St Morris Reserve Upgrade draft Concept Plan - consultation closed

The Council’s decision to construct an open detention basin at St Morris Reserve has created the opportunity for the Council to renew and upgrade all of the open space and recreational assets within St Morris Reserve.

The St Morris Reserve Concept Plan is being developed by the Council to identify at a ‘conceptual level’, all of the potential uses that could be incorporated into St Morris Reserve as part of the proposed upgrade (in addition to the proposed detention basin) and to determine the best location for these uses in the context of the whole Reserve.

The Concept Plan will be delivered in two stages:

• Stage 1 (current stage) which comprises the preparation of the draft Concept Plan which the Council has just released for community consultation and engagement
• Stage 2 involves the preparation of the final Concept Plan based on the feedback received through the community consultation and engagement.

The elements that will be included in the final Concept Plan will be determined following the current consultation with the community.

The following dot points have been included to assist the community in understanding the future vision for the Reserve and to provide the community with a foundation in which to provide comment:

• existing playground equipment will be replaced with new equipment in the current location. The decision for the playground to be kept out of the detention basin is to ensure accessibility at all times
• upgrade will provide opportunities for new recreational elements to integrate with the form of the detention basin (ie nature play area, soccer and/or football goals)
• relocation of the existing basketball facilities as the existing location will be excavated for the detention basin
• new all accessible path around the Reserve and into the detention basin, with an all-abilities access ramp into the large open green space
• new Entry Statement located at the north eastern entrance at the corner of Seventh Avenue and Green Street
• opportunity to install a new shelter with a barbeque and/or an all-accessible toilet
• location of the inlet and outlet close together so that limited portions of the Reserve are impacted during frequent storm events
• retention of 24 trees, removal of 39 trees and approximately 106 new trees planted as part of the Project. None of the proposed trees to be removed for the Project are Regulated Trees
• design of the turf subgrade to limit risk of the Reserve becoming boggy following detention engagement.

For further information, download: Concept Boards

The proposed detention basin at St Morris Reserve is designed to ensure that stormwater is captured and overland flow is diverted from entering the City of Norwood Payneham & St Peters from Glynburn Road up to the 1% Annual Exceedance Probability (AEP) rainfall event (i.e. a rainfall event that occurs on average once every 100 years).

Stormwater will be transferred to the open detention basin at St Morris Reserve by the installation of a new 900mm pipe running along Glynburn Road and Third Avenue between the existing open drainage channel on the eastern side of Glynburn Road and Williams Avenue, St Morris and a new 750mm pipe along Green Street, St Morris between Third Avenue, St Morris and the detention basin in St Morris Reserve. A new 450mm pipe will discharge stormwater from the detention basin to the existing drainage along Gage Street, St Morris.

One of the benefits of an open detention basin is that different elements within the Reserve such as the playground, landscaping and the introduction of water sensitive urban design can be incorporated into the overall design.

For the majority of the year, the detention basin will simply perform as an area of open space. What elements are incorporated into the open space detention basin is the subject of the current consultation. The location of the detention basin does not form part of the current consultation.

As the construction works need to be undertaken in the drier months of the year, for this reason it is planned that the construction of the detention basin will commence in late 2023.

During the construction of the detention basin the grassed area of the St Morris Reserve will be fenced off. The playground area will remain open and continue to be accessible during the construction period. Following the completion of the detention basin, the construction of the playground and the associated recreational elements will commence.

For more information, please refer to the Trinity Valley Stormwater Upgrade Project drop down heading on this page. 

Project background

The City of Norwood Payneham & St Peters has been identified as a location for flood risk following extensive storm events and subsequent flood mapping investigations.

In order to improve the resilience of the community against this natural hazard, the Council is undertaking flood mitigation works. Included as part of this work is the establishment of a detention basin at St Morris Reserve, which will help to reduce the impact of flooding and reduce the burden of recovery for the community, should a flooding event occur.

Project details

The Council has commenced work on the Trinity Valley Stormwater Upgrade Project which will be delivered in four distinct construction phases: Each of the four phases (Construction Phase 1 to 4) of the Project provide a flood risk reduction benefit as standalone projects, with maximum benefit to flood risk reduction occurring at the completion of all four phases.

To find out more about the flooding prevention infrastructure or the Trinity Valley Stormwater Upgrade Project, download maps of the construction locations below: 

• Construction Phase 1 – overland flow interception and detention storage within Albemarle Avenue, Trinity Gardens
• Construction Phase 2 – overland flow interception and detention storage within Jones Avenue, Trinity Gardens
• Construction Phase 3 – overland flow interception and detention storage (ie Detention Basin within St Morris Reserve) and the upgrade of St Morris Reserve
• Construction Phase 4 – overland flow interception and trunk drainage capacity upgrades within the suburbs of Stepney and Maylands.

