Stormwater Sizing Criteria

A unified stormwater sizing framework provides a comprehensive approach to managing runoff from developed sites. Instead of designing a practice for a single purpose or a single design storm, this framework requires designers to address five distinct management objectives, from water quality treatment for small, frequent storms to safe passage of extreme floods. A single stormwater practice, such as a pond or bioretention facility, can be designed with a multi-stage outlet structure to meet all applicable criteria simultaneously. This ensures that a development project adequately protects against channel erosion, preserves groundwater recharge, mitigates local flooding, and reduces pollutant loads delivered to downstream waters.

The five primary sizing criteria are the Water Quality Volume (WQv), the Groundwater Recharge Volume (Rev), the Channel Protection Volume (Cpv), the Overbank Flood Control discharge (Qp), and the Extreme Flood control discharge (Qf). Each criterion corresponds to a specific design storm and a specific management goal. By nesting the required storage volumes and controlling the peak discharge rates for multiple storm events, a single, efficient stormwater management system can be designed to meet the full spectrum of regulatory and environmental protection goals.

The Five Sizing Criteria at a Glance

The unified sizing framework is built around five core criteria, each targeting a different storm event and management objective.

Water Quality Volume (WQv)

The Water Quality Volume (WQv) is designed to capture and treat the runoff from the vast majority of storm events that occur in a given year. These smaller, more frequent storms are responsible for transporting the bulk of the annual pollutant load from a developed site. The primary goal of the WQv is to improve downstream water quality by removing suspended solids, nutrients, metals, and other contaminants commonly found in urban runoff.

Several methods exist for determining the WQv, but the most common and recommended approach is based on capturing the runoff from the 90th percentile rainfall event. This is the 24-hour storm depth that is greater than or equal to 90% of all 24-hour storms on an annual basis. While this value varies regionally, it has been shown to capture approximately 90% of the total annual runoff volume, and thus a similar percentage of the annual pollutant load.

Alternative sizing options include:

• One-Inch Rule: A simplified approach that mandates capturing and treating the runoff from a 1.0-inch rainfall event. This may be oversized for arid regions and undersized for wetter climates.
• Half-Inch/First-Flush Rule: Based on the concept that the “first flush” of runoff carries the highest pollutant concentrations. This method requires capturing the first half-inch of runoff from the site area. However, research has shown this approach can be inadequate for highly impervious sites. A study in Austin, TX, found that this rule captured 100% of the total solids load for sites with 10% impervious cover, but only 75% for 50% impervious sites and 43% for 90% impervious sites.
• On-Site Load Calculation: A performance-based approach where the goal is to reduce post-development pollutant loads (e.g., phosphorus, nitrogen) to a specific target, such as pre-development levels or a watershed-based TMDL requirement. This often requires more complex modeling.

The default method for calculating WQv uses the 90% rainfall event (P1) and a volumetric runoff coefficient (Rv) derived from the Simple Method. A Simple Method runoff calculator can assist with this calculation. The formulas are as follows:

WQv = (P1)(Rv)(A) / 12

Where:

• WQv = Water Quality Volume (acre-feet)
• P1 = 90th percentile rainfall event depth (inches)
• Rv = Volumetric runoff coefficient (unitless)
• A = Site area (acres)

Rv = 0.05 + 0.009(I)

Where:

• I = Percent impervious cover (e.g., 35 for 35%)

A minimum Rv of 0.2 is often applied for sites with very low imperviousness to ensure a baseline level of treatment.

The 90% rainfall event varies by location. The following table provides values for selected U.S. cities.

Groundwater Recharge Volume (Rev)

Urban development, through soil compaction and the addition of impervious surfaces, dramatically reduces the amount of rainfall that can infiltrate into the ground. This loss of groundwater recharge depletes aquifers and reduces the dry-weather baseflow of streams, which is critical for maintaining aquatic habitat and ecosystem health. The Groundwater Recharge Volume (Rev) requirement aims to mitigate this impact by mandating that a portion of the developed site’s runoff be infiltrated back into the soil.

The required Rev is typically calculated as a fraction of the WQv, with the specific fraction determined by the site’s pre-development hydrologic soil group (HSG). More permeable soils (Group A) are required to infiltrate a larger volume than less permeable soils (Group D).

