H - Revised Integrated Water Management Plan by Cardno (inc MUSIC file) (13.09.19)

3.3.6 Tree Health Considerations
Following tree selection, and subject to preliminary design including soil media selection, the hydrology of the
system can be assessed. The aim of this stage of the design is to determine if the selected tree can maintain healthy
growth within the constraints of the design. The water balance toolkit, outlined in Section 4.2, examines the water
consumption of the tree against the stormwater inflow and field capacity of selected soils to determine the likely health
of the tree. This looks at the wilting threshold of the soil to indicate whether the tree will require additional water, which
may be in the form of irrigation (Section 3.3.12) or re-design of the concept to produce more favourable conditions.
Refer to the Water Balance User Guide (Appendix D) to appreciate some of the options available to improve tree
conditions.


3.3.7 Hydraulic Design Considerations
Following catchment delineation, a suitably qualified designer should confirm that stormwater runoff can be delivered
to the proposed WSUD Tree Pit system under gravity (i.e. via an overland kerb opening, pipe connection, or a
surcharge system, etc…).
The designer should also confirm that stormwater flows can discharge from the system under gravity. If surrounding
soils are suitable exfiltration can be relied upon to drain the system otherwise underdrainage is required. A sufficient
profile depth should be allowed for if underdrainage is required. Refer to Underdrainage Requirements in this section,
for more information.
Stormwater inlets, outlets, overflow weirs, etc. should be designed by a suitably qualified professional and in
accordance with appropriate engineering practice and industry design guidelines.
WSUD systems are usually designed to convey and treat stormwater runoff from an urban catchment based on a
‘treatable flow rate’ which is typically set as the 3 month and 1 year ARI peak flow. Design flows through systems can
be calculated using the rational method or an appropriate hydrological computer model.
The Urban Rational Method is discussed in detail in the Australian Rainfall and Runoff (AR&R) Book VIII, Section 1.5.5.
A detailed procedure of hydraulic design calculations for WSUD systems are provided in sections 5 and 6 of WSUD
Engineering Procedures: Stormwater and sections 3.4 and 3.5 of Bioretention Technical Design Guidelines


3.3.8 Underdrainage requirements
Frequently inundating a tree pit with stormwater creates the need for suitable underdrainage to prevent a tree pit soils
from becoming waterlogged.
Depending on the configuration, treated water may drain into the surrounding soils or via perforated pipes that connect
into the downstream drainage system.
Figure 5 has been adapted from the Bioretention Technical Design Guidelines (Water by Design, 2012) and provides a
basic decision tree to assist designers in selecting the appropriate drainage profile within a WSUD tree pit system.
Drainage profile options include the following;
   • Submerged Zone - Partially lined system with saturated drainage layer including slotted pipe underdrainage
   • Fully Lined system - with slotted pipe underdrainage
   • Unlined system with slotted pipe underdrainage
   • Unlined system without underdrainage


Detailed guidelines for designing the drainage profile for WSUD systems is provided in Section 3.2 of Bioretention
Technical Design Guidelines




3. Melbourne Water, 2005. WSUD Engineering Procedures: Stormwater, CSIRO Publishing, VIC
4. Water by Design, 2012. Bioretention Technical Design Guidelines. Healthy Waterways Brisbane, QLD.
5. Water by Design, 2012. Bioretention Technical Design Guidelines. Healthy Waterways Brisbane, QLD.

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