In 2021-2022, the City of Norwood Payneham & St Peters was successful in receiving $9.9 million from the Federal Government’s Preparing Australia Program to deliver the Trinity Valley Stormwater Upgrade Project.

The purpose of the Program is to target local projects across the nation, with a focus on improving resilience against natural disasters, which includes flooding. In order to be eligible to receive the grant funding the Council was required to match the grant funding, resulting in a total of $20 million allocated to delivering the Trinity Valley Stormwater Upgrade Project.

Why is the Council upgrading St Morris Reserve?

The Council’s decision to construct an open detention basin at St Morris Reserve as part of the Trinity Valley Stormwater Upgrade Project has created the opportunity for the Council to renew and upgrade all of the open space and recreational assets within St Morris Reserve and include new features into the Reserve, such as:

• a new playground
• a new walking track
• additional trees and planted zones
• new recreational facilities
• new fencing
• improved park entrances & connections.
  
  

What is the Trinity Valley Stormwater Upgrade Project?

The Trinity Valley Stormwater Upgrade Project is a $20 million project designed to reduce the risk of flooding in the suburbs of St Morris, Trinity Gardens, Maylands and Stepney – which is known as the Trinity Valley.

The new stormwater drainage infrastructure will reduce surface flood waters during storm events by capturing stormwater and temporarily storing it in a detention basin at St Morris Reserve, as well as underground detention tanks within the road network.

What is the flood risk?

The upgrades to stormwater drainage infrastructure stems from the City-wide floodplain mapping project which identified locations which are at risk from flooding and considers a range of future scenarios, including infill development and the ongoing impacts of climate change.

Why is this drainage work important and necessary?

This work is important because it will help to improve the resilience of the community against this natural hazard. Once completed it will help to reduce the impact of flooding and reduce the burden of recovery for the community, should a flooding event occur.

What will this drainage work achieve?

The drainage work is designed to:

• increase the capacity of the stormwater infrastructure;
• alleviate future flooding where possible;
• reduce the flood risk for dwellings from future 1-in-100 year (1% AEP) stormwater events;
• utilise Council reserves to act as detention areas - reducing the risk of flooding to properties; and
• address upstream flooding to reduce pressure on the downstream system.

What is a detention basin?

A detention basin (also referred to as a retarding basin) is an excavated area installed to protect against flooding and, in some cases, downstream erosion by storing water for a limited period of time and then releasing the water at a slower rate. 

Why is a detention basin proposed at St Morris Reserve?

The St Morris Reserve detention basin enables flows to be diverted upstream thereby providing flood mitigation benefit to existing flow paths within the Trinity Valley drainage network downstream.

Detailed investigations have resulted in the Council adopting a top-down approach to addressing the upstream flooding in order to reduce the pressure on the downstream system and in turn reduce the extent/scale of downstream flood mitigation infrastructure required.

How big is the proposed detention basin at St Morris Reserve going to be?

The detention basin covers the majority of the St Morris Reserve with the exception of the north-eastern corner of the site, which is being reserved for the playground and associated amenities.

In total the detention basin will cover an area of approximately 5,500m² of the Reserve.

How deep will the detention basin at St Morris Reserve be?

The detention basin will be designed with batters around the permitter with freeboard incorporated into the design to ensure the defined 1-in-100 year (1% AEP) flows are contained within the detention basin.

For much of the time, the detention basin will not have any stormwater in it.

For the defined 1-in-100 year (1% AEP) flood event, the maximum flood depth above the base of the detention basin varies from 0.4m up to 1.5m. The 1.5m depth is at the detention basin base outlet (deepest point). Depths vary throughout the detention basin based on design surface grading. Excavation below the existing surface varies throughout the detention basin with some sections excavated to a depth of approximately 2.4m below the existing surface to the lowest detention basin base level.

What are the benefits of the detention basin?

Hydraulic modelling has been undertaken to determine the flood benefits of the proposed drainage design in conjunction with the St Morris detention basin.

It is expected that based on the hydraulic modelling undertaken, the proposed detention basin at St Morris Reserve will result in a reduction in the flood levels on properties within the Trinity Valley.

When combined with the upgrade of the remainder of the Reserve and improved landscape design, the reactivation and reimaging of St Morris Reserve will deliver multiple benefits including but not limited to flood mitigation benefits, and an improved asset which will benefit the community through both passive and recreational uses.

What alternative options did the Council consider?

