Duplication of the Southern Expressway | Project Impact Report
13. Surface water and groundwater
13.1 Overview
Water, whether surface or subsurface, is a vital resource and effects on its quality are important for the sustainability of the area and the integrity of the environment in general. Stormwater runoff and its quality is a key issue for the project as the corridor is intersected by several significant creeks, a river and a number of minor watercourses.
The impacts of the project on surface water and groundwater were assessed by:
reviewing the existing surface water and groundwater environments in the project corridor and wider catchments reviewing the current stormwater quality treatment and management systems in the project corridor assessing impacts to surface water and groundwater as a result of the project assessing impacts from surface water and groundwater as a result of the project identifying features that could be integrated into the design to minimise identified impacts identifying suitable management and mitigation measures to address impacts during construction and operation.
13.2 Legislative and policy requirements
Local and regional stormwater drainage assets, including those located in roadways or road reserves, are managed by local government. Existing infrastructure crossings along the project corridor are under the care and control of the City of Marion and City of Onkaparinga. Other government agencies also have an interest in the drainage and water quality aspects of the project, and they guide ‘best- practice’ with regard to state and local government objectives for the area of interest.
Table 13.1 summarises key legislation, policies and guidelines relevant to stormwater quality associated with the project.
Table 13.1 Relevant acts, policies and guidelines (water quality)
Act, policy or Description Relevance to project guideline Natural Defines the legislative framework for managing A water affecting activity permit is Resources South Australia’s natural resources, including required for activities specified in the Management Act planning and management of water resources. Act, such as depositing or placing an 2004 (NRM Act) object or solid material in a The Act requires the Adelaide and Mount Lofty (SA) Ranges Natural Resources Management watercourse, lake or floodplain of a (AMLR NRM) Board to prepare and maintain a watercourse, subject to exclusions regional NRM Plan setting out the long-term outlined in the relevant regional NRM Plan. vision for managing natural resources in the region DTEI has a Best Practice Operating Procedure in place with AMLR NRM Board for the management of permits required for projects
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Act, policy or Description Relevance to project guideline Environment Deals with protection of the environment and Construction and operation of the Protection Act polluting activities, and provides for protection project must comply with the Act 1993 (SA) of air, land, water and ecosystem values from including Section 25 pollution by defining standards of care for Schedule 1 of the Act details industry and the community, promoting the ‘activities of environmental principles of ecologically sustainable significance for which authorisations development are required’; depending on the final Administered and enforced by the SA project design and construction Environment Protection Authority methods, an authorisation (in the Part 4, Section 25 states: form of a licence) may be required for earthworks and drainage activities as A person must not undertake an activity defined by Schedule 1 which pollutes, or might pollute the environment unless the person takes all reasonable and practicable measures to prevent or minimise any resulting environmental harm. Environment Governs the regulation of water pollution and is Construction and operation activities Protection (Water legislated under the Environment Protection Act associated with the project should Quality) Policy Defines key environmental values and comply with relevant maximum 2003 (SA) associated water quality targets for all surface pollution levels specified in this policy water and groundwater resources Defines community and industry obligations for preventing or minimising different water polluting activities Specifies maximum levels for pollutant concentrations, provides for regulation of industrial pollution and associated source monitoring where required Does not deal specifically with sources of diffuse pollution, such as motor vehicles Australian and Stipulates the limits of contaminants below Water quality guidelines and New Zealand there is a reasonable expectation of objectives developed according to the Guidelines for maintaining identified environmental values to environmental values particularly Fresh and Marine sustain current and future values or uses for around the major watercourse will Water Quality natural and semi-natural watercourses define the minimum requirements for 2000 stormwater treatment Guides water quality monitoring and assessment programs required to enable a sustainable, adaptive approach Water Sensitive Provides planning, technical information and Generally considered as best practice Urban Design guidance on inclusion of WSUD elements for in SA and have been recognised by (WSUD) various types of applications the Environment Protection Authority Guidelines Aims to: (EPA) and NRM Boards (DPLG 2009) Have been adopted during project . improve the urban landscape design . reduce pollutant export . control stormwater flows The local councils (in particular City of Onkaparinga) have objectives for WSUD and stormwater management
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Duplication of the Southern Expressway | Project Impact Report
Act, policy or Description Relevance to project guideline Protecting Directs design of stormwater infrastructure and Defines DTEI’s minimum standards Waterways management of potential construction impacts and acceptable practices that must Manual (Transport for DTEI projects be adopted for design, construction SA 2002) Includes pollutant reduction targets for and operational phases stormwater
13.3 Assessment methodology
The desktop and qualitative approach included:
desktop review investigating all watercourses, including condition and location, along the project corridor desktop review of the size and effectiveness of existing stormwater treatment infrastructure along the Southern Expressway i.e. sedimentation ponds and wetlands capacity assessment of existing open drains based on a nominated design flow DRAINS (stormwater drainage system design and analysis program used to assess peak flow) modelling for 1 in 1, 10 and 100 year average recurrence interval (ARI) peak storm level events MUSIC (program that investigates stormwater runoff and treatment effectiveness) modelling for two scenarios: existing roadway and concept design desktop review of groundwater conditions in the corridor and wider area.
