One of the most common sources of confusion in stormwater design is the difference between water quality treatment and water quantity control. They are separate requirements, governed by different regulations, designed for different storm events, and often reviewed by different agencies. Failing to address either one will stop your project in plan check.

Water Quality: What Is It?

Stormwater water quality treatment removes pollutants from runoff before it enters the receiving water. The pollutants of concern include sediment (total suspended solids), heavy metals (copper, zinc, lead), petroleum hydrocarbons, nutrients (nitrogen, phosphorus), trash, and bacteria. These pollutants wash off roads, parking lots, rooftops, and construction sites with every rain event.

The Clean Water Act, through the NPDES Municipal Separate Storm Sewer System (MS4) permit program, requires municipalities to control pollutant discharges in stormwater. Municipalities pass this requirement down to developers through local stormwater ordinances and conditions of approval.

What the Regulations Require

Most post-construction stormwater quality requirements specify treatment of the "water quality volume" or "water quality flow" — the runoff generated by a small, frequent storm that represents the majority of annual runoff. Common standards:

• 85th percentile 24-hour storm — used in California (C.3) and many other states. This storm is approximately 0.75 to 1.0 inches of rainfall, depending on location. It captures about 85 percent of the annual runoff events by volume.
• First 1 inch of rainfall — a simplified approach used in some jurisdictions.
• Water quality flow rate — some permits specify the flow rate to be treated rather than the volume. Typically the peak flow from the 85th percentile storm or the first 0.2 inches per hour of rainfall intensity.

How You Meet It

Treatment BMPs for water quality include bioretention, permeable pavement, media filters, tree well filters, hydrodynamic separators (for pre-treatment), vegetated swales, and proprietary cartridge filters. The BMP must be sized to treat the design storm volume or flow rate and must meet the performance standards specified in the permit.

Water Quantity: What Is It?

Stormwater water quantity control (also called flow control, peak flow management, or hydromodification management) limits the rate and sometimes the volume of stormwater runoff from a developed site. The concern is that development increases impervious area, which increases both the rate and volume of runoff, causing downstream flooding and stream channel erosion.

What the Regulations Require

Quantity control requirements are typically set by the local flood control district, public works department, or MS4 permit. Common standards:

• Peak flow matching — post-development peak discharge for the 2-year, 10-year, 25-year, or 100-year storm must not exceed pre-development peak discharge. The specific recurrence intervals vary by jurisdiction.
• Zero net increase — post-development peak flow and/or runoff volume must not exceed pre-development values for any design storm. This is the most restrictive standard.
• Hydromodification management — a more nuanced requirement (used in California C.3) that controls both the flow rate and duration of post-development flows to prevent erosion in receiving streams. HM requirements apply to the range of flows from 10 percent of the 2-year peak to the 10-year peak.

How You Meet It

Detention systems (basins, vaults, underground chambers) sized to store the difference between post-development and pre-development runoff volumes for the design storm, with an outlet structure that releases the stored water at the allowable rate. Retention systems (infiltration, harvesting) that remove runoff volume entirely can also satisfy quantity requirements by reducing the effective impervious area.

Why They Are Separate

A water quality BMP is designed for a small, frequent storm (the 85th percentile, roughly a 1-year event). A detention vault is designed for a large, infrequent storm (the 10-year or 100-year event). The design storms differ by a factor of 5 to 20 in intensity. A bioretention basin sized for water quality treatment does not provide meaningful peak flow control for the 100-year storm, and a detention vault sized for the 100-year storm does not provide effective pollutant removal for small storms (because the residence time is too short and the flow-through velocity is too high).

The Integrated Approach

Some agencies allow or encourage integrated designs where a single facility provides both water quality treatment and peak flow control. For example:

• A bioretention basin with an oversized aggregate reservoir below the planting soil that provides detention storage for larger storms, with a flow-controlled outlet.
• A surface detention basin with a permanent pool (wet pond) that provides sedimentation-based treatment during dry weather and detention during storms.
• A retention/infiltration system that handles both the water quality volume (through infiltration) and the peak flow control (by reducing the impervious area contribution to the hydrograph).

The integrated approach saves space and cost, but it requires careful hydraulic modeling to verify that both the water quality and quantity requirements are met simultaneously. The water quality BMP must still treat the design capture volume effectively, and the detention component must still attenuate the design storm peak flow.

Who Reviews What

On many projects, water quality and water quantity are reviewed by different agencies:

• Water quality is typically reviewed by the city or county's clean water program (the MS4 permittee), or by a regional water quality control board.
• Water quantity is typically reviewed by the public works department, the flood control district, or the city engineer.

Submit the right analysis to the right reviewer. A C.3 stormwater control plan satisfies the water quality reviewer but may not address the flood control district's peak flow requirements. A detention calculation satisfies the flood control district but does not demonstrate C.3 compliance. Most projects need both documents.