Design note 3 - Drainage links and routing

In MUSIC, water flows from one point  to another through links. From a node, a primary drainage link must first be  connected then secondary links can optionally be used to direct selected flows  to another destination.

Primary drainage links

When only a primary drainage link is used, all flows from the node are directed through that link. For primary links from source nodes (catchments), this includes by default:

  • Base flow
  • Pervious storm flow
  • Impervious storm flow

Deep seepage and evapotranspiration are assumed to be routed out of the model as losses and these are not routed with a primary link.
MUSIC assumes flows diverted around a treatment and those passing through it are recombined at the outlet. Primary drainage links from a treatment include by default:

  • Low flow bypass
  • High flow bypass
  • Pipe flow
  • Weir overflow

Secondary drainage links

Secondary drainage links can be used to divert and redirect flows from source nodes and treatment nodes in MUSIC. They are used in combination with a primary drainage link to divert flows to a different downstream destination. This is useful for separating and directing specific components of the total flow to or around downstream treatment nodes.

Secondary links should be used to more realistically represent actual conditions where flows are split or diverted. Any flows normally within the model that are diverted with a secondary link need to be directed back into the model. This should occur at or upstream of the outlet where assessment of overall flow and treatment occurs, if the Treatment Train Effectiveness is to be used (otherwise it will not be available). An example of a flow split to divert base flows around a treatment and back to the catchment outlet is shown in Figure 2.
Secondary drainage links separate and redirect flows from Source Nodes that are otherwise combined. These flow components include:

  • Base flow
  • Pervious storm flow
  • Impervious storm flow
  • Deep seepage
  • Evapotranspiration

Figure 2 : Directing only surface flows to treatment while base flows are assumed to directly enter the waterway without treatment.

Secondary drainage links separate and redirect flow components from treatment nodes that are otherwise combined. The range of outflow components depends on the type of treatment node and may include:

  • Outflow (pipe outflow)
  • Overflow (weir overflow)
  • Low flow bypass
  • High flow bypass
  • Reuse
  • Infiltration
  • Evapotranspiration

This can be useful for cases such as:

  • Pumped flow from a tank or pit directed to a treatment or storage. The pump is represented in the model using a constant water demand and the ‘reuse’ flows directed to the downstream asset.
  • Diversion of ‘Weir Overflow’ and ‘High Flow Bypass flows from a sediment basin around the macrophyte zone of a wetland. This is required when a separate ‘Sediment Basin’ node is used upstream of a wetland system.

Routing

Routing can be omitted to reduce the complexity of the model. Routing is not required for modelling small catchments where the time of concentration is less than 9 minutes (times are rounded to the nearest 6-minute interval in MUSIC where this is used as the modelling timestep). Not using routing will generally result in the performance of treatment systems being underestimated so is conservative for most designs.

If routing is to be used, it should be calculated and applied consistently across a model to ensure timing of peak flows and possible coincident peaks are modelled correctly.

There are two types of routing to consider:

  • Water flows from the furthest upstream point in a catchment to its outlet or the point at which flows reach a confluence with flows from other catchments. This is represented using the link immediately downstream of a catchment node. Hydrologic routing should be included where appropriate to reflect the Time of Concentration of the Catchment.
  • Water flows from one confluence point or treatment to another. Hydrologic routing should be included where appropriate to represent the travel time between these points.

The time of concentration will depend on the size of the catchment and the corresponding length of the longest flow path within it.
Travel times depend on the length of the flow path and may also be influenced by whether flows occur through piped drainage, lined channels or natural depressions and waterways.

It is recommended ‘Translation’ is used for representing flows in pipes and concrete channels and ‘Muskingum Cunge’ is used for overland flows and flows in natural channels and waterways. A ‘theta’ value of 0.25 is recommended for natural channels.
Routing through primary and secondary drainage links can be specified by accessing the properties dialogue box, see Figure 3. The link properties dialogue box can be accessed by selecting a link and clicking the Edit Properties button on the Create Model tab or by double clicking on the link. Within the link properties dialogue box, a user can attribute routing properties and outflow components associated with the link.


Figure 3 : Drainage link inputs.

Routing is recommended for analyses sensitive to peak flows, including:

  • Flow rate diversions such as high flow bypasses and pumping to stormwater harvesting.
  • Assessment of peak flows.

Key requirements:

  • Routing is omitted from model; or
  • Routing is calculated based on a suitable methodology (Rational method or hydrologic/hydraulic model) and applied consistently across the model; and
  • Calculations and/or modelling and a summary of outcomes for routing times of concentration and travel times are provided where used.
  • Translation is used for pipes and lined channels.
  • Muskingum-Cunge routing with theta of 0.25 is used for overland flows and flow in natural channels.

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Page last updated: Wednesday, 18 December 2019

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