Before the Council made a decision to establish an open detention basin at St Morris Reserve, it considered and compared a number of underground tank options with two above ground options.

The comparative analysis undertaken by the Council applied a multi-criteria assessment on the underground tank and the above ground options assessing each option based on cost effectiveness, safety in design, maintenance rating, environmental consequences and passive recreational opportunities.

Following completion of the technical review and investigations, including a geotechnical investigation, it was determined that the above ground open detention basin was the best option. The Council did not pursue with the underground tank options mainly due to the costs and potential maintenance constraints associated with these options.

Are there any other detention basins in the City of Norwood Payneham & St Peters?

The Council currently has a detention basin in Felixstow and construction on a 670 cubic meter stormwater detention tank underneath the Burchell Reserve tennis courts will commence shortly. The proposed detention basin at St Morris Reserve will be the third detention basin in the Council area.

Why is the detention basin at St Morris Reserve not included in the consultation?

The reason why the Council is not consulting on whether or not St Morris Reserve should perform as a detention basin and what size the detention basin should be is that any change to the location or size of the detention basin will have a significant impact on the capacity of the drainage system, resulting in an increased risk of flooding to private properties.

The ability to have infrastructure that can detain water and slow down the release of the stormwater has significant benefits for the whole community.

What will the detention basin look like?

The detention basin will be designed with batters around the permitter with freeboard incorporated into the design to ensure the defined 1-in-100 year (1% AEP) flows are contained within the detention basin.

The detention basin will simply look like an area of green open space. What elements are incorporated into the open space detention basin is the subject of the current consultation.

How often/frequent will there be water in the detention basin?

The design intent is to utilise the existing stormwater capacity in full (i.e. stormwater pipes fully utilised) before floodwaters are redirected into the detention basin. Subject to further detailed design, the aim is to only direct flows into the detention basin once the existing stormwater pit / pipe network is being fully utilise and at capacity. Further investigation and confirmation of inundation frequency will be undertaken at the detailed design stage of the project.

On a day-to-day basis, the green open space in the detention basin will be dry and useable.

The detention basin will be designed to gently slope down towards the swale. The swale will be designed as a Water Sensitive Urban Design Zone, which will help to keep the basin dry.

During large rainfall flooding events, parts of the basin will be flooded temporarily.

The flood levels are determined by the likelihood of a flooding event. For example, a 1-in-100-year flood is a flood event that has a 1 in 100 chance (1% probability) of occurring in a given year, a 20 year flood is a flood event that has a 1 in 20 year chance (5% probability) and a 1 in 5 year flood event has 1 in 5 year chance (20% probability) of occurring in a given year.

How is this project being funded?

The Council will receive $9.9 million from the Australian Government through the Preparing Australian Communities Local Stream Program to deliver the Trinity Valley Stormwater Upgrade Project. The Grant funding will be provided to the Council over four (4) financial years. The funding commenced in 2021-2022 and will conclude in 2024-2025.

The grant funding will be matched by the Council bringing the total budget for the project to approximately $20 million.

What is the Preparing Australian Communities Program?

The Preparing Australian Communities Local Stream Program is one component of the Federal Government’s Preparing Australia Program which is aimed at supporting locally identified and locally led projects that will improve the resilience of communities against natural hazards, such as bushfires, flood and tropical cyclones.

The Program is based on the impact risk level of local areas informed by the Australian Climate Service and State and Territory Governments, of which the City of Norwood Payneham & St Peters has been identified for flood risk. The intended outcome of the grant program is to support communities to undertake disaster risk reduction initiatives that provide public benefit. In the case of the City of Norwood Payneham & St Peters through reducing the risk of flooding:

How will the Trinity Valley Upgrade Project be constructed?

• Construction Phase 1 – overland flow interception and detention storage within Albemarle Avenue, Trinity Gardens;
• Construction Phase 2 – overland flow interception and detention storage within Jones Avenue, Trinity Gardens;
• Construction Phase 3 – overland flow interception and detention storage (i.e. Detention Basin within St Morris Reserve) and the upgrade of St Morris Reserve; and
• Construction Phase 4 – overland flow interception and trunk drainage capacity upgrades within the suburbs of Stepney and Maylands.

What is the Council seeking community feedback on?

The Council is seeking the Community’s feedback on what types of open space and recreational assets should be included at St Morris Reserve. In particular:

• the types equipment that should be included in the playground;
• whether or not a basketball court or multi-purpose court should be included and where it should be located;
• the location of new tree planting;
• extent of landscaping;
• what types of recreational and exercise facilities should be included; and
• what type of amenities (i.e. toilet, shelter, barbeque) should be included in the Reserve.
  
