Boulder County Flooding:
Estimating Population at Risk and Proposing Evacuation Points
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, BoulderFloodInfo.net, http://www.bouldercolorado.gov/index.php?option=com_content&task=view&id=3493&Itemid=1253
A flood safety video is linked from: http://floodsafety.com/colorado/boulderflood/
2) What Data is Available Online and in
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): http://www.bouldercounty.org/gov/data/pages/gisdldata.aspx
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, http://www.bouldercolorado.gov/index.php?option=com_content&task=view&id=3551&Itemid=1280
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
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.
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
When you copy the census data from the Z-disk, copy and paste the
entire folder into "BoulderFlooding"
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
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
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
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.
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
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
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
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.
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.