Construction Specifications

Effective stormwater management relies on the successful transition from design to construction. While plans may accurately reflect design intent based on established sizing criteria, the long-term performance of a stormwater practice is ultimately determined by how well it is built. Construction specifications provide the necessary detail to ensure that materials, methods, and sequencing result in a facility that functions as designed throughout its service life. These specifications form a critical part of the local review process and are the basis for construction inspection.

This page provides a condensed overview of key construction specifications for common stormwater practices. Local requirements and site-specific geotechnical recommendations may impose additional or more stringent standards. All references to ASTM and AASHTO standards should apply to the most recent version.

Stormwater Pond Specifications

Stormwater ponds, including wet ponds, extended detention ponds, and stormwater wetlands, are embanked structures that require careful construction to ensure stability and water tightness. For detailed calculations, refer to the pond design calculator.

Embankment and Earth Fill

Proper construction of the pond embankment is critical to prevent structural failure.

• Site Preparation: Areas for the embankment, borrow sites, and structural works must be cleared, grubbed, and stripped of topsoil. All trees must be cleared within 15 feet of the toe of the embankment.
• Fill Material: Fill material must be taken from approved borrow areas and be free of roots, stumps, wood, rubbish, stones greater than 6 inches, and other objectionable materials. For the embankment core and cutoff trench, fill material must conform to Unified Soil Classification GC, SC, CH, or CL and have at least 30% passing the #200 sieve.
• Placement: Before placement, the foundation area must be scarified. Fill is to be placed in continuous horizontal layers not exceeding 8 inches in thickness (before compaction). The most permeable material should be placed in the downstream portions of the embankment, outside the core.
• Compaction: Each lift of fill must be thoroughly compacted. A common standard is for the fill material to contain sufficient moisture so that it will not crumble when formed into a ball, but not be so wet that water can be squeezed out. The minimum required density is typically 95% of the maximum dry density as determined by AASHTO Method T-99 (Standard Proctor), with a moisture content within 2% of the optimum. A qualified geotechnical engineer should certify all compaction testing.

Core Trench and Cutoff

A cutoff trench is excavated along the embankment centerline to create an impervious barrier. It must be excavated to a minimum depth of 4 feet below existing grade and have a minimum bottom width of 4 feet. The trench is backfilled with suitable clay material (GC, SC, CH, CL) and compacted to ensure maximum density and minimum permeability.

Principal Spillway

The principal spillway, consisting of a riser and barrel, must be installed concurrently with the embankment fill, not excavated into it later. Backfill around the pipe is critical and must be placed in 4-inch lifts and compacted with hand-directed equipment. No heavy equipment should operate within 4 feet of the structure until at least 24 inches of compacted fill is in place over it.

• Reinforced Concrete Pipe (RCP): Must meet ASTM C-361 and have bell and spigot joints with rubber gaskets. RCP conduits must be laid in a concrete bedding or cradle for their entire length. The bedding must consist of high-slump concrete placed under the pipe and up its sides to at least 50% of its outside diameter, with a minimum thickness of 6 inches. Gravel bedding is not permitted.
• Corrugated Metal Pipe (CMP): Must have watertight coupling bands or flanges with rubber or neoprene gaskets. Dimple bands are not considered watertight. The material must be appropriate for site soil and water conditions (e.g., polymer coated, aluminum coated) and meet relevant AASHTO specifications (M-245, M-274, M-196).
• Plastic Pipe (PVC/HDPE): Must meet relevant ASTM or AASHTO standards (e.g., ASTM D-1785, AASHTO M294 Type S). Joints must be completely watertight.

Anti-Seep Collars and Drainage Diaphragms

To prevent seepage along the exterior of the spillway barrel, anti-seep collars must be installed as specified on the plans. Collars must be composed of the same material as the pipe and connected with completely watertight seals. As an alternative, a drainage diaphragm or filter diaphragm may be used, but its design and construction must be supervised by a registered professional engineer.

Emergency Spillway

The emergency spillway must be constructed to convey large storm events safely without damaging the embankment. It is typically excavated from undisturbed earth and stabilized with a dense stand of vegetation or rock riprap placed over a geotextile fabric to prevent erosion.

Outlet Protection

Discharge from the principal spillway barrel must be directed through a section of outlet protection to prevent scour at the toe of the embankment. This typically consists of rock riprap placed over a Class “C” non-woven geotextile fabric.

Vegetation

All exposed surfaces of the embankment, emergency spillway, and borrow areas must be stabilized immediately after construction. This involves placing a 4-inch layer of topsoil and then seeding, liming, fertilizing, and mulching in accordance with local standards to establish a dense, erosion-resistant turf.

