Variation of Manning’s Roughness Coefficient with Diameter, Discharge, Slope and Depth in Partially Filled HDPE Culverts

Abstract

High Density Polyethylene (HDPE) culverts are increasingly used in road and railroad crossings because of their low construction cost. The changed hydraulic condition of flow inside a culvert due to a constricted river cross-section results in increased velocity and possibly decreased water depth, causing a fish passage barrier. Velocity within a culvert is a function of the cross-sectional area, slope, discharge and Manning's roughness of the culvert material. Previous researchers have found that the Manning's roughness for partial flows is greater than full flow but inadequate data were collected for flows less than 20% full. Therefore, the objective of this research was to collect water depth data in HDPE culverts and to derive a relation between Manning's roughness and relative depth. With a 4ft x 2ft flume of 60 ft length, three test culverts of diameter 1ft, 2ft and 3.5ft were tested with the discharge ranging from 0.2 to 10.3 cfs at bed slopes of 0.2%, to 2%. A total of 11,000 depth data points were collected and 125 average depths were reported. The results could not validate previous research on relative Manning's roughness curves, though the statement by Pomeroy that the Manning's roughness should be less than full flow roughness under 18% relative depth was validated. The relation between relative depth and relative Manning's roughness for flow less than 20% was found to be fairly linear and less than the full flow roughness published by HDPE manufacturers, whereas for 20% to 40% relative depth, the roughness was found to be equal to full flow roughness. Above 40% and below 75% relative depth, Manning's roughness decreases fairly linearly to less than full flow roughness.