13.4 Design standards
13.4.1 Drainage design
Drainage for the project has been evaluated as part of the concept design development. The drainage design principles adopted for elements in the project will comply with relevant design standards.
The general philosophy for drainage design considers the following principles and assumptions:
Drainage systems (including culverts, drainage networks, kerb and channel and open drains) shall cater for 1 in 10 ARI storm events, with a 1 in 100 year ARI storm event checked for overland flooding. The system shall have sufficient capacity to accommodate the design drainage flow in accordance with drainage requirements and without causing damage or nuisance to surrounding landowners and properties.
Discharge from the project to any council downstream drainage pipe or channel system (regional drainage system) should not increase flows in that system to the extent that the design standard of the existing system is compromised.
Existing regional drainage catchments and flow patterns should be maintained where practicable and drainage flows shall not cause scour, damage or nuisance to surrounding landowners and properties. The design basis shall not permit redirection, concentration or diversion of drainage flows for the project except with the written consent of council.
No runoff from any part of the project shall be discharged out of the road corridor except in an underground or surface drainage system.
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The pavement drainage approach and philosophy will generally follow that of the existing Southern Expressway in order to maintain continuity throughout the infrastructure project.
The design and layout for stormwater treatment ponds will follow the rationale and design features for artificial urban wetlands. The treatment flow adopted in the design will be based on a 3 month to 1 year ARI peak discharge rate from the Southern Expressway. By designing systems to cope and treat such flow, on average up to 98% of all annual rainfall and daily runoff events from the Southern Expressway would receive treatment to best-practice standards. Any flows of a higher rate than the 1 year ARI would still pass through a stormwater treatment pond and therefore receive some treatment. This is the maximum and most efficient treatment design approach.
13.4.2 Stormwater treatment (water quality) objectives
Stormwater treatment (water quality) objectives have also been investigated as part of the concept design (Table 13.2).
The adoption of these objectives will be reviewed during the detailed design phases of the project.
Table 13.2 Stormwater treatment (water quality) objectives
Stormwater treatment objectives Operational Reduce total pollutant load from the contributing roadway catchment. Urban Stormwater phase Best Practice Environmental Management Guidelines (Victorian Stormwater Committee 1999) have the objectives of. . 80% retention of total suspended solids . 45% retention of total phosphorus . 45% retention of total nitrogen . 90% retention of gross pollutants, and retention of litter greater than 50 mm for up to the 3 month ARI peak flow . no visible oils and grease for flows up to the 3 month ARI peak flow. Spill Accommodate the volumes of both minor and major spills in the design of pond systems. containment Provide measures to contain them within a pond. A minimum volume of 100 kL shall be accommodated in each pond system to enable a spill on any section of Southern Expressway to be isolated.
13.5 Existing conditions
13.5.1 Drainage and catchments
13.5.1.1 Surrounding catchments
The project corridor crosses several catchments which are intersected by major watercourses and flow paths, all of which eventually discharge to Gulf St Vincent. The two largest catchments are Field River and Christie Creek catchments. The upper part of the catchments are in the steep western slopes of the Adelaide Hills to the south-east of Adelaide’s southern region. These upper parts include significant areas of rural and native vegetation; expanding urban development areas occupy a large portion of the undulating plains west of the hills.
