The Village of Winnetka's Flood Plain
When we speak of the flood plain, we are referring to the land that is typically adjacent to a body of water that has its ground elevation at or below the BFE (100-year flood frequency elevation). In Winnetka, there are two sources of flood plain, the Skokie River and Lake Michigan. The BFE for the 100-year flood event for the Skokie River is 625.3 NAVD 1988 (or 625.5 NGVD 1929). The mapped BFE for Lake Michigan is 585 NAVD 1988. The BFE for both water bodies is generally consistent throughout the limits of Winnetka. The BFE for the Skokie River does vary, south of Hill Road. The flood plain is made up of two components, the flood way and the flood fringe. The flood way is a more restrictive development area than the flood fringe. All of the residential areas within the Village of Winnetka that are located within the flood plain are located within the flood fringe.
Flood Plain's Natural and Beneficial Functions
The original function of the flood plain and waterway served to provide for a variety of needs. Rivers provided for transportation, water supply, a source of energy, and even a source of waste disposal. However, after hundreds of years, the character of the rivers and flood plains has changed and the natural function of the riparian ecosystems (the vegetated areas adjacent to the streams) has been significantly altered by human action.
Flood plain resources, including wetlands, have experienced increased pressure from development. Expanding urbanization of our watershed deliver increased amounts of surface water runoff into the receiving water bodies. The straightening and channelization of streams allow for increased water velocities, which with the increase in storm water runoff from past developments have created increased bank erosion, loss of wildlife habitat and detrimental changes in water quality. However, over the past several years, the Village of Winnetka has taken significant steps toward minimizing storm water runoff from developments, as well as requiring that storm water quality management techniques be implemented for each development. Every permit issued in the Village of Winnetka that results in an alteration of the existing drainage pattern requires that some form of storm water management and water quality control be implemented as part of the design. All permits issued in the flood plain require that no net fill be placed in the flood plain in order to maintain the existing capacity of our flood plain storage without having to increase its boundaries.
The Federal Emergency Management Agency has classified the natural resources of flood plains into three groups: water resources; biological resources; and cultural resources. Water resources include both the maintenance of natural flood and erosion control characteristics as well as the maintenance of water quality. Benefits from these include the reduction of flood velocities and peaks; the reduction of erosion potential and impacts; the stabilization of soils; the reduction of sediment loads and the amount of sediments entering the stream; the filtering of nutrients and impurities from the storm water prior to entering the stream; and protecting the physical, biological and chemical integrity of the water. Biological resources include the support and diversification of flora and the provision of fish and wildlife habitat. Cultural resources include the provision of recreational opportunities, scientific study and outdoor education areas, and an improved economic base for the community by improving property values and stimulating natural resource activities and businesses.
Village of Winnetka Flooding
Although the Village of Winnetka has not witnessed a 100-year flood event in recent history, we have experienced several 100-year plus storm events, most recently in April of 2013, July of 2011 and September of 2008. There have also been several river bank overtopping events, but not up to the 100-year BFE. As a point of reference, if one were standing near the intersection of Hibbard Road and Ash Street, the elevation of a 100-year flood event would be approximately 2.3 feet deep, well above the knee of an adult. A third, and probably the most common source of flooding, occurs during heavy rainstorm events when the capacity of the storm sewers has been exceeded due to the increase of storm water flow. This situation can readily worsen if the storm sewer inlet structures or other storm water conveyance systems are covered or blocked with leaves, or other debris, as well as ice. Once storm water starts ponding on the street surface, the potential for that water to infiltrate into the sanitary sewer system, increases. As a result, basement back-ups can occur for those homes with a gravity sanitary sewer.
Do not dump or throw anything into ditches, swales, streams or other water conveyance systems, including storm inlets and manholes. Dumping or altering grades is in violation of the Village’s ordinance Sections 9.16.020.B.11; 14.04.130.A.1; 15.32.080.G and L; and 15.68. Even grass clippings, mulch and branches can accumulate and block drainage. To read the entire ordinance on-line, view the Village's Code of Ordinances.