Water Resources of Illinois: n-Values Project

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Study Reach.--The channel reach under consideration is dredged in an urban setting. A study reach, approximately 600 ft long, is selected that extends upstream from the State Route 13 bridge as shown on the quadrangle map on the top left. Three surveyed cross sections (surveyed by the Illinois Department of Natural Resources in June 2005) are available for describing the channel geometries of the study reach (see plots above). The channel alignment, approximate variations in channel width and bank conditions, and locations of cross sections are shown in the aerial photo on the top right.

Gage Location.--Lat 37°43'52", long 88°53'21", in SW1/4 SW1/4 sec.16, T.9S., R.3E., on right bank at downstream side of bridge on State Route 13, 1.8 miles downstream from Buckley Creek, 2 mi east of the town square in Marion, Williamson County, and at river mile 32.9. The USGS streamgage-number is 05597500.

Drainage Area.--31.7 sq mi.

Gage Datum and Elevations of Reference Points.--Datum of gage is 415.82 ft. A wire-weight gage (WWG) is located on the downstream side of State Route 13 bridge. Reference points (RP) for the n-value study are located along the channel on the right bank. RP-2 is a nail in a tree located 795 ft upstream from the WWG, elevation=428.10 ft. RP-1 is a nail in a tree located 443 ft upstream from the WWG, elevation=429.21 ft. All elevations are in NGVD 1929 convention.

Stage, Discharge Measurements, and Computed n-Values.--Water-surface elevations were measured by tape down from the RP’s and by reading the WWG at the time of discharge measurement. Discharge was measured using conventional current-meter methods. Flow through a box culvert 500 ft east of the gage on Route 13 starts when stage in main channel reaches about 9.3 ft. Because of this lateral diversion, events with stage higher than 9.0 ft were not used in the n-values study. 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 state 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
1/27/1999	23.3	40.3	1.48	0.64	0.000664	0.056

Description of Channel.--This channel has been straightened and dredged. The channel bed consists of clay-sand-fine gravel mixtures. The bank face is a clay and sand mixture, covered with exposed tree roots and stumps. The top of the banks are covered with trees, brush, and weeds. Both banks are low and will overflow at a stage of about 11 ft. The channel shape can be described as rectangular and channel alignment is straight. The channel is subject to scour and fill. Often, the channel reach is cluttered with logjams, debris jams, and beaver dams.

Floods.--Dec. 17, 2001, Discharge 9,430 ft³/s, gage height 13.63 ft; May 11, 1996, 9,270 ft³/s (gage height, 13.60 ft), determined by indirect measurement (contracted opening with culvert overflow).

Estimated n-Values using Cowan’s Approach.--0.041 - 0.055