In Iowa City, the Iowa River and its five tributaries – Clear Creek, Ralston Creek, Rapid Creek, Snyder Creek, and Willow Creek – transport snowmelt and rain within their banks. Throughout the year, water overtops the banks and spreads out into floodplains. The area of floodplain is greatest around the Iowa River (over 900 acres) and includes nearly 300 structures. Along Ralston Creek, the floodplain is less than half that size (about 400 acres) but it contains roughly 450 structures.
Floodplains are categorized according to the likelihood of flooding. The most likely flood is within the 1% Special Flood Hazard Area (SFHA - sometimes referred to as the 100-year floodplain). Less likely, but more devastating, is the 0.2% SFHA (sometimes referred to as the 500-year floodplain). The 0.2% flood elevations along the Iowa River are as much as four feet higher than the 1% flood elevations.
In flood hazard areas, new buildings, additions, remodels and repairs require a floodplain development permit as well as a building permit. In Iowa City, new buildings are elevated or flood-proofed to one foot above the 0.2% flood elevation. Existing buildings are elevated or flood-proofed to one foot above the 0.2% flood elevation if the original floor area is being increased by 25%, or if the project value equals at least 50% of the structure’s value.
Floodwaters are dangerous because they tear out trees and carry large debris that can destroy buildings. A car can be lifted and carried away in one foot of flowing water, and six inches of flood water can knock down an adult. Children should be instructed at a young age to stay away from floodwater in streets and streams. Floodwater also carries contaminants and can make garden produce unsafe to eat.
The safest way to protect your buildings and property from flood damage is to build or locate outside of a flood hazard area. If your home or business is in a flood hazard area, you can minimize the likelihood of damage by elevating the lowest finished floor and essential utilities (furnace, water heater, electrical service).
Localized flooding is often the result of failing or unmaintained infrastructure. You can report clogged inlets, culverts, and stormwater detention basins that have collected debris by going to ICGov.org, and using ICGovXpress. City staff will inspect, and contact whoever is responsible for maintaining this segment of the drainage system. If you don’t have a computer or experience difficulty using ICGovXpress, call 319/356-5120.
If your property is in a flood hazard area, you should have flood insurance. In Iowa City, floodplain premiums for structures in the 1% SFHA are automatically reduced by 15% because Iowa City participates in the Community Rating System. In Iowa City, floodplain premiums for structures in the 1% SFHA are automatically reduced by 20% because Iowa City participates in the Community Rating System.
Johnson County uses Alert Iowa to provide emergency information to residents with a voice or text message. You can register for Alert Iowa at www.johnson-county.com or by calling 319-356-5700. The Iowa Flood Information System (IFIS) provides up-to-the-minute information on Coralville Dam release rates as well as river gauge readings and flood stages on iowafloodcenter.org.
Is your home, apartment, or business in a floodplain? View the Floodplain Rental Property Map and Flood Hazard Areas Map. If you need a more detailed view, you can use the Johnson County GIS viewer. If you have questions or experience difficulty finding information, call 319-356-5132 for assistance.