Infiltration Rain Garden

DESIGN PRINCIPLES
■ Literature suggests rain garden areas of about 10-20% of upstream impervious area. For GVRD, calculate rain garden area by continuous flow modelling. Optimum rain garden size isabout 50sq.m. draining 250sq.m. of impervious area. ■ Smaller, distributed rain gardens are better than single large scale facilities. ■ Locate rain gardens a minimum 30.5m from wells, 3m downslope ofbuilding foundations, and only in areas where foundations have footing drains and are not above steep slopes. ■ Provide pretreatment and erosion control i.e. grass filter strip to avoid introducingsediment into the garden. ■ At point-source inlets, install non-erodable material, sediment cleanout basins, and weir flow spreaders. ■ Bottom width - 600mm (Min.) to 3000mm (desirable). Length-widthratio of 2:1. ■ Side slopes - 2:1 maximum, 4:1 preferred for maintenance. Maximum ponded level - 150 300mm. ■ Draw-down time for maximum ponded volume - 72 hours. ■ Treatment soil depth - 450mm (Min.)to 1200mm (desirable); use soils with minimum infiltration rate of 13mm/hr. ■ Surface planting should be primarily trees, shrubs, and groundcovers, with planting designs respecting the various soilmoisture conditions in the garden. Plantings may include rushes, sedges and grasses as well as lawn areas for erosion control and multiple uses. ■ Apply a 50-75mm layer of organic mulch for both erosioncontrol and to maintain infiltration capacity. ■ Install a non-erodible outlet or spillway to discharge overflow. ■ Avoid utility or other crossings of the rain garden. Where utility trenches must beconstructed below the garden, install trench dams to avoid infiltration water following the utility trench. ■ Drain rock reservoir and perforated drain pipe may be avoided where infiltration tests bya design professional show a subsoil infiltration rate that exceeds the inflow rate.

An Infiltration Rain Garden is a form of bioretention facility designed to have aesthetic appeal as well as a...