Vermilion River at Pontiac, IL




Study Reach.--The channel reach is in an urban setting. The study reach, approximately 940 ft long, is located from the pedestrian footbridge in Humiston Riverside Park to 130 ft downstream of the gage on the Vermilion Street bridge, as shown in the quadrangle map on the top left. Four surveyed cross sections (surveyed by the U.S. Geological Survey in June 2003) are available for describing the channel characteristics of the study reach. The alignment of the study reach, approximate variations in channel width and bank conditions, and locations of surveyed cross sections are shown in the aerial photograph on the top right. Cross-sectional geometries, as described by four surveyed cross sections, vary gradually and continuously from upstream to downstream (see plots above).

Gage Location.--Lat 40°52'40", long 88°38'10", in SE1/4 SW1/4 sec.22, T.28N., R.5E., Livingston County, Hydrologic Unit 07130002, near the center of the span on the downstream side of the bridge on Vermilion Street in Pontiac, 0.1 mi upstream from State Highway 116, 0.8 mi upstream from Turtle Creek, and at river mi 60.3. The USGS streamgage station number is 05554500.

Drainage Area.--579 sq mi.

Gage Datum and Elevations of Reference Points.--Datum of the gage is 619.45 ft. A reference point (RP-N1) is the center mark of three file marks located on the downstream side of the footbridge, elevation of the center mark = 645.086 ft. A wire-weight gage (WWG) is attached to the downstream side of the Vermilion Street bridge. Two staff gages are located 130 ft downstream from Vermilion Street. Elevation of the brass screw on the upper staff gage is 631.207 ft and the elevation of the brass screw on the lower staff gage is 626.619 ft. All elevations are referenced to NGVD29.

Stage, Discharge Measurements, and Computed n-Values.--Water-surface elevations are measured from RP-N1 on the footbridge, the WWG and the staff gages before and after each discharge measurement. Discharge measurements are made using the conventional current-meter method. 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
5/21/2003 488.0 610.8 4.07 0.84 0.000080 0.033
3/31/2004 1900.0 970.6 6.02 2.05 0.000251 0.034
6/1/2004 3550.0 1365.3 7.88 2.73 0.000286 0.032

Description of Channel.--This channel is in an urban setting. Bed material consists of a gravel and cobble mix with sparse patches of drift debris. The earthen banks have sparse spreads of bushes and grasses. Some areas along the bank are lined with riprap. Channel geometry is trapezoidal in shape, becoming narrower in the downstream direction. The bottom width of the channel ranges from 180 ft at the upstream end to 100 ft at the downstream end. The banks are about 10 ft high. Top width varies from nearly 256 ft at the upstream end to about 138 ft at the downstream end. The channel has a mild meander.

Floods.-- Maximum discharge, 13,200 ft3/s, Jan. 9, 2008, gage height, 19.18 ft.

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Page Contact Information: David Soong
Page Last Modified: February 4, 2013