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MUSIC Modelling Guideline 2024 - FAQs

Melbourne Water

01/11/2024

Melbourne Water has updated the MUSIC Modelling Guideline as of July 2024.

To help support the update, the following FAQs have been developed based on questions received throughout the engagement process. 

 

General Questions

Where can I access the new guidelines and rainfall data? 

On Melbourne Water's website, on the Guidelines page, under Developer guides and resources - half way down the page. 

What are the updates provided in this release? 

This release is an update to the existing guideline provided by Melbourne Water. The last update provided was in 2018.  

The update includes: 

  • New templates with revised rainfall data which better represent rainfall patterns, including an additional template band. 
  • Clearer guidance on sediment ponds to minimise the risk of designing inappropriate treatment systems. 
  • New guidance on high flow bypasses, overflows, wetlands with multiple inlets.  
  • The guidance also clarifies Melbourne Water’s position that proprietary stormwater treatment devices may not replace nature-based systems in Development Services Schemes. 

Who was consulted for these updates? 

This update was developed by peer review, scientific research, and stakeholder engagement across Melbourne’s stormwater industry, and incorporates recent advancements in science and industry. 

Is Melbourne Water seeking feedback on Release A of the MUSIC modelling guideline?

Melbourne Water always welcomes feedback. While this update to the guideline has been finalised, we are happy to consider your feedback in the future. We will have an engagement process for the next update. In the meantime, you can share any insights or suggestions you have, and we'll ensure they are reviewed for future updates. You can email us at enquiries@melbournewater.com.au.

Why does Melbourne Water provide a guideline and associated templates for MUSIC? 

It is important that new developments comply with requirements for water quality and waterway health. In Victoria, these requirements are set by the Environmental Protection Authority (EPA). 

The purpose of this guideline is to provide guidance on modelling approaches and input parameters for MUSIC models that are submitted to Melbourne Water. 

The MUSIC Templates are rainfall datasets which cover 10-year periods thatrepresent rainfall patterns for specific areas across Melbourne. They have been selected to include larger and smaller storms, and wetter and drier years. Melbourne Water provides several templates and guidance on which to use at which location. This ensures modellers are appropriately representing the local rainfall conditions in their areas. 

Why doesn’t Melbourne Water provide guidelines for other stormwater tools? 

Designers must clearly document and explain the design intent for any treatment asset with Melbourne Water early in the concept design stage. It is usually expected that MUSIC will be used for the design of stormwater quality assets. Designers may alternately choose methods or models other than MUSIC (for example, EPA SWMM) to demonstrate performance targets are achieved. The intended approaches should be discussed and agreed with the relevant authority in advance. The acceptance of such methods may require additional review effort and time (which may include external review) and acceptance of alternatives is wholly at the discretion of the relevant authority. 

What is Melbourne Water’s process for proprietary devices?

Melbourne Water is not a regulator and does not undertake reviews or provide endorsements of proprietary stormwater treatment devices and their performance. 

It is acknowledged that proprietary devices play an important role and form part of the solutions for stormwater management. However, they may not be a replacement of nature-based solutions in Development Services Schemes managed by Melbourne Water.  

Outside of Development Services Schemes the selection and adoption of proprietary stormwater treatment devices is to be discussed and agreed upon with the relevant responsible body, for example, councils, road authorities, body corporates, golf courses etc. 

Melbourne Water supports a national approach to proprietary stormwater treatment device validation and is looking toward accepting a national approach that works with Melbourne Water science-based decision-making process and requirements. 

How do we make sure extracts from a MUSIC planning applications, which are difficult to assess?

“Some consultants prefer to include extracts from a MUSIC report rather than the whole report with planning applications.  This makes it difficult to check the inputs.  What should we do?”

We recommend to always request a copy of the model file (*.sqz or mxproj), the summary report (*.mrt) file (and if in doubt the climate template).  

The summary report can be opened in Excel for your own review of all parameters. 

It is also recommended to request a report the MUSIC Auditor and any flagged parameters should be either revised or justified to your satisfaction to prompt the consultant to check things themselves. Other tools may also offer functionality with similarities. 

Is there guidance on Integrated Water Management Strategies? 

“Any guidance on Integrated Water Management Strategies outside the project site like reusing, harvesting, irrigation etc. to achieve overall water treatment objectives?”

This is outside scope for the MUSIC modelling guidelines.  

DEECA have published a range of IWM guidelines.

See also the recently released Whittlesea guidelines for some additional guidance on stormwater harvesting 

Works outside the project site may fit within the realm of stormwater offset schemes and DEECA are preparing guidance on these for Councils. 

