RFI Eng response notice items 4-19 MCU150111

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The proposed central basin/water quality control pond (WQCP) is to be constructed and has an area of 2000m². The
WQCP is not a receiving environment. Council’s planning scheme identifies waterways/receiving environments
approximately 1.6 km downstream of this site. The WQCP is a site control and landscape feature.

Modelling of the pond was discussed with Mr Mircea Stancu from eWater, the company that developed the modelling
software. It was his view that ponds can legitimately provide stormwater treatment. This is evident by the fact that there is
a pond treatment node in MUSIC. He agreed that pre treatment before the pond is recommended.

An opinion was also sought from the Manager of Water by Design (author of the Guideline reference above) regarding a
receiving environment. It was his opinion that a receiving waterbody is a lake, river or bay that accepts stormwater
discharge and is the environment that we are seeking to protect. A small pond would not necessarily represent the
‘ultimate receiving water body’.

The Manager highlighted two potential concerns regarding ponds outlined below:

   – Where ponds are owned by Council there may be ongoing maintenance costs.
This, however, is not an issue as the pond will be owned by the developer and all maintenance costs would be borne by
the site operators.
   – The pond may experience a deterioration in water quality.

This is also not seen as an issue as the pond will receive significant pre-treatment to address water quality concerns. (A
copy of the correspondence is attached in Attachment C)

As indicated in report B14158.W-03A and attached drawings the majority of the catchment to the central basin will have
pre-treatment, either at the central or southern bio retention basins. Some local catchments (catchment F3) adjacent to
the control pond will not be pre-treated but this area is mainly landscaping.

Due to the high turn over in the WQCP (approximately 14 days for the 20th percentile) and high pre-treatment, the risk of
algal blooms and poor water quality is low. If low quality water is identified the site operator can follow practices outlined
in the Stormwater Management Construction and Maintenance Plan (B14158.W-09A).

It is our view that the proposed pond is a small site control and as such it should not be considered as part of the ultimate
receiving waterbody. Therefore as per discussions with eWater the pond can be considered as part of the treatment train.

Further MUSIC analysis shows that the pond only provides a small percentage of treatment (6% TSS, 7% TP and 1% TN
load reduction) and can be seen as the final polishing of stormwater only.

In conclusion we have not removed the WQCP from the model.

Item 11 Provide a Concept Maintenance Plan to be approved as part of the MCU. The Concept Plan must include
objectives for detail design as well as operation/maintenance of the Stormwater Treatment Train including all
end of line receiving waters on the site – such as the Pond. The Concept Maintenance Plan is to detail trigger
points to manage risks of algal or cyanobacteria blooms within the pond prior to an algal or cyanobacteria
outbreak occurring.

Our Response

As discussed with Council the requirement for a Concept Stomwater Construction Management and Maintenance Plan is
typically in relation to operational works application. Also the average turnover time for the pond water is approximately
14 days (20th percentile hydraulic Residence Time) as such there is reduced likelihood of cyanobacteria outbreaks
occurring.

However, as agreed the Concept Stormwater Construction Management and Maintenance Plan (Report No B14158W-
09A)) has been provided for further advice regarding management of potential algal blooms.