Water Resources of Illinois: n-values Project

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Study Reach.--The channel reach is constructed in an urban setting, as shown in the quadrangle map on the top left. The study reach, about 310 ft long, begins at the downstream side of Broadway Avenue and extends downstream to an abandoned bridge. Seven surveyed cross sections (surveyed by the U.S. Geological Survey in August 2004) are available for describing the channel geometries in the study reach. The channel alignment, approximate variations in channel width and bank conditions, and locations of cross sections are shown in the aerial photograph on the top right. Cross-sectional plots at four river stations (RSs), as shown above, are selected to illustrate the variation in cross-sectional geometry.

Gage Location.--Lat 40°06'53", long 88°12'33", in SW1/4 SE1/4 sec.08, T.19N., R.9E., Champaign County, Hydrologic Unit 05120109, on the left bank at the upstream side of the Race Street bridge in Urbana, 0.7 mi upstream from the Saline Branch, at river mi 0.1. The USGS streamgage station number is 03337250.

Drainage Area.--6.86 sq mi.

Gage Datum and Elevations of Reference Points.--Datum of the gage is 694.00 ft. Two reference points were created for the n-value study. RP-N2, is the top of the nut on the concrete anchor on the 2nd concrete upright from the right on the guardrail on the downstream side of the Broadway Avenue bridge, elevation = 709.544 ft. RP-N1 is the top of the nut on the concrete anchor in the upstream side of the bridge deck on the abandoned bridge 312 ft downstream from Broadway Avenue located on the right bank near the junction of the bridge deck and the retaining wall, elevation=704.872 ft. All elevations are referenced to NGVD29.

Stage, Discharge Measurements, and Computed n-Values.--Water-surface elevations were measured from the two RP-Ns before, during and after each discharge measurement. Discharge measurements were made using the conventional current-meter method. Because the stage rises and falls quickly, measurements were made using quarter counts and the time for each section was recorded to the nearest minute. When possible, multiple discharge measurements were made during a rise and recession to provide data for calculating n-values over a range in stage. The computed n-values are listed in the following table. Whenever possible, the computed n-values are associated with a photograph taken at the time of the measurement. The photographs are arranged from low to high discharge in order to illustrate the contributing factors of n-values at a particular discharge.

Date of Observation	Discharge (ft3/s)	Average Cross Section Area (ft2)	Hydraulic Radius (ft)	Mean Velocity (ft/s)	Slope (ft/ft)	Coefficient of Roughness n
6/10/2004	108.0	56.5	2.20	2.15	0.006082	0.078
6/10/2004	129.0	59.7	2.28	2.42	0.006106	0.072
6/15/2004	136.0	62.8	2.36	2.37	0.006549	0.080
6/10/2004	157.0	65.9	2.43	2.64	0.005819	0.068
6/15/2004	158.0	67.0	2.46	2.57	0.006327	0.075
8/25/2004	164.0	70.5	2.55	2.53	0.006186	0.078
8/25/2004	174.0	71.8	2.58	2.63	0.006218	0.076
6/15/2004	190.0	73.7	2.62	2.80	0.006002	0.071
6/10/2004	205.0	73.7	2.61	3.06	0.005524	0.061
6/15/2004	236.0	83.5	2.83	3.05	0.005748	0.068
6/15/2004	320.0	97.8	3.14	3.50	0.005361	0.062
6/15/2004	391.0	102.5	3.23	4.06	0.005801	0.057

Description of Channel.--This channel is constructed with portions of the bank covered with gabion boxes. Bed materials are primarily coarse gravel and cobbles with some sand deposits at the downstream end. The channel is subject to the accumulation and dissipation of urban debris, such as bicycles, shopping carts and assorted garbage. The banks are steep and vary in height from 10 to 13 ft. The left bank material varies from stacked concrete rubble at the extreme upstream end of the study reach to a 65 ft length of offset layers of gabion boxes at mid-reach to rubble and grouted riprap at the extreme downstream end. The right bank is lined with offset layers of gabion boxes, covered with occasional sparse and scattered weed growth. Weeds, flexible long-stem grasses, and grape vines grow abundantly on the left side of the channel and left bank. The width of the bed is about 20 ft and the top width is generally about 35 ft. Cross sections are of trapezoidal shape but with unequal slopes on each side. The study reach is straight.

Floods.-- Maximum gage height, 11.68 ft., July 9, 2003.