  

When is the construction work at St Morris Reserve expected to occur?

The Council is currently in the concept design stage and is seeking community feedback prior to finalising the design. Following the community consultation period, the design team will undertake detailed design.

The Council expects construction at St Morris Reserve to commence in late 2023.

The Council will keep residents informed about proposed timelines through letter drops, Social Media and website updates. If you have any questions about the process or design, you are welcome to contact Council staff by phone (08 8366 4555) or email - townhall@npsp.sa.gov.au .

How many trees are proposed to be removed as part of these works?

Due to the excavation required, 39 trees will need to be removed to allow for the new stormwater works. None of these trees are significant or regulated. The Council is proposing to retain 24 trees and plant over 100 new trees as part of the Reserve upgrade. This will assist in providing new habitat for fauna, additional shade and contribute to further greening the Reserve.

What is the total capacity of the detention basin?

The detention basin will provide capacity for approximately 3,000m³.

How long will water remain in the detention basin after a storm?

No two rain or flood events are the same. Every flood event varies based on differences in rainfall intensities, durations and frequencies.

For more frequent flood events, the detention basin will be engaged for less than 7 hours, whereas during rare flood events the detention basin could potentially be engaged for longer periods up to 31 hours (worst case scenario).

Will any water remain in the detention basin after it fills and then drains?

The base of the detention basin is designed to be free draining with approximately 1.5% to 2% cross-fall on the base, to avoid water remaining in the detention basin. However due to potential construction inaccuracies there may be some minor ponding areas where consistent grading is not achieved.

It’s likely that some residual water will remain in the detention basin after it fills and drains via the drainage outlet on the north-western corner of the site at Seventh Avenue. This is due to the design levels of the site limiting the cross-fall grade. However the degree of ponding is expected to be minimal.

Will there be an increase in mosquitos?

The design has incorporated a cross-fall grade along the base of the detention basin to mitigate the risk of standing water and promote a free-flowing drainage outlet. This will act to reduce the risks associated with standing water.

I’m worried about the loss of open space. Will the Reserve become muddy and boggy and unusable?

The detention basin area is intended to remain as a usable green open space for the public to enjoy. The only exception is during significant rainfall events. During these periods it is unlikely that most people would want to use a Reserve.

The detention basin will be designed as a backup piece of infrastructure. The Council will not be diverting stormwater during all rainfall events into the detention basin. It will only be used during more significant rainfall events. The free draining base and a diversion of low flows around the detention basin mitigate the risk of the Reserve becoming muddy, boggy or unusable.

Once the detention basin is established will the Reserve smell?

Where prolonged and or high intensity rainfall occurs, there may be periods where portions of the site become muddy and boggy. An operational and maintenance regime will be put in place to manage situations where the detention basin is engaged for long periods of time. This should include removal of any debris or other materials that could lead to odour issues.

How will the Council ensure safety for users of the Reserve? Will the detention basin be fenced off?

The detention basin will only be engaged during the more significant rainfall / flood events. It would seem unusual for a child to be in the playground or the public to be using the Reserve during a significant rainfall flood event. The detention basin is also designed to convey flows away from the designated playground area.

Over the years, detention basins have evolved to not exclude the community from a piece of infrastructure that has a 1% chance of being fully engaged during any year. Instead, these drainage reserves are being activated and converted into feature reserves that include and engage the community.

Fencing off the area would result in adverse effects to the intent of the design to engage and activate the Reserve. The Council will incorporate signage to promote and build understanding for users outlining the function of the Reserve and the role of the detention basin and the risks associated in rare flood events.

Is there any potential for the detention basin to overflow and flood properties around St Morris Reserve?

The detention basin has been designed to cater for a 1-in-100 year (1% AEP) flood event. A freeboard is also being incorporated as per standard engineering practice to mitigate the risk associated with the detention basin overflowing and subsequently flooding properties adjacent to the Reserve. The inclusion of a freeboard provides some level of assurance that the detention basin will not result in spilling over in flood events that may exceed the 1-in-100 year (1% AEP).

How and when will I know the outcome of the community consultation?

Community consultation commenced on Monday 30 January 2023 and will conclude on Sunday 26 February 2023. As part of the consultation process, the Council will conduct two Community Information Days on Tuesday 7 February from 5.30pm to 7.30pm and Saturday 11 February from 11am to 1pm.

Following the conclusion of the Community consultation period, the feedback received from the community will be collated and presented to the Council for its consideration.  This is anticipated to occur in April 2023.  Following the Council’s consideration, the community will be advised of the outcome of the consultation and the Council’s decision.