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13.5.1.2 Existing road and pavement drainage
A typical section of the existing Southern Expressway includes two or three traffic lanes and sealed shoulders, with the general carriageway having a one-way cross fall.
The pavement drainage systems include kerb and gutter or a concrete table drain along the edge of the road shoulder or the verge, depending whether the road is in-cut or fill. Side entry or grated inlet pits are located in series as required and connected to the underground drainage system.
Other systems considered part of the pavement drainage system include batter cut-off drains at the top of embankments, rock-lined swales at the bases of fill batters, and spill points to allow excess flows to exit the roadway during a major rainfall event.
Underground drainage systems (pits and pipes) along the corridor collect and convey runoff to the nominated outfall locations. Most stormwater is discharged into and intercepted by stormwater treatment ponds, and then flows to major watercourses or regional drainage systems.
Subsoil drains have been installed below the road pavement, particularly through deep cut areas, for the existing Southern Expressway to protect the integrity of the pavement and associated infrastructure.
13.5.1.3 Regional drainage
The Southern Expressway corridor also crosses a number of underground drainage systems or drainage lines (Figure 13.1). The systems generally discharge into one of the major watercourses along the project corridor.
During the design for the existing Southern Expressway, these regional systems were assessed to determine their levels of capacity. The design and construction included detention basins at various locations along the corridor to take regional drainage.
13.5.1.4 Bridges and major culvert crossings over watercourses
Bridges or culverts cross the major watercourses intersecting the existing Southern Expressway at:
Sturt River Grant Creek Christie Creek.
Other major culvert crossings are located at:
Glenthorne Creek Field River Panalatinga Creek Hackham Creek.
13.5.2 Water quality
Water quality information for all watercourses in the project area is limited. Most was sourced from Adelaide and Mount Lofty Ranges NRM Board, which conducts the majority of the water quality monitoring in the region.
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13.5.2.1 Sturt River
Sturt River flows from the western slopes of the Adelaide Hills to the south-east of the city (AMLR NRM Board 2008). The river has a steep gradient and retains significant areas of native vegetation; the lower portion of the catchment is completely urban in nature. The Southern Expressway crosses Sturt River near the northern end of the corridor where the river flows into Warriparinga Wetland. Downstream, the river is a concrete lined channel, which collects runoff from the urban catchment through council drainage systems (AMLR NRM Board 2008).
The salinity of the river is higher than expected for freshwater and there is a relatively high concentration of nutrients. Catchment inflows usually occur during the high rainfall winter months and the water quality is therefore expected to vary over the year.
Existing risks to water quality in Sturt River and receiving waters are from the use of local roads and the Southern Expressway, and runoff from upstream farms, and predominantly focus on:
pollutants emitted from vehicles during fuel combustion accidental spills of pollutants from vehicle accidents farm runoff containing fertiliser, manure, animal waste, leaf litter and lawn clippings road runoff containing cigarette butts, litter and debris, heavy metals such as lead and copper, and oil.
Erosion upstream of Warriparinga Wetland has previously raised concern about the condition of Sturt River. Table 13.3 shows results of water quality monitoring by the AMLR NRM Board over the last 10 years, at a testing site located immediately upstream of Warriparinga Wetland inlet, a comparison of the results with Environment Protection (Water Quality) Policy guidelines and the EPA water quality classification.
Table 13.3 Water quality summary for Sturt River
Criterion Level EPA guidelines EPA classification Salinity (mg/L) 1881 1000–3000 Moderate Turbidity (NTU) 13 Max. 20 Good Nitrate/nitrite (mg/L) 0.42 Max. 0.5 Moderate Phosphorus (mg/L) 0.84 Max. 0.5 Poor pH 7.67 6.5–8 Good
13.5.2.2 Field River
Field River originates in the agricultural southern Mount Lofty Ranges and meets the sea at the southern extremity of urbanised Hallet Cove (AMLR NRM Board 2008b). Near its mid-section, the Southern Expressway crosses Panalatinga Creek, Field River and Grants Creek. Both the larger Panalatinga Creek and smaller Grants Creek lie in the Field River catchment and flow into the river.
