Site Summary Saugatuck River Near Redding, Connecticut

Introduction:

Team members:

Charles Galgowski, Design Engineer, NRCS, Tolland, Connecticut

Todd Bobowick, Resource Conservationist, NRCS, Torrington, Connecticut

Thom Garday, Hydraulic Engineer, NRCS, NWMC, Little Rock, Arkansas

Aaron Pugh, Hydrologist, USGS, NWMC, Little Rock, Arkansas

USGS Stream Gage Stations visited:

Objectives:

• Measure and record representative bankfull channel dimensions in stable riffle sections and the dominant rock size in the bed.

• Relate the bankfull elevations in the stable section(s) back to the USGS Gage & ultimately relate the bankfull elevations to a discharge via the station's rating table.

• Determine the subsequent return interval for the bankfull discharge via station flow records.

• Classify the stream reaches using Rosgen's parameters of slope, sinuosity, width to depth ratio, entrenchment ratio, and dominate bed material size.

• Form relationships of bankfull channel dimensions - cross sectional area, top width, and mean depth for all gages visited to the corresponding channel forming discharges i.e. develop regional curves and regime equations for the streams hydraulic geometry. (This will take at least 12 gages in the same physiographic province to form meaningful relationships).

Summary:

May 11, 2001; The four team members met at the gage site approximately 100 feet south of the intersection of State Routes 53 and 107. A profile of the stream thalweg, water surface, and bankfull indicators were surveyed, along with three cross sections and Wolman pebble counts in two of the cross sections.

Bankfull discharge is ~276 cubic feet per second (cfs). The estimation was obtained by taking a least squares second order curve fit (parabolic, with a correlation = 0.986) through the bankfull indicators along the profile and the three cross sections and determining the stage at the gage at station 215 feet. Discharge was then read off the USGS rating table #23 corresponding to a gage height of 3.28 feet. The recurrence interval of the bankfull discharge is based upon the least squares linear curve fit of the Weibull Extreme Value plotting positions of 38 peak annual discharges (1962 to 1999). The bankfull recurrence interval is 1.10 years. This can also be expressed by saying that there is a 90% chance that the 21 square mile watershed will exceed 276 cfs within any consecutive 12-month period.

Woman Pebble Counts were conducted in cross sections (XS) one and three; 42 samples in XS #1, ~54 feet downstream (DS) from State Route 53 Bridge (22 ft wide by 8 ft tall) and 35 samples measured in XS #3, 310 feet DS from the same bridge. The D₅₀ or median size of the particle size distributions in the two bankfull cross sections varied in proportion with the product of the water surface slopes in the vicinity of the two cross sections and the hydraulic radius. The particle/sediment size distribution is based on the cumulative frequency of the number of samples in a particular class size versus the median dimension of the class size. The secondary axis of the particle represents the particle’s diameter and was recorded into a class size. For XS #1, D₅₀ plots out at ~61 mm (very coarse gravel). In the vicinity of XS #1, the bankfull slope (S) is 0.01165 ft/ft and the hydraulic radius (R) is 1.9 ft. The average shear stress (t = g RS) in XS #1 is 1.37 lb_f/ft², (where g = 62.4 lb_f/ft³). The minimum particle size estimated to move (based on 2 times the average shear stress and converted from inches to millimeters) is 70 mm. The minimum particle size estimated to move compares well with the median size (D₅₀) measured. For XS #3, D₅₀ plots out at ~23 mm (coarse gravel). In the vicinity of XS #3, the bankfull slope is 0.0047 ft/ft and the hydraulic radius is 2.0 ft. The average shear stress in XS #3 is 0.59 lb_f/ft². The minimum particle size estimated to move (based on 2 times the average shear stress and converted from inches to millimeters) is 30 mm. The minimum particle size estimated to move compares well with the median size (D₅₀) measured.

Bankfull Stage at the gage is 3.28 feet. Gage datum is 285.46 feet above mean sea level. The bankfull stage 117 feet upstream from the gage (cross section #2) is 5.10 feet gage height, which is 2.6 feet above the bed. At bankfull stage water is flowing at a mean velocity of 3.9 feet per second through this cross section, Manning’s roughness is 0.068, according to all of the assumptions of steady uniform flow. The bankfull cross sectional flow area is ~70.9 square feet, bankfull top width is 39.6 feet, and the mean or hydraulic depth is 1.8 feet in cross section #2.

The bankfull stage 42 feet downstream from the gage (cross section #1) is 2.78 feet gage height, which is 2.9 feet above the bed. At bankfull stage water is flowing at a mean velocity of 3.8 feet per second through this cross section, Manning’s roughness is 0.065, according to all of the assumptions of steady uniform flow. The bankfull cross sectional flow area is ~73.5 square feet, bankfull top width is 38.0 feet, and the mean or hydraulic depth is 1.9 feet in cross section #1.

The bankfull stage 298 feet downstream from the gage (cross section #3) is 0.9 feet gage height, which is 3.7 feet above the bed. Water is moving at a mean velocity of 3.5 feet per second. The bankfull cross sectional flow area is ~78.5 square feet, bankfull top width is 37.5 feet, and the mean or hydraulic depth is 2.1 feet in cross section #3. Manning’s roughness is 0.046; the reduction in bankfull slope indicates a lower sediment transport capacity, which is reflected in the smaller D₅₀ of the bed material in cross section #3.

According to Rosgen’s criteria for stream classification, the Saugatuck River upstream and downstream from the gage is a B4c/1. The dominant bed material is coarse to very coarse gravel with exposed bedrock in the pools and along the banks in places. It is moderately entrenched, as it expands to widths of 83 to 88 feet as the depth doubles. Sinuosity is ~1.2 and the average width to depth ratio is 20.

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