watermark

SC6.17 Landscape design guidelines for water conservation planning scheme policy

Contents

1 Introduction

1.1 Relationship to planning scheme

1.2 Purpose

1.3 Terminology

2 Soil infiltration

2.1 Landform

2.2 Soil

2.3 Mulch

3 Plant selection

4 Irrigation

5 Landscape maintenance

1 Introduction

1.1 Relationship to planning scheme

This planning scheme policy:

(a) provides information the Council may request for a development application;
(b) provides guidance or advice about satisfying an assessment criteria which identifies this planning scheme policy as providing that guidance or advice;
(c) states a standard for the assessment criteria identified in the following table:
Column 1–
Section or table in the code
Column 2–
Assessment criteria reference
Column 3–
Standard in the planning scheme policy
   
CO19
All
   
CO19
All
CO20
All
AO12.1
All
AO12.2
All

1.2 Purpose

This planning scheme policy provides information required for a development application, guidance and advice on satisfying essential criteria and standards for assessment criteria for irrigation design and landscaping for water conservation to reduce the amount of potable water used for landscaping purposes, through water sensitive landscape design. These principles seek to:

(a) protect existing natural features and ecological processes;
(b) maintain the natural hydrologic behaviour of catchments;
(c) protect water quality of surface water and groundwater;
(d) minimise demand on the reticulated water supply system;
(e) integrate water into the landscape to enhance visual, social, cultural and ecological values.

1.3 Terminology

In this planning scheme policy unless the context or subject matter otherwise indicates or requires, a term has the following meaning:

deep infiltration: infiltration of stormwater to deep soil layers and aquifers

evaporation rate: the quantity of water, expressed in terms of depth of liquid water, which is evaporated from a given surface per unit of time. It is usually expressed in millimetres per day, month, or year.

evapotranspiration: combination of evaporation from free water surfaces and transpiration of water from plant surfaces to the atmosphere

field capacity: the soil condition that results when macropores are empty of water and micropores are full of water. This state usually occurs 24 to 48 hours after rain or irrigation. Sand holds very little water at field capacity because it has few micropores. Clays and organic soils hold significantly greater quantities of water because they have more micropores.

hydro-zone: areas within a site of differing soil moisture, evaporation rate and exposure to the local weather conditions

hydro-zoning: landscape design that locates plants according to hydro-zones

infiltration rate: the rate at which infiltration takes place expressed in depth of water per unit time, usually in millimetres per hour

irrigation efficiency: the percentage of water applied that can be accounted for in soil moisture increase for consumptive use

macropore: larger soil pores, generally having a minimum diameter between 30 and 100 micrometres, from which water drains readily by gravity

micropore: relatively small soil pore, generally found within structural aggregates and having a diameter less than 30 micrometres. Micropores hold most of the water that can be used by plants.

percolation: the movement of water, under pressure, through the gaps in rock or soil. It does not include movement through large openings such as caves

pores: the gaps that exist between soil particles. They include macropores and micropores.

rhizobia: bacteria of the genus Rhizobium capable of forming nitrogen-fixing nodules on the roots of leguminous plants

shallow infiltration: infiltration to topsoil and subsoil layers

wilting point: the soil condition that results when the soil dries out to the point where plants cannot extract any remaining water. Soil holds onto water via capillary forces; as more water is removed, these forces become larger, making it increasingly difficult for plants to extract water. Plant leaves and stems wilt when the plant can no longer extract water.

2 Soil infiltration

(1) Direct rainfall infiltration to soils is to supply 50% or more of the plants’ annual water needs. If rainfall infiltration provides between 50% and 100% of the plants’ water needs, an irrigation system incorporating rainwater harvesting is to be used and is to supply at least 50% of the remaining water for plant irrigation. If 100% of the plants’ water needs can be met by rainfall, an irrigation system is not required.
(2) For landscaping located on a podium, a minimum of 75% of the plant’s water needs is to be sourced from other than the reticulated water supply.

2.1 Landform

(1) Run-off from impervious surfaces such as roofs, driveways and car parks, and overland flows from turf and garden beds is to be captured and allowed to infiltrate into subsoils by reshaping existing landforms through the use of swales, contour banks, soaks, percolation pits and basins, rain gardens and bioretention filters.
(2) To reduce run-off and erosion and encourage rainwater infiltration into soil, a landscape embankment is not to exceed grades of 1 in 4.
(3) The regrading of land is not to occur within the tree protection zone of existing trees to be retained.

2.2 Soil

Editor’s note—Guidance on the design of these measures can be found in the Council’s publications.

(1) Priority is to be given to using existing site soil as imported soil is a limited resource. Existing topsoil is conserved by either not disturbing the soil during construction or by stockpiling it prior to construction commencing. Subsoil is to be cultivated to a minimum depth of 150mm for garden beds and turfed areas unless this will adversely affect the roots of established trees.
(2) The minimum topsoil depth is:
(a) 100mm for non-irrigated turf areas;
(b) 200mm for irrigated turf areas;
(c) 400mm for garden beds;
(d) 1000mm over an area 1500mm x 1500mm for trees in deep-planting areas;
(e) 400mm or 1.5 times the root-ball depth, whichever is greater, over an area of twice the root-ball diameter for trees supplied in pots or bags.
(3) If additional soil is required to meet these minimum depths, soil is to meet AS 4419-2003 Soils for landscaping and garden use.
(4) The soil quality is sufficient to allow plants to grow effectively. Soil amelioration measures to improve the infiltration of existing soils, the soil’s macropore and micropore balance and ensure a stable soil ecosystem, include the following:
(a) scarifying crusted topsoil layers;
(b) aerating topsoil layers;
(c) deep ripping subsoil layers;
(d) using hand tools only within the tree protection zone of a tree for retention;
(e) applying gypsum to sodic clay topsoils and subsoils;
(f) installing a 50mm layer of lucerne hay between the topsoil and mulch layer;
(g) adding worms to the topsoil;
(h) applying soil rhizobia in solution to the topsoil;
(i) inoculating plants with mycorrhizal fungi;
(j) incorporating soil wetters, crystals and wettable foams.

