Study Reach.--The channel reach under consideration is a straightened, dredged ditch located at the downstream side of an abandoned railroad bridge and extending to the County Road 200N bridge, as shown in quadrangle map on the top left. The study reach selected, 1300 ft long, extends from a point 300 ft upstream from the Belknap Road bridge to 1000 ft downstream. The study reach represents a transition from a natural meandering reach to a straightened, dredged reach. Four surveyed cross sections (surveyed by the Illinois Department of Natural Resources, in May 2003) are available for describing channel geometries in the study reach (see plots above). The channel alignment, approximate variations in channel and width and bank conditions, and locations of cross sections are shown in the aerial photo on the top right.
Gage Location.--Lat 37°20'11", long 88°55'26" in NE1/4 NW1/4 sec.6, T.14S., R.3E., Johnson County, on downstream side of Belknap-Forman county highway bridge, 1.2 mi southwest of Forman and at river mi 8.1. The USGS streamgage-station number is 03612000.
Drainage Area.--244 sq mi.
Gage Datum and Elevations of Reference Points.--Datum of gage is 308.47 ft. Reference point for n-value study, RP-N2, is two file marks on the steel plate at 31st upright post from the right on guardrail on downstream side of bike path trestle, elevation = 345.422 ft. A wire-weight gage (WWG) is attached to the handrail of the highway bridge at downstream end. Upper staff gage located 1000 ft downstream of gage on right bank, elevation of brass screw = 328.247 ft. Lower staff gage located 1,000 ft downstream of gage on right bank, elevation of brass screw = 324.594 ft. RP-N1 is at two file marks located on vertical I-beam on upstream side of bridge located about 1 mi downstream of gage on County Road 200N, elevation = 347.930 ft. All elevations are in NGVD 1929 convention.
Stage, Discharge Measurements, and Computed n-Values.--Water-surface elevations at the upstream RP-N2, WWG, and staff gages and the downstream RP-N1 were measured before and after each discharge measurement. Discharge measurements were 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 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 |
|---|---|---|---|---|---|---|
| 1/13/2005 | 2640.0 | 1070.5 | 10.27 | 2.47 | 0.000225 | 0.046 |
| 5/5/2003 | 2740.0 | 1251.1 | 11.13 | 2.20 | 0.000314 | 0.066 |
| 1/26/1999 | 2790.0 | 1205.0 | 10.91 | 2.32 | 0.000386 | 0.066 |
Description of Channel.--Overall, the channel is a deep, dredged ditch. Localized channel scouring, bed deposition, and bank failures are observed in the original trapezoidal channel sections. The streambed is composed of clay-sand mixtures, gravel, and scattered boulders. The bottom width of the channel is approximately 30 ft. The bank heights are about 20 ft. with a top width of about 100 ft. Bank slope is steep. The bank faces are covered with dense grasses, with brush and trees on top of the banks and on the flood plains. There is little flood plain on the left-hand-side of the channel and flood plains on the right-hand-side of the bank are limited by a levee approximately 200 ft away from the bankline. Overflowing to the flood plains occurs at a stage of about 22 ft. Also, backwater from the Ohio River may reach the channel under consideration when the staff gages at Metropolis and Lock 53 exceed approximately 30 ft. Therefore, over-the-bank events and those susceptible to backwater effects were not used. The study reach is considered as straight.
Floods.--Maximum gage height 42.29 ft Mar. 12, 1935 (9,630 ft3/s), site then in use, current datum. Maximum gage height at present site, 37.65 ft, current datum, on Mar. 11, 1964 (8,040 ft3/s).
Estimated n-Values using Cowan’s Approach.--0.043 - 0.055
Bank: vegetation – flexible grasses, young trees with heavy under growth (make sure this is the condition for in-channel flows .-- this is true for the upper end of the reach, further down the trees are at the top of the banks )
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://il.water.usgs.gov/proj/nvalues/db/sites/03612000.shtml
Page Contact Information: David Soong
Page Last Modified: Thursday, 06-Mar-2008 10:49:35 CST
