Design note 3 - Wetlands and Ponds

Refer to Melbourne Water's MUSIC Guidelines - chapter 10 for Wetlands and Chapter 11 for Ponds.

Wetlands

See Melbourne Water’s MUSIC Guidelines, Chapter 10.

Wetlands that are to be handed over to the City should generally be designed and modelled in accordance with Melbourne Water’s Constructed Wetland Design Manual (2017) as updated. Specific requirements include:

  • The macrophyte zone extended detention depth (EDD) must be less than or equal to 350 mm.
  • At least 80% of the macrophyte zone at normal water level (NWL) must be less than or equal to 350 mm deep to support shallow and deep marsh vegetation. The bathymetry should provide approximately equal amounts of shallow marsh (100-150 mm deep) and deep marsh (150-350 mm deep).
  • The macrophyte zone must be offline from all waterways and drains.

Wetland inundation frequency analysis

A key consideration in the design of a wetland is ensuring that water levels are suitable for planting. This is primarily achieved by limiting the extended detention depth (EDD) and permanent pool depth within the shallow and deep marsh zones to ensure the total depth of water will support emergent macrophyte plants. Further consideration of inundation patterns as well as the frequency and duration of spells where the water levels will exceed various thresholds can also help inform the planting design.

An inundation frequency analysis should be undertaken to support the design of each wetland. This can be used to demonstrate compliance with a range of requirements, all of which must be met. Reasonable efforts must be made to resolve any non-compliance. Any remaining non-compliance must be discussed with the City and is subject to agreement and written approval.

Users may use the Wetland Analysis Tool on the MUSIC Auditor website to undertake an inundation frequency analysis. Refer Melbourne Water’s Guidelines for Constructed Wetlands for further background and guidance.

  • The following requirements are intended to reduce (but do not eliminate) the risk of plants drowning due to excessive depth, frequency and duration of inundation, they are minimum requirements and may be supported with input from an experienced ecologist and other metrics:
    • The effective water depth (permanent pool depth plus depth above NWL) must not exceed half the average plant height for more than 20% of the time, see Figure 4.
    • The average water level (exceeded 50% of the time) should not be more than 50 mm above normal water level.
    • No more than one spell of 10 days or more occurs where the water level is greater than or equal to 300 mm above NWL (in a reference period of 10 years).
  • The following requirement is intended to ensure water spends enough time in the wetland to be adequately treated:
    • The macrophyte zone must provide a 90th percentile residence time of at least 72 hours and it is recommended this is no more than 80 hours.

Figure 4 : Comparing plant heights and effective water depth for requirement 1.

An example of Wetland Analysis Tool outputs is shown in the Figures 4 through 9 illustrating the following:

  • A typical inundation frequency graph which can be used to obtain water levels exceeded 20% and 50% of the time.
  • Plant selection to choose plants for the shallow and deep macrophyte zones.
  • A summary report indicating compliance or non-compliance with each requirement.
  • A spells analysis showing that the resulting spells for significant depths above NWL are acceptable.

Read water level exceeded 20% of time which is 0.131 m


Figure 5 : Typical inundation frequency graph for assessing requirement 1.


Figure 6: Typical plant selection to select suitable plants.


Figure 7: Example results demonstrating compliance with all requirements.


Figure 8: Example results demonstrating non-compliance.


Figure 9: Typical spells analysis output indicating compliance with requirement 3
(Longest spell is 5 days, not more than one spell of 10 days or more).


Wetlands with Separate vs. Combined Treatment Nodes

In most cases, a wetland should be modelled with an integrated sediment pond. In certain cases it may be appropriate to represent the sediment pond a separate upstream node.
A single wetland node should be used where the sediment pond outflow will be significantly influenced by the water level in the wetland or wetland outlet. To determine whether the wetland is likely to significantly influence the sediment pond the following should be considered:

  • Is the difference between the sediment basin and wetland macrophyte zone extended detention levels (X) less than or greater than half the total extended detention depth of the macrophyte zone (Y/2). Refer to Figure 10 for illustration.
    • If X <= ½ Y, a single ‘Wetland’ node should be used to represent the system in MUSIC without an upstream ‘Sediment basin’ node. The sediment pond must be represented within the wetland node using the Inlet Pond Volume parameters
    • If X > ½ Y, separate treatment nodes may be used to represent the sediment basin and the macrophyte zone.

Figure 10 : Sediment pond and wetland extended detention levels.

If separate sediment basin and wetland nodes are to be used to represent a treatment asset, the weir overflow from the sediment basin will likely need to be directed around the wetland treatment nodes using a secondary drainage link.

For borderline cases where some interaction occurs, further analysis may be undertaken to confirm the wetland water level will not exceed half the extended detention depth (½ Y) for more than 20% of time and submitted for consideration. Where any doubt exists, an integrated sediment pond shall be adopted unless agreed in writing by City.


Key Requirements

Represent wetlands and upstream sediment ponds with an integrated wetland node with inlet pond, except where the difference in extended detention levels between the sediment pond and wetland macrophyte zone is less than or equal to half the extended detention depth of the wetland.


Wetlands with Multiple Inlets

The MUSIC wetland node is intended to represent a single inlet pond flowing into a macrophyte zone. While wetland assets with multiple inlets are occasionally proposed, with treatment flows split between multiple sediment ponds before flowing through the attached macrophyte zone, this configuration should be minimised. Rather, wetlands should be appropriately placed using a distributed WSUD approach. Alternatively, good design can be employed for upstream drainage assets that directs them to a common sediment pond.

Care must be taken when designing a multiple inlet wetland to ensure the total inlet pond volume in MUSIC is allocated to each inlet pond proportional to the volume of stormwater runoff generated from the catchment. Further, the flow path length through the macrophyte zone must also be sized proportional to the volume entering the system. Ideally the macrophyte zone surface area at NWL, zone distribution and macrophyte coverage should also be proportional.

To model a wetland treatment with multiple inlets in MUSIC, the following approach is recommended with reference to the preceding section on separate and combined sediment ponds.

  • If the wetland is represented by a single treatment node, the inlet pond volume input to the wetland treatment node must equal the sum of the volume of all associated inlet ponds up to NWL.
  • If the wetland is represented by separate wetland and sediment basin treatment nodes, a sediment basin node should be provided for each inlet pond consistent with the design of each inlet pond.

Stage-storage discharge relationships

A custom stage-storage-discharge relationship is required to be used for wetlands intended to be handed over to the City at the functional and detailed design stages. The following should be submitted to provide evidence of design and calculations for this:

  • Screenshot of output from earthworks modelling software of stage-storage relationship.
  • MUSIC model with defined stage-storage-discharge relationships.
  • Area and volume at NWL and EDD.
  • Calculations used for stage-discharge relationships for both wetland outflows and overflows.
  • Peak top water level for the 1 EY storm event (1 in 1 year ARI) and maximum flows through wetland as per design.
  • Check of node water balance to confirm that mass balance error is negligible.

Ponds

See Melbourne Water’s MUSIC Guidelines, Chapter 11.


Next section: Bioretention

Previous section: Swales, gross pollutant traps and sediment ponds

Back to table of contents





Page last updated: Wednesday, 18 December 2019

Print