2.3 Mulch

(1) Organic mulches applied to landscaped garden beds and trees:
(a) meet AS 4454-2012 Composts, soil conditioners and mulches;
(b) are applied to a depth of 75–100mm;
(c) are of a coarse texture to allow water penetration to prevent nitrogen drawdown of the soil;
(d) are aged prior to application;
(e) do not use plastic sheeting, typically used for weed suppression, as this prevents rainwater from infiltrating soils and inhibits gaseous exchange between the soil and air;
(f) use a biodegradable mulch mat on any waterway embankment.
(2) Inorganic mulches, such as recycled concrete or brick cobbles, are limited to feature or themed landscapes, or in windy areas where organic mulches may be blown away.

3 Plant selection

Plants species are to be selected by a suitably qualified and experienced horticulturist, landscape architect or designer to suit each of the site’s hydro-zones, so that direct rainfall supplies all or most of their water needs.

4 Irrigation

(1) If irrigation is required, the storage and irrigation system is to be used to supply harvested stormwater to garden beds and turfed areas.
(2) The system is to be designed by a person suitably qualified person accredited as a certified irrigation designer by the Irrigation Association of Australia, holding a Diploma of irrigation, or with equivalent experience.

4.1 Site water harvest capacity

The quantity of water which can be harvested from roofs, driveways, car parks and other impervious surfaces is to be determined, as follows:
(a) Step 1—Obtain monthly median rainfall data from the Bureau of Meteorology for Brisbane from the pluviograph station(s) closest to the site or use the data in Table 1.
Table 1—Monthly rainfall data for Brisbane Airport
Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total Annual
Median rainfall per month (mm)
127.4
119.8
124.4
53.3
72.3
35.6
40
33.8
33
73.5
73.9
107
1138.4

Source: http://www.bom.gov.au/climate/averages/tables/cw_040223.shtml

(b) Step 2—Identify the site's impervious areas that could act as stormwater harvesting surfaces and calculate the total m2 for each surface type (including roofs, driveways and car parking areas). Unless pumps are to be incorporated, stormwater harvest surface level is to be higher than the irrigation area.
(c) Step 3—Calculate the harvestable volumes of stormwater for each surface type as follows:

Potential stormwater harvest volume = median rainfall value x harvest area (in m2) x run-off coefficient

4.2 Sizing of water tanks and cisterns

(1) Water harvested from roofs is to be stored in an above-ground rainwater tank or below-ground cistern. Due to public health issues, water harvested from driveways, car parks and hardstand should divert directly to turf and garden areas and are not to be stored for later use, unless a treatment system is installed.
(2) The required size of a tank or cistern is to be established as follows:
(a) Step 1—Determine the daily volume required by dividing the annual volume (calculated in Step 3 above) by 365 days per year.
(b) Step 2—Determine the total tank storage required by using the roof area and volume of water required per day for the appropriate value shown in Table 2. For example, if 100L per day harvested from a roof area of 200m2 is required, a storage capacity of 7,000L is to be provided.
(c) Step 3—Determine the size and capacity of the tank/s required.
Table 2—Tank storage requirements
Tank size (kL)
Volume required (L/day)
Roof area (m2)
100
150
200
300
400
500
600
100
10
8
7
7
7
7
7
200
34
23
19
16
14
14
14
400
-
-
-
47
39
34
31

Note—Table adapted from Appendix 4 of Cunliffe, DA 1998, Guidance on the use of rainwater tanks, National Environmental Health Forum, Adelaide.

Note—Tank sizes are based on Brisbane's typical annual rainfall of 1200mm per year.

4.3 Irrigation design

The irrigation design is to include scheduling which complies with the Council’s watering regulations and devices and techniques that contribute to the efficient use of water such as:

(a) automatic controllers incorporating multiple start times, rain delay programming, and evapotranspiration programming;
(b) automatic shut-off devices, such as rain and moisture sensors;
(c) low-volume irrigation delivery, through systems using drippers, bubblers or micro-jets;
(d) pressure-regulating devices;
(e) high-efficiency nozzles;
(f) deficit watering programs that irrigate to achieve 80% field capacity when available water reaches 40–45% of field capacity;
(g) stationing of sprinkler systems to match each of the site's hydro-zones;
(h) hand watering via hoses or buckets for small garden areas.

5 Landscape maintenance

(1) The landscape design is to continue to deliver water-efficient outcomes throughout its life through the preparation of a landscape maintenance plan by a suitably qualified horticulturist or landscape designer (or irrigation designer for any irrigation system component of the landscape management plan).
(2) The landscape maintenance plan is to include:
(a) regular top dressing and aeration to turfed areas;
(b) top-up mulching;
(c) ongoing weed suppression and removal;
(d) regular incorporation of compost into garden beds;
(e) ongoing maintenance of plants, such as pruning to maintain vigour or size, or the removal of damaged or diseased plant material;
(f) a schedule that lists tasks, locations, frequency, season of visit and number of visits per year.
(3) Where an irrigation system is required, the landscape maintenance plan is to include:
(a) conducting leak and blockage checks in irrigation lines and emitters;
(b) reprogramming the irrigation controller and restationing of emitters as plants grow, die out and are replaced;
(c) inspecting moisture sensors;
(d) maintaining tanks and cisterns;
(e) checking water pumps.
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