Flood Hazard Analysis of
Delaware County, Ohio

by Michael Volk
Earth Science 551
Emporia State University


Situated at the northern fringe of Columbus, Ohio, Delaware County is a fast-growing and rapidly-changing area. Between 2000 and 2004, Delaware County's population increased nearly 30% to 142,503 residents. Despite this influx of new residents, the county retains a rural character.

For a geographic perspective of the county, click here. Delaware County is located toward the center of the state. A more detailed map of the county can be found here.

There are several major drainage basins within the county. This, coupled with a growing population and abundance of farmland, creates the need for comprehensive flood hazard assessment.

This project seeks to:

  1. Define regions of the county affected by a 100 year flood

  2. Describe land uses affected by a 100 year flood

This geographic information is presented in a layered fashion to ensure maximum amounts of information can be easily and effectively interpreted by the reader. All original data sets are ESRI shape files that were converted into IDRISI raster format, resized, and reclassified. Custom palletes and feature labels were applied.


  1. Areas affected by a 100 year flood.

    A 100 year flood defines the area where there is a 1/100 chance per year of a flood event. Determining the areas a 100 year flood would affect first requires a delineation of the county's hydrographic features.

    Map I shows the hydrographic features of Delaware County. The Scioto and Olentangy rivers and Big Walnut and Alum creeks are major north-south flowing rivers in the county. Several large reserviors, including the O'Shaughnessy, Delaware, and Hoover, are also present.

    Map I showing the hydrographic features of Delaware County.

    Map II shows the 100 year flood areas for the county. Using IDRISI's HISTO function, hydrographic features cover 3.4% of the image (which includes land outside the county boundary), while the 100 year flood covers 7.1% of the image.

    Map II showing areas affected by a 100 year flood.

    The most noticable features are the expansion of the Delaware Reservoir and Scioto River. Another difference is the expansion of Big Walnut Creek and its branching into several streams. Judging from the number of right-of-ways (ROW's), a minimal number of developed areas would be affected from the 100 year flood. There are relatively few ROW's in the northern section of the county. The county's two main towns, Delaware and Westerville, appear to be well-protected from the flood, as their ROW's are not overtaken by the flood area.

  2. Land uses affected by a 100 year flood.

    Geographic data can be used to determine which land uses would be most affected by a 100 year flood. Before deciphering which zoning would be affected, it is important to first define the zoning in the county.

    Map III presents data on the types of land use in Delaware County.

    Map III showing types of land use within Delaware County.

    The southern portion of the county is much more suburbanized than the rural north. The Delaware Reservoir, Scioto River, and Alum Creek are bordered mainly by wooded and agricultural areas. The O'Shaughnessy and Hoover reservoirs and the Olentangy River border significant areas of development. Although the Alum Creek Reservoir is bordered by the suburb Westerville to the southeast, the reservoir is surrounded by a wooded buffer zone of significant size.

    Map IV presents data on the land uses that would be submerged under the 100 year flood. Each roman numeral denotes the local land uses that would be flooded.

    Map IV showing land uses affected by the 100 year flood.

    Numeral I represents flooded, wooded areas. The extensive wooded area surrounding Delaware Reservoir would be covered by the flood. The wooded area is large enough to prevent widespread damage to adjacent agricultural and developed lands. Wooded areas would also be flooded along the Olentangy River in downtown Delaware. Less widespread areas of wooded land would be flooded around the Alum Creek and Hoover reservoirs.

    Numeral II represents agricultural land that would be flooded. Portions of this land would flood along the Scioto River and its newly-formed feeder streams. Other affected areas include the northern portion of Alum Creek Reservoir, Alum Creek, Big Walnut Creek, and along the new streams that flow into the Hoover Reservoir.

    Numeral III represents developed land that would flood. Developed areas would be flooded along the new feeder streams that flow into the Scioto River and O'Shaughnessy Reservoir. Developed areas to the west and south of Delaware would also flood closest to the Olentangy River and along its newly-formed feeder stream. Very few developed areas in Westerville would be affected. To the east of the Hoover Reservoir, developments along a new feeder stream would flood. North of the same reservoir, developed areas along the largely expanded Big Walnut Creek would be under water.


    Flood hazard analysis is an important tool used to predict exactly where and which land uses would be affected by a significant flood. In Delaware County, it is apparent based on the 100 year flood zone that wooded, agricultural, and developed lands would be harmed by a flood. The county's main towns, Delaware and Westerville, would be minimally affected despite their close proximity to major water sources. Many areas covered by a 100 year flood are not adjacent to permanent water sources.

    The use of such geographic data is an important component to comprehensive flood risk mitigation. Using this geographic information, county officials can make informed decisions to protect the land and residents of Delaware County.

    Data Sources

    1. Delaware County, Ohio, Regional Planning Commission (DCRPC). DCRPC GIS Services, World Wide Web homepage <http://www.dcrpc.org/HOW_DO_I/gismap.htm> [retrieved on 27 April 2006].

    2. Geography Network. Geography Network Explorer, World Wide Web homepage <http://www.geographynetwork.com/data/downloadable.html> [retrieved on 05 April 2006].

    3. Ohio Department of Transportation (ODOT). Ohio Counties, World Wide Web homepage <http://www.dot.state.oh.us/map1/cntymap.HTM> [retrieved on 26 April 2006].

    4. United States Census Bureau. State & County QuickFacts, World Wide Web homepage <http://www.census.gov> [retrieved on 27 April 2006].

      Webpage created May 1, 2006.