Floodvale Drain Case study

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.

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.

DESIGN DETAILS

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.

CONSTRUCTION

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).

The original rack was situated at 90 degrees to the flow and was often completely blocked as the design lacked any directional encouragement to disburse the retained material. The newly constructed concrete apron was designed to cater for the angled trash rack which also enabled access to the upstream side of the rack for a bobcat or similar plant.

The new Stainless Steel Trash Rack was designed and manufactured by Ecosol, significantly different to the previous. The new rack was angled in excess of 35 degrees to allow the material to be conveyed to the capture bay situated offline of the watercourse. The rack was stepped across the length to allow adequate by–pass during peak events. The frame construction consisted of 100mm x 100mm SHS uprights with 75mm x 50mm SHS cross members. The directional bars were spaced at 75mm intervals at 45 degrees to assist with the conveyance of pollutants and large quantities of pine needles. This model was based on previous Ecosol designs with proven results.

Whilst this site would still suffer from localised flooding during peak events as previously discussed, this design would eliminate issues such as premature blocking, scouring of the surrounding areas and the necessity to clean after each and every event. The Ecosol design ensured effective retention of pollutants with safe efficient access. The requirement for the removal and disposal of debris was likely to change from monthly to quarterly.

Grouted sand stone scour protection was incorporated upstream of the concrete apron to protect against undermining of the apron and access ramp. The surrounding areas were backfilled and reinstated with councils choice of vegetation. A 30m section of cyclone boundary fence was also replaced

CONCLUSION

The project was completed by mid-December 2014. Council proposed to remove the sediment and debris that had accumulated over many years in the three culverts by the early stages of 2015. They also anticipated that further clearing downstream of the McPherson street bridge would also commence by February 2015.

This work would likely allow the rack to function to its full potential.
The innovative design and upgrade of this site has delivered a host of positive outcomes for the City of Botany Bay, including:

  • Longevity – Due to the products stainless steel construction
  • Life Cycle Cost Benefits – Due to improved cleaning intervals and almost zero requirement for maintenance
  • WHS – Safe, efficient access for council or contracted staff
  • Flood Mitigation – Due to design efficiencies
  • Amenity & Functionality – Vastly improved aesthetics and operation, ensuring a reprieve from local business for council.

Should you require any information about this project or on any of Ecosol’s other products or services please do not hesitate to contact us on 1300 706 624 or via email at info@ecosol.com.au. Alternatively please visit our website at www.ecosol.com.

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