How to Make a Flood Map

Find Hazardous Areas Using Topography and Historic Water Levels

Mar 10, 2009

Living near rivers that have historically flooded can be hazardous, especially if the areas around the rivers are flat. A flood map identifies vulnerable areas.

Spring is here, and with melting snow comes swollen rivers. Many low-lying areas in America's Midwest have to deal with this hazard every year as the spring season brings either more rain, melting snow, or - like in the upper Midwest - both. One of these such cities is Grand Forks, North Dakota, located in the Red River Valley. In 1997, the Red River Valley experienced a high level of precipitation, with increased raining in the fall, a record high snowfall in the winter and more rain in the spring. Come April 22, 1997, it was more water than the Red River could handle.

Making a Flood Map

A topographic map of the area of concern can usually be found at the public library or a local university library. Historic and recent hydrological data is provided for every city in the U.S. near a river by NOAA's National Weather Service. Information from this website will show the present water level of the river as well as historical river crests.

Start out by laying down a copy of the topographic map in a well-lit area - preferably on a light table - and looking for the contour lines and what elevation they represent. Locate points of interest, such as houses, schools and churches. Refer to the hydrological data from the National Weather Service for historical crests. This information will be given in feet. Add the height of the historical crests to the elevation of the river given by the contour lines on the topographic map. For example, The elevation of the Red River is usually about 779 feet above sea level. In 1997, the Red River crested at 54.35 feet, so the elevation of the Red River during its maximum crest was 833.35 feet above sea level.

Identify Areas Vulnerable to Flooding

Locate the contour line that represents the elevation of flooding that has been calculated. Use the colored pencils to outline and shade areas on the map that will be effected by various water levels. Use the historical crest data to identify areas on the map that have been flooded in the past. Unless flood mitigation development has occurred, these areas will probably be affected by flooding in the future. Take note of any points of interest that will be flooded if the river crests to historical water levels. Also take note of any roads that will be flooded, blocking routes of evacuation.

Areas most vulnerable to flooding will be those in close proximity to the river. Likewise, any areas in the river valley may also be affected, depending on how high the flood waters reach. During the April 1997 Red River flood, the entire city of Grand Forks, ND had to be evacuated because of flood waters. There is hardly any elevation change between the banks of the Red River and the city of Grand Forks. Once the water level had exceeded the river banks, the water just kept spreading out and almost the entire city was flooded.

Source

The copyright of the article How to Make a Flood Map in Hydrogeology is owned by Alexandra Matiella Novak. Permission to republish How to Make a Flood Map in print or online must be granted by the author in writing.