The Southern Expressway overpasses Field River where Panalatinga Creek intersects Field River at the Young Street. Much of the catchment has been urbanised and almost 50% is residential development (AMLR NRM Board 2008b). The land surrounding the creek is used for commercial and residential purposes for several kilometres east (upstream) of the expressway. West of the expressway the riparian zone is greater; however there is extensive industrial, urban and recreation development, until the creek meets the gulf.
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Stormwater mains Existing regional drainage systems Watercourses Waterbodies Figure 13.1
Duplication of the Southern Expressway | Project Impact Report
Water quality is sampled upstream of the Southern Expressway at a site near the intersection of Main South Road and Panalatinga Road (AMLR NRM Board 2010). The most recent data from this site is from 2008. Results of 10 years of water quality monitoring are summarised in Table 13.4.
Table 13.4 Water quality summary for Field River
Criterion Field river EPA guidelines EPA grade Salinity (mg/L) 522 1000–3000 Good Total Kjeldahl nitrogen as N 0.89 Max 5 Not available Nitrate/nitrite (mg/L) 0.13 Max. 0.5 Good Phosphorus (mg/L) 0.08 Max. 0.5 Good Suspended solids (mg/L) 19.49 Max 20 Not available Copper (mg/L) 0.01 Max 0.01 Not available Zinc (mg/L) 0.03 Max 0.05 Not available Lead (mg/L) 0.01 Max 0.005 Not available
13.5.2.3 Christie Creek
Christie Creek (and catchment) is located south of Adelaide and bounded by Field River catchment to the north and Onkaparinga River catchment to the south. It originates in the agricultural southern Mount Lofty ranges and reaches the sea at urbanised Christies Beach (AMLR NRM Board 2008c).
The Southern Expressway overpasses Christie Creek at the midpoint between O’Sullivan Beach Road and Flaxmill Road. Much of the catchment has been urbanised and almost 50% is residential development (AMLR NRM 2008c). The land surrounding the creek is used for commercial and residential purposes for several kilometres east (upstream) of the expressway. West of the expressway the riparian zone is greater; however there is extensive industrial, urban and recreation development, until the creek meets the gulf.
Results of water quality monitoring conducted by the AMLR NRM Board over the last 10 years, at a testing site downstream of the Southern Expressway at a site near Galloway Road, are summarised in Table 13.5.
Table 13.5 Water quality summary for Christie Creek
Criterion Christie Creek EPA guidelines EPA grade Salinity (mg/L) 2711 1000–3000 Moderate Turbidity (NTU) 52.53 Max. 20 Very poor Total Kjeldahl nitrogen as N 0.86 Max 5 Not available Nitrate/Nitrite (mg/L) 0.58 Max. 0.5 Poor Phosphorus (mg/L) 0.07 Max. 0.5 Good pH 8.36 6.5–9 Good Suspended solids (mg/L) 50.15 Max 20 Not available Copper (mg/L) 0.01 Max 0.01 Not available Zinc (mg/L) 0.03 Max 0.05 Not available Lead (mg/L) 0.00 Max 0.005 Not available
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13.5.3 Stormwater treatment devices
Stormwater runoff from the Southern Expressway is treated using both sedimentation ponds (installed during construction of the first stage of the project) and wetland systems (installed during the construction of the second stage of the project).
Table 13.6 summarises existing stormwater treatments devices along the corridor.
Table 13.6 Existing stormwater treatment system
Location Treatment device and site description reference 1 Warriparinga Triangle – existing treatment pond system adjacent to Sturt River at intersection with Main South Road 2 Seacombe Road drainage system – existing stormwater diversion system (not a treatment system) 3 O’Halloran Hill sedimentation pond 4 Adams Road sedimentation pond, and Glenthorne Creek culvert crossing 5 Glenthorne Creek sedimentation pond 6 Lander Road sedimentation pond (two tiered ponds) 7 Field River culvert crossing, Field River, Field River wetland 8 Panalatinga Creek culvert crossing, Field River, Young Street wetland 9 Wetland near Grant Creek 10 Moore Road wetland – flow path and culvert 11 Brodie Road (north) wetland 12 Brodie Road (south) wetland – path and culvert; Brodie Road (south) wetland 13 Christie Creek wetlands (north) 14 Christie Creek wetlands (south) 15 Near Beach Road – wetland and detention pond 16 Honeypot Road (north) wetland 17 Honeypot Road (south) wetland 18 Hackham Creek wetland
13.5.4 Groundwater
The depth, location and extent of regional groundwater in the Southern Expressway corridor vary in response to the terrain and geological formations. Along the majority of the project corridor, the regional groundwater is expected to occur at depths of greater than 10–20 metres.
