State Highway 70 crosses Honcut Creek near Live Oak about 3 miles upstream from its confluence with the Feather River by a series of three bridges separated by short approach embankments. The California Department of Transportation is planning to replace or widen the three bridges. This report evaluates flow characteristics of the existing and proposed crossings. The distribution of flow across the floodplain and water-surface elevation of Honcut Creek at the site are affected by levees, a natural channel constriction, agricultural improvements on the floodplain, and high water levels on the Feather River.
The average recurrence interval for overbank flow is less than 2 years, and for flows equal to the flood of January 12, 1980 (discharge, 10,100 cubic feet per second), is about 2. 5 years. A flood with an average recurrence interval of 50 years would discharge about 19,000 cubic feet per second. Flooding caused by high stages on the Feather River has occurred seven times during the period 1964-80.
For flows exceeding 20,000 cubic feet per second on Honcut Creek, and assuming present bridge conditions, backwater at the approach section about 400 feet upstream from the bridge is about 0.4 foot. If bridge l (left bank bridge) is eliminated, backwater conditions at the approach section would increase to 0.5 foot. Backwater effects extend upstream more than 3,000 feet when flows exceed about 5,000 cubic feet per second.
The present arrangement of the bridges and approach embankments occupies about 66 percent of the channel (between the levee and high ground). Measurements of the average velocity of flow at a bridge range from l to 3.2 feet per second, depending on the flood stages of the Feather River. The maximum point velocity of flow measured during the 1979-80 flood season was 5. 6 feet per second. In general, flow velocities will be less than about 6 feet per second because flooding and high stages on the Feather River create ponded conditions at the bridge site. For present channel and bridge conditions, overbank flows are distributed among the three bridges in a proportion of about 10, 40, and 50 percent for flows from 2,850 to 8,480 cubic feet per second.
