Performance Criteria: Open Channel Systems

Open channel systems are vegetated open channels that are explicitly designed to capture and treat the full WQ_v within dry or wet cells formed by checkdams or other means. Design variants include:

• Dry Swale (Figure 1)
• Wet Swale (Figure 2)
• Grass Channels (Figure 3)

This section presents criteria for:

• Feasibility
• Conveyance
• Pretreatment
• Treatment
• Landscaping
• Maintenance

A summary of the design criteria is provided in Table 1. The table separates required or minimum design elements from design criteria that would act primarily as guidance.

There is some debate regarding the extent to which grass filters meet water quality goals. They are included in this section, but may be eliminated from some cold design manuals.

For more information, consult "Design of Stormwater Filtering Systems"Center for Watershed Protection, 1996.

Open channel systems shall not be designed to provide stormwater detention (Q_p) or channel protection (Cp_v) except under extremely unusual conditions. Filtering practices must generally be combined with a separate facility to meet these requirements

Please note that judgement is needed to separate minimum design elements from guidance. When crafting a local or state design manual, the jurisdiction will need to go through a process involving stakeholders to select which design elements are necessary in all cases.

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Open Channel Feasibility Criteria

Open channel systems should have longitudinal slopes less than 4.0% to qualify for WQ_v treatment.

Open channel systems, designed for WQ_v treatment, are primarily applicable for land uses such as roads, highways, residential development, and pervious areas.

Open Channel Conveyance Criteria

The peak velocity for the 2 year storm must be non-erosive.

Consult Soil and Water Conservation Engineering (Schwab et al.) for data on erosive velocities.

Open channels should be designed to safely convey the ten year storm with a minimum of 6 inches of freeboard.

Channels should be designed with moderate side slopes (flatter than 3:1) for most conditions. Side slopes should not be steeper than 2:1.

The maximum allowable temporary ponding time within a channel should be less than 48 hours.

Open channel systems which directly receive runoff from impervious surfaces may have a 6 inch drop onto a protected shelf (pea gravel diaphragm) to minimize the clogging potential of the inlet.

An underdrain system should be provided for the dry swale to ensure a maximum ponding time of 48 hours.

Open Channel Pretreatment Criteria

Pretreatment of 0.1 inch of runoff per impervious acre storage should be provided. This storage is usually obtained by providing checkdams at pipe inlets and/or driveway crossings.

A pea gravel diaphragm and gentle side slopes should be provided along the top of channels to provide pretreatment for lateral sheet flows.

Open Channel Treatment Criteria

Dry and wet swales should be designed to temporarily store the WQ_v within the facility to be released over a maximum 48 hour duration.

Open channels should have a bottom width no wider than 8 feet to avoid potential gullying and channel braiding.

Dry and wet swales should maintain a maximum ponding depth of one foot at the "mid-point" of the channel, and a maximum depth of 18" at the end point of the channel (for storage of the WQ_v).

Grass channels should be designed to retain the water quality volume in the practice for a minimum of 10 minutes, with no greater than a 1.0 fps velocity.

Please note that the grass channel design is the only practice with a "rate-based" design. The designer determines the peak flow rate from the water quality storm event, and then uses Manning's equation to ensure that the velocity required to retain flow can be achieved with the channel's cross section and slope.

Open Channel Landscaping Criteria

Wet swales are not recommended for residential developments as they can create potential nuisance or mosquito breeding conditions.

Landscape design should specify proper grass species and wetland plants based on specific site, soils and hydric conditions present along the channel.

Plant lists should be included in the design manual. In addition, the manual may provide plant lists for salt-tolerant vegetation for roadside runoff.

Open Channel Maintenance Criteria

Open channel systems and grass filter strips should be mowed as required during the growing season to maintain grass heights in the 4 to 6 inches range. Wet swales, employing wetland vegetation, do not require frequent mowing of the channel.

Sediment build-up within the bottom of the channel or filter strip should be removed when 25% of the original WQ_v volume has been exceeded.