In addition to the main (deeper) groundwater systems, perched groundwater is sometimes encountered nearer to the surface where highly permeable soils and the highly impermeable soils, such as Hindmarsh Clay, interact. These are often called perched watertables. The volume of water associated with perched watertables are relatively small and usually limited to local areas; they are not considered in a regional context.
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Duplication of the Southern Expressway | Project Impact Report
Based on the geology along the project corridor, groundwater can be encountered in alluvial soils in the vicinity of watercourses such as Sturt River, Glenthorne Creek, Field River, Panalatinga Creek, Christie Creek, Hackham Creek and numerous other flow paths that traverse the corridor. The depth to groundwater in these locations is highly dependent on flow regimes and rainfall.
13.6 Effects of the project
A number of effects to surface water and groundwater have been identified as likely to occur during both the operation and construction phases of the project.
13.6.1 Operations
The effects and impacts of the project during the operation phase include the potential for increased:
flooding and flows into existing drainage systems erosion and scour pollutant loads and stormwater contamination (resulting in water quality impacts to receiving environments) transfer of pollutants or chemicals downstream, and spills.
The effects would be caused by additional and concentrated stormwater discharges associated with:
interruption of existing water regimes, and additional flows to watercourses increasing the roadway catchments (impervious area), which can increase pollutant loads and cause downstream systems to experience greater drainage flows.
13.6.2 Construction
Potential effects during construction generally include:
discharge of water associated with dewatering activities litter accumulation from construction packaging and waste material hydrocarbon and toxicant contamination from spills and leakages temporary and permanent changes in direction of surface and stormwater flows resulting in pooling in undesirable locations including excavations increasing stormwater flows and velocities as a result of greater impermeable surface area loss of surface vegetation and exposure of bare earth to the effects of wind and rain on the surface increased and uncontrolled stormwater flow on the surface of exposed earth leading to erosion and transportation of sediment laden runoff into existing stormwater infrastructure, watercourses and other downstream areas increased turbidity levels in receiving waterbodies, stream flow and potential for excessive sediment accumulation in the bed of watercourses and in existing stormwater pipe networks, from poor erosion and sediment control during site disturbance and movement of construction vehicles.
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13.6.3 Stormwater
13.6.3.1 Drainage systems
The impact of the duplicated carriageway on existing drainage systems is generally related to encroachment of the new carriageway and road formation over the existing system and includes:
direct impacts on system elements i.e. removal and/or alteration to drains, wetlands, detention basins and sediment ponds due to the alignment
changes to the cut and fill extents along the corridor (and associated changes to stormwater flows and the drainage system elements required to manage)
alterations to interchanges and potential increases in footprints (and associated changes to stormwater flows and volumes and the drainage systems elements required to manage).
These impacts in turn create impacts to drainage system flows and associated capacities.
Figure 13.2 shows existing and proposed stormwater treatment ponds for the project.
13.6.3.2 Watercourses
Impacts to watercourses are related to the need to realign watercourses and extend existing culverts and bridges; and to increased volumes entering watercourses and altered flow regimes associated with the duplicated carriageway.
Physical changes to watercourses (e.g. extensions to culverts) have the potential to:
increase the risk of erosion and scouring associated with increase velocities and flows cause loss of riparian habitat in and adjacent to watercourses.
The level and severity of these impacts in the watercourses across the corridor will be dependent on the final design of the duplicated carriageway. Measures to mitigate and manage these impacts are discussed in Section 13.7.
13.6.4 Water quality
The project will increase traffic volumes along the Southern Expressway and consequently increase emissions, exhaust fumes, tyre and brake wear, and the risk of accidental spills. These pollutants will be washed off roadways with stormwater runoff and eventually end up in watercourses and other receiving environments downstream. Typical roadway runoff contains a range of constituents (Table 13.7).
