Rain Gardens- what are they?
A Rain Garden is a planted drainage system that allows rain water runoff from impervious urban surfaces like roofs, driveways, roads and car parks, to be absorbed and treated through a natural system prior to entering stormwater drains. Rain Gardens reduce runoff by allowing stormwater to soak into the ground as opposed to flowing over it. Storm water entering the pit or swale, filters through the substrate, trapping sediment, nutrients and pollutants prior to entering sub-surface drains that pipe water off to storm water drains. This helps with reducing erosion, water pollution and flooding. Rain Gardens can reduce the amount of pollution reaching creeks and rivers by up to 30%. The purpose of a Rain Garden is to improve water quality entering nearby water bodies, this is achieved in the following ways:
Flow Attenuation: Rain Gardens reduce flow rates of storm water. By reducing the flow rate, erosion is reduced, sediments and other pollutants carried by the storm water have a chance to settle out before entering drains. This helps in the prevention of pollution reaching our creeks, rivers and streams.
Rain Gardens planted with the right type of plants will improve water quality. Rain Gardens trap litter, oils & grease. The sediments carried by storm water are trapped and the nutrients carried in them are absorbed by the plants. They are an important pre-treatment for storm water.
(Diagram courtesy of the ABC website)
Plants Ideal for Rain Gardens
Native plants are recommended for Rain Gardens because they generally don't require fertilizer and are more tolerant of the local climate, soil, and weather conditions. Plants that are best suited include riparian species and small trees as these are accustomed to Rain Garden conditions and will assist in the up take of excess water flowing into the system. As water filters through soil or media the plant root systems enhance infiltration and maintain soil permeability, provide moisture redistribution, and sustain diverse microbial populations involved in the bio-filtration process. The process of transpiration by the plants returns water vapour to the atmosphere.
Two sedges that are commonly used in Rain Gardens and Drainage Swales are Carex appressa and Juncus usitatus, these 2 species have proven to be very effective.
Other suitable species would be: Isolepsis nodosa; Carex fasicularis; Juncus krausii; Gahnia sieberiana and clarkei; Lomandra hystrix and Lomandra longifolia.