Questions from the field is a selection of questions NPCA technical engineers received from calls, emails and plant evaluations.
MR SWIFT ASKS: We are working on a steep storm drain system where flow velocities in the pipes are predicted to be as much as 18 ft/s, and the flow volume will be between 60 cfs and 70 cfs. Should we expect issues at the manholes with entrance velocities this high? Would abrasion of the manhole walls be a concern?
NPCA TECHNICAL EXPERTS ANSWER: While studies have investigated the influence of flow velocity on concrete durability, a definitive threshold above which structures become increasingly vulnerable to long-term degradation has not been established. The Concrete Manual, A Water Resources Technical Publication by the U.S. Bureau of Reclamation, is a well established reference documenting studies on concrete spillways, wherein cavitation and destructive erosion of the concrete were not considered significant when flow velocities were limited to 40 fps. Similar studies conducted by the U.S. Army Corps of Engineers and California’s Los Angeles County also support 40-50 fps as a limit, but these all assume relatively smooth flow dynamics.
Flow through a manhole, however, is anything but smooth unless it can be kept 100% in the flow channel. The previously cited studies do not explicitly address cases with a high potential for cavitation and disruption in a manhole or junction structure, nor do they quantify how well higher-quality concrete resists abrasion. Therefore, a practical limit for most drainage systems is typically 10-15 fps in storm sewers, particularly those that tend to carry higher bed loads. For higher velocity flows, some form of calming or scour protection would be prudent.
One recommendation would be to install a drop manhole, which would also entail decreasing the slope of the influent storm line to a minimum slope of about 0.3%, possibly while also transitioning up to a larger diameter pipe to accommodate high flow volumes at lower velocities, in an effort to induce a hydraulic jump in the pipe before it empties into the manhole. Assuming there is sufficient
headroom, the influent pipe would enter the manhole at a slightly higher level than the effluent, dissipating energy as the flow drops to the manhole invert before exiting through the effluent.
If slowing the flow by calming the slope is not an option, another option is to provide a smooth flow path through the manhole by benching the invert, as is done with most sanitary sewer manholes.
Benching would reduce any offsets and, therefore, reduce or prevent cavitation. This solution would work best for flows that enter and exit the manhole in nearly a straight line, rather than flows that change direction significantly.
A third option would be to protect the concrete from high velocity flows. For example, if flow from the influent pipe impacts the opposite wall of the manhole before dropping to the invert and exiting through the effluent pipe, that wall can be lined with a removable, replaceable wear plate made of plastic or steel. Many types of durable coatings are also available, but the coating manufacturer’s instructions must be followed precisely. Otherwise, the coating may not properly adhere to the concrete.
One final option might be to do nothing and schedule an inspection within a year. While 18 fps is certainly higher than 15 fps, it is still well under 40 fps. If excessive wear is observed in the concrete, a wear plate or epoxy coating could be applied at that time. Precast concrete is made with lower water-to-cementitious ratios, high-quality materials and advanced manufacturing techniques, making it inherently stronger and more durable than cast-in-place concrete. This greater durability increases the concrete’s ability to withstand higher-velocity flows.