Chapter 17: Flooding and Drainage

17.3 Assessment of potential construction impacts

Construction works have the potential to change flood behaviour and impact on the surrounding environment. In addition, flooding has the potential to impact on areas within and near construction sites for the project (ie potential inundation of project sites).
An assessment of the flood risks to the project and the surrounding environment, along with development of appropriate mitigation measures, has been carried out in accordance with the Floodplain Development Manual (OEH 2005), the requirements of the previous WestConnex program of works environmental approvals and industry guidelines.

Construction of the project would involve a range of activities at sites of both permanent and temporary occupancy. The construction activities associated with the project that could result in impacts if not mitigated include:

  • Enabling and temporary works, including construction power, water supply, ancillary site establishment, demolition works, property adjustments and public transport modifications (if required)
  • Construction of the road tunnels, interchanges, intersections and roadside infrastructure
  • Haulage of spoil generated during tunnelling and excavation activities
  • Fitout of the road tunnels and support infrastructure, including ventilation and emergency response systems
  • Construction and fitout of the motorway operations complexes and other ancillary operations buildings
  • Realignment, modification or replacement of surface roads, bridges and underpasses
  • Implementation of environmental management and pollution control facilities for the project.

An assessment of construction impacts associated with water extraction, flooding and drainage is provided in the following sections.

17.3.1 Flooding and drainage

This section considers flood behaviour resulting in potential detrimental increases in the potential flood affectation of the project infrastructure and other properties, assets and infrastructure. Flooding during construction of the project could potentially impact areas within and near the construction sites. Flood related impacts during construction could include:

  • Inundation of excavated tunnels
  • Damage to facilities, infrastructure, equipment, stockpiles and downstream sensitive areas caused by inundation from floodwaters
  • Increased risk of flooding of adjacent areas due to temporary loss of floodplain storage (due to displacement of water) or impacts on the conveyance of floodwaters.

Tunnel portals would be located at the Wattle Street interchange, the Rozelle interchange, along Victoria Road near the eastern abutment of Iron Cove Bridge (associated with the Iron Cove Link) and at the St Peters interchange. Tunnel portals at the Wattle Street interchange and the St Peters interchange are being built by the M4 East and New M5 projects respectively.

Tunnel portals would be constructed using cut-and-cover techniques (refer to Chapter 6
(Construction work) for a description of this construction technique). Tunnelling would also occur via temporary access tunnels that would connect the Parramatta Road West civil and tunnel site (C1b), the Darley Road civil and tunnel site (C4), the Pyrmont Bridge Road tunnel site (C9) and the Campbell Road civil and tunnel site (C10) with the mainline tunnels.
Ingress of floodwater into the tunnel shafts or cut and cover excavations during construction would pose a risk to personal safety for those working in the tunnel. Where these facilities occur within the floodplain, such as at Darley Road at Leichhardt and Rozelle, protection measures such as bunding or floodwater barriers would be provided to ensure floodwaters do not enter shafts or portals.
Other flood impacts during construction, such as flooding of site facilities or stockpiles and erosion of cleared areas, are expected to be minor and would be adequately managed through the management and mitigation measures identified in section 17.5.

These would include adjusting the construction ancillary facility designs and planning sites to recognise the identified flood conditions and minimise the potential for off-site flood impacts. The indicative layouts of the construction ancillary facilities have been developed to provide setback from high risk flooding areas (ie high flood hazard areas and overland flow paths) to minimise impacts on existing flow paths, where feasible. All formwork, access tracks and other temporary works would be located outside of the existing Whites Creek channel.

While there is the potential for temporary structures (used to support permanent structures, materials,  plant, equipment or people) to reduce the available waterway area beneath the replacement bridge, the longer spans of the bridge would be designed to mitigate potential impact on flood behaviour. It is also likely that the replacement bridge would comprise pre-cast members, meaning that the waterway would not be obstructed by any additional temporary structures associated with an alternative cast in situ type approach. This approach would also result in a comparatively shorter timeframe for
installation of the bridge.

