Functional design

The functional design stage contains the next level of detail following the approved concept design.

It involves designing all functional elements of the system, including:

calculations for levels and pipe sizes of all inlet and outlet structures
modelling for hydrology, hydraulics and water quality
design refinement to account for detailed information such as the survey

It's important to have a project hold point at the functional design stage, so that the modelling assumptions and designed water depths can be checked.

Design objectives

The functional design usually includes the following objectives:

pollutant removal
community safety
maintenance and operational access and safety
cost-effective asset management

Functional design package

A functional design package consists of calculations, models and drawings, and demonstrates that the treatment system will:

achieve pollutant reductions
meet safety requirements
be feasible and practical to construct
allow cost-effective maintenance

Submission requirements

Developers designing wetlands that will be handed over to Melbourne Water for operation and maintenance must use the Constructed Wetlands Design Manual.

The functional design package must contain all relevant elements of the concept design as well as the following requirements. Depending on the scale and type of treatment, some or all of these may be relevant for the design and required by the approving authority.

These requirements can also be used or adapted by councils to prepare a brief for a project tender or a list of requirements for a development application or planning permit.

Requirement	Details
A statement listing: any parts or behaviour of the design that do not conform with relevant authority requirements justification for how the proposed alternative approach achieves similar or better outcomes	pollutant reductions comply with current urban stormwater best practice environmental management guidelines safety outcomes maintenance costs sustainability and robustness
Functional design report that is technically reviewed and undersigned by the designer	This report should identify: Major changes made since the concept design phase Design flow rates, water levels and flow velocities for relevant design events and key treatment elements as well as assumptions and methods including catchment details Function of inlets, outlets, overflows and bypasses and how extended detention will be controlled Hydrologic and hydraulic modelling (where applicable) including key model inputs and justification for design and parameterisation in accordance with relevant modelling guidelines Description of design to support maintenance including access and sediment dewatering Design response and findings of geotechnical testing with full report as an appendix Summary of relevant consultation
Copies of relevant hydrologic, hydraulic and water quality models	RORB, HEC-RAS and MUSIC
Overall plan drawings	Scale plans showing proposed surface levels (usually in m AHD) within and surrounding the stormwater treatment. The plans must indicate: top of extended detention depth normal water level edge of each planting zone maintenance access tracks sediment dewatering areas existing and proposed services based on physical site proving or detection Indicative long sections showing planting zones, topsoil, liner, media and drainage layers, peak design event water levels, outlet and bypasses and the location and depth of any underground services. Landscape concept plans for surrounding areas where relevant Plan of maintenance responsibility boundaries and who is responsible for maintaining what parts Geo-referenced GIS (MapInfo) layers showing catchment boundary for each element
Letters from any other parties agreeing to be responsible for identified maintenance
Works cost estimate	Clearly itemised items to be funded by each party

Assessing functional design

When reviewing a design, check that all of the design objectives have been met. Checklists are available to help you assess the functional design of a WSUD treatment:

Case studies and checklists

Common mistakes and issues

Drawings

wetland open water greater than 20% of surface area
batters too steep, not in accordance with safety requirements
overflow weirs not considered
stormwater inlets at the downstream end of the wetland, leading to little or no treatment
no transition layer in raingardens
geotextile used to separate raingarden layers
raingarden underdrain pipes perforated or with filter sock
raingarden underdrain spacing too far apart
raingarden filter depth too shallow

Calculations

calculation that there is no velocity in a wetland or sediment pond, or no knowledge how to calculate it
no knowledge about plant inundation frequency requirements
incorrect sediment pond sizing. Sediment pond size should be calculated in accordance with the WSUD Engineering Procedures manual rather than in MUSIC
unrealistic hydraulic efficiency (λ) value used for sediment pond or wetland shape
sediment pond depth below the permanent pool level to top of accumulated sediment (d*) set to zero. d* must be 0.5m below normal water level
oversizing of sediment ponds to reduce wetland or raingarden size
no sediment dryout area considered
no consideration for high velocities exiting pipes into sediment ponds
overflow weirs not considered or sized

MUSIC

use of separate sediment pond node in MUSIC, even though same NWL as macrophyte zone
average permanent pool too deep
use of unjustified infiltration rates for sediment ponds and wetlands (= most water being lost through infiltration rather than being treated)
use of Brisbane rainfall runoff parameters instead of Melbourne
unrealistic fraction impervious for the catchment
unrealistic wetland CSTR parameter
wetland notional detention time less than 72 hours
waterways modelled as swales
proprietary gross pollutant trap products modelled to remove nutrients
overflow weir widths not as per drawings
claiming ponds or lakes as treatment systems

Last updated:

9 February 2022