Flooding

According to the Federal Emergency Management Agency (FEMA), “Floods are one of the most common hazards in the United States. Flood effects can be local, impacting a neighborhood or community, or very large, affecting entire river basins and multiple states.”

Floods cause human casualties and damage by inundating structures and roads. The can also create “fluvial erosion hazards” which occurs when floodwaters erode stream banks moving the stream channel.

Some floods develop slowly, sometimes over a period of days. But, flash floods can develop quickly – sometimes in just a few minutes and without any visible signs of rain. Flash floods often have a dangerous wall of roaring water that carries rocks, mud, and other debris and can sweep away most anything in their path. Flooding can occur behind levees and floodwalls when those structures are overtopped or breached. And, dam failures can cause particularly devastating floods. Not all flooding happens near a stream or river. For example, overland flooding can occur in low areas, such as when road underpasses are flooded by torrential rains.

In order to limit flood-related injuries and property damage, surface-water hydrologists study stream flow patterns and areas prone to flooding. You’ve probably heard someone refer to a 50-year or 100-year flood – these numbers are called “recurrence intervals” and indicate the statistical chance of a flood occurring in any one year. For example, a 100-year flood has a 1/100 or 1% chance of occurring in any year. People sometimes misunderstand this terminology and think that once a 100-year flood has happened, another will not happen for 100 years. This is not true, and there are areas that have seen multiple 100-year or greater floods in the same year – sometimes, within a month of each other!

Recurrence intervals are based on a statistical analysis of streamflow data, which highlights the importance of maintaining gaging stations and establishing long-term data sets. High-water marks, which establish the elevations of flood waters, are another important type of flood-related information. High-water marks are used in conjunction with recurrence intervals develop the floodplain maps that FEMA uses to administer the National Flood Insurance Program. State and local agencies use these maps to regulate development in floodplains. Floodplain maps also help developers and city planners build structures in a way that minimizes the damages that can occur from flood events. The Indiana Department of Natural Resources has developed an online tool to make floodplain mapping information readily available online through the Indiana Floodplain Information Portal.

Early flood warnings are another proven way to help reduce damages and save lives during flood events.  The U.S. Geological Survey (USGS) maintains more than 220 stream gages across Indiana that provide emergency managers and the public with real-time water level data 24 hours a day, seven days a week and 365 days a year. The National Weather Service (NWS) uses stream gage data and advanced predictive techniques to provide flood forecasts at many gaging stations. These forecasts can provide estimates of flood water levels up to seven days in advance. The NWS also issues flood watches and flood warnings to give people a “heads up” about the possibility of a flood event.

Water Resource Professionals in Indiana Focusing on Flooding and Stream Channels

  • David Knipe (Indiana Dept. of Natural Resources)
  • Jeff Woods (U.S. Geological Survey)
  • Bob Barr (Indiana Univ. – Purdue Univ. Indianapolis)
  • Vankatesh Merwade (Purdue Univ.)

Related Resources

Federal Emergency Management Agency’s Flood Map Service Center: https://msc.fema.gov/portal.

Indiana Department of Natural Resources Floodplain Mapping: http://www.in.gov/dnr/water/2454.htm#flood%20plain%20mapping.

Indiana Department of Homeland Security Flood Information: http://www.in.gov/dhs/2791.htm.

Indiana Fluvial Erosion Hazard Program: http://feh.iupui.edu/.

National Flood Insurance Program: https://www.floodsmart.gov/floodsmart/.

National Weather Service Significant River Flood Outlook: http://www.hpc.ncep.noaa.gov/nationalfloodoutlook/.

Purdue University’s GIS Services Soil-based Natural Floodplain Maps:  http://mapsdev.lib.purdue.edu/floodmap/floodmap.htm.

U.S. Geological Survey’s Daily Streamflow Conditions for Indiana: http://waterdata.usgs.gov/in/nwis/rt/.