Table 13.7 Potential roadway runoff constituents Pollutant Description Potential sources Gross pollutants Litter, debris, organic matter Wind blow, vehicles, organic matter Sediment Particulate Land surface erosion, atmospheric deposition (dust), maintenance, road verges Nutrients Nitrogen, phosphorus Atmospheric deposition, fertilisers, organic matter, animal faeces Metal (toxicants) Lead Tyre wear, motor oil, grease Iron Rusting metal, moving engine parts
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Duplication of the Southern Expressway | Project Impact Report
Pollutant Description Potential sources Copper Metal plating, bearing and wear, moving engine parts, brake lining wear Cadmium Tyre wear, catalytic converters Chromium Metal plating, moving parts, break lining wear Nickel Diesel fuel and petrol exhaust, lubricating oil, metal plating, brushing wear, brake lining wear Manganese Moving engine parts Mercury Diesel fuel exhaust Zinc Vehicle tyre wear Oil and grease Visible oils Leaks from vehicles and trucks
13.6.5 Water quality treatment
13.6.5.1 Treatment devices
The impacts to stormwater (and stormwater treatment devices) of the duplicated carriageway are generally related to the encroachment of the new carriageway and road formation over the existing drainage system and include:
direct impacts on existing stormwater system elements and treatment devices i.e. removal and/or alteration to drains, wetlands, detention basins and sediment ponds due to the alignment changes to the cut and fill extents along the corridor (and associated changes to stormwater flows and system elements required to manage) alterations to interchanges and potential increases in footprints (and associated changes to stormwater flows and volumes, and systems elements required to manage).
13.6.6 Groundwater
The potential for the construction and operation phases of the project to affect groundwater depends largely on underlying geology and soil composition. Groundwater quality could deteriorate through contamination of soil or direct contamination of groundwater during excavation.
There are no immediate threats to groundwater in the region primarily due to depth and hydrogeological characteristics along the project corridor.
The potential effects of the construction and operation of the duplicated Southern Expressway on groundwater will be further investigated in detailed studies including a Phase II contaminated land investigation and a geotechnical investigation (Chapter 18 – Geology, soils and contamination).
13.7 Mitigation measures to minimise effects
The management of drainage, stormwater and water quality is intrinsically linked. Addressing one element often has beneficial outcomes for another e.g. use of vegetated swales as part of a drainage design can help improve stormwater quality.
Mitigation measures to minimise effects to surface water and groundwater are outlined individually for drainage, water quality, stormwater and groundwater.
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13.7.1 Drainage
Drainage systems for the project will be designed in accordance with the relevant design standards. Development of detailed drainage design measures will consider:
appropriate choice of final road design alignment limiting the modifications required to the alignment of Field River where the road alignment is in close proximity incorporation of fish passages in the design of culvert extensions required at Field River underground systems and major flow paths to cater for major storm flows pavement drainage systems including kerbs, pit inlets and pipes to capture and drain minor storm flows detention basins to reduce increased capacity during peak flows, where necessary allocation of sufficient space for sediment traps where necessary avoidance of pump systems, to reduce the risks of failures during flood events drainage designed so the formation is adequately dewatered and drained, and subsurface drains installed where surface drains cannot provide adequate drainage allowing for construction of drainage systems in a way to prevent erosion of embankments and downstream areas incorporation and selection of appropriately sized and designed erosion control techniques to all outlets incorporation of spillway openings in specified locations to maintain overland flows paths for events that exceed the drainage capacity.
The locations at which the pavement drainage systems discharge into the land corridor and the regional drainage lines have been carefully selected to minimise the impact on sensitive watercourse environments. The approach has been to maximise the capacity of stormwater control basins and wetlands to manage both flow and quality. The locations of existing systems (both pavement drainage and treatment wetlands) generally follow the approaches taken in the existing expressway.
13.7.2 Watercourses
To mitigate impacts to watercourses, the final design of the project will consider:
providing erosion control techniques at the downstream side of additional or extended water course crossings incorporating pools on the downstream side of additional or extended watercourse crossings to allow base flow to slow and provide for fish passage and restore habitat and improve/enhance the environmental values revegetating watercourse areas directly affected by the project areas with indigenous plant species to restore habitat and environmental values realigning channels (where appropriate) to minimise downstream impact and facilitate required flows.
