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

Culverts are used to convey small to medium sized streams under transportation routes and are increasingly expected to maintain passage for fish and other aquatic animals. Because of excessive velocity or inadequate water depth, culvert barrels can present a barrier for fish movement. Previously culverts were designed primarily for flood conveyance purposes, but nowadays with increasing concern for aquatic ecology, culverts are being designed and assessed for fish passage purposes as well. Velocity within a culvert is a function of the cross-sectional area, slope, culvert material, and discharge. Mangin (2010) presented a review of the variation in Manning's roughness coefficient in partially filled culverts showing generally higher roughness at lower flows. However, very little data has been collected for flows less than 20 percent filled. Therefore, the aim of this research was to collect water depth under low flow conditions and develop a relation between Manning's roughness coefficient and the water depth in partially filled culverts. Since similar efforts focusing on roughness in partially filled corrugated metal culverts are underway at the Federal Highway Administration and two universities, we have focused our study on roughness variation in high-density polyethylene (HDPE) culverts. HDPE culverts are increasingly popular with road engineers due to their high strength, low cost, and low hydraulic roughness. The laboratory set up consisted of installing one of three 55 foot long culverts within a 4ft x 2ft x 60ft (W x H x L) flume. Flow depths were collected at discharges ranging from 0.2 to 9.5 cfs, at four slopes ranging from 0.2% to 2%, and in culverts of three different diameters (12in, 24in, and 42in). In total, more than 11,000 water depth measurements were collected and are reported. The analysis of the obtained data showed that the value of Manning's roughness coefficient for depths less than 20% of the culvert diameter is indeed a function of discharge, slope and diameter. The results most closely match the predictions by Pomeroy (1967), such that roughness is a maximum at 40% depth and decreases with shallower flows. These results are expected to aid engineers in the analysis and design of HDPE culverts such that they are both economical and provide adequate passage conditions for fish.