Increased roughness in reinforced concrete box culverts

Adam Samuel Hill

The purpose of this experimental investigation was to determine the extent to which trapezoidal-shaped corrugations placed within a barrel of a reinforced concrete box culvert decreased water velocity within the barrel. Flow measurements were conducted in a tilting flume in Albrook Hydraulics Laboratory at Washington State University in Pullman, WA, using a half-scale simulation of the corrugations expected to be used in the field. Discharges ranged from 24.1-144 L/s and slopes ranged from 0.3-10.1 percent. Tests were also conducted in a flume in Sloan Teaching Laboratory using a quarter-scale simulation, where discharges ranged from 13.9-97.1 L/s. The flume slope was zero. Depths were measured using a point gage at seven different locations in both flumes. Sixty-eight tests were used to determine the Manning 'n' value. Manning's 'n' is inversely proportional to the submergence ratio and to the aspect ratio. Experimental errors in the determination of Manning's 'n' ranged from 4.3-10 percent. Manning's 'n' values for replication tests were within 3.2 percent of the original test. Undular jumps did not occur in experimental testing; however, hydraulic jumps did form in some experiments upon initially reaching the corrugations. The jumps are caused by the change in critical slope due to the increase in roughness. Three different flow situations were observed during upstream supercritical flow experiments. Velocities within the corrugations decreased 44-66 percent compared to upstream supercritical velocity. Broken-back Culvert Analysis Program (BCAP) outputs compared reasonably well with experimental results within the corrugations if inputs were set so the hydraulics at the break were the same as the upstream experimental data. The program suggests a hydraulic jump will form within the corrugated outlet section of a broken-back culvert. Velocity data were collected using an Acoustic Doppler Velocimeter (ADV) and compared with a study by Ead et al. (2000). The velocity profile was found to fit a log law profile using the Prandtl equation for rough turbulent flow for data points above the corrugation crest. The shear velocity was 2.9 times higher and the Nikuradse equivalent sand roughness was 100 times higher than values found by Ead et al. (2000).

Increased roughness in reinforced concrete box culverts

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