San Ramon Bypass Channel Overflow Weir, Contra Costa County, California: Hydraulic Model Investigation
bypass, California, channel, hydraulic models, overflow weir, weir, upstream, open channel flow, slope, Gradient, prototype, high velocity
Technical Report HL-90-14
National Technical Information Service
Tests were conducted on a 1:25-scale model of the San Ramon Bypass Channel, Contra Costa County, California, to develop an overflow weir to remove flows from the channel in excess of the 100-year frequency discharge. The overflow weir would remove flows upstream of the covered channel reach to maintain open channel flow conditions through the covered reach and to not exceed channel capacity downstream of the covered reach. The model reproduced approximately 900 ft of the San Ramon Bypass Channel, 100 ft of the Sans Crainte Creek channel, and 400 ft of the catch channel. The model was constructed so that the slopes of the channels could be adjusted to reproduce various energy gradients equivalent to those resulting from different prototype Manning's n roughness factors. The slopes of the model were initially adjusted to produce an energy gradient resulting from a Manning's n roughness factor of 0.012 in the prototype. Based on initial water-surface profiles measured in the model without the overflow weir installed, the weir height was set at 13.25 ft above the center line invert elevation and the weir length was set at 200 ft. With the overflow weir installed in the model, water-surface profiles and discharges over the weir were recorded with various flow conditions. The weir length was shortened from the upstream end by 25, 50, and 75 ft. Test results indicated that a weir length of 125 ft set at 13.25 ft above the center-line invert provided satisfactory results. Discharges over the weir were recorded with various channel discharges for each weir length. The slopes of the high-velocity channels were adjusted to reproduce the energy gradient resulting from a Manning's n roughness factor of 0.014 in the prototype. These tests were conducted with an overflow weir length of 125 ft and weir height of 13.25 ft. Water-surface profiles and discharges over the weir were recorded with various channel discharges. The divider wall at the downstream end of the weir was streamlined by reducing the angle 15 deg where the flow was being deflected and placing a 3-in. radius on the nose of the divider to split the flow. This resulted in satisfactory flow conditions at the weir for all discharges tested.