We will be providing more guidance on this in our next update to the MUSIC guideline 

 

Questions about existing or in progress schemes and PSPs 

How does this impact existing schemes and DSS assets?

Melbourne Water have processes in place to revise and update drainage schemes to account for changes in guidelines and effects on outcomes. It will take time for schemes to be updated as you are aware. Melbourne Water are working to improve and streamline processes for development and revision of schemes. 

What is MW’s position with respect to treatment performance requirements for catchments where the PSP-allocated drainage reserve is too small to achieve BPEM treatment?

Melbourne Water evaluates this on a case-by-case basis. When the PSP-allocated drainage reserve is too small to achieve BPEM treatment, we explore other options in the scheme or on site, to the satisfaction of council and Melbourne Water.  

Will I have to redo models developed following the old guidance?

  • Developments with approved permits that used historical MUSIC guidelines to prepare models and asset concepts, are to model proposed assets with the updated new guidelines, however asset design decision making is to consider constraints/ arrangements that have occurred as a result of the granting of the permit.  
  • Developments that have not submitted a formal application for a permit will need to develop Stormwater Management plans and corresponding MUSIC models based on the new guidelines 
  • All new Development Services Schemes where the corresponding PSP has not been through Public Consultation Process will be designed based on new guidelines.  
  • All other active Development Services Schemes will be reviewed over time and the Stormwater Quality treatment Models assessed against new guidelines. 

  

Questions about training and information 

Where can I access the recording of the information session? 

You can find the recording here (on the Clearwater website).

Who can I talk to if I have questions? 

Please email enquiries@melbournewater.com.au with any questions. 

Are we providing training on the new guidelines? 

Melbourne Water will consider providing training on the updated manual and MUSIC applications once we've received feedback from the industry on the documents. If there are difficulties in understanding the guidance, we will look into ways to build capacity and ensure everyone is comfortable with the changes. Your input will be crucial in shaping how we approach this, so please share your thoughts and experiences once you've reviewed the materials. 

 

Questions about rainfall and climate data 

Have we investigated other methods to represent the rainfall?

(i.e. Have we considered fully synthetic 6-min data based on statistical models) 

Yes, these have been considered. Synthetic rainfall brings its own challenges and needs to be representative of all the relevant statistical characteristics of the rainfall, which needs to be checked thoroughly. 

Stochastic methods do exist such as the DRIP model (which depends significantly on a good underpinning historical rainfall data set) and downscaling of global climate models (GCMs). 

Different approaches offer different benefits and disadvantages.  

The approach we have used is generally consistent with the published method used by SILO (for infilling daily data) and uses the best available rainfall data for a given timestep with missing data, drawing on the station with the highest statistical correlation. This preserves the historical integrity of the dataset. However, it can leave some gaps where no stations within the area have no data. The data sets are chosen according to thorough consideration of the statistics of the long-term rainfall data for each region as well as data quality and the period chosen. 

There may benefits in combining different infilling methods in future and we are open to further discussions with industry. 

How can we model or take into consideration the impacts of climate change?

“Are there any thoughts on how we can model or take into consideration the impacts of climate change?  More importantly is taking the impacts of climate change into consideration important with respect to water quality modelling or does that increase in rainfall intensity have minimal impact?”

The potential effects of climate change can be modelled in a number of ways.  

Climate data can be downscaled from global climate models. Usually these datasets are only daily which is of limited use for WSUD but can give some overall impressions. 

Historical data can also be modified to represent the anticipated changes in rainfall volumes and intensity.  

Work to date considering changes in rainfall volumes and daily intensity suggest that changes in volumes far outweigh intensity effects. Most future climate projections for Victoria are for warming and drying and this translates to less rainfall, more evapotranspiration and more effective WSUD (so better treatment and smaller asset sizes). The limiting cases for stormwater quality are then current conditions and the scenarios that project minimal change or even cooling and more rainfall (there is a broad range of scenarios, all are valid and possible). 

Additional information

The outcomes also point to the need for representing multiple potential climate scenarios given both historical and future variability and a move to report ranges and uncertainties. We anticipate the next major revision of rainfall templates may introduce modelling with multiple climate projections or periods to account for the range of possible outcomes (Similar to the ARR2019 design event ensembles). 

 

Questions about sediment basins 

Why have you decided to set the K-value for stand-alone sediment ponds to zero? 

