Reference Document MCU22-0102 Stormwater Management Plan

4.4. SITE WSUD TREATMENT MEASURES
The overall site has been designed in two (2) parts due to the location and depth of the existing
150mm diameter sewer pipe that traverses through the north-eastern corner of the site.

Pits 1-4 collect the stormwater that falls on the ‘Exit Side’ driveway area. Stormwater entering Pit
P4 will connect into the SQIDEP approved SPELBasin (Pit P5) pre-treatment chamber, filter
through the Biofilter Media, into the slotted collection pipes and pass into the outlet chamber
and then discharge into Pit P6. Pit P4 is fitted with a high-level overflow pipe that connects
directly into Pit P6. This high-level overflow pipe is set 50mm above the top of the incoming
pipe (from Pit P3) to allow the SPELBasin to treat all smaller storm events first without
overflowing. All stormwater that enters into Pit P6 will connect into the site LPD point Pit P9 and
discharge from the site to the existing 600mm dia RCP stormwater barrel drain via a new
Stormwater Manhole (to be constructed in accordance with Council standard drawings).

The locations of the existing services present on the site (ie: sewer, water and
telecommunications) have been checked to determine that the proposed stormwater pipes will
be able to pass under these existing services and achieve the required minimum depth/offset
clearances.

On the ‘Entry Side’ of the site, a portion of the driveway area will drain into Pit P7 which connects
directly to Pit P8. The remainder of the entry driveway will drain towards the 8M long x 300mm
wide grated strip drain that is located just inside the entry driveway area. This strip drain will be
an Aco K300, ie: 300mm wide x 450mm MIN deep strip drain. The entire length of the strip drain
will be fitted internally with 8 No. SQIDEP approved SPEL Hydrochannels. The end of the strip
drain connects into Pit P8. Pit P8 is fitted with a lower level SPEL Stormsack which will remove all
Gross Pollutants from the entire entry driveway area prior to connecting into the final site LPD
discharge Pit P9.

Unfortunately, with the existing site conditions and proposed site design, it was not possible to
achieve compliance with the Stormwater Quality and Protection of Receiving Waters P07, as
natural stormwater quality treatment methods are not suitable or actually achievable for this
particular site. It was not possible to install grassed swale areas due to the proposed high-level
landscaping (ie: medium/large trees are not suitable for grassed swale areas) and the driveway
pits are not located in the proximity of landscaped areas. The site is not large enough to
incorporate a wetland treatment system. The site was not suited to a non-controlled, non-
bunded bioretention basin- as a natural bioretention basin did not operate as efficiently and did
not achieve the same level of quality compliance for the area available as the proprietary
SPELBasin did. Little efficiency is gained from using raingardens- as all roofed water is captured
and used in the Car Wash process (ie: this water is eliminated from the stormwater system and
transfers into the trade waste/sewer system), and driveway areas could not connect into a
raingardens due to site level differences. The natural slope of the existing site (of 1.2M
diagonally) and the need to match in with the existing surface levels on all 4 sides of the site also
further eliminates possible natural stormwater quality treatment processes from being used.

We ask that Council consider the great level of work that has been completed for the holistic
stormwater and waste water use and treatment cycle for this overall site, the difficult existing
site features that naturally eliminate a large percentage of the possible natural quality treatment
processes and the high level of compliance that is being achieved by the use of the SPELBasin,
SPEL Hydrochannel and SPEL Stormsack. We request that Council adopts the “wherever possible”
part of this P07 performance outcome.
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