13.3 BMP T1030 - Continuous Inflow Biofiltration Swale

13.3.1 Description

In situations where water enters a biofiltration swale continuously along the side slope rather than discretely at the head, a different design approach–the continuous inflow biofiltration swale–is needed. The basic swale design is modified by increasing swale length to achieve an equivalent average residence time.

13.3.2 Applications

Continuous inflow biofiltration swales can be used to ensure compliance with Minimum Requirement 6 - Stormwater Treatment.

A continuous inflow biofiltration swale is to be used when inflows are not concentrated, such as locations along the shoulder of a road without curbs. This design may also be used where frequent, small point flows enter a swale, such as through curb inlet ports spaced at intervals along a road, or from a parking lot with frequent curb cuts. In general, no inlet port shall carry more than about 10 percent of the flow.

A continuous inflow swale is not appropriate for a situation in which significant lateral flows enter a swale at some point downstream from the head of the swale. In this situation, the swale width and length must be recalculated from the point of confluence to the discharge location in order to provide adequate treatment for the increased flows.

13.3.3 Design Criteria

Same as specified for BMP T1010 - Basic Biofiltration Swale except as noted below and shown in Table 4 - 16: Design Criteria for Continuous Inflow Biofiltration Swales

Table 4 - 16: Design Criteria for Continuous Inflow Biofiltration Swales

Design Parameter

BMP T1030 - Continuous Inflow Biofiltration Swale

Longitudinal Slope

0.015 - 0.025 (feet per foot)

Maximum Velocity at Qbiofil

1 ft / sec

Maximum water depth

2”- if mowed frequently;

4” if mowed infrequently

Manning Coefficient

Soil and Cover

Manning's Coefficient

Grass-legume mix on compacted native soil

0.20

Grass-legume mix on lightly compacted soil

0.22

Grass-legume mix on lightly compacted amended soil

0.35

Bed width (bottom)

2 - 10 ft

Freeboard height

1.0 ft

Minimum hydraulic residence time at Qbiofil

18 minutes (initial assumed for design - see notes in BMP T1030)

Minimum length

100 ft

Maximum sideslope (Z)

3 H:1 V

4H:1V preferred

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The design flow for continuous inflow swales must include runoff from the pervious side slopes draining to the swale along the entire swale length. Therefore, they must be online facilities.

If only a single design flow is used, the flowrate at the outlet should be used. The goal is to achieve an average residence time through the swale of 9 minutes as calculated using the online water quality design flowrate multiplied by the ratio, K. Assuming an even distribution of inflow into the side of the swale double the hydraulic residence time to a minimum of 18 minutes.

Interior side slopes above the water quality design treatment elevation shall be planted in grass. A typical lawn seed mix or the biofiltration seed mixes are acceptable. Landscape plants or groundcovers other than grass may not be used anywhere between the runoff inflow elevation and the bottom of the swale.