The likelihood of flooding and a summary of the potential impacts of construction sites (as shown in Figure 17-1) and associated construction activities on flood risk are provided in Table 17-4. These are based on preliminary construction plans and indicative layouts, which would be refined in the future as the detailed design and site construction planning is further developed.
Table 17-4 Construction ancillary facilities and flooding

C6 The Crescent civil site  Construction of Whites Creek bridge

  • Widening and improvement works to Whites Creek
  • Construction of culverts from Rozelle Rail Yards
  • Buildings and laydown area
  • Parking
  • Whites Creek catchment
  • On the edge of Rozelle Bay
  • Located outside 100 year ARI flood extent but within PMF flood extent
  • AECOM flood modelling (2016)

Potential displacement of water by hoardings, buildings, stockpiles and other structures.

C9 Pyrmont Bridge Road tunnel site

  • Temporary access tunnel for construction
  • Buildings and laydown area
  • Workshop
  • Parking
  • Acoustic shed and spoil handling area
  • Temporary sub-station
  • Johnstons Creek catchment
  • Overland flow in 10 year ARI event, depths of over 1m limited to Bignell Lane
  • Johnstons Creek Catchment Flood Study (WMA Water 2015), Leichhardt Flood Study (Cardno 2014a)

Potential displacement of water by bunding of ramps to prevent floodwater ingress, as well as presence of temporary noise walls, buildings/hoardings, acoustic shed, offices and other structures.

Localised flooding and drainage

All construction works would have the potential to impact local overland flow paths and existing minor drainage paths. Disruption of existing flow paths, both of constructed drainage systems or those of overland flow paths, could occur as a result of:

  • Disruption of existing drainage networks during decommissioning, upgrade or replacement of drainage pits and pipes
  • Interruption of overland flow paths by installation of temporary construction ancillary facilities
  • Sediment entering drainage assets and causing blockages
  • Overloading the capacity of the local drainage system.

These are typical impacts faced on most construction projects and would be addressed by adopting industry standard mitigation measures. Consideration of these impacts would be included during future detailed design and construction planning phases, along with consideration of the typical mitigation measures described in section 17.5 and Appendix F (Utilities Management Strategy). Assessment and mitigation of sedimentation is provided in Chapter 15 (Soil and water quality).

17.3.2 Hydrological impacts

Water balance
The SEARs make reference to a detailed water balance for ground and surface water. Due to the staging and variable nature of construction activities, presence of a potable water supply and highly disturbed nature of the receiving waterways, the water balance assessment has been limited to estimation of rainwater and groundwater reuse volumes and daily treated wastewater discharge volumes. Volume estimates are provided in Appendix Q (Technical working paper: Surface water and flooding). A summary of the findings for water use and water discharges is provided below.

The total volume of water required during construction of the project is estimated to be around 900 megalitres. The use of non-potable water would be preferred over potable water where possible.
Non-potable water demands include:

  • Surface activities such as dust suppression, wheel washing and plant washing
  • Underground activities such as road header dust suppression, rock bolting and plant washdown.

Stormwater and other non-potable sources such as treated tunnel groundwater and treated ‘dirty’ construction water would be reused for non-potable water demands during construction. It is not proposed that surface water would be extracted from the local urban waterways.

The extent to which non-potable water sources can be used would be variable and governed by workplace health and safety considerations, economic feasibility, the functional specifications of the design and the availability and quality of non-potable water.

Construction wastewater (including stormwater, groundwater and construction water) would be generated from all temporary construction ancillary facilities with the exception of the Northcote Street civil site (C3a), which would be used for parking and construction support only.

