Different Sites Require Different Stormwater Solutions

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A SWMP shouldn’t give the same solution for every site.

By Kelly McKendry, Director – Water & Environment, PEAKURBAN

It is par for the course that when preparing a Development Application in Queensland a Stormwater Management Plan (SMP or SWMP) is generally required. However, a SWMP shouldn’t give you the same solution for every site. For instance, stormwater detention may not be required for all projects, or what’s considered ‘adverse impact’ or ‘actionable nuisance’ might be different for different sites.

Approaching the preparation of an SWMP from not just a technical perspective but also a commercial one, can deliver valuable outcomes for all stakeholders. For example, developers can achieve more developable land and Council end up with less basins to maintain, reducing ongoing maintenance costs.

The four main aspects that should be addressed in a SWMP are:

1. Flooding: from a regional system or local overland flow.

2. Quantity: addressing increases in flow due to the development and potential mitigation measures such as detention requirements.

3. Quality: meeting the State Planning Policy SPP (in Queensland) and the local authority’s requirements for improving the quality of runoff from our catchment.

4. Lawful Point of Discharge (in Queensland): the location and conditions under which stormwater can flow from a property.

Flooding and Overland Flow

Having a site with flood hazard or overland flow planning overlays does not necessarily mean that part of your site is excluded from development. Depending on the LGA and it’s planning scheme you may be able to put stormwater basins and bioretention within the flood extent and/ or it may be possible to do earthworks within the flood extent if there are no, or acceptable, impacts. Filling within floodplains may also be possible, however there is likely to be some specific rules around what you need to do in terms of compensatory cut and fill to achieve a no nett loss of floodplain storage.

Quantity (Detention)
Urban development and increases in the impervious nature of a site will have the effect of increasing the rate and volume of stormwater running off a site. However, a detention basin is not always the answer.

A ‘site based’ SWMP should not be limited to consideration of what is happening on the site only. This is a mistake made on many developments and by many engineers. The potential changes in stormwater runoff from the site need to be considered in the context of:

  • the existing land use of the site;
  • the location of the site within the catchment;
  • the downstream drainage infrastructure and its capacity; and
  • the drainage system into which the site is discharging.

If, for example, discharge from the site can be conveyed to a creek or river system without causing nuisance or damage then detention may not be required. Similarly, if the site is located in a catchment were the increased rate of discharge or timing of the discharge from the site does not alter or worsen the peak of flood flows within the waterway then again detention may not be required (despite the Council planning scheme indicating it is).  A focus on local flow management, velocity control and scour may be more appropriate.

Stormwater Quality
In Queensland the State Planning Policy (SPP) must be considered for those triggered by the type of development (reconfiguring a lot, material change of use and operational works) and area of land to be developed (generally greater than 2500m2). Most local Planning schemes have now been amended to mirror the requirements of the SPP but may also contain additional requirements for the treatment of stormwater from new development.

The SPP standard for stormwater treatment is to meet a load based reduction target for the common pollutants (known as the 80/60/45/90 WQOs in SEQld). Again these targets may differ in different geographical locations and as a result of the local Planning Scheme.

Most engineers have forgotten that there are more Water Sensitive Urban Design (WSUD) options than just bioretention basins to treat stormwater.

Some of the WSUD elements available for use in a treatment train include:

  • Swales and buffers
  • Bioretention swales
  • Sediment basins
  • Bioretention basins
  • Streetscape bioretention and tree pits
  • Constructed Wetlands
  • Infiltration measures
  • Stormwater harvesting
  • Passive irrigation
  • Rainwater tanks and re-use

Choosing the correct and most appropriate elements can reduce capital cost, maximise developable land, benefit the environment and improve life cycle costs.

Lawful point of discharge (LPOD)
If uninformed, or worse misinformed, lawful point of discharge can become the Achilles heel of a development project resulting in stalled projects, held up due to ‘discharge approval’ not being obtained

The Queensland Urban Drainage Manual (QUDM) has long been held as the reference for Lawful Point of Discharge (LPOD) in Queensland. However, it is cautioned that each Local Authority may (and do), apply a definition of and demonstration of meeting LPOD differently.

There are now two versions of QUDM that are currently adopted and referenced in local planning schemes and associated codes and policies. The versions have slightly different wording around determining if a lawful point of discharge is achieved.

The older version applies a ‘two point test’; with confusion over the definition or demonstration of meeting LPOD, referencing a “no worsening” criteria by definition and elsewhere speaks of “no actionable nuisance”. Often more influential was the second part of the test which required lawful control over the land onto which discharge was proposed. Often downstream landholder discharge agreements or easements and drainage reserves were required for approval.

The current version of QUDM inserts what has been coined a ‘do you need it’ test. The premise now is that if discharge from a site can be demonstrated to be released in a manner that will not substantially damage third party property then no further steps are required to meet a LPOD. This opens the way to allow development applications to demonstrate stormwater is to be managed in such a way that there will be no substantial damage to downstream property and thus development can theoretically proceed without the authority having lawful control over a discharge location (or permission to discharge).

However, QUDM itself is not a legal document in its own right but a mere guideline for best practice. A local authority may choose to reference one or the other version of QUDM or include specific requirements on meeting LPOD within their local planning scheme and instruments. This is certainly the current case in Queensland.  Whilst, LPOD is still a critical issue to be addressed in a SWMP, how the stormwater solution is framed and designed will influence if you need LPOD at all.

In summary

  • don’t expect that a SWMP will always recommend the same infrastructure requirements or solution for different sites;
  • Flood and overland flow overlays may not be a show stopper.  Make sure  you get the right advice;
  • The right advice could save you building a detention basin and can assist in maximising developable land;
  • Be open to alternative, and clever, use of WSUD to treat stormwater;
  • LPOD is still an issue and you should be advised of the specific requirements based on the site location.

When a project kicks off, getting expert advice early will optimise the development potential of a site. You almost always need a SWMP but not all SWMPs answers should be the same.

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