Design note 3 - Appendix A: Stage-Storage-Dsicharge Relationship

Custom stage-storage-discharge relationships are required when modelling wetlands at the functional and detailed design stages. Guidance on preparing these is provided below.
The stage-storage relationship defines the physical topographic conditions of the proposed asset.


Extended detention depth

It is important when creating a new custom stage-storage relationship that the user first enters the correct extended detention depth in the treatment properties dialogue. MUSIC then expects values will be provided in the stage-storage-discharge relationships for at least 2.0 m above this EDD value.

Figure 12: Set Extended Detention Depth before entering ‘Custom Outflow and Storage Relationship’.


The stage-storage relationship should be entered prior to defining the stage-discharge relationships for pipe and weir flow.

A stage value of 0 represents the wetland NWL and the corresponding storage volume at NWL represents the wetland permanent pond volume. The specified stage-storage relationship must be consistent with the submitted functional design.

Figure 13 is a screenshot of an example stage-storage relationship dialogue box from MUSIC. The relationship is generally defined by the output of an earthworks model.

Figure 13 : Example Stage-Storage Relationship.

Where 1 is the Height increment and decimal places settings, 2 is the option to open data from a text file and 3 is the option to paste copied data from a text file.

The data output from the earthworks model should be saved with a *.csv file extension. It is recommended the data is viewed in a text editor (for example: Notepad) to ensure the data does not contain additional formatting or formulas. The data must be presented in the following format for input to MUSIC:

  • No header line
  • Comma or space delineated
  • Set Height increment (metres) to 0.1 metres and Decimal places to at least 4 decimal places (Recommended) (Note 1 Figure 13)
  • Stage-Storage relationship must extend at least 2.0 metres above the extended detention level (rounded up to the nearest 100 millimetres) – to ensure significant changes in storage are recorded and rounding errors avoided
  • Use standard number format (MUSIC will convert these to scientific format)
  • A starting height of 0.00 metres represents NWL of the wetland
  • Starting volume is the storage below NWL (that is: permanent pool volume)

Once formatted correctly, the *.csv file containing the earthworks model data can be imported using the open button (Note 2, Figure 13) or copied to the clipboard and pasted using the paste button (Note 3, Figure 13).

In the example provided in Figure 13, an extended detention depth of 0.35 m was adopted and therefore the stage-storage relationship must be defined up to a height of at least 2.35 metres (2.0 metres above the EDD = 2.0 + 2.35 = 2.35 metres).

Often the data from the associated earthworks model will not contain storage values for all stages up to 2.35 metres above NWL, unless the wetland is located within a retarding basin. Generally, the stage-storage curve can be extrapolated linearly from the highest two stage-storage data points to the target stage value (2.0 m + EDD), see Figure 14. Since the wetland should rarely reach these water levels this should usually be a reasonable approximation.

Figure 14 : Extrapolation of storage to target stage value.

Stage-Discharge Relationship

The stage-discharge relationship defines the outflow behaviour in terms of both outflow (pipe flow) and overflow (weir flow).

In its calculations, MUSIC interpolates between the stage (water level) and total discharge, which is the sum of pipe and weir discharges. To support this, all possible combinations of stage and total discharge should be unique and the total discharge should always increase with stage. The curve cannot be flat or increase then decrease as there would then be two possible stages which have the same discharge.

Figure 15 and Figure 16 show example stage discharge curves for low (‘pipe’) flows and high (‘weir’) flows, respectively.

Figure 15 : Example Stage Discharge Relationship for Pipe Flow. Where 1 denotes whether the imported data will replace or add to the pipe flow based on the equivalent pipe diameter in MUSIC (usually use replace), 2 denotes the stage-discharge table and 3 denotes the settings for Height increment and decimal places.

Figure 16 : Example Stage Discharge Relationship for Weir Flow. Where 1 denotes whether the imported data will replace or add to the pipe flow based on the equivalent pipe diameter in MUSIC (usually use replace), 2 denotes the stage-discharge table and 3 denotes the settings for Height increment and decimal places.

Inputs for the stage-discharge curves can be calculated using standard orifice and weir equations. Weirs and orifices should use accepted equations for weir and orifice design as prescribed on the Melbourne Water website. The designer must consider the type of outlet structure and the resulting outflow behaviour and adopt the appropriate equations accordingly.

The Custom High (Weir) Flow in must be set such that the calculated outflows replace standard flows (Note 1, Figure 15 and Figure 16).

The adopted relationships for low and high flows can be verified by observing the curves of the respective data and noting changes at the stage corresponding to the adopted EDD of the wetland (Note 2, Figure 15 and Figure 16).

For the high (weir) flow relationship, it is expected that the outflow will be zero up to EDD, then increase at an increasing rate as the stage increases above the EDD level.

Submission requirements and assessment criteria are summarised in Table 7.

Table 7 : Submission Requirements and Assessment Criteria.

Submission requirement Assessment check
Screenshot of output from earthworks modelling software of stage-storage relationship No vertical, horizontal or negative slopes in the stage storage relationship (for example: same stage with multiple storage or same storage at multiple stages)
Consistency with stage-storage table from MUSIC
Values are provided for EDD plus 2.0 metres in stage storage discharge relationships
MUSIC model with defined stage storage discharge relationships (Weir) overflow is zero up to EDD, then increases at an increasing rate
(Pipe) outflow increases from NWL upwards.
No negative slopes in the specified relationships
The sum of (pipe) outflow and (weir) overflow curves produces unique values for each stage.
Areas and volume at NWL and EDD Design treatment area and volume are consistent with the stage-discharge relationship and proposed asset design
Reconcile adopted stage-storage relationship with area and volume reported at NWL and EDD
Calculations used for stage-discharge relationship for both wetland outflows and overflows Reconcile stage-discharge relationship in MUSIC for (pipe) outflow and (weir) overflow with provided calculated values
Check (weir) overflows engage at the expected EDD level
Check assumptions for stage discharge relationships including pipe diameters, weir widths and outlet coefficients
Review proposed dimensions, equations and coefficients are appropriately selected
Peak top water level for the 1design storm event (for example: 1 EY) and maximum flows through wetland as per design Check the highest stage has both a storage and discharge value associated with it

Next section: Appendix B: Selection of Rainfall Templates

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Page last updated: Tuesday, 17 December 2019