RFI - Technical note on bio basin retaining walls

TECHNICAL NOTE ON BIO-BASIN RETAINING WALLS AT
30 POTTERY STREET, POMONA

I refer to the civil engineering drawings prepared by Tod Consulting for the proposed
subdivision at 30 Pottery Street, Pomona and provide the following information in
support of the preliminary bioretention basin design prepared for the Lot
Reconfiguration application lodged with Council.

As shown on drawing 19660 - C1003 and C1004 rev A retaining walls have been
used to form the bioretention basin instead of landscaped batters.

The bio basin area is orientated diagonally across the contours of the land. To allow
for the inlet pipe to the basin to have sufficient cover and to allow for the 300mm
extended detention depth the filter media will need to be set approximately 1.3m
below the natural surface at the north east corner of the drainage reserve. If a 1 in 4
batter was applied from this level to the north, it would be chasing the natural slope
(approx. 15%) up the hill resulting in a batter width of approx. 13m. The basin filter
level will also be influenced by the roofwater inlet in the south east corner of the
basin that could result in the filter level being pushed lower than the scenario above
which would further increase the batter slope on the high side of the basin. On the
south side of the basin we have shown a retaining wall mainly to ensure water is
retained in the basin. It is planned to minimise the height of this wall to approx. 500-
600mm above the filter media. If a bund detail with 1 in 4 batter was used in lieu of
this wall the width of the bund would be in the order of 5m, meaning the overall width
of the drainage reserve would need to be in the order of 25m instead of 7m.

We trust the above provides sufficient background information as to why retaining
walls have been adopted in this case.




Doug Reynolds, Director (RPEQ 9072)
07 5449 9600 | [email protected]
TOD consulting engineers