The formula for Rev is:

Rev = (S)(Rv)(A) / 12

Where:

• Rev = Recharge Volume (acre-feet)
• S = Recharge Factor, based on HSG (inches)
• Rv = Volumetric runoff coefficient (from WQv calculation)
• A = Site area (acres)

The recharge factor (S) is selected from the table below.

The Rev is considered to be “nested” within the WQv; it is the portion of the water quality volume that must be infiltrated, not an additional volume. This requirement can be met using infiltration practices, bioretention cells with open bottoms, or permeable pavement. In some cases, non-structural practices promoted through stormwater credits, such as rooftop disconnection to pervious areas, can help satisfy the requirement.

If a site contains multiple soil types, a composite Rev should be calculated based on the proportional area of each HSG. The recharge requirement may be waived or relaxed by the local review authority for sites designated as stormwater hotspots, sites with contaminated soils, or sites with geology unsuitable for infiltration (e.g., karst topography, marine clays).

Channel Protection Volume (Cpv)

Development increases both the volume of stormwater runoff and the frequency of runoff events. This altered hydrology dramatically increases the erosive power of flows in downstream channels. Research has shown that uncontrolled runoff from developed areas can cause stream channels to enlarge to two to five times their original size, leading to severe erosion, habitat loss, and property damage. This erosion is a major source of sediment pollution in urban watersheds.

Historically, many communities required only that the post-development 2-year peak discharge be controlled to pre-development levels. However, extensive research and field observation have demonstrated that this approach is insufficient. While it controls the peak of a single design storm, it extends the duration of erosive, bankfull flows for a much wider range of smaller storms. This increased duration of erosive force is the primary driver of channel degradation.

To provide adequate channel protection, the recommended criterion is to provide extended detention (ED) of the 1-year, 24-hour storm event. The entire volume of runoff from this storm is stored and released gradually over a 24-hour period. This strategy mimics the pre-development hydrograph more closely by releasing flows at a rate low enough to avoid causing downstream erosion. For temperature-sensitive coldwater streams, such as those supporting trout populations, a shorter detention time of 6 to 12 hours may be specified to prevent thermal impacts from sun-warmed water in a pond.

The storage volume required for the Cpv can be calculated using hydrologic models like NRCS TR-55 or TR-20. The procedure involves routing the 1-year, 24-hour storm hydrograph through a storage facility and sizing the outlet to achieve the required detention time. A stormwater pond design calculator can assist with these calculations.

Overbank Flood Control (Qp)

The Overbank Flood Control criterion (Qp) is intended to protect downstream properties from the increased frequency and magnitude of out-of-bank flooding caused by development. These are flow events that exceed the capacity of the stream channel and spill into the adjacent floodplain. While less frequent than channel-forming events, these floods can cause significant property damage.

The most common requirement is to control the post-development peak discharge from the 10-year, 24-hour storm (Qp10) to its pre-development rate. Some jurisdictions may specify a different design storm, such as the 2-year or 25-year event, depending on local conditions and regulatory goals.

When determining the pre-development condition for hydrologic modeling, a conservative standard should be used. For vegetated areas, including former agricultural land, the standard is typically “meadow in good condition.” This prevents a scenario where a high-runoff pre-development condition (like a row crop) results in an artificially low storage requirement.

In some cases, “overcontrol” may be required, where the post-development peak flow is reduced to a level below the pre-development rate (e.g., 50% of the pre-development 10-year peak). This can be an effective strategy in watersheds where the cumulative impact of many developments, each controlling to the pre-development peak, could lead to coincident peaks and exacerbated downstream flooding.

Extreme Flood / Safe Passage (Qf)

The Extreme Flood criterion (Qf) addresses very large, infrequent storm events, typically the 100-year, 24-hour storm. The primary objectives are to:

1. Prevent catastrophic flood damage to property and infrastructure.
2. Protect the physical integrity of the stormwater management practice itself.
3. Ensure that the practice can safely pass the extreme flood volume without failure.

Unlike the other criteria, Qf does not always require peak flow attenuation. The default requirement is often to ensure safe conveyance of the 100-year storm through the site and any stormwater practices. This means designing spillways, bypass channels, and downstream conveyance systems to handle the 100-year flow without uncontrolled overtopping or structural failure.

