Site Summary Baron Fork at Dutch Mills, Arkansas

October 19, 2000 the team drove approximately (~)25 miles south on State Highway 59 to bridge over the Baron Fork at Dutch Mills. The Baron Fork originates in Northwest Arkansas in the southwest corner of the Springfield Plateau Province. The Baron Fork watershed at 40.6 square miles drainage receives ~46 inches of precipitation annually, most in the form of rain. Runoff from the watershed averages just over 15-1/4 inches. The channel bottom below the bridge is mostly scoured bedrock with few sediments observed, except in the banks of the ~1-year flow channel.

A Pebble Count was taken, 50 times in the gravel bar composing the right-hand bank of the 1-yr flood channel, ~500 feet downstream from the DS bridge piers. The median size of the particle’s secondary axis is 25 mm, this agrees fairly well with the sediment size predicted (29 mm diameter particles), that will move during bankfull flows.

The bankfull discharge is predicted to be 1,382 cubic feet per second (cfs). This rating was obtained by projecting the average reach slope of 0.0040 ft/ft from the bankfull indicator in the cross section at station 509 back to the gage at station 0 feet. The bankfull indicator is the flat bench on the right bank. This feature is evident upstream from the gage as the floodplain. The recurrence interval for bankfull, based upon the Weibull Extreme Value plotting positions of 42 peak annual discharges is 1.1 years. This can also be expressed by saying that the stream has a 91% chance of exceeding 1,380 cfs within a consecutive 12-month period.

Bankfull Stage at gage is 5.20 feet, gage datum is 986.47 feet, bankfull elevation is 991.67 feet above mean sea level. The bankfull stage in the cross section at station 509 feet downstream from gage is 3.15 feet gage height and 4.5 feet above the bed. Water is flowing at a mean velocity of 4.8 feet per second at bankfull stage through this cross section, Manning’s roughness is 0.034, according to all of the assumptions of steady uniform flow. The cross sectional flow area below 3.15 feet gage height is 289 square feet, bankfull top width is 126 feet, and the mean or hydraulic depth is 2.3 feet through this cross section. Above the 1.1-yr return stage in the cross section is a break in slope at the 1.35-yr return stage, (5.49 feet gage height = 3,800 cfs). A bench roughly 43 feet wide, helps convey discharges from 3,800 cfs to 5,216 cfs (1.63-yr return interval) to 12,140 cfs (3.75-yr return interval). The elevation of the floodplain on the left is 12 feet gage height, return interval is 3.75-years and it helps relieve shear stresses for discharges greater than 12,140 cfs.

According to Rosgen’s criteria for stream classification, the Baron Fork at Dutch Mills, below the gage is a B4/1. The mean bed material size is 25 mm, with areas of exposed bedrock, it is moderately entrenched, (Entrenchment Ratio = 1.5), as it expands to widths over 193 feet as the depth doubles. Sinuosity is ~1.18 and the width to depth ratio is 54.

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