Geographic Information Systems

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Maps By Mitch

New Construction within the FEMA designated Flood Zone

February 10, 2012

Welcome to "Maps By Mitch". A "blog" of sorts to highlight various maps and the techniques used to create them. I hope to have a new "map" posted on a monthly basis.

Since the historic flooding of the Susquehanna River and its tributaries in early September 2011 damaged thousands of homes and structures we began to wonder: "How much new construction has occurred within the Federal Emergency Management Agency (FEMA) flood zone?  And since the county uses the U.S. Army Corps of Engineers (USACE) provided flood model (which was built in 2001) to determine flood risk based on a river gauge, how much construction has occurred that may affect its reliability?"  This months map attempts to examine this question.

FEMA refers to a flood zone also as a Special Flood Hazard Area (SFHA). SFHA's are defined as the area that will be inundated by the flood event having a 1-percent chance of being equaled or exceeded in any given year. The 1-percent annual chance flood is also referred to as the base flood or 100-year flood (FEMA, 2010). SFHAs are labeled in many ways; however, Columbia County is defined as Zones A, AO, AE, X, and 0.2-percent-annual-chance. Zones A, AO, and AE are used to define the base or 100-year flood. This does not mean that a flood of that size will happen once every 100 years. It means that a flood of that size has a 1 percent chance of happening every year. Therefore, the zone defined by 0.2 percent annual chance, or the 500 year flood, has a 0.2 percent chance of happening yearly (Perlman, 2011). Zone X defines areas that are considered to be outside of the 500 year flood zone. For more information please visit The USGS.

To reiterate, the purpose for this scenario is to show the number of structures that have been built within the 100-year and 500-year flood zones from 2002-2011.  To view the maps larger, please click image(s).

To determine the number of structures that have been built since 2002, a few steps needed to be taken. First, I added the Federal Emergency Management Agency's (FEMA) flood layer. This layer is comprised of zones A, AO, AE, X, and 0.2 percent annual chance which were defined earlier.

Because the FEMA flood layer is comprised of several zones, I used the Select by Attribute tool to select the polygons that made up the 100-year and 500-year flood zones.

Once I had the 100-year and 500-year flood zones extracted and displayed as individual feature classes, I added the county parcel layer which contains various attributes about each parcel.  The attribute that I was interested in was the year a structure was built. So I used the Select by Attribute tool to select all of the parcels that where built between 2002 and 2011. Once those parcels were selected I ran the Select by Location tool, using the current selection, to locate the parcels that not only were built between 2002 and 2011, but also have their centroid within either the 100- or 500-year flood zone. The next four images demonstrate this process.



Then, I created a new feature class entitled "New Builds" which I used to plot the actual location of the structures. I also created two new attribute fields called flood zone and municipality, which allowed for symbolizing "New Builds" based on the flood zone and municipality that it fell within.  Those structures that are within the 100-year flood zone are symbolized as red dots and those that are within the 500-year flood zone are black dots.

Once finished with the previous step, I exported the attribute table from "New Builds" into Microsoft Excel where I was able to analyze the final data.

Since 2002, there has been a total of ninety-four new structures built within a flood plain. Of the ninety-four, fifty-one were built within the 100-year flood zone. The five municipalities that have had the most structures built within either the 100-year or 500-year flood plain are: Scott Township (40 structures), Bloomsburg (29 structures), Greenwood Township (4 structures), Hemlock Township (4 structures), and Fishing Creek Township (3 structures).

-FEMA. (2010, August 11). Fema, flood zones. Retrieved from
-Perlman, H. (2011, December 22). Floods: Recurrence intervals and 100-year floods. Retrieved from


3/7/2012 12:37:01 PM
Very interesting! Thanks for doing this, Mitch! We might be calling you for more details!
- Susan Schwartz
Press Enterprise

3/15/2012 3:51:45 PM
Hopefully this points to a cahnge in strategy for FEMA. In the past they did not request the point cloud as part of the deliverable, nor did they coordinate with local partners. This new direction should help to increase the value of FEMA's mapping efforts and allow local partners to chip in funds to improve the quality and/or expand the mapping area.

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