Study Reach.--The channel reach is an artificially straightened river, as shown in the quadrangle map on the top left. The study reach, approximately 450 ft long, is a segment extending from the USGS gage house to the upstream side of the I-74 bridge. Three surveyed cross sections (surveyed by the U.S. Geological Survey in September 2004) are available to define channel geometries at this site (see plots above). The alignment of the study reach, approximate variations in channel width and bank conditions, and locations of the surveyed cross sections are shown in the aerial photograph on the top right.
Gage Location.--Lat 40°28'16", long 89°01'48" (NAD of 1927), in NE1/4 NW1/4 sec.7, T.23N., R.2E., McLean County, Hydrologic Unit 07130009, on the left bank at the Bloomington-Normal Sanitary District sewage-treatment plant, 450 ft upstream from the Interstate 74 bridge, 0.4 mi west of Bloomington, and at river mi 48.8. The USGS streamgage station number is 05580950.
Drainage Area.--34.4 sq mi.
Gage Datum and Elevations of Reference Points.--Datum of the gage is 725.11 ft. A staff gage is installed at the upper end of the reach, elevation of the brass screw on the staff =730.020 ft. A reference point (RP-C1L), is two file marks on top of the outer edge of the lower bracket of the crest-stage gage (CSG-1) located 450 ft downstream from the staff gage, elevation=732.966 ft. All elevations are referenced to NGVD29.
Stage, Discharge Measurements, and Computed n-Values.--Water-surface elevations were determined by reading the staff gage at the upstream end, by taping down from RP-C1L at the downstream end and by running levels to high water marks. Discharge measurements were made either using the conventional current-meter method or a tethered boat Acoustic Doppler Current Profiler (ADCP). 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 |
|---|---|---|---|---|---|---|
| 7/10/2003 | 1290.0 | 317.9 | 4.04 | 4.09 | 0.000236 | 0.020 |
| 7/10/2003 | 1360.0 | 327.0 | 4.10 | 4.19 | 0.000236 | 0.021 |
Description of Channel.--The study reach represents a channelized, straightened river in a suburban environment. The streambed material is sand and gravel with small scattered cobbles. Tall grasses and sparse brush line both banks. Appreciable amounts of algae grow in the channel during summer months. The channel has moderately steep-sloping banks about 6 ft high. The top width varies from 70 ft to 90 ft. The channel bed has a flat bottom about 65 ft wide. The cross-sectional shape is trapezoidal. The channel is straight.
Floods.--Maximum discharge, 6,600 ft3/s, Dec. 3, 1982, gage height, 14.02 ft, from floodmark.
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://il.water.usgs.gov/proj/nvalues/db/sites/05580950.shtml[an error occurred while processing this directive]?
Page Contact Information: David Soong
Page Last Modified: February 4, 2013
