are the quadrangle and quadrangle names.

USGS Quads over Boulder County

3) What Data Do You Need and How to Use It

Information on population is contained in the attribute files for the block group and block centroid maps.  Boulder County floodplains are available from the city's and county's data download site (listed above).  So the first question can be addressed with existing data if these files can be combined.

A wider range of data will be of use to in proposing evacuation points.  This analysis and map will probably involve using Google Earth, floodplain maps,road maps and, perhaps, air photos.

For each of the data sources you use, please be sure to read any available metadata files.  The metadata files will contain information about coordinate systems, map projections, variable types and variable values.

4) Organizing Your Data

One of the most difficult aspects of this project is keeping track of all of the data files you will use and transform.  I would suggest that--at a minimum--you keep these in ONE folder on your memory stick.  However, you might find it useful to keep each type of data in its own subfolder within one folder.

So, on youe memory stick you might create a folder called "BoulderFlooding"

When you copy the census data from the Z-disk, copy and paste the entire folder into "BoulderFlooding"

Z-disk copy folder

Similarly, when you unzip the data from Boulder County and the City of Boulder, make sure that the extracted files go into "BoulderFlooding" folder, perhaps into subfolders labeled "BoulderCounty" and "CityofBoulder".

5) Strategies for Answering the Questions

For this project I would like you to come up with a defensible strategy for both questions.  However,  it is worth considering possible strategies in general terms.

Who is at risk?

Here the basic strategy is to use the distribution of Boulder County floodplains to select census units.  Once census units have been selected by location within the floodplains, the number of people at risk can be totaled.  But what census units should you use?  The Census Bureau provides information aggregated at a variety of spatial scales including: 1) census tracts; 2) census block groups; and 3) census block centroids. 

The 100-year floodplain map provided by the County includes estimates for all drainages, not just Boulder Creek.  Although it is unlikely that all of these drainages would approach their 100-year peak in a single flood event, I think it is worth estimating the range between the minimum and maximum number of people who might be affected by a 100-year flood.

An important consideration are differences between the City and County estimates of the 100-year floodplain.  Why might these differ?  Can you develop a strategy that accounts for your uncertainty about which one is correct--and still estimate the minimum and maximum number of people who might be affected by a flood?

In using the floodplain map to select census units, it is important to consider whether you will examine only units falling in the floodplain or, instead, census units that are in and close to the floodplain.  It is possible to use a buffer to select census units close to the floodplain, and this my be a more realistic way to estimate how many people are at risk.

What Are Good Aid and Evacuation Points?

A number of characteristics might make for good aid and evacuation points--areas out of the 100-year floodplain, close to major roads or arteries, with large tracts of open space or public land where people could gather, yet close enough to the floodplains that people could walk to them.  The data you need to choose sites includes the Boulder County road map and open spaces.  You can also use Google Earth and DOQQ orthophotos to help identify school lots, parks, and other public spaces that might serve as aid and evacuation points. 

6) Going Further with Orthophotos

You may find it useful to add air photos to your maps. 

First, copy the air photos to your project folder.  The images are located in: Z:\Geog Files\Foote\BoulderCountyDOQQs. These are grouped according to the names found on the map in step 2.

Copy the entire folder for the files you wish to use. Again, put them in the "BoulderFlooding" folder on your memory stick. These files and folders are quite large (approx. 200MB for the BoulderDOQQs, for example), so be sure you have space on your memory stick before you copy them.

Z-disk DOQQ copy

You can add these DOQQ images to your ArcGIS map. If you add them to your map "as is" you will get a warning that they are in a different coordinate system. The DOQQs use geodetic latitude-longitude for their, the files from Boulder County and the City of Boulder are in State Plane Coordinates (Colorado North Zone). ArcGIS will place them close to their true position.

However, the DOQQs need to be projected if they are going to fit perfectly on the other files. If you would like to project the DOQQs, I have added instructions here.

7) Exporting Maps from ArcMap

To prepare maps for a PowerPoint presentation, you need to export them from ArcMap.  There are two methods:

1) File | Export Map.  This option allows you to save into a wide range of graphics file formats.  JPG, GIF, and PNG are compressed file formats that help to keep PowerPoint files small.  However, you will probably find that you will have to touch up the maps a bit before putting them into your presentation.  In this case, exporting BMP or TIFF files may be a good option since they offer more data for editing in Photoshop, Illustrator or other paint or draw programs. 

2) Use the Print Screen button on your keyboard to take a "picture" of the entire screen and put it on the Windows clipboard.  Then open a paint program like MSPaint or Photoshop, start a new file, and paste the clipboard image into the file.  Once in the Photoshop file you can crop out the parts of the image you don't need and touch up what's left.  If you try the Print Screen option, be sure to make your map as large as possible on the screen before you press the Print Screen button.  The large the screen area, the better the map image will appear.

You will have to experiment with the export options to get good results.

8) What to Turn In

For this project I would like you to create a PowerPoint presentation of no more than five slides to present your results to the Boulder City/County Office of Emergency Management. 

At a minimum, two of the PowerPoint slides should be maps that provide cartographic/graphical answer the two questions.

The PowerPoint presentation can be handed in on CD or memory stick and, if your TA is willing, by email.

The grading rubric for this project can be found here.

Last revised 2012.3.8.  KEF. 

Note: Terraserver instructions have been deleted from this file in transition to Arc 9.3.  Text and graphics are in TerraserverDownloadInstructions.html in this folder.

DEM instructions have been deleted from notes, but screen captures are still in folder.