Full peak discharge control of the 100-year storm to pre-development levels may be required in situations where downstream structures (e.g., bridges, culverts) lack adequate capacity or where development encroaches on the 100-year floodplain. An alternative to site-by-site 100-year control is for a community to adopt a model stormwater management ordinance that prohibits development within the ultimate 100-year floodplain (the floodplain that would exist at full watershed build-out). When the floodplain is preserved, the need for costly on-site 100-year detention is often eliminated.

Worked Mini-Example

This example demonstrates the application of the unified sizing criteria for the “Swann Center,” a 38.0-acre residential subdivision. The site has a post-development impervious cover of 36.3% and is located in a region where the 90% rainfall event (P1) is 0.9 inches. The site contains a mix of HSG B and C soils.

Step 1: Water Quality Volume (WQv)

First, calculate the volumetric runoff coefficient (Rv):

Rv = 0.05 + 0.009(I)
Rv = 0.05 + 0.009(36.3) = 0.38

Next, calculate the WQv:

WQv = (P1)(Rv)(A) / 12
WQv = (0.9 in)(0.38)(38.0 ac) / 12 = 1.08 acre-feet

Step 2: Groundwater Recharge Volume (Rev)

The site is 60% HSG B soils (S=0.25) and 40% HSG C soils (S=0.13). A composite calculation is required.

Rev (B soils) = (0.25 in)(0.38)(38.0 ac * 0.60) / 12 = 0.18 ac-ft
Rev (C soils) = (0.13 in)(0.38)(38.0 ac * 0.40) / 12 = 0.06 ac-ft
Total Rev = 0.18 + 0.06 = 0.24 acre-feet

This volume is a subset of the WQv and must be infiltrated.

Step 3: Channel Protection Volume (Cpv)

Using the TR-55 methodology (based on the Maryland Department of the Environment’s procedure for extended detention), the storage volume required to detain the 1-year, 24-hour storm (2.5 inches) for 24 hours is calculated. The post-development 1-year peak inflow (Qi) is 35.0 cfs. The analysis yields a required storage volume (Vs) and an average release rate (Qo).

Cpv = 1.65 acre-feet
Average Release Rate = 0.84 cfs

Step 4: Overbank Flood Control (Qp10)

The goal is to control the 10-year post-development peak flow (129.96 cfs) to the pre-development rate (50.38 cfs). Using TR-55 routing calculations, the required storage volume is determined.

Qp10 Storage = 2.83 acre-feet

Step 5: Extreme Flood (Qf)

For this site, 100-year peak control is not required. The design must ensure that the post-development 100-year storm (216.3 cfs) can be safely conveyed through the site without causing damage or uncontrolled flooding.

Summary of Sizing Requirements

How the Criteria Combine in One Practice

The unified sizing criteria are not additive in a simple sense; rather, they are “nested” within a single stormwater management facility, such as a detention pond, wetland, or bioretention area. This is achieved through a multi-stage outlet structure that controls the release of water at different elevations corresponding to the different required storage volumes.

In a typical stormwater pond:

• The lowest part of the pond provides the permanent pool (if any) and the Water Quality Volume (WQv). A small orifice or weir at the top of this stage is sized to drain the WQv over a specified time (e.g., 24 hours) to provide treatment. If infiltration is part of the design, the Rev is achieved within this WQv stage.
• Above the WQv elevation is the storage for Channel Protection (Cpv). A second, higher orifice is sized to release the Cpv over 24 hours (or the locally specified time) to prevent downstream erosion.
• The largest volume of temporary storage, above the Cpv elevation, is dedicated to Overbank Flood Control (Qp). A larger outlet, such as a weir or culvert, is sized to control the 10-year peak discharge to pre-development rates.
• Finally, an emergency spillway is set at the top of the Qp storage pool. This structure is designed to safely pass the Extreme Flood (Qf) event, typically the 100-year storm, without overtopping the dam and causing a catastrophic failure.

This integrated design allows one facility to perform multiple functions efficiently, from pollutant removal for small storms to safe conveyance of extreme floods. Similar principles apply to other practices, like using a dry swale design calculator to size a facility that combines conveyance, water quality treatment, and infiltration. Correct sizing is only the first step: the volumes only perform as designed when the practice is built to its construction specifications and kept functional through routine maintenance. The other chapters of the Design Manual cover those stages.

Frequently Asked Questions