In response to industry requests, clearer guidance is provided in the guideline on how standalone sediment ponds, without any further secondary or tertiary treatment, should be accounted for in treatment performance reporting with respect to nitrogen. 

Melbourne Water have prepared a literature review documenting the importance of vegetation for nitrogen removal and the evidence indicating the relatively limited effectiveness for nitrogen removal for sediment ponds and ponds. A summary of our findings is presented below. The full review will be published soon. 

A comprehensive literature review with a focus on sediment ponds was undertaken. Its outcome was that there remains considerable uncertainty around the extent and reliability of nitrogen removal (both temporary and permanent) in sediment ponds designed according to Melbourne Water guidelines and operated under local conditions. This is due to their limited detention times and capacity to support long-term nitrogen removal via biological processes, coupled with a lack of reliable long-term data.  

As there is insufficient evidence to confirm that permanent nitrogen removal is facilitated by sediment ponds, Melbourne Water cannot accept nitrogen removal modelled for stand-alone sediment ponds when these are not part of a treatment train, including downstream secondary or tertiary treatment. 

A range of nitrogen and other pollutant removal processes are facilitated by stormwater control measures (SCMs), depending on their design. Since nitrogen is present in stormwater in both particulate and dissolved form, it is necessary to consider the processes that can act on both nitrogen and sediment in SCMs: 

  1. sedimentation – removal of sediment and associated pollutants via settling; particles may gradually become buried within the sediment or remobilised by re-suspension or decomposition processes 
  2. re-suspension – disturbance and resuspension of sediment and associated pollutants by high velocity flows, wave action or bottom-dwelling organisms 
  3. decomposition and mineralisation – conversion of organic nitrogen to simpler and eventually inorganic components 
  4. nitrification – bacterial conversion of ammonium to nitrate 
  5. biological assimilation – uptake and retention of inorganic nitrogen in the tissues of plants or microbes 
  6. denitrification – bacterial conversion of nitrate to N2 gas 
  7. sorption/adsorption – a reversible process of attachment of ammonium to the surface of negatively charged particles 
  8. ammonia volatilisation – conversion of aqueous ammonia to gaseous ammonia followed by loss to the atmosphere 

Nitrogen is very active and readily transforms between forms. The transformation processes that can occur in water-plant-soil SCMs are complex but, as they are aerobic environments, the overall direction of transformation is from particulate organic nitrogen to dissolved organic and, ultimately, dissolved inorganic form. In addition, the majority of nitrogen (>75%) in stormwater is present in dissolved form.  As a result, the SCMs that deliver the most reliable, long-term removal are those that facilitate the removal of dissolved inorganic nitrogen, particularly nitrate. 

Denitrification has previously been assumed to be an important nitrogen removal process in SCMs but recent studies have shown that, not only is denitrification less important than previously assumed, but that nitrogen fixation (the reverse of denitrification) can exceed denitrification in some open-water SCM sediments. Long-term nitrogen removal by sediment ponds is therefore solely reliant on the burial of incoming organic matter before it is transformed to soluble forms via decomposition and mineralisation. Given that only a small proportion of nitrogen in stormwater is in particulate organic form and the uncertainty about the extent to which deep burial occurs, the ultimate fate of most of the nitrogen that enters a sediment pond is thus expected to be release via the pond outlet. It is for this reason that Melbourne Water does not accept sediment ponds on their own as a viable system for long-term nitrogen removal. 

Why have we not applied the same zero k approach to sediment basins when connected to wetlands? 

Sediment pond performance is most obviously an issue for the use of sediment ponds where they are the sole treatment for a catchment. Where sediment ponds are part of a treatment train, they will support improved performance in the wetlands. 

The much higher proportion of water covered by dense vegetation in constructed stormwater wetlands facilitates a greater range and extent of removal processes, including vegetation-related filtration, biological assimilation and coupled nitrification-denitrification.  The addition of a pre-treatment sediment pond to a constructed wetland increases the overall volume of the system, meaning it can detain a greater volume of water per event.  Some nitrogen will be temporarily removed from incoming stormwater through the settling of particulate organic matter in the sediment pond. However, as noted above, settling is largely only a temporary store of nitrogen because most of the nitrogen that enters the system as particulate organic matter will eventually be converted to dissolved (and thus mobile) form. When this eventually moves from the sediment pond to the macrophyte zone of the wetland, it can be locked up in longer term stores (for example recalcitrant biomass) or even permanently removed from the system via denitrification. Melbourne Water therefore currently accepts the contribution of sediment ponds when they are part of a constructed wetland system or similar because, while they do not directly contribute to net removal of nitrogen, they allow the system to detain a larger volume of water and thus increase the opportunity for eventual removal within the macrophyte zone. 

