Virginia Stormwater Dewatering Requirements and Sediment Basin Skimmer Guidelines

Proper dewatering of construction sites in Virginia is critical to prevent sediment pollution in waterways. State regulations and permits set clear requirements for how water must be drained from excavations and sediment basins. This guide outlines Virginia’s dewatering rules, required drain times for sediment basins (especially those using skimmers), skimmer device types and their performance, and best practices for compliance.

General Dewatering Regulations in Virginia

Virginia law mandates that any water pumped or drained from construction sites must be treated to remove sediment before leaving the site. The Virginia Erosion and Stormwater Management Regulation specifies:

“Effluent from dewatering operations shall be filtered or passed through an approved sediment trapping device, or both, and discharged in a manner that does not adversely affect streams or off-site property.”

Construction General Permit (CGP) Requirements

Construction projects disturbing one acre or more must be covered under Virginia’s Construction General Permit (CGP). The permit explicitly allows uncontaminated construction dewatering only if it is filtered, settled, or similarly treated before discharge. Releasing turbid, sediment-laden water directly into storm drains or streams violates both the permit and Virginia’s stormwater regulations.

Local ordinances, such as those in Arlington County, prohibit the discharge of sediment or muddy water into storm drain systems and may impose fines or stop-work orders for violations. Virginia’s regulations align with federal Clean Water Act requirements to ensure water quality compliance.

Sediment Basin Drain Time Requirements

Sediment basins must dewater slowly to allow sediment particles to settle. Best practices recommend a minimum drawdown time of 48 hours. Research indicates that longer dewatering times (4–7 days) maximize sediment removal, whereas excessively long detention periods (>7 days) can lead to stagnation, odor issues, or mosquito breeding.

Inspectors check for proper outfall devices, such as skimmers or small orifices, that regulate water release. Virginia’s DEQ and Erosion and Sediment Control Handbook require sediment basins to have outlet structures that gradually draw down water, with floating skimmers being the preferred method.

Skimmer Types and Their Impact on Dewatering

Floating Weir Skimmers

Floating weir skimmers remove water from the surface, where sediment concentrations are lowest. This design ensures clearer water discharge and improves sediment capture efficiency, with studies showing up to 90% fine sediment retention.

Advantages:

• Releases the cleanest water last
• Controls flow rate for optimal sediment removal
• Reusable and easy to install

Considerations:

• Requires monitoring for debris and potential clogging
• May need removal in freezing conditions

Perforated Riser Outlets

Perforated risers drain water through small holes or slots, withdrawing from multiple depths. While they provide moderate sediment removal (~80% efficiency), they lack the surface withdrawal benefit of floating skimmers.

Orifice-Controlled Outlets

Orifice outlets restrict discharge through a single small opening, controlling flow but withdrawing from lower water levels. This can lead to silty initial discharge. While effective in slowing dewatering, they can clog and require regular maintenance.

Compliance Considerations for Dewatering Practices

To ensure compliance with Virginia’s regulations, contractors should:

• Follow the Approved E&S and SWPPP Plan: Install and use sediment controls as specified.
• Filter or Settle Water Before Discharge: Use sediment basins, dewatering bags, or filter systems before releasing water.
• Control Pumping Rate: Avoid overwhelming filters or sediment traps by adjusting pump rates.
• Monitor and Adjust During Dewatering: Regularly inspect discharge points for clarity and adjust controls as needed.
• Conduct Regular Inspections: Sites must be inspected at least every five business days or after significant rain events to ensure BMPs are functioning.
• Monitor Turbidity in Special Waters: New CGP requirements may mandate turbidity sampling for discharges into impaired or protected water bodies.
• Ensure Proper Training: On-site personnel should understand dewatering system operation, including skimmer use and maintenance.

Best Practices for Dewatering Compliance

Beyond meeting regulatory standards, adopting best management practices (BMPs) can further reduce sediment discharge risks. These include:

• Using Redundant Treatment Systems: Combining skimmers with sediment basins and geotextile filtration enhances efficiency.
• Installing Check Dams or Vegetated Buffers: These slow and filter runoff before it reaches receiving waters.
• Employing Chemical Treatment When Necessary: In high-risk areas, flocculants or coagulants can accelerate sediment settling.
• Seasonal Adjustments: In colder months, preventing skimmers from freezing and optimizing basin depths improve performance.\
• Documentation and Record-Keeping: Maintaining logs of dewatering activities, inspections, and water quality tests helps demonstrate compliance during regulatory reviews.

Compliance with Virginia’s dewatering requirements involves using the right equipment and managing it effectively. By adhering to mandated practices—such as filtering all pumped water, designing sediment basins for slow drawdown, and maintaining proper oversight—contractors can ensure regulatory compliance and protect local waterways from sediment pollution. Implementing enhanced BMPs and staying updated on evolving regulations can further improve site performance and environmental responsibility.

Sources:

¹ Virginia Administrative Code 9VAC25-875 (Virginia Erosion and Stormwater Management Regulation) – law.lis.virginia.gov
² Virginia Construction General Permit (9VAC25-880) – 2019 and 2024 revisions – law.lis.virginia.gov
³ Virginia Stormwater Management Handbook and Erosion & Sediment Control Standards – Sediment basin design criteria and minimum drain times – extension.psu.edu
⁴ Pennsylvania State University Extension – Controlling the Dewatering of Sedimentation Basins (performance of skimmers vs. other outlets) – extension.psu.edu
⁵ Indiana Stormwater Manual – Floating Skimmer vs. Perforated Riser efficiency comparison – in.gov
⁶ Arlington County, VA – Construction Pollution Prevention Guidelines (Dewatering best practices) – arlingtonva.us