Melbourne Water MUSIC Guidelines – Update
Melbourne Water updated their Guidelines and mandated their application in July 2024. The new document incorporated a number of minor tweaks and changes including an update to the rainfall across the region. The more significant decision was around the level of water quality improvement a sediment basin might bring to a treatment train.
The latest MUSC guidelines builds on previous iterations of the guidelines previously issued by Melbourne Water. The current guidelines came into effect on the 1st July 2024. ALDE have outlined the significant changes from the previous version of the guidelines (2018) and what implications the guidelines may have on water quality treatment assets and their sizing. The implications have been listed the items under “major implications”, “moderate implications” and “items to note” depending on the changes to the Land Development industry.
Major Implications
The removal of Nitrogen reduction in standalone sediment basins will impact current MW DSS’s which rely on several standalone SB prior to entry to waterway assets as part of the larger scheme treatment train. This will result in a shortfall in scheme treatment performance. It is assumed that this legacy item will be accepted by Melbourne Water and that further upsizing of wetlands to accommodate this shortfall will not occur in already gazetted PSP’s, however this needs to be confirmed by Melbourne Water. This was discussed during the webinar of presentation of the new guidelines; consultants have requested MW/E2 Design Lab to provide evidence (i.e. research paper etc.) of the rationale behind this decision.
The guideline appears to recommend the diversion of base flows within the model around treatment systems that are fed by pit and pipe networks. Clarification from Melbourne Water is required to confirm if this is a requirement or not as the wording given in the guidelines is unclear. This is a change from the previous guidelines where it was not recommended. Given that majority of treatment assets constructed in greenfield areas are fed by the subdivision pipe network, the diversion of the base flows around the treatment assets impacts the performance of the assets, and will increase the treatment footprint required to meet best practice targets. Initial testing suggests that treatment sizing’s will increase by approximately 5% for constructed wetlands and may have similar impact on other treatment types. The increase in treatment area is exacerbated if a sediment basin is combined within the wetland node (see moderated implications), with treatment targets for TSS becoming the critical load reduction target. Based on some preliminary modelling, using one node to represent a constructed wetland (sediment pond + wetland) seems to lead to a more significant decrease in TSS and TP (rather than TN).
Rainfall templates have been reevaluated and modified:
- An additional rainfall template has been added (“86224 Dandenong”);
- the “Melbourne Airport” rainfall template has been renamed “86282 Melbourne Airport infilled” and the 10-year period considered has changed from 1971-1980 to 1987-1996;
- the “Melbourne City” rainfall template has been renamed “86071 Melbourne Regional”, maintaining the 10-year period considered (from 1952 to 1961);
- the “Koo Wee Rup” rainfall template has been renamed “87075 Bullengarook East”, the 10-year period considered has changed from 1971-1980 to 1990-1999 and part of the rainfall map has been amended (with the creation of the new Dandenong rainfall template);
- the “Mount St Leonard” rainfall template has been renamed “86142 Toolangi”, maintaining the 10-year period considered (from 1995-2004);
Moderate implications
The guidelines provide clarity on when to model a sediment basin within the wetland node and when to model it separately within its own node, with the former having limitations with regards to representing bypass arrangements. Preliminary investigations suggest a reduction in 5% in treatment performance results when modelling the sediment basin within the wetland node. This will likely result in a larger grade separation between the sediment basin and macrophyte zone being selected in most designs to allow the sediment basin to be modelled as a separate node in MUSIC. This grade separation between the sediment basin and macrophyte zone will increase in most cases from the minimum 100 mm to 175 mm.
There is little clarity on how to model a wetland with two (or more) sediment basins under the assumptions that those are to be part of the same wetland node, but their outfall into the macrophyte area is at different locations. This is left with the consultant to decide the best approach and justify the rationale of the decision with MW.
Flows lost via infiltration in treatment nodes must be accounted as contributing to outflow pollutant loads. Majority of systems are generally lined so this is not applicable in most circumstances, however with infiltration systems becoming increasingly more common, this will be of more importance going forward.
Items to note
Based on the discussion at the Webinar presentation of the guidelines, it is understood that Climate Change has no or negligible impact on the water quality treatment for the different WSUD solutions. It is believed that further investigations should be conducted on this matter especially considering the latest AR&R 4.2 guideline advice regarding the impacts of climate change which are already affecting Australian catchments and its impact to exacerbate droughts and floods.
The guidelines explicitly states the requirements for wetland spells analysis. This advice is additional from the previous guidelines and provides clarity on the approach and requirements.
Noting that the pollutant concentration data, which is to be used if the user selects split surface model, has been updated from the previous guidelines. It is still recommended that mixed used nodes are used in most cases with the default MUSIC concentration data still recommended.
The guidelines provide no endorsement of proprietary devices or how to model such devices, it also states that such devices are not considered a replacement of nature-based solutions in DSS’s.
The section “version compatibility” acknowledges that the amended guidelines were prepared for MUSIC 6.3; however the underlying science of MUSICX is unchanged and the guidelines are applicable to MUSICX as well.
Flow routing is recommended for analyses that are sensitive to peak flows. These include flow rate diversions (for example, wetland high flow bypasses and pumping to stormwater harvesting assets), however avoiding the use of flow routing is considered a conservative (and probably the suggested) approach.
Bioretention systems are recommended to be modelled with and filter media hydraulic conductivity of 100mm/hr in MUSIC, which is the MUSIC default.
The Concept of TIA (Total Impervious Area)/DCI (Directly Connected Imperviousness) has been included in the guidelines, in accordance with the AR&R19. Use of TIA is recommended in order to avoid the potential of under-sizing drainage assets.
The new guidelines highlight how the treatment areas should be considered and represented in MUSIC as 100% impervious surface area to ensure that the direct rainfall becomes inflow.
Normal range and typical values of some zones outlined in table 2 of the guidelines still not consistent with fraction imperviousness values outlined in the EDCM.
