Storm Sewer

Start with the project conditions that cannot be negotiated. Those usually include required flow capacity, cover depth, service environment, maintenance access, utility conflicts, and construction sequencing.

Then compare products in a practical order:

1. Confirm the hydraulic target.
2. Check burial depth and loading.
3. Review joint performance requirements.
4. Evaluate installation handling, lengths, and field access.
5. Confirm coatings, materials, or corrosion resistance needs.

Contech supports that process with design resources and engineering review so the selection is tied to the site rather than a generic material preference.

The next step is to assemble the job-specific inputs before you lock in material or profile. That means flow targets, burial depth, loading, soil or corrosion concerns, utility conflicts, and any joint-performance requirement.

Once that information is defined, use Contech’s Start a Project page or contact your local team to review the layout, compare product paths, and get project-specific guidance. From a pipe manufacturer’s perspective, that is how you avoid avoidable redesign late in the process.

Cost savings can often be found by evaluating alternate pipe materials, using more efficient wall designs, reducing hydraulic energy losses, replacing large concrete manholes with prefabricated risers or junction structures, and simplifying joint testing. The main takeaway is to involve manufacturers or technical specialists early in design so alternative systems can be considered before the project layout and specifications are locked in.

Flexible buried pipe systems use soil-structure interaction to carry loads primarily through ring compression rather than bending. This allows the surrounding soil envelope to help support the pipe. As a result, flexible structures can carry significant loads with much less wall material than rigid structures. In the webinar example, a 60-inch corrugated metal pipe weighed roughly 49 pounds per foot and could allow up to 50 feet of cover, while a comparable reinforced concrete pipe weighed over 1,500 pounds per foot with much lower cover limits depending on installation conditions.

Profile wall pipe uses geometry, such as corrugations or ribs, to increase stiffness and structural efficiency without requiring a thick solid wall. This can reduce material use and cost while still meeting performance needs. For example, a corrugated metal pipe profile can achieve stiffness comparable to a much thicker solid wall section. Some profile wall products also provide smooth interiors for improved hydraulic performance.

Composite wall pipes use more than one material so each material contributes its strengths. Reinforced concrete uses steel for tensile capacity and concrete for compression. Truss pipe uses PVC elements with mortar for stiffness. Steel reinforced polyethylene pipe combines the corrosion resistance and smooth hydraulics of polyethylene with the structural strength of steel. This approach can support larger diameters, higher covers, and improved durability depending on the application.

Important factors include pipe diameter, depth of cover, surface loads, hydraulic requirements, material cost, local regulations, joint tightness, corrosion resistance, and site-specific risks such as differential settlement, seismic activity, or wildfire exposure near outfalls. Sanitary sewer applications generally require a higher performance level than storm sewers, especially for tight joints, air testing, and resistance to hydrogen sulfide corrosion.

Sewers are typically designed under outlet control, so hydraulic losses directly affect how efficiently water moves through the system. Lower energy losses can help avoid unnecessary pipe upsizing, improve system capacity, and reduce project cost. The two main types of losses discussed were friction losses within the pipe and form losses at structures such as manholes, junction boxes, bends, and other obstructions.

Friction losses are energy losses caused by resistance along the interior surface of the pipe. Smoother pipe interiors generally have lower friction losses and lower Manning’s n values. Many sewer designs use a Manning’s n value of about 0.012, so selecting a pipe that meets or improves on that hydraulic performance can help maintain capacity without increasing diameter.

Smooth-interior pipe products reduce resistance to flow, which can improve hydraulic capacity. Examples discussed include PVC, spiral rib metal pipe, double wall steel pipe with a smooth liner, and some steel reinforced polyethylene products. These can provide smoother hydraulic performance than traditional corrugated profiles, depending on the material and wall design.

Yes. Corrugated metal pipe can be used in storm sewer applications, especially when the pipe profile is selected for hydraulic performance. Traditional annular corrugated metal pipe has a higher Manning’s n and is often used for culverts, while helically corrugated pipe in smaller diameters can have lower roughness. Spiral rib metal pipe and double wall metal pipe are options when smooth-wall hydraulic performance is desired.

Metal pipe can provide smooth-wall-like hydraulics through specific wall profiles. Spiral rib metal pipe uses a box-rib corrugation that creates a smoother interior and can provide a Manning’s n around 0.012. Double wall metal pipe has a corrugated exterior for strength and a smooth interior liner for hydraulic efficiency.

Thermoplastic pipes can differ in material stiffness, wall profile, and how they respond to soil loads. The webinar noted that polypropylene and HDPE may experience circumferential shortening after installation, which can slightly change the interior profile and increase roughness. PVC, because of its higher yield strength, higher modulus of elasticity, lower thermal expansion, and different wall design, was presented as more likely to remain smooth after backfill is applied.

Common storm sewer materials include reinforced concrete pipe and corrugated HDPE pipe, but alternatives may include PVC, corrugated metal pipe, spiral rib metal pipe, double wall metal pipe, and steel reinforced polyethylene. The best option depends on hydraulics, cover, loads, joint needs, corrosion environment, and local specifications.

For pipes around 48 inches and larger, conventional concrete manholes or junction boxes can become very large, expensive, heavy, and slow to obtain. Prefabricated access risers attached to the pipe can reduce the need for large concrete structures, simplify installation, reduce equipment requirements, and lower material costs. They can also reduce hydraulic form losses because the flow path remains more continuous.

Inline risers should be considered when sewer pipe diameters become large enough that standard manhole structures become costly or impractical. For example, a 72-inch sewer may require a 96-inch or larger manhole structure. In the webinar case study, replacing large concrete risers with prefabricated inline risers significantly reduced material cost and allowed installation with typical excavation equipment.

Lateral connections can be prefabricated into the riser or pipe body at the required elevations and orientations. This allows the contractor to install the main pipe and riser as a manufactured component, then connect the laterals in the field. This approach avoids custom field fabrication and can simplify construction sequencing.

Prefabricated junction structures can combine multiple large incoming pipes, directional changes, and access points into a single manufactured component. This can replace a large cast-in-place junction box, reducing formwork, concrete placement, construction time, and schedule risk. One example described involved multiple 60-inch lines converging into a large prefabricated chamber with access.

Yes. Prefabricated wet wells can be manufactured with required connections, knockouts, and access features before arriving at the job site. The webinar showed large-diameter wet wells that could be dropped into place, reducing field construction work. Properly designed caps or load-distribution elements are used to transfer surface loads into surrounding soil rather than directly into the structure.