The total volume of wastewater generated during construction would vary according to rainfall, construction activities taking place, the amount of groundwater infiltrating into the tunnel, and the length of the tunnel that has been excavated.
Indicative daily discharge rates, ranging from 10 kilolitres per day at The Crescent civil site (C6) to 2,400 kilolitres per day at Rozelle civil and tunnel site (C5) are provided in Appendix Q (Technical working paper: Surface water and flooding). A qualitative assessment of the impacts of the discharges to the receiving waterways and bays is provided in the following section.


The discharge of treated construction water would generate a minor increase in the base flow rates of the receiving waterways. Anticipated discharges are likely to be continuous. The locations of discharge points into Dobroyd Canal (Iron Cove Creek), Hawthorne Canal, Easton Park drain and Alexandra Canal, all modified waterways, are within reaches that are tidally influenced. As the flow variability and water level at the discharge locations is dominated by tides, and given the urban setting and artificial nature of the waterways, it is unlikely that discharges during construction would
significantly impact on natural processes at these locations. The ultimate discharge point to Johnstons Creek from Pyrmont Bridge Road tunnel site (C9) could potentially be slightly upstream of the tidal limit. Given the artificial nature of the waterway, the indicative additional baseflow is unlikely to impact on any natural processes within the waterway.
Iron Cove and Rozelle Bay would also receive direct discharges from the project. As they are large tidal waterbodies associated with the Parramatta River Estuary and Sydney Harbour, the discharge volumes would not impact on flow variability or water levels within the bay. Given this, and the highly disturbed nature of these receiving waterways, the construction site discharges are not considered to pose an impact on any natural processes within Iron Cove or Rozelle Bay.

Waterway works

It is proposed to divert the Easton Park drain at Rozelle into a new channel to convey flows through the Rozelle Rail Yards. Once these diversions works are complete, the former Easton Park drain would be decommissioned. Given the artificial nature of the waterway, the decommissioning of the existing drain would be unlikely to impact on natural processes.

Works would also be undertaken on Whites Creek as part of the redevelopment of the City West Link and The Crescent intersection and proposed naturalisation of Whites Creek. Given the artificial and tidal nature of the waterway, whilst water levels are likely to be controlled locally to facilitate the construction works (for example, using a coffer dam); this is unlikely to impact on any natural processes within the waterway. Potential water quality impacts are assessed in Chapter 15 (Soil and water quality).

17.4 Assessment of potential operational impacts

This section describes the flooding and drainage impacts associated with the project during operation and includes:

  • Operational flood risks at locations where the potential flooding impacts required that a quantitative assessment of flood risk be undertaken (see section 17.2.3)
  • Consideration of emergency management and response procedures
  • Potential impacts of future climate change on the operation of the project
  • Impacts on existing drainage infrastructure
  • Hydrological impacts including stormwater runoff and discharge into waterways.

17.4.1 Operational flood risks

The Rozelle interchange, Iron Cove Link and Darley Road motorway operations complex (MOC1) would be partially located within the PMF flood extent, which has the potential to impact on the interchange and tunnel portals. The design of the interchange would prevent flooding of the portals for events up to the PMF or the 100 year ARI event plus 0.5 metres freeboard (whichever is greater). Freeboard is a safety factor for greater protection against different types of flooding and is typically expressed in metres above a flood level for flood protective or control works. Therefore, mitigation measures are required to prevent any floodwater ingress during these events.

Preventing floodwater ingress has the potential to displace floodwaters where the interchange blocks existing flow paths, or reduces available floodplain storage. This may result in potential impacts on surrounding properties. This is particularly the case at Rozelle Rail Yards, as this area functions as a floodway and provides a significant amount of storage of floodwater in larger events such as the 100 year ARI and PMF.

17.4.2 Emergency management and response procedures

Council emergency management and response procedures relating to flooding have not been assessed in detail as they are still under development as part of the Inner West Council’s Floodplain Risk Management Study and Plan. The Inner West Council is working toward formation of a Floodplain Risk Management Committee. Consideration would be given to council emergency and response procedures during detailed design, dependent on the timing for finalisation of these by the relevant council.

