History of Flooding in Northbrook
When it rains in Northbrook, storm water flows to one of the five different water courses (streams and rivers); the Skokie Lagoons/River, the Middle Fork of the North Branch Chicago River, the West Fork of the North Branch Chicago River, the Techny Drain and the Des Plaines River. The Techny Drain is tributary to the West Fork. The areas that drain to these particular water courses are also referred to as drainage basins. Water normally reaches these water courses either by storm sewers or ditches. In flooding situations where the sewers and ditches are at capacity, water will then flow overland to these drainage basins utilizing the path of least resistance.
The Village has two distinct sewer systems that are not interconnected, one that conveys storm water and the other to convey sanitary waste water for treatment. The Village’s storm sewer system is a network of pipes, swales, ditches, streets, streams and rivers. Streets are designed to provide temporary detention during high intensity rainstorms as long as vehicle access is not impeded. Storm water enters this system from yards, fields, streets, parking lots, footing tiles, downspouts and exterior drains.
There are also three regional detention basins along the West Fork and one along the Middle Fork that are maintained by the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC). In addition, the Village has constructed a regional detention basin along the Techny Drain-South Fork, just west of Glenbrook North High School. These regional detention basins are very beneficial because they provide on-line detention of 1,725 acre feet of storage which is equivalent to over 565 million gallons of storm water. As the levels in the streams rise during flooding, water is diverted into the basins to help keep the water levels from further increasing downstream.
On September 13-14, 2008 approximately 6” of rain was recorded at the Public Works Center rain gauge, 7.9” at the Water Filtration Plant and the 7½” at MWRDGC Glenview rain gauge. Prior to this rain event, 2” of rain fell on the Monday before and caused abnormally saturated ground conditions and high stream levels. Flooding was wide spread throughout the entire region. This storm also produced the most rainfall ever recorded for the City of Chicago. The five main streams and rivers flowing through the Village were at such high levels that our storm sewer outfalls at the rivers were totally submerged. This caused our storm sewer system to be essentially sealed shut. The water levels were so high that numerous manhole covers on the storm sewer system were blown off. This was observed on Illinois Road (east of the METRA tracks), Dundee/Skokie intersection, Midway/Whitfield-South intersection and at Sanders/Wood Oaks Junior High. The MWRDGC regional basins along these watercourses were all either at capacity or near full. Once the rain subsided and the rivers and streams began to recede, Northbrook felt an immediate impact in most areas in draining with the overland flooding and standing water. The frequency of flooding for our region is also increased by the predominant impermeable clay soils and relatively flat topography. 242 calls came into the Public Works Center during the rain event related to overland flooding. Of that amount, 186 of these calls pertained to flooded streets.
The three previous major flood events to this in the Village occurred in 1982 (two) and 1987. 7.75 inches of rain fell over a 24 hour period on August 14, 1987 and 6.5 inches fell over a 24 hour period on July 22, 1982. The 1987 rainfall amount was recorded by a rain gauge located at the Village's Public Works Building while the 1982 rainfall amount was recorded at Sportsmans Park Golf Club. Just slightly over 5.25 inches of rain fall over a 12 hour period during the August 22, 2002 rainstorm. This interpolated to a 35 year frequency storm. On October 13, 2001 3.75 inches of rain fell over a 6 hour period while the rainfall volumes only interpolated to a 18 year frequency storm, saturated ground conditions produced flooding levels that were very close to the 100 year event. The 24 hour rainfall amount for the "theoretical" 100 year storm is 7.58 inches. The 100 year storm has a 1 in 100 statistical chance of occurrence in any given year. (The boundaries of the 100 year floodplain (shaded areas) are shown on the enclosed map.) Please be advised that a detailed topographical survey based on the FEMA datum will be required to determine the exact limits of the 100 Year floodplain for those properties that are either located partially in, or adjacent to the floodplain boundaries shown. A review of Village records indicated that following the 1987 flood event, water damaged personal items were picked up at 818 locations. However, only 73 of these locations were in identified floodplains.