Document prepared 8/30/2006

Intent

This technical design guide will help garden is right for your site. The use Development (LID) techniques, suchreduce the size of or entirely eliminatstormwater ponds and conveyance sydevelopment costs while providing poenvironmental benefits such as pollutgroundwater recharge of stream and abe updated periodically as the techno Rain gardens are shallow, small, heavstormwater retention systems locatedthe stormwater runoff is generated. Tthe landscaping by gentle slopes, irrediverse vegetation (preferably native)draw down ponded water within 24 hVegetated channels can convey overfgarden or a naturally landscaped area For more detail, see Chapter 10 of theManual) at: http://www.co.pierce.wa.the Puget Sound Action Team and Wahttp://www.psat.wa.gov/Publications/(253) 798-4671 or timothy.lowry@co Design Criteria

Location

Place outside of buffers for wet

Locate rain gardens on slopes l

Setback 100 feet from slopes gr

Setback 10 feet from structures

Collect stormwater from an are

Allow 1 foot of separation (throengineered soil (see diagram)

Blend irregular curves of the ra

Provide an overflow pathway

SMALL RAIN GARDENTECHNICAL GUIDE

you decide if a rain of Low Impact as rain gardens, can e the need for stems. They reduce sitive aesthetic and

ion treatment and quifers. This guide will

logy develops.

Wood Chip MulchAbove Ponding Elevation2 to 3 inches depth

RAIN GARDEN SECTION

Seasonal High Groundwater Elevation

41

Engineered Soil(Compost Amended Soil)

18 inch (minimum)

Well Draining Soil12 inch (minimum)

Maximum Water Depth3 inches

High Water Elevation

Bottom Area of Rain Garden

Compost Mulch Below Ponding Elevation2 to 3 inches depth

ily vegetated on the same lot where hey are integrated into

gular curvature, and . They are designed to ours of a storm event. low to another rain .

Pierce County Stormwater Management and Site Design Manual (the Stormwater us/xml/services/home/environ/water/cip/swmmanual/B-SWMChap10.pdf, shington State University Pierce County LID Manual (the LID Manual) at: LID_tech_manual05/lid_index.htm, or contact Timothy Lowry, P.E. at .pierce.wa.us

lands, streams, floodplains, and floodways

ess than 20%

eater than 40%, landslide hazard areas, and erosion hazard areas

and property lines, measured from the top of the water line

a less than 2,000 square feet of combined pervious and impervious surface

ugh well draining soil) between seasonal high groundwater and the bottom of the

in garden into the surrounding landscape

Document prepared 8/30/2006

Operation

Collect on-lot stormwater runoff only; including roofs, residential driveways, landscaped areas, vegetated channels, and other rain gardens

Dewater the surface pool within 24.

Property owners must own and maintain rain gardens

Prior to site development, provide an operation and maintenance plan in the design submittal

Periodically replace wood chip mulch and compost mulch so rain gardens continue to treat pollutants. Owners must maintain 2-to-3 inches of wood chip mulch on garden banks above the high water elevation and 2-to-3 inches of compost mulch below the high water elevation.

500 SF

1.5’ 166 2' 157

2.5' 148 3' 139

3.5' 130

Engi

neer

ed S

oil

Dep

th

4' 121 Notes: 1) Collection areas greater Bioretention Technical Guid2) One foot of separation beengineered soil is required.

to Calcu

Sizing

Rain garden water depth cannot exceed 3 inches above the bottom area (as measured from the top of the compost mulch layer)

Calculate the bottom area of the rain garden using the Rain Garden Sizing Chart. Calculated bottom areas may be accommodated in multiple rain gardens.

Design flat bottoms to maximize infiltration

Design gentle side slopes, no steeper than 4H:1V and blend into surrounding landscape

Soil Engineered soils (compost amended soils) can be produced by mixing compost to well-dimported sources. This design sheet focuses on rain gardens located in good-to-well draito infiltrate vertically and horizontally. Sites with poorly draining soils should use the BioGuide.

Provide a soils report prepared by a geotechnical professional, licensed sewage dessoil characteristics underlying each rain garden as described in section 10.5.5.3 of t

Preserve uncompacted soil at the bottom, sides, and outlet pathway during construc

Select an engineered soil mix such as 30-35% compost and 65-70% loamy sand. (Sof the Stormwater Manual for additional criteria and soil mix options).

Select compost that is permitted by (or exempt from) Tacoma-Pierce County Healtand is Class A compost per WDOE Interim Compost Quality Guidelines

Planting

Select and place plants according to moisture zones described in the Bioretention Pthe LID Manual

In addition to grasses, plant a minimum of 3 tree species and 3 shrub species (prefesystem

Stormwater Modeling Credit

To downsize conventional stormwater systems (ponds, treatment and conveyance),guidance is provided in Appendix III-C of the DOE Stormwater Management Manhttp://www.ecy.wa.gov/pubs/0510031.pdf and in Chapter 7 of the LID Manual

Rain Garden Sizing Chart late Rain Garden Bottom Area (SF)

Stormwater Collection Area 1000 SF 1500 SF 2000 SF

332 583 833 314 553 791 296 523 749 278 493 707

260 463 665 242 433 623

than 2000 SF must be engineered using the e tween groundwater and bottom of Well draining soil is required in this area.

raining soil from on-site or ning soils that allow stormwater retention System Technical

igner or soil scientist discussing he Stormwater Manual

tion and operation

ee sections 10.5.5.3 and 10.5.5.5

h Department (TPCHD) rules

lant List found in Appendix 3 of

rably native) for a diverse habitat

stormwater modeling credit ual for Western Washington at