There are no local State Emergency Services (SES) flood plans for the area. The NSW State Flood Plan, which is a sub plan of the State Emergency Management Plan, has been reviewed as part of this assessment. The design has taken into consideration the general recommendations set out in the NSW State Flood Plan with regards to managing flooding. The flood assessment has been undertaken in accordance with the Floodplain Development Manual (NSW Government 2005) and has sought to minimise adverse flood impacts. During the detailed design stage, relevant flooding information would be provided to council and SES to assist in informing the Floodplain Risk Management process.

17.4.3 Potential impacts of future climate change

Future climate change could lead to sea level rise and a potential increase in rainfall intensity and frequency. This could affect flood behaviour over the life of the project. As a result, an assessment of the potential impact of climate change on flood behaviour near the project has been undertaken. For further detail on the potential impacts of future climate change refer to Appendix Q (Technical working paper: Surface water and flooding) and Chapter 24 (Climate change and risk adaption).

17.4.4 Impact on existing drainage infrastructure

There is limited existing drainage infrastructure at many of the project sites that would be impacted or need to be modified. For the operational sites, the surface water runoff would be managed to minimise flood impacts on adjoining properties. Where the operational sites propose to connect directly to existing drainage infrastructure, flow rates from the sites would match existing flow rates where possible so as not to overload the existing drainage system or cause adverse flood impacts on adjoining properties. Further details on the relocation and adjustments to drainage infrastructure can
be found in Appendix F (Technical working paper: Utility Management Strategy). The impacts the project may have on the social and economic costs to the community as consequence of flooding are considered to be minimal with the adoption of the mitigation measures provided in section 17.5.

17.4.5 Hydrological impacts

Surface water balance

Stormwater runoff volumes generated within the project footprint would increase as a result of an increase in impervious surfaces associated with surface road widenings, ramps and ancillary surface infrastructure. The footprint included within the modelling and the change in impervious area is provided in Appendix Q (Technical working paper: Surface water and flooding).

Modelling was undertaken to estimate changes in annual stormwater runoff volume to receiving waterways caused by the project. The modelling results are provided in Appendix Q (Technical working paper: Surface water and flooding). The results indicate that annual runoff volumes would be slightly increased as a result of the project, with increases occurring at Rozelle Bay and White Bay with no change to Whites Creek. A slight decrease in runoff volume would occur at Iron Cove.

Treated tunnel water flows from the operational water treatment plants at Darley Road (MOC1) and Rozelle would ultimately discharge to Hawthorne Canal and Rozelle Bay respectively, leading to an increase in base flow rate to those waterways. It is estimated that up to 725 megalitres per year and 693 megalitres per year of treated groundwater would be discharged to Hawthorne Canal and Rozelle Bay respectively.

It is estimated that up to 50 megalitres per year of tunnel drainage from about one kilometre of the northbound and 600 metres of southbound tunnel would be captured by the New M5 drainage system and conveyed to the New M5 operational water treatment plant at Arncliffe, prior to discharge to the Cooks River. A post development mean annual water balance is provided in Appendix Q (Technical working paper: Surface water and flooding).


The flow variability within the receiving waterways is dominated by tides at the discharge locations. Therefore, the minor increases in stormwater flow within Rozelle Bay and Whites Bay and increase in base flow to Hawthorne Canal and Rozelle Bay is not considered to pose a material impact on the flow variability or natural processes within the receiving waterways. As Hawthorne Canal is hard-lined, increased discharge volumes would not impact on bed or bank stability or the geomorphology of the
waterway. Scour and/or dissipation measures would minimise any sediment disturbance impacts at the outlets to the receiving bays and waterways. Further information on scour impacts and proposed management measures is discussed in Chapter 15 (Soil and water quality).
The impacts associated with discharges from the Arncliffe operational water treatment plant were assessed as part of the New M5. The additional tunnel drainage flow (around 1.6 litres per second) associated with the project would be negligible compared to flows within the Cooks River. It is therefore considered that impacts on levels and velocities in the Cooks River would be negligible. The existing scour protection and/or energy dissipation measures would minimise any sediment disturbance impacts near to the outlet.