13.7.3 Water quality treatment
Water quality impacts will be mitigated primarily through the approach and design of the final stormwater system for the project.
The project’s water quality objectives will be reviewed and finalised in detailed design (Section 13.4.2). Subsequently, the drainage and stormwater design will be finalised to demonstrate the design meets the project requirements in water quality targets for stormwater.
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Based on the existing system, a continuation of a ‘treatment train’ approach is being considered as a reasonable and practical way to address water quality issues associated with the project.
A treatment train is a series of treatment measures that cumulatively address water quality issues. The approach is appropriate due to the nature of the project and receiving environment, and the fact that different treatment measures are required to treat different pollutants (Transport SA 2002).
Treatment train measures included in the concept design are capable of addressing issues of sedimentation, organic matter, litter and nutrients, They include:
buffer strips diversion drains vegetated swales sedimentation basins retention basins constructed wetlands.
Treatment measures to address issues associated with hydrocarbons and accidental spills are also being considered.
The proposed approach to mitigating impacts to surface water associated with stormwater will consider:
new treatment wetlands to cater for the ultimate Darlington Interchange design on the south- western side of Sturt River new treatment wetlands at various locations along the corridor as space allows provision of gross pollutant traps to any new wetland reinstatement and extension of the existing stormwater diversion system to facilitate and cater for the duplicated carriageway redesign of existing outlets to manage additional runoff from the carriageway to prevent erosion extension of the existing treatment ponds and wetlands to incorporate macrophyte zones expansion of existing sedimentation ponds to accommodate additional runoff from the project redesign of the existing outlet system to manage additional runoff from the carriageway and prevent erosion.
13.7.4 Groundwater
Mitigation and monitoring measures to reduce the likelihood and consequence of potential impacts to groundwater in the study area will be developed and incorporated into the project construction environmental management plan (CEMP). The measures may include:
baseline monitoring of current groundwater quality and level in the project corridor routine monitoring of groundwater quality and level during project construction to identify if water quality or level is influenced if groundwater quality or level is identified as declining then management measures being developed to reduce or ameliorate this impact implementation of measures to reduce the likelihood or consequence of soil and surface water contamination which could indirectly contaminate the groundwater.
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13.7.5 Construction
Mitigation and monitoring measures to reduce the likelihood and consequence of potential impacts in the project area during construction (Table 13.8) will be incorporated into the project CEMP.
Table 13.8 Construction impacts and their management and mitigation
Project stage Potential impact Mitigation/management measures Design and Site soil erosion and . Complete a water quality risk assessment in accordance Construction sediment input into local with DTEI requirements as part of the final design process underground drainage . Develop a soil erosion and drainage management plan networks (SEDMP) before construction based on the final design Site soil erosion and and EPA and DTEI guidelines, codes of practice and other sediment input into relevant documents regional flow paths . Prepare CEMP in accordance with DTEI Contractor’s Site soil erosion and Environmental Management Plan Guidelines for sediment input into minor Construction watercourse tributary (first . Take an adaptive approach to surface water management; order) identify monitoring of treatment systems, actions, Site soil erosion and maintenance, corrective and rectification plans in the sediment input into other Environmental Management Plan major watercourses . Include a range of treatment measures placed in sequence . Glenthorne Creek . Design sedimentation ponds to achieve minimum . Grant Creek performance criteria of
. Hackham Creek 1 year peak flow rate
90% trapping efficiency of 125 micron sediment fraction . Maintain safe and adequate flood capacity of systems, flow paths and watercourses . Progressively stabilise soil and areas disturbed by earthworks, using vegetation, matting and other techniques . Limit the area stripped of its vegetation at any one time and undertake this progressively . Maintain flow paths, progressively stabilise and do not obstruct by temporary structures . Base sediment pond design on best practice approach . Adopt SEDMP techniques in accordance with DTEI’s guidelines Protecting and Waterways Manual, Transport SA (2002)
13.8 Conclusion
Through the adoption of specific design requirements for drainage and stormwater (water quality) and concentrated management through construction, the impacts to surface water and groundwater as a result of the duplicated Southern Expressway are not considered to be significant.
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