Why we need risers?
If you’ve ever walked down a city street and stepped over a manhole with the label “STORMWATER” nestled in the pavement, chances are you’ve skimmed the surface of a detention system. If you were to lift up that manhole lid and peer down into its’ depths, you might be surprised to find a whole underground network of pipes and products designed to prevent flooding and treat stormwater. However, one can’t see an underground grid from the city streets they walk on, which presents a problem to municipalities when maintenance and inspection is an important part of keeping stormwater clean and up to regulation. Risers play a significant role in maintaining detention systems. They allow inspection crews to enter and visually inspect detention systems to evaluate functionality and determine if maintenance is needed. If a pipe is clogged, or sediment has accumulated in front of the outlet orifice, the riser allows the maintenance personnel to climb down a ladder with a pump to vacuum out the system.
Where to put risers?
Often, an engineer will consider the above ground aesthetics or a desired location for the ring and cover to be located due to other constraints. However, it is typically the manufacturer’s responsibility to make sure the engineer’s desires make sense with respect to the structural integrity, cost, and functionality of the chosen location.
For the purpose of system inspection, it is recommended that at least one riser per run of pipe is shown over any given detention system. Risers are often staggered on systems with multiple runs (see example below). Staggering the risers is done to improve aesthetics by avoid having a series of manholes next to each other.
If access to a specific location is required, the most convenient entrance is from the same run of pipe. For similar reasons of access convenience, risers should be located both before and after weirs in detention systems, and almost always close to the outlet of the detention. Generally, Contech can accommodate most riser locations as long as they do not interfere with welding constraints. One of the most common conversations about risers that Contech engineers encounter is when risers are requested on the elbow of a system, as this would result in overlapping welds which weaken the structure. This is why Contech engineers will recommend placing a riser just upstream of a joint, with the rule of thumb being 1’ away from any welds.
How to size a riser?
While sizes of risers may vary, the most common diameter provided is a 36” round. A three-foot riser is small enough so that it is not very noticeable by the general public but large enough for a person to enter the system. If a ladder is required, Contech recommends a diameter larger than 24”, otherwise the riser can be as small as acceptable for inspection purposes. Larger diameter risers are also common on large diameter systems. Contech will check the structural integrity of the metal pipe and riser connection for all risers, and especially those with larger diameters to ensure that the riser does not need reinforcing, and if so, ensure that reinforcing is shown and implemented.
Risers offer a convenience that remain unseen, but are essential to the lifespan and longevity of a stormwater system. Although risers are the tip of the iceberg so to speak, in the larger world of underground detention, it is nice to know that they can be sized large or small, and used as much or as little as needed, for each project.