Environmental water availability

No surface water is proposed to be extracted directly from adjacent waterways or bays during the operational phase. Discharge volumes are likely to slightly increase as a result of the project. All operational discharges would be to unregulated, artificial and tidally influenced waterways or bays. Therefore, no impacts on environmental water availability or flows are likely to occur.

17.5 Management of impacts

The flood mitigation standards established for the project infrastructure have been achieved by demonstrating that there is no impact on properties in the 100 year ARI. Therefore, it is not anticipated that floor level impacts would occur, however this would be confirmed during detailed design. If changes to flooding in larger events such as the PMF were found to impact tunnels or critical infrastructure, further flood mitigation measures would be adopted.

Public safety is one of the driving factors for assessing and mitigating flood impacts. This is reflected in the hydrologic standards that have been set for both construction and operation of the project as set out in section 17.1.3. In terms of flooding, public interest has specifically been taken into account by:

  • Providing PMF flood immunity to tunnel portals and other critical infrastructure such as motorway control centres and substations
  • Providing drainage channels within the Rozelle Rail Yards that have 100 year ARI capacity, leaving the overbank areas flood free up to the 100 year ARI and opening the area up to recreational uses
  • Widening of Whites Creek which reduces 100 year ARI flood levels along Whites Creek.

Environmental management measures relating to flooding and drainage for the construction and operation of the project are provided in Table 17-5. Specific management measures for each construction ancillary facility are provided in Appendix Q (Technical working paper: Surface water and flooding). The environmental management measures listed in Table 17-5 should be read in conjunction with the environmental management measures provided in Chapter 15 (Soil and water quality).
Table 17-5 Environmental management measures – flooding and drainage
Impacts on flood behaviour from construction and operation
FD01 A Flood Mitigation Strategy will be prepared by a suitably qualified and experienced person in consultation with directly affected landowners,
DPI-Water, OEH, SES, Sydney Water and the relevant local councils. It will include but not be limited to:

  • Identification of flood risks to the project and adjoining areas, including consideration of local drainage catchment assessments and climate change implications on rainfall, drainage and tidal characteristics
  • Identification of design and mitigation measures to protect proposed operations and not worsen existing flooding characteristics during construction and operation, including soil erosion and scouring
  • Identification of drainage system upgrades
  • The 100 year ARI flood level will be adopted in the assessment of measures which are required to mitigate flood risk to the project, as well as any adverse impacts on surrounding property
  • Changes in flood behaviour under PMF conditions will also be assessed in order to identify impacts on critical infrastructure and significant changes in flood hazards as a result of the project
  • Consideration of limiting flooding characteristics to the following levels:

– A maximum increase in inundation time of one hour in a 100 year ARI rainfall event
– No inundation of floor levels which are currently not inundated in a 100 year ARI
rainfall event
– A maximum increase of 10 mm in inundation at properties where floor levels are currently exceeded in a 100 year ARI rainfall event
– A maximum increase of 50 mm ininundation at properties where floor levels will not be exceeded in a 100 year ARI rainfall event
– Or else provide alternative flood mitigation solutions consistent with the intent of these limits

  • Consideration of the EIS documents.

FD02 Hydrologic and hydraulic assessments will be carried out for all temporary project components (including ancillary facilities) and permanent design features that have the potential to affect flood levels in the vicinity of the project.

