Water Resources of Illinois: n-Values Project

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Study Reach.--The channel reach under consideration is meandering and dredged channel. The study reach selected, about 1950 ft long, is located between the Childerly Park pedestrian bridge and Elmhurst Road bridge, as shown in quadrangle map on the top left. Ten surveyed cross sections (surveyed by the Illinois Department of Natural Resources, in April and May 2003) are available for describing the channel geometries. The channel alignment, approximate variations in channel width and bank conditions, and locations of surveyed cross sections are shown in the aerial photo on the top right. Four representative cross sections at river station (RS) 16410, 16146, 15616, and 15363 are plotted above.

Gage Location.--Lat 42°09’06”, long 87°57’26”, in NE1/4 NW1/4 sec.4, T.42N., R11E., Cook County. Hydrologic Unit 07120004. On left bank at downstream side of bridge on Short Aptakisic Road, 1.0 mi downstream from unnamed tributary, 2.5 mi west of Wheeling, and at mile 5.0. The USGS streamgage-station number is 05528500.

Drainage Area.--19.6 sq mi.

Gage Datum and Elevations of Reference Points.--Gage datum is 658.60 ft. Reference points (RP-N) for n-value studies are listed below. All elevations are in NGVD 1929 convention.

Stage, Discharge Measurements, and Computed n-Values.--Water-surface elevations were measured from RP-Ns before and after a discharge measurement. Discharges were measured using the standard current-meter method. The computed n-values are listed in the following table. Whenever possible, the computed n-values are associated with a photo taken at the time of the measurement. The photos are arranged from low stage to high stage in order to illustrate contributing factors of n-value at a particular stage.

Date of Observation	Discharge (ft3/s)	Average Cross Section Area (ft2)	Hydraulic Radius (ft)	Mean Velocity (ft/s)	Slope	Coefficient of Roughness n
7/12/2000	42.4	27.8	1.31		0.001400	0.043
9/19/2001	256.0	85.4	2.71	3.01	0.001400	0.032

Description of Channel.--This channel is dredged. Cross sections of the study reach are a trapezoidal shape. The channel meanders mildly. The bed material consists of silt, gravel and rocks. Channel bottom width is about 20 ft with an average top width of approximately 50 ft. The left bank is mostly covered with grass, where as the right bank is heavily covered with willows and other brush. The banks are about 10 ft in height and begin to overtop into the flood plain at about 4 ft of stage.

Floods.--Maximum discharge, 887 ft³/s on July 22, 1982, gage height, 7.94 ft.

Estimated n-Values using Cowan’s Approach.--0.035 ~ 0.054