Treatment MeasuresRain Gardens : Design Details

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Design Approach

(These guidelines are an adaptation of Technical Publication #10 by Auckland Regional Council due to variances in climate and soil)

Determine Storage Volume
The use of 1/3 of the 2 year, 24 hour ARI, with the minimum tc > 10 minutes.
Required Surface Area
A =

(WQV)(d)

k(h+d)(t)

Where:

A = Surface area (m2)
WQV = Treatment Volume (m3)
d = planting depth (m) [usually 1m]
k = coefficient of permeability (m/day)
h = average height of water (m) [usually around 0.075m]
t = time to pass through soil bed [use 1 day conservative]

Concept drawing of example Rain Garden

Enlarge Concept drawing of example Rain Garden
Diagram provided by Contour Designs Pty Ltd

Design Caracteristics (provided as example only)

Diagram of example design characteristics of a Rain Garden

General Comments

When using Rain Gardens, it is advisable to:

Use more and smaller rain gardens than using few larger ones
Contributing catchments should be limited to ~0.1 ha
Design as off-line systems
Aesthetically designed systems will encourage greater public support and acceptance

Soil Composition

Consideration to soil characteristics at each site should be a priority. It is important to understand the physical characteristics of the soil for permeability rates; prevent water logging and therefore vegetation survival. Plant species should be selected on the basis of expected periods of wetting and drying times within the treatment structure (ie. Similar to vegetation selection considerations for wetlands).

For maximum permeability performance, the soil should be of a sandy loam, loamy sand or a loam:sand mix (40:60). Clay content of the soil should be below 25% and permeability should exceed 0.3 m per day. The soil should be clean and free of woody material larger than 25mm in diameter and loosely compacted.

A mulch layer on the surface assists in the retention of soil moisture and resists surface sealing. This organic component can assist in the structure of the soil and supports soil biota at its interface.