The results of the assessment will inform the preparation of the Flood Mitigation Strategy (FD01) as well as the design development of temporary and permanent works.
FD03 Measures developed to manage potential flood impacts, as identified in the Flood Mitigation Strategy, will be incorporated into the design of temporary and permanent project components and construction and operational management
systems as relevant.
FD04 All entries (portals) into the tunnels will be designed so that they are located above the peak level of the PMF or the 100 year ARI design flood plus 0.50 metres, whichever is greater. The same hydrological standard will be applied to tunnel ancillary facilities such as tunnel ventilation and emergency response facilities, electrical substations and water treatment plants where the ingress of floodwaters will also have the potential to flood the tunnels.
FD05 Bridge crossings over existing waterways and proposed drainage channels will be designed for the underside of bridge structure to be above the peak 100 year ARI design flood level.
FD06 The need to maintain flood conveyance will be factored into construction planning associated with the new bridge structure over Whites Creek.
FD07 Parts of the site that will be adversely affected by floodwaters, such as tunnel dive shafts, portals and cut and cover sections, will be protected from floodwater ingress during construction. The flood level adopted for design of temporary protection
will be informed by consideration of both mainstream and local overland flows, the potential risk to the environment, safety and the potential disruption and damage to project works.

FD08 The Pyrmont Bridge Road tunnel site (C9) will be designed with consideration of and to appropriately manage the existing surface water flow path on Bignell Road.

FD09 The permanent surface water conveyance solution within the Rozelle Rail Yards will be implemented as soon as possible.
FD10 Flood contingency measures will be prepared and implemented where construction ancillary facilities and vulnerable temporary facilities (including fuel storages, water treatment plants and substations) are located in the 20 year ARI design flood extent.

Impacts on stormwater drainage systems
FD11 Further hydrological and hydraulic modelling based on the detailed design will be undertaken to determine the ability of the receiving drainage systems to effectively convey drainage discharges from the project once operational. The modelling must be undertaken in consultation with the relevant council(s). It will include, but not be
limited to:

  • Confirming the location, size and capacity of all receiving drainage systems affected by the operation of the project
  • Assessing the potential impacts of drainage discharges from the project drainage systems on the receiving drainage systems
  • Identifying all feasible and reasonable mmitigation measures to be implemented where drainage from the project is predicted to adversely impact on the receiving drainage systems.

FD12 Where drainage systems are to be upgraded or replaced during the project, existing systems will be left in place and remain operational during the process wherever possible.
FD13 Runoff generated from project construction and operational facilities will be managed to mitigate risk of overloading the receiving drainage system.
FD14 Entry points to the stormwater used by or immediately downgradient from the project sites will be inspected regularly for blockages and cleaned as required to maintain performance.

Impacts on flood behaviour from future climate change
FD15 Hydrological and hydraulic assessments of the permanent design will consider the climate change related flood risk to the project and flood impacts from the project, and will confirm requirements for any management measures. The assessment will be undertaken in accordance with the Practical Considerations of Climate Change – Floodplain Risk Management Guideline (DECC 2007).

Impacts on property and infrastructure
FD16 Where peak levels in the 100 year ARI design flood are predicted to increase at any residential, commercial and/or industrial buildings due to construction or operation of the project, a floor level survey will be carried out. If the survey indicates flood impacts in excess of the limits set in FD01, further refinements will be made to the temporary or permanent designs as required to minimise impacts.
FD17 A flood review report will be prepared after the first defined flood event affecting the project works for any of the following flood magnitudes – the five year ARI event, 20 year ARI event and 100 year ARI event – to assess the actual flood impact against those predicted in the design reports or as otherwise altered by the FMS. The Flood Review Report(s) must be prepared by an appropriately qualified person(s) and include:

  • Identification of the properties and infrastructure affected by flooding during the
    reportable event
  • A comparison of the actual extent, level, velocity and duration of the flooding event against the impacts predicted in the design reports or as otherwise altered by the FMS
  • Where the actual extent and level of flooding exceeds the predicted level with the consequent effect of adversely impacting of property(ies), structures and infrastructure, identification of the measures to be implemented to reduce future impacts of flooding related to the M4-M5 Link project including the timing and responsibilities for implementation.

Flood mitigation measures will be developed in consultation with the affected property, structure and/or infrastructure owners, OEH and the relevant council(s).