Stormwater runoff

Stormwater runoff is generally regarded as excess rain and snowmelt that moves across the surface of the ground rather than being absorbed into the soil. This excess water typically ends up in a nearby lake, stream, or river. Managing this runoff is critical to flood control and avoidance.

What is a watershed?

The land area that drains to a low point in the landscape, such as to a river, lake, or wetland is a watershed. All land is part of a watershed. In other words, all land drains into a lake, river, stream or other water body.

There are different scales of watersheds, too. The Mississippi River has a huge watershed that includes land in 33 states. You might live in the Mississippi River watershed, but at the same time you live in a watershed of a smaller, local stream or river that eventually flows into the Mississippi. Think about where the rainfall drains in a yard after it rains. It might flow to the street, then into a storm sewer, and into a local stream or lake. This is a sub-watershed within a larger watershed, such as the Mississippi River Watershed.

Learn more about threats to the watershed and simple things people can do to protect their local watershed in After the Storm: Co-Produced by the U.S. EPA and The Weather Channel.

Detention Ponds

Detention ponds are often used to temporarily store or detain excess stormwater runoff from a development. Using this approach, runoff is directed to constructed or natural basins from which it is released continually at a restricted rate until the water elevation in the pond reaches its designed dry-weather stage. Typically, the allowable release rate from the site after development must be equal to or less than the existing runoff rate prior to development.

Overland Flow Paths

An overland flow path is often referred to as the “major conveyance system.” These systems are drainageways that convey flows from large storms when the capacity of minor conveyance system (storm sewer) is exceeded. Overland flow paths can consist of roadways, ditches or swales.

Storm Sewers

Storm Sewers are used to convey stormwater runoff from small storms to a desired outlet. A storm sewer system is often referred to as the “minor conveyance system.” This system may include infrastructure such as curb, gutter, culverts, roadside ditches and swales, storm sewers, tiles, subsurface drainage systems, and other practices intended to capture and convey stormwater runoff from small storm events. Minor conveyance systems, or storm sewers, prevent stormwater water from ponding on roadways, sidewalks, and properties during the more frequent, small storms.

Urban Flooding

Urban Flooding is defined as flooding in developed areas where water enters a building through wall openings, floor connections, through seams and cracks, or accumulation of water on public property or rights of way.

The Illinois General Assembly passed the Urban Flooding Awareness Act, which went into effect August 3, 2014. The Act tasked the Illinois Department of Natural Resources (IDNR) with preparing a report about urban flooding in Illinois. The report had two primary objectives:

  1. Determine the extent, cost, prevalence, and policies related to urban flooding in Illinois
  2. Identify resources and technology that may lead to mitigation of the impact of urban flooding.

More information, including the final report, is available on IDNR’s website: Urban Flooding Awareness. Additional information about urban flooding can also be found at the University of Maryland.