Infiltration Trench Specifications

The function of an infiltration trench depends entirely on maintaining the porosity of the underlying soils and the void space within the stone aggregate. Construction methods must be chosen to avoid compaction and prevent clogging with sediment.

• Sequencing: Infiltration trenches must not receive runoff until the entire contributing drainage area has been fully stabilized. All upstream erosion and sediment controls must be in place and functional. The trench should be one of the last items constructed on a site.
• Site Preparation: Heavy equipment traffic must be kept away from the proposed trench location to prevent soil compaction. Excavation should be performed with equipment that minimizes compaction, such as a backhoe operating from the side of the trench. If the trench walls or bottom become smeared and sealed by equipment, they must be scarified or roughened to restore porosity.
• Stone Aggregate: The stone reservoir should be filled with clean, washed, uniformly graded stone aggregate, typically meeting AASHTO M-43 Size No. 2 or No. 3. The stone should be placed in maximum 12-inch loose lifts and compacted using a plate compactor.
• Filter Fabric: The bottom and sides of the trench must be lined with a Class “C” or better non-woven geotextile fabric. The fabric should be wide enough to overlap by a minimum of 6 inches at the top after the stone is placed. Where rolls of fabric overlap, the upstream roll must lap at least 2 feet over the downstream roll.
• Observation Well: An observation well is required to monitor drawdown times. It should be constructed of 6-inch diameter Schedule 40 PVC pipe, perforated in the lower section, and installed vertically to the bottom of the trench. The well should be capped with a locking, vandal-proof cap set 6 inches above the final ground surface.

Sand Filter and Open Channel Specifications

Sand filters and open channels like grass swales rely on specific filter media and properly established vegetation to function correctly.

Sand Filters

• Sand Media: The sand for the filter bed must be a clean, washed concrete sand conforming to AASHTO M-6 or ASTM C-33, with a typical size range of 0.02 to 0.04 inches. Unacceptable substitutions include “rock dust” or sands with high calcium carbonate content.
• Underdrains: An underdrain system is placed at the bottom of the filter bed in a layer of gravel. The pipes are typically 6-inch rigid Schedule 40 PVC with 3/8-inch perforations, spaced 6 inches on center. A minimum of 3 inches of gravel (e.g., AASHTO M-43) must cover the pipes.
• Sequencing: As with infiltration practices, sand filters must not receive runoff until the contributing drainage area is fully stabilized to prevent premature clogging of the sand bed.
• Testing: Underground sand filters may require a water tightness test before filter media is placed. The chamber should be filled with water to ensure there are no leaks.

Open Channels (Grass Channels and Dry Swales)

• Soil Preparation: For dry swales, a permeable soil mixture is required, typically 20 to 30 inches deep, meeting specifications similar to bioretention soil (e.g., USDA loamy sand or sandy loam). The interface between the prepared soil bed and any underlying gravel layer should be roto-tilled to create a gradual transition. For detailed calculations, see the dry swale design calculator.
• Vegetation: Channels must be seeded with flood- and drought-resistant grass species appropriate for the region. Vegetation must be fully established before the practice is put into service.
• Check Dams: If check dams are specified, they must be embedded at least 3 feet into the side slopes of the channel to prevent bypass flow. Materials can include pressure-treated 6×6 or 8×8 timbers or durable, rot-resistant natural wood such as Black Locust, Cedar, or White Oak.

Construction Sequencing and Inspection Points

A rigorous inspection schedule is essential to verify that stormwater practices are built according to the approved plans and specifications. Inspections should occur at key milestones during construction.

Common Construction Defects

Failure to adhere to specifications can lead to predictable failures. Common defects include:

• Improper Embankment Compaction: Leads to slumping, settlement, or catastrophic failure of a pond embankment.
• Leaking Spillway Joints: Causes internal erosion (piping) through the embankment, which can lead to failure.
• Compaction of Infiltration Surfaces: Reduces or eliminates the infiltration capacity of trenches and basins, causing them to function as ponds and fail to meet water quality or recharge goals.
• Poor Construction Sequencing: Allows construction sediment to enter and clog infiltration trenches, sand filters, and bioretention areas before they are stabilized, resulting in premature failure.
• Incorrect Media: Use of “dirty” or improperly graded stone, sand, or soil media reduces void space and infiltration rates.
• Incorrect Elevations: Setting weirs, orifices, or spillways at the wrong elevation fundamentally alters how the practice controls runoff, impacting both water quality treatment and peak flow attenuation.
• Inadequate Vegetative Stabilization: Results in erosion of embankments, spillways, and channel linings.

Frequently Asked Questions