Site Summary Norwalk River at South Wilton, 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:

The objectives of the USGS stream station site visits are to:

• 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:

The Norwalk River originates in southwestern Connecticut and lie within the Highland Region of the New York Bight, which encompasses areas of northern New Jersey, the Hudson Highlands region of southern New York and upland parts of Connecticut. The topography is characterized by rugged, hilly to mountainous terrain bearing evidence of Pleistocene glaciation. Rocky outcrops visible on hillsides and stream banks consist mostly of Ordovician and Cambrian aged gneiss and schist.

May 7, 2001; The four team members met at the gage site approximately 300 feet east of the intersection of State Route 7 and Kent Drive. A profile of the stream thalweg, water surface, and bankfull indicators were surveyed above the gage, along with two cross sections and Wolman pebble counts in both cross sections.

Bankfull discharge is approximately (~)612 +/- 50 cubic feet per second (cfs). The estimation was obtained by taking a least squares first order curve fit (correlation = 0.997) through the bankfull indicators along the profile, in the two cross sections and using the discharge measurement notes at the USGS gage at station 0 (feet). Discharge was read off the USGS rating table #29 corresponding to a gage height of 3.25 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 34 peak annual discharges (1955, 1963 to 1999). The bankfull recurrence interval is 1.24 years. This can also be expressed by saying that there is a 80.3% chance that the 30 square mile watershed will exceed 612 cfs within any consecutive 12-month period.

Woman Pebble Counts were conducted in both cross sections; 63 samples were measured in XS #1, ~291 feet upstream (US) from Kent Road Bridge and 31 samples measured in XS #2, 504 feet US from the same bridge. Particle 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. For XS #1, D₅₀ plots out at ~57 mm (very coarse gravel). The bankfull slope (S) in the reach is 0.0066 ft/ft and the hydraulic radius (R) is 2.4 ft. The average shear stress (t = g RS) in XS #1 is 1.0 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 50 mm. The minimum particle size estimated to move (50 mm) compares well with the median size (D₅₀) measured (57 mm). For XS #2, D₅₀ plots out at ~1.5 mm (very coarse sand). The bankfull slope is again 0.0066 ft/ft and the hydraulic radius is 2.1 ft. The average shear stress in XS #2 is 0.86 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 40 mm. The comparison between the minimum particle size estimated to move and the median size (D₅₀) measured is not good, it is my belief that the estimation of particle size is good and that the Wolman Pebble Count was biased towards smaller mineral particles. Part of the reason is in the number of samples taken in XS #2 (31 samples), but mainly due to samples measured outside the bankfull cross sectional area in the deposition zone. (The author is at fault here).

Bankfull Stage at the gage is 3.25 feet. Gage datum is 115.69 feet above mean sea level. The bankfull stage 277 feet upstream from the gage (cross section #1) is 5.0 feet gage height, which is 4.5 feet above the bed. At bankfull stage water is flowing at a mean velocity of 4.1 feet per second through this cross section, Manning’s roughness is 0.053, according to all of the assumptions of steady uniform flow. The bankfull cross sectional flow area is ~149 square feet, bankfull top width is 60 feet, and the mean or hydraulic depth is 2.5 feet in cross section #1.

The bankfull stage 490 feet upstream from the gage (cross section #2) is 6.4 feet gage height, which is 3.6 feet above the bed. At bankfull stage water is flowing at a mean velocity of 4.5 feet per second through this cross section, Manning’s roughness is 0.043, according to all of the assumptions of steady uniform flow. The bankfull cross sectional flow area is ~135 square feet, bankfull top width is 64 feet, and the mean or hydraulic depth is 2.1 feet in cross section #2.

According to Rosgen’s criteria for stream classification, the Norwalk River upstream from the gage is a C4_/1. The dominant bed material is coarse to very coarse gravel with exposed bedrock in the pools and riffles in places. It is slightly entrenched, as it expands to widths of 172 to 354 feet as the depth doubles. Sinuosity is ~1.1 and the average width to depth ratio varies from 24 to 30.

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