2014-03-25 Planning & Organisation Committee Agenda Pages 81-167

PLANNING & ORGANISATION COMMITTEE MEETING AGENDA                                  25 MARCH 2014


Stormwater Quality/Water Sensitive Urban Design (WSUD)
The entire development is located in the Lake Weyba catchment. The majority of the
development discharges to Lake Weyba via Eenie Creek, with some development within Precinct
1 discharging directly to Lake Weyba.

Water quality sampling undertaken by the applicant shows that the water quality in Lake Weyba
and Eenie Creek currently does not meet the water quality objectives in terms of total suspended
solids and total nitrogen concentrations. This is an indication that the system is already under
stress from existing uses within the catchment area.

The proposed Water Sensitive Urban Design strategy incorporates rainwater tanks, bioretention
basins, bioretention swales, vegetated swales and litter baskets. The proposed stormwater
quality treatment train has been modelled as exceeding current best practice load based
reduction targets. Rainwater tanks form a critical component of the proposed stormwater quality
treatment train. The tanks also form part of a broader strategy to manage the impacts on the
ecology of the downstream system by reducing changes in the volume of stormwater discharged
and frequency of stormwater discharges.

Although the proposed stormwater quality treatment train exceeds best practice load based
reduction targets, it is uncertain how much of a change this represents from the existing situation.
This uncertainty is because a large portion of the site is currently used for agriculture and water
quality sampling from which typical loads for agricultural land have been determined is variable
depending on the season.

Stormwater quality treatment devices, other than those proposed to be located within existing
road reserve areas, are proposed to remain in body corporate ownership and represent a
significant responsibility for the body corporate to maintain.

Erosion and Sediment Control

There is a real and significant threat that Lake Weyba waters and Keyser Creek will become
contaminated with sediments after significant rain events, due to the extensive earthworks
undertaken to alter the land form to make it suitable for urban development.

A Preliminary Erosion and Sediment Control Strategy was submitted with the application and this
strategy proposes best practice erosion and sediment control practices to be followed during the
construction phase of the development. The document shows that if the areas of the site
proposed to be developed were disturbed all at the same time and no control measures
implemented then the soil loss from the site would be 3,963 tonnes per year. The Engineering
Report estimates that the total amount of earth proposed to be moved during the cut and fill
operation is 148,684 m3, which based on a conservative bulk density of
1.5 tonnes/m3 is the equivalent of 223,000 tonnes of soil. The depth of cut is proposed to exceed
2.5 metres on the ridges and fill is proposed to exceed 2.5 metres on areas currently subject to
flooding.

The discharge criteria used is for controlled releases and the basins are only proposed to be
sized for the 85th percentile, 5 day rainfall event. Analysis undertaken of the effectiveness of
basins sized for the 5 day, 85th percentile rainfall event shows that more than half of the total
runoff volume from a construction site is uncontrolled releases and typically has concentrations
over an order of magnitude greater than controlled releases. This analysis shows that reliance on
basins alone to prevent degradation of the downstream environment will be ineffective.




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