5.4.7
5.4.7.1 BMP Description
A perforated stub-out connection (BMP T5.10C in Volume V, Chapter 4 of the Ecology Manual) is a length of perforated pipe within a gravel-filled trench that is placed between roof downspouts and a stub-out to the local drainage system (see Perforated Stub-out Connection figure in Volume V, Chapter 4 in the Ecology Manual).
5.4.7.2 Performance Mechanism
Perforated stub-out connections are intended to provide some infiltration during drier months. During the wet winter months, they may provide little or no flow control.
5.4.7.3 Applications and Limitations
Perforated stub-outs are not appropriate when seasonal water table is less than 1 foot below trench bottom.
In projects subject to Vol I–4.2.5 Minimum Requirement #5: On-site Stormwater Management, perforated stub-out connections may be used only when all other higher priority on-site stormwater management BMPs are not feasible, per the criteria for each of those BMPs.
Select the location of the connection to allow a maximum amount of runoff to infiltrate into the ground (ideally a dry, relatively well drained, location). To facilitate maintenance, do not locate the perforated pipe portion of the system under impervious or heavily compacted (e.g., driveways and parking areas) surfaces.
Have a licensed geologist, hydrogeologist, or engineering geologist evaluate potential runoff discharges towards landslide hazard areas. Do not place the perforated portion of the pipe on or above slopes greater than 20% or above erosion hazard areas without evaluation by a professional engineer with geotechnical expertise or qualified geologist and jurisdiction approval.
For sites with septic systems, the perforated portion of the pipe shall be down-gradient of the drain field primary and reserve areas (see Vol II–5.3.1 Determine Dispersion Feasibility and the Kitsap County Board of Health Ordinance 2008A‑01, as now or hereafter amended). This requirement can be waived if site topography will clearly prohibit flows from intersecting the drain field or where site conditions (soil permeability, distance between systems, etc.) indicate that this is unnecessary.
Perforated stub-out connection BMPs may only be used to satisfy (partially or fully) MR #5: On-site Stormwater Management using the List approach (see Table II-5.11). It may not be used for the LID Performance Standard nor to meet runoff treatment or flow control requirements.
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
||
|
Perforated Stub-Out Connection |
X | ||||||
5.4.7.4 Site Considerations
The stub-out connection shall be sited to allow a maximum amount of runoff to infiltrate into the ground (ideally a dry, relatively well drained, location). Site considerations for the applicability of perforated stub-out connections include:
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Setbacks and Restrictions – The perforated portion of the system shall meet the siting and infiltration rate requirements for infiltration BMPs presented in Vol II–5.3.2 Determine Infiltration Feasibility and for infiltration trenches presented in Vol II–5.4.16 Infiltration Trenches.
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Site Prohibitions – The perforated pipe portion of the system shall not be located under hard or heavily compacted (e.g., driveways and parking areas) surfaces.
5.4.7.5 Design Information
Perforated stub-out connections consist of at least 10 feet of perforated pipe per 5,000 square feet of roof area laid in a level, 2‑foot wide trench backfilled with washed drain rock. Extend the drain rock to a depth of at least 8 inches below the bottom of the pipe and cover the pipe. Lay the pipe level and cover the rock trench with filter fabric and 6 inches of fill (see Perforated Stub-Out Connection figure in Volume V, Chapter 4 in the Ecology Manual).
Any flow reduction is variable and unpredictable. No computer modeling techniques are allowed that would predict any reduction in flow rates and volumes from the connected area.
See BMP T5.10C in Volume V, Chapter 4 of the Ecology Manual for guidance on Runoff Model Representation.
5.4.7.6 Minimum Construction Requirements
During construction, it is critical to prevent clogging and over-compaction of the subgrade. The minimum construction requirements for infiltration trenches in Vol II–5.4.16 Infiltration Trenches apply.
5.4.7.7 Operations and Maintenance Requirements
General O&M guidelines and procedures for infiltration BMPs apply to perforated stub-out connections; see Volume V, Appendix V‑A of the Ecology Manual.