Hydraulic Capacity of Deteriorated Culverts and Impact on Asset Management

Although corrugated metal pipe (CMP) is widely used as an inexpensive means for culverts crossing streams and providing drainage along and across roadways, and is a very important component of many transportation systems in the US, historically, very little is known regarding hydraulic capacity of CMP culverts after enduring many years of service. Though the decision to repair, renew or replace existing old culvert pipes should be based on structural integrity and hydraulic efficiency, it is currently decided based on only the structural integrity. Hence in this research the hydraulic efficiency of an old culvert is measured and compared with that of a new one. Two 8-inch diameter CMP culverts, a new one and an approximately 30-year-old culvert that was removed from a rural roadway in New Jersey, were tested for their hydraulic carrying capacities. The mean value of Manning Coefficient was determined to be 0.017 for the new helical pipe and 0.021 for the old annular pipe. The maximum value of the Reynolds and Froude Numbers were determined to be 38,562 and 0.64 respectively for the new pipe, and 36,144 and 0.85 for the old pipe, which indicates that the measured flows were turbulent and remained in the sub-critical range. Based upon the experimentally determined values, the full-flow capacity of each pipe was determined to be approximately 630 gpm (1.40 cfs), and 540 gpm (1.20 cfs), respectively, which corresponds to a 14% reduction of capacity due to the combined effects produced by their innate physical differences and those that accompany deterioration after 30 years of service. Based on the section loss the old culvert was determined to be at condition state 3 and a survival probability of 74.5% with an average section loss of between 10% and 30%. These results also point out the importance of inspections, since the reduction in hydraulic capacity appears, based upon our limited number of tests, to lag the loss of structural integrity, and thus tends to support the current practice of assessing the condition state of old culverts based only on their structural integrity as being conservative.