We will continue to be guided by the science and make evidence-based decisions as new knowledge is understood.

Have you considered the sediment loading rates coming to sediment ponds?

“Have the new standards (or will later revisions of the standards) considered the sediment loading coming to sediment ponds? There are many discussions in industry around the appropriateness of adopting one rate for the entire Melbourne metropolitan area.”

There are many discussions in industry around the appropriateness of adopting one rate for the entire Melbourne metropolitan area.

It is recognised that pollutant concentrations including sediment concentrations and loads need to be revised to reflect latest science and data. 

Providing region, rainfall or soil type specific sediment loading rates would require significant data monitoring to allow spatial variations in sediment loading to be well quantified. This would likely require a substantial program of monitoring that is not currently planned. 

Generally, recent information is pointing to lower TSS concentrations - but is also more focussed on smaller catchments (such as lots) while construction phase and road sources may be more significant. There is also significant variation within the data. 

Further refinement of concentrations and loading rates is desirable as data and understanding improves. 

 

Questions about modelling methods

When modelling a raingarden/bio-retentions, are the weir and pipe flow both treated in the node? 

That is correct. 

The overflow (weir) flow only receives treatment within the surface storage based on k-C*.  

The outlet (pipe) flow first receives treatment within the surface storage (based on k-C*) and then through the filter media according to a set of equations based on the filter media, inflow concentrations and/or other factors. 

When should we use the generic discharge vs custom stage-storage approaches?

“On stage-storage discharge curves: is it fair to say we should only really use the generic orifice diameter, etc. parameters really early on in the design process to guide what's necessary to achieve the ~72hr detention time then move to a custom stage-storage approach?”

The default 'equivalent orifice' and weir in MUSIC is adequate for most smaller assets and concept design. We agree that for wetlands and ponds the simplified discharge and 'vertical sides' assumptions can break down fairly quickly and it is preferable to move to using custom stage-storage-discharge relationships. These are essentially required for any Melbourne Water wetlands - but are quite a bit more cumbersome for modelling unfortunately. 

In terms of custom stage storage, is there guidance on how to factor in if the wetland is located within a retarding basin?  

Extend the stage-storage-discharge curves to include the retarding basin. The relationships between levels and outlet capacities can be calculated in Excel or other tools and included within the pipe and weir stage-discharge relationships  

Note that flows usually discharge relatively freely once they reach the retarding basin outlet so often this has little effect but may be important if the retarding basin drains relatively slowly. 

Can water be directed to permeable paving for treatment or does it simply treat rain that falls on the area? 

Either are possible. For example a permeable car park may commonly treat the adjacent road.  

We recommend maintaining a high treatment to catchment area ratio (TCAR) between the permeable pavement and total catchment of preferably 50% and at least 20%. The sediment loading (and corresponding clogging) increases proportional to this ratio and it is usually preferable not to concentrate infiltration volumes too strongly into a small area.  

Permeable pavements are best used adjacent to trees and vegetation that may potentially draw upon the infiltrated water. 

How was porous pavement water quality typically modelled before this update? 

Different consultants likely come up with a range of different approaches to approximate these or referred to interstate guidelines as a basis.  

Most use the media filtration node in some form, or potentially the infiltration node for assets without underdrainage. 

There is some guidance on soakaway design in ARQ, 2006 in the section on infiltration by John Argue but this is more focussed on design events rather than stormwater quality. 

Other models such as SWMM already support modelling of permeable pavement. 

See also DesignPave from the Concrete Masonry Association of Australia (CMAA) for additional design considerations. 

 

Questions about asset management 

Can we consider the changing performance of assets over time?

“Any future thoughts on modelling or taking into consideration the degradation of assets over time (and the associated decline in treatment performance) based on their assumed asset lifespan between resets?”

This would be a separate asset management and condition modelling exercise which could be informed by MUSIC modelling to understand accumulation of litter and sediment and loading rates – It’s beyond the scope of this modelling guidance.

How does the MUSIC model impact the analysis of asset lifecycle performance?

MUSIC provides a prediction of the potential (ideal) performance of an asset. It does not include assessment or prediction of degradation behaviour or impacts on performance. 

It does provide predictions of gross pollutant and sediment loadings that can potentially be used to inform estimates of maintenance requirements (although we would recommend the collection and use of more robust underlying data sets and consideration of local context to best support this as these can vary widely and the data within MUSIC is significantly dated).