Integration of a Storm Water Project into a Public Space -an example of a RAIN GARDEN in a commercial car park . 1. What’s wrong with Traditional Storm Water Management?
Storm water piped untreated to the sea
In the early 1980’s environmental monitoring programmes identified an accelerating degradation of the region’s estuarine environments as a direct result of storm water contaminants. These include: • zinc from tyres and roofing material • copper from vehicle brake pads • oil, grease, fuel • industrial and domestic chemicals and detergents. The Auckland region’s storm water issues could cost the region several billion dollars to fix over the next 20 years. The ARC, Territorial Authorities, Local Network Operators and Watercare Services Limited are already investing a huge amount of effort and money in storm water management, however a greater collective effort is required; http://www.arc.govt.nz/environment/water/stormwater/stormwater_home.cfm
Sterile harsh environment
2. What are the Benefits, Constraints and Hazards of a Rain Garden? Hazards and Mitigation
-Trip Hazard. To be minimised by having clearly defined edges and ensuring leaves etc. that fall on the car park are removed regularly. -Insects stings or mosquito bites. General maintenance of the gardens should include the lookout for and removal of any wasp nests. Mosquitos can be minimised by ensuring that water does not pond for more than 24 hrs by using appropriate planting soil. -There may be up to 220mm of water ponding on the surface which could potentially be enough for a small child to drown. Dense planting should ensure this is not an area that children would choose to walk into.
-Provides visual amenity and softens hard edges of buildings and carparks. -Filters water before discharging to streams and allows natural water tables to recharge.
-Take up space. Approx. 7% of the area they are treating. -Cannot be used where the natural slopes and soils would make them unstable. -Can only contain plants that can tolerate an extreme range in soil moisture and water level.
3. How do Rain Gardens Work?
Rain gardens are small-scale storm water devices that collect run-off from a small area such as a carpark. The plants and soil filter pollutants in the run-off as it soaks into the rain garden resulting in cleaner water being discharged to the water table below or an outlet pipe. While rain gardens take the form of small depressions their porous soils prevent water from ponding for more than 24 hours. The catchment area should not be too large, usually should be less than 1000sqm. It is better to have more rain gardens than oversized ones. If the soil is unstable an impermeable layer should be used to line the trench. A permeable layer is able to be used if the soil is stable. The size required for a rain garden (ref TP10, 7.8) is dependent upon -The area of land that is to be treated =AF -The percent permeability of the land the water comes from. (concrete, grass or bush ie a car park is 100% impermeable) -The amount of rain that falls in this particular area. Diagrams in TP108 show the maximum rainfall expected in 24 hours in a typical 2 year period for different locations. 80mm for Beachlands= RAIN -The depth of the planting soil. (The filter, measured in m) = D -The permeability of the soil. This is calculated to be 0.3metres/day.=K -The average height in m of the water head. (The average is assumed to be half the full depth of water that can pond in the water garden) =H -The time taken to pass through the soil bed. (one day is used to be conservative)=T
Max. Ponding Depth 220mm
Planting Soil (filter) 1m min
Permeable or Impermeable Liner
Optional Sand Layer 300mm
Gravel Layer 400mm Underdrain
Plan View of Car Park Showing Walkway from Park to Commercial Centre en
Thus to treat 1000sqm of water on an impermeable (car park) surface the calculation is AF=1000sqm x 0.3 x RAIN x D / (K x (H+D) x T =1000 x 0.3 x 0.08 x 1 / (0.3 x ( 0.11 +1) x 1 =81 sqm
4. Plants Specified
Planting Soil 1000mm deep
Carex testacea sedge
Coastal sedge up to 40cm h igh with shiny orange foliage. Prefers full sun and exposure. Tolerates dry soil conditions.
ti kouka, cabbage tree Palm-like in appearance with large heads of linear leaves and panicles of scented flowers. Sun to semi-shade. Prefers damp to moist soil. Grows eventually to 12m+ height.
Cortaderia fulvida toetoe
Branching from the base and forming a clump to 4m h igh. Long strapshaped leaves with redorange coloured veins. Prefers good drainage and semi-shade.
Entelea arborescens whau
Fast growing shrub or small tree (to 5m height) with large bright green heart-shaped leaves. Spiny seed capsules follow clusters of white flowers in spring.
harakeke, flax Clump-forming flax with large stiff leaves, to 3m. Full exposure and sun.
Gravel 400mm Underdrain
mikoikoi, native Iris Clump forming native irise with narrow, upright leaves. Small white fl owers in spring. Sun or shade.
5. What if it Doesn’t Rain or it Rains Too Much? Some watering of plants may be needed (preferably from collected stored water tanks) but generally the plants chosen can tolerate normal dry periods. Filtered water is removed from the base of the rain garden if it is not draining away fast enough to the water table below. Thus more water can be treated than just what is able to be absorbed into the ground. Overflow pipes will take away water (unfiltered) if the water exceeds the 220mm depth limit in a severe weather event. This water will be discharged into the street swales and ultimately into the natural waterways
3D View of Car Park Showing Walkway from Park to Commercial Centre