Policy for stormwater in new developments
Managing stormwater in new development is key to reducing the health, safety and amenity impacts of flooding and to protecting community assets.
Stormwater in Sorell Council is managed through planning and plumbing approvals systems and is guided by:
- Stormwater in new development policy
- Southern Beaches Onsite Wastewater and Stormwater Specific Area Plan
Stormwater management in Sorell Council can be challenging in some locations that were originally subdivided without necessary infrastructure or in low lying areas that are flood-prone.
Stormwater disposal can be via reticulated systems managed by Council or onsite absorption and may require onsite retention or detention. This Decision Tree Stormwater in New Development Policy guides you through what method is likely to be appropriate to your site.
In the townships of Sorell and Midway Point, stormwater can usually be disposed of by piping to the underground drainage system or to kerb and gutter. Stormwater retention through rainwater tanks may be required either by covenants on titles or where the underground drainage system is already at capacity. Larger developments may have additional requirements on managing the quality and quantity of stormwater.
Please refer to this guidance document if your property has a covenant requiring rainwater tanks.
In the Southern Beaches and rural living areas, stormwater is often managed via rainwater tanks and onsite absorption. In some areas, a combination of rainwater tanks for detention and underground drainage or open drains can be used. Flooding is a significant issue in low lying areas and additional requirements apply to stormwater on a flood-prone site. This flow chart guides you through what method is likely to be appropriate to your site.
Stormwater can also be disposed to natural watercourses subject to adequate controls to avoid erosion as specified in the policy.
In serviced areas, your plumbing can provide a connection to the underground main that is owned by Council. The connection must be in accordance with the current Tasmanian Standard Drawings (SW25, SW26, SW27) which are available here. One connection per lot is allowed.
Before you can connect your stormwater to the kerb and gutter, you’ll need a permit from Council to excavate the footpath and road reserve using tusing the Notice of Intention to Carry Out Work form available here.
Works must be in accordance with the current Tasmanian Standard Drawing TSD-SW29-v2.
Typically, a connection to a roadside table drain is not possible as they are generally only sized to convey runoff from the road carriageway. If a table drain is at least 450mm deep and 1200mm wide for the entire catchment below the lot and there are no downstream flood issues then a table drain connection will be considered.
Before you can connect to a roadside table drain, you’ll need a permit from Council to excavate any footpath and road reserve using the Notice of Intention to Carry Out Work form available here.
The connection must be provided with a concrete subsoil endwall installed flush to the shape of the table drain.
An onsite stormwater detention (OSD) system temporarily stores stormwater runoff and reduces the rate of runoff from your site onto other properties or into Council’s drainage system. OSD is important to help control stormwater flows and minimise flood risks, particularly in short, intense rainfall events. OSD is provided in tanks either above or below ground.
Where TasWater drinking water is not available, OSD is achieved through rainwater tanks with stored water used for domestic plumbing.
The overflow from a rainwater tank is typically contained within the lot through onsite absorption. The common method of onsite absorption is through subsurface trenches while soak pits and subsurface drainage cells can also be considered. Overflow may be suitable for connection to piped services or roadside drains that are at least 450mm deep and 1200mm wide.
The Southern Beaches Onsite Wastewater and Stormwater Specific Area Plan (SAP) helps manage issues of limited or lacking infrastructure, the small-size of many lots, flooding and variability in soils which can impact onsite services.
The acceptable solution requires new development is to be capable of connecting by gravity to a public stormwater system. Given variability in the presence, type and condition of stormwater infrastructure and localised pockets of flooding, determining what development is ‘capable’ will not always be straightforward. Development will not be ‘capable’ of connection where infrastructure is undersized or lacking or where overland flood risk is identified.
The performance criteria covers a range of site and design issues that will generally require an engineers report to be addressed.
Under the directors determination for plumbing work in Tasmania a stormwater absorption trench needs to be designed by a suitably qualified engineer or an architect with knowledge of soil permeability. The stormwater absorption trench falls within category 4 Permit Plumbing work that requires a plumbing permit prior to installing the stormwater absorption trench. The stormwater absorption trench is a Performance Solution and is not a deemed to satisfy solution under the National Construction Code Volume 3, therefore the absorption trench design needs to be accompanied by a form 35 and or 55 from the suitably qualified engineer or architect.
At the planning approval stage the minimum design requirements consist of:
- Concept stormwater plan including all pipes and lot connection or OSD system for all buildings and impervious surfaces
- Maintenance schedule for a OSD system
- Any necessary flood hazard report
In Midway Point and Sorell, driveways and uncovered car parking areas exceeding 100m2 per site must drain to one or more grated pits and channels and drained to the lot connection.
In an unserviced area, sealed driveways and uncovered car parking areas exceeding 100m2 per site must drain one or more grated pits and channels and drained to the road.
In an unserviced area, gravel driveways and uncovered car parking areas may drain to the road in a manner that does cause give rise to siltation or sediment runoff or may be retained on site.
Many low lying parts of Council are subject to flood risk. Additional stormwater flows can increase flood risk to downstream properties and this may require onsite stormwater detention (OSD) as discussed above. Onsite stormwater absorption may not be possible if all or some of a lot is flood-prone. Where groundwater is elevated and the land is too flat to drain flood water, surface water may pond on a lot for weeks at a time.
Council’s policy for stormwater in new development provides criteria by which development that requires OSD and absorption on a flood-prone site. Where feasible, the preferred outcome is for owners to extend the underground drainage system so that onsite absorption is no longer required. If an extension is not feasible, an engineering will need to design a system that can capture a 1% AEP (i.e., 1 in 100 year) rainfall event and allow that to discharge at a rate no greater than a 5% AEP (i.e., 1 in 20 year) rainfall event. The engineer will also need to demonstrate that a H1 flood hazard category will be achieved.
Developing a site that is fully or partly subject to flood-risk is particularly challenging and may not be possible. In addition to obtaining engineering advice, we encourage potential applicants to do extensive research on best practice guidelines and tips such as the Melbourne Water Flood Resilient Guide to Retrofitting Your Home. We also encourage designs to avoid site excavations due to risks of creating new areas of ponding.
New subdivision lots and multiple dwelling units in Sorell are subject to a developer charge as set out in the Council policy and fees and charges. The charge is part of a significant forward program of stormwater upgrades and follows the preparation of the Sorell Stormwater System Management plan by Entura.
Stormwater quality requirements are set out in table 3 of the polcy. Council’s policy provides the flexibility to meet these requirements through a direct financial contribution in lieu of onsite works.
Where capacity does not exist in the underground drainage system Council’s policy requires larger development to either directly upgrade the system, limit post-development peak flows to pre-development rates or contribute to future system upgrades by Council. These requirements also apply if there is a downstream flood risk that will be increased by the development.
Development of a single dwelling or equivalent is exempt from stormwater quantity management requirements, as is development with onsite stormwater management.
Discharging to a natural watercourse can create erosion issues through high velocity stormwater leaving a stormwater pipe.
Erosion risk can be easily mitigated through energy dissipation devices between the end of the pipe and the top of the bank of the watercourse. Suitable energy dissipation include:
- rip-rap; or
- bio-retention swales; or
- headwalls with baffle blocks or equivalent dissipater.
In the town of Sorell, a development charge is imposed on new subdivision lots, multiple dwellings, non-residential development greater than 350m2 and where a site coverage of 50% or more is proposed. The charge is $5,016 per new lot, unit or development. This charge will fund a 10 year program of capacity upgrades and reflects the capacity utilisation cost of new development.