After reading the article “Sliplining success: hit or myth” (page 125 of the Spring 2005 Manual) and analyzing the possibilities, I noticed an error in the “Maintaining flow capacity by sliplining” section and felt this conclusion could be misleading. The statement, “we derive a Q1/Q2 value of 1.15” is incorrect; the value of Q1/Q2 is 0.87. The resultant conclusion is the same but this should have been written Q2= 1.15 × Q1.
The logic in the conclusion, as I see this, could have disastrous consequences if a pipe system is not analyzed completely. Essentially this conclusion completely disregards the energy of a pipe system and only focuses on the flow volume. This is especially important when talking about stormwater flows were understanding the energy of a system is often more important than flow. When considering replacement of a damaged pipe, the peak design flow is usually known and will not vary. Increasing flow is not the issue. By replacing the pipe with a smaller diameter the area decreases and the velocity for this peak flow must increase (Q=VA). If the velocity increases too much you have the potential to blow a manhole lid, erode an outlet designed for a much smaller velocity, or create a backwater effect with flooding upstream. I see this as a safety problem to consider that would far out way any benefits associated with costs.
To summarize, the main reason this conclusion seemed misleading is because it is very possible that a maintenance worker could implement this type of repair based on the statements in this article to save time and money without consulting an engineer, or without consideration to the engineering that was involved in the original design of an entire system.
— Carl Schmitz, associate civil engineer, Santee, Calif.
It was with great interest that I read the article “Good culverts gone bad” (March 2005, page 38) and the resultant letters to the editor authored by Rich Gottwald, president of the Plastic Pipe Institute, and Dan Edwards, chief engineer, NCSPA. In reading the two letters it is clear that current thinking remains firmly entrenched in the concept of maximizing design service life. In this context, “design service life” is readily replaced with “maintenance-free service life.” However, one can argue that there is very little reliability in the determination of design service life. Methodologies used to determine the expected service life of any of the major drainage pipe types have large variability.
The environmental conditions these pipes are exposed to, as well as backfill materials, backfill compactive effort, and maintenance activities all play a part in the service life that can be expected. It is dangerous to expect any buried structure to reach 50, 75, or 100 years in service without routine inspection and maintenance. The propagation of such long service lives, by both owners and manufacturers, for these buried pipes only exacerbated the problem. The argument can be made that there needs to be a paradigm shift from “maintenance-free service life” to proper asset management, including inspection and maintenance of these pipes.
I have complete confidence that both HDPE and CSP pipes can be successfully installed and easily reach 100 years in service with routine maintenance. Without such maintenance, regardless of what the research suggests, I have very little confidence in making the 100-year claim.
— Kevin White, P.E., senior hydraulic engineer, Wilbur Smith Associates