Designing a Rain Garden

Owen Guy

Stormwater biofiltration systems (also known as biofilters, bioretention systems and rain gardens) are becoming an increasingly popular way of dealing with nutrient and sediment run-off in new and existing residential development projects. Rain gardens serve as an essential catchment for nutrient and sediment runoff and act as a ‘Biofilter’ for the removal of these particulates from stormwater. It is a requirement within the design of most new developments that infrastructure is appropriately designed to minimise nutrient and sediment run-off from surrounding landscapes.

Nutrient and sediment runoff can have detrimental effects on local waterways and oceans. A nutrient run-off is well known for causing algal blooms and facilitates the growth of noxious aquatic weeds. Sediment run-off is known for producing increased turbidity in waterways, salt deposition and the blocking of water passage. These implications of nutrient and sediment run-off are well-known causes for the decline in local ecosystems and can lead to severe losses in local endemic species.

How does a rain garden work

To ensure optimal performance objectives; visually pleasing within the urban landscape, a practical filtration system of pollutants and effective storm and surface water run-off. Designs are adapted to suit the local environment (e.g. soil type, rainfall intensity) and stormwater management requirements.

Sandy soil profiles are used to facilitate the growth of plants and the removal of sediment and nutrients. In most circumstances, rainwater is diverted via slope or pipes to these gardens to maximise the filtration of water. The sediment and nutrient run-off are collected within the sandy soil profile and serve as a nutrient source for plants growing within the medium. The plants act as a sink and remove nutrients from the water as it drains from the filtration medium. The water that has passed through the filtration medium is generally diverted into stormwater pipes and returned to the local river systems or ocean.

When designing and selecting materials for rain gardens, it is important to consider the guidelines provided by the Facility for Advancing Water Biofiltration (FAWB) 2008, species selection and the maintenance of rain gardens. This article provides a brief overview of material selection, design, species selection and maintenance of a rain garden.

Material Selection

The most important aspect of the FAWB guidelines is hydraulic conductivity. There is a delicate balance between a filtration medium that drains too fast, and one that drains too slow. The FAWB guidelines suggest a hydraulic conductivity of 100-300 mm/hr; however, conductivities of 300-500 mm/hr during construction are generally deemed acceptable.

Higher hydraulic conductivity is usually slowed within 2-3 months after the construction of a rain garden as sediment begins to block and clog the surface of the filtration medium. This should be taken into consideration when materials are at the lower end of the hydraulic conductivity spectrum, as it is likely a material that has a hydraulic conductivity on 100 mm/hr will drop over time and become prone to water logging.

When selecting materials as a filtration medium for a rain garden, special attention should be paid to the particle size distribution of the medium. Materials with high percentages of fines are generally poorly draining and very prone to packing; conversely, materials high in coarser particles are likely to be very free draining. As specified in the FAWB guidelines, a gap-graded medium with the majority of particles being 0.05-1.0mm should be selected for a filtration medium. These particle sizes have been favoured as they provide an appropriate hydraulic conductivity and particles are unlikely to settle and become graded over time.

Materials should be analysed at a laboratory to ensure that they are appropriate for use as a biofiltration medium. This simple step can provide essential insight into the potential performance of a rain garden once constructed and allow for effective decisions to be made early on in the design process.

SESL can conduct analysis for rain gardens in line with the FAWB guidelines and provide advice on the suitability of the material, potential ameliorations and design considerations.

Run-off and drainage design

When considering the purpose of a rain garden, the general idea is to have run-off water from surrounding areas diverted to the area so that it may pass through the filter medium. This can be achieved with the implementation of surface grading and stormwater drainage to focus water flow to these areas during rainfall events.

In saying this, too much of a good thing can be damaging or ineffective to a rain garden. Practical measures should be put in place to divert water flow during heavy rainfall events. This is especially important in tropical areas that experience flooding, monsoons or extreme rainfall in short periods.

Rain garden materials can be prone to washing away, especially during heavy rainfall events. Filtration media should be protected from runoff and surface erosion through the use of non-floatable mulches or jute mesh to prevent disruption of the surface material. This is especially important during the establishment of plant species, as roots are still developing and providing anchorage to the plant and stability to the soil.

Species Choice

Another important aspect of designing and constructing a rain garden is the choice of plant species to be established within the filtration layer. When considering the environment that is created within a rain garden, there is significant variation in conditions depending on season and region.
Rain gardens experience periods of rainfall, occasional inundation with water and at other times, complete drought. Due to the free draining nature of the filtration medium, rain garden soils tend to dry out quicker than the average landscape soil. Additionally, rain gardens should not be fertilised, as nutrients are sourced from run-off and sediment which are generally low in essential nutrients for plant growth, so filtration mediums are typically nutrient poor.

As such, the choice of locally sourced drought tolerant species with relatively low nutrient requirements are usually best suited for use in rain gardens. Generally speaking, grasses and rushes that originate from the ephemeral margins of wetlands are typically suited to the conditions experienced within a rain garden. These species have adapted to the fluctuation of water levels and periodic wet/dry periods similar to those experienced in rain garden environments.  

Rain garden Maintenance

Generally speaking, rain gardens are very low maintenance landscaping features. If designed correctly they are essentially self-watering, self-fertilising and self-maintaining gardens that require very little intervention to succeed. During establishment, it is good practice to provide some extra care to newly planted species through irrigation and weeding. These practices will allow for the rapid establishment of plants and set them off on the right foot.

Rain gardens should be given regular check-ups to ensure the rain garden is performing as desired. Monthly visual checks should be undertaken to observe that plants are growing well, and no signs of nutrient or water deficiency are observed.

The in-situ (on site) hydraulic conductivity should also be monitored annually. Similar to air filters, over time the filter medium of rain gardens becomes clogged with particulates and become less effective, this can be measured by the hydraulic conductivity and allows for management decisions to be made to maintain the filtering aspect of the medium. Raingarden filtration medium can be ‘refreshed’ by adding some cleaner coarse sand or skimming off the top layer of sediment to expose less clogged filtration medium.

Rain gardens are becoming increasingly popular in landscape design and provide a method for capturing nutrient and sediment run-off. With careful design and construction considerations, these landscapes are effective in the removal of sediment and nutrients from water before reaching stormwater drains. SESL has been involved in multiple projects and assisted in the design, construction and assessment of materials for rain gardens and can assist in any of these aspects.