CITY OF BOTANY BAY, FLOODVALE DRAIN
The Floodvale Drain catchment is situated within the City of Botany Bay LGA, flowing from Pagewood at the northern extent to Botany Bay via the Penrhyn Estuary in the south. This includes portions of the suburbs of Pagewood, Eastgardens, Botany and Banksmeadow. Combined the catchment drains an area of approximately 375 ha (3.75km2). The catchment is heavily urbanised and is predominately comprised of industrial development. The total length of Floodvale drain is approximately 2.9km comprising 2.1 of closed conduit and 0.8 of open channel.
Previous flood studies had detailed 1% AEP Peak Flood Discharges at the McPherson Street intersection of between 7.2 and 8.2 m3/s.
The existing failed infrastructure consisted of a direct screen rack with dilapidated retaining structures which encroached on each of the outer culverts. This along with the heavily silted culverts meant that almost each and every rain event caused the area to surcharge resulting in the flooding of the heavily trafficked intersection of McPherson Street & Coal Pier Road.
The Ecosol designed solution offered numerous advantages over the current infrastructure. Whilst it was noted that the site would still be subject to flooding during peak events (due to service infrastructure and nature of the site). The new design would significantly reduce any such instance. Furthermore the site would become more accessible and maintenance friendly. This was made possible with the inclusion of the innovative angled stainless steel rack design, concrete access ramp and holding bay.
On the 21st of July 2014 The City of Botany Bay awarded Ecosol Pty Ltd the contract to construct the designed solution for the problematic McPherson Street site in Banksmeadow.
Construction and progress of the project was always going to be dictated by the weather and overall site conditions. Following early inclement weather the project started on the 16 September 2014. The construction zone was rife with the challenges typical of that found in this LGA, particularly in regards to drainage infrastructure. Along with alluvial material it was also tidal and required work close to the water table,all with varying levels of contamination.
On the 14th October a 1:100yr storm event stopped in for a brief visit. This particular event saw the water level rise from the invert of the culvert to some 1.5m. Undetered by this event and with the implementation of clever construction contingencies to manage the elements works continued.
Clever decision key to overall project success.
The new design set about using the culverts to their full potential. The dilapidated and encroaching retaining structure were removed, a portion of the existing damaged base slab was also removed along with the ineffective cumbersome filtration rack.
With the newly presented blank canvas the improved design was under construction. For ease of construction and due to tidal effects the form work for the retaining wall footings were not constructed in-situ, instead they were constructed off-site using Pecaform sacrificial form work. This allowed for the efficient steel fixing of the retaining walls and footings once they were excavated to the desired design levels. The footings and walls were constructed simultaneously by shot-crete. The newly constructed walls were aligned as much as possible with the extremities of the culverts (increasing the width from 4.5m – 7m).