The 100-year storm is an amount of rain so heavy that the chances of experiencing such a storm are one in 100. (Note that those are the chances every year. It is possible to have this unusual storm two years in a row or even more frequently.) The amount of rain that creates a 100-year storm in most parts of the Huron River watershed is nearly five inches within 24 hours.

Why flooding occurs
In the past, building development has occurred on the floodplains, which required trying to control the stormwater to keep it out of the floodplains. This practice causes water to overflow riverbanks in other locations – often creating floods of a greater magnitude and danger.

Building on floodplains increases the risk of property damage and life threatening situations. Diverting stormwater into channels and easing its path to bodies of surface water forces water to flow faster. This greater velocity destroys habitats and causes greater erosion including the loss of topsoil which often creates a need for increased fertilizer use.

There are also other factors that increase flooding:

• The removal of stabilizing vegetation around stream banks and rivers
• Erecting structures that deflect or inhibit the flow of floodwaters. This modifies flow paths and can spread flooding problems and increase erosion.
• Constructing bridges, culverts, buildings, and other structures that encroach on the floodplain. These developments reduce the storage area available for floodwaters and cause an increase in flood elevations.
• Building drainage systems that feed stormwater quickly into the receiving body.
• Straightening meandering watercourses to hasten drainage. This transfers flooding problems downstream and also alters habitat.
• Filling and dumping in floodplains. Floodwaters can transport this debris, which may interfere with the movement of the floodwater, causing increased flood elevations.

With increasing development in the floodplain, open spaces, and wetlands, our land has lost the ability to soak up rain. Areas that were once effective sponges storing precipitation are now being replaced by buildings and pavement that have made the land increasingly impervious. As a result, floods have become far larger and frequent. Local governments can prohibit building in the floodplain.

Preventing flooding
Taking the time to word your community’s zoning ordinances so that they reflect the importance of preserving water quality is an important task. There are many angles to do this from including: