5.4.13
5.4.13.1 BMP Description
Rainwater harvesting (BMP T5.20 in Volume V, Chapter 11 of the Ecology Manual) is the capture and storage of rainwater for beneficial use. Roof runoff may be routed to cisterns for storage and non-potable uses, such as irrigation, toilet flushing, and cold water laundry. The potable use of collected rainwater may be used for single-family residences with proper design and approval from Kitsap County Public Health District.
5.4.13.2 Performance Mechanism
Rainwater harvesting can be used to achieve reductions in peak flows, flow durations and runoff volumes. The flow control performance of rainwater harvesting is a function of contributing area, storage volume and rainwater use rate.
5.4.13.3 Application and Limitations
Rainwater harvesting systems can be designed to provide on-site stormwater management and flow control and can be an effective volume reduction practice for projects where infiltration is not permitted or desired (Table II-5.17). Rainwater harvesting has higher stormwater management benefits when designed for uses that occur regularly through the wet season (e.g., toilet flushing and cold-water laundry). The use of harvested rainwater for irrigation during the dry months provides less benefit.
Rainwater harvesting functions can be used alone or can be combined with detention pipes, vaults, and cisterns to improve on-site stormwater management and flow control performance.
| BMP | MR #7: Flow Control | ||||||
|---|---|---|---|---|---|---|---|
|
List |
LID Performance Standard |
Basic |
Enhanced |
Oil Control |
Phosphorus |
||
|
Rainwater Harvesting |
X |
|
X | ||||
5.4.13.4 Site Considerations
Rainwater harvesting can be used for new or retrofit projects. Depending upon site constraints, cisterns may be installed at grade, underground, under a deck, or in a basement or crawl space (applicants shall be mindful of required setbacks and “no-build” buffers). Cisterns may be used individually or connected to each other in a series for increased storage capacity. When the captured water is solely for outdoor use, the use of rainwater harvesting should be limited to four homes per acre and lower densities.
5.4.13.5 Design Information
In order to use the guidance for Runoff Model Representation in BMP T5.20 in Volume V, Chapter 11 of the Ecology Manual, the design shall show 100% reuse of the annual average runoff volume. The designer shall use an approved continuous runoff model to calculate the annual average for drainage area.
System designs involving interior uses shall have a monthly water balance that demonstrates adequate capacity for each month and reuse of all stored water annually.
Restrict the use of this BMP to four homes per acre housing and lower densities when the captured water is solely for outdoor use.
See Section 6.7 of the LID Technical Guidance Manual (Hinman and Wulkan 2012) and BMP T5.20 in Volume V, Chapter 11 of the Ecology Manual for detailed design guidance and criteria.
5.4.13.6 Minimum Construction Requirements
Rainwater harvesting systems shall be constructed according to the manufacturer’s recommendations, the Kitsap County’s Technical Building Codes (Chapter 14.04 KCC), and all applicable laws.
5.4.13.7 Operations and Maintenance
Refer to Section 6.7 of the LID Technical Guidance Manual (Hinman and Wulkan 2012) for details on maintenance activities and schedules for the various rainwater harvesting components.