Links

A reach refers to a stretch of river, or physical section, between an upstream and downstream location. A link, on the other hand, is a logical connection within a river systems model. Routing describes the change in timing and shape of flow as water moves down a river.

Links (or reaches) can have routing configured on them. For links (or reaches) that do not have routing configured, they are used to define the order of execution in the model.

Using links in Source

You can configure some aspects of links in a similar way to nodes. Refer to Renaming nodes and links, Searching for nodes and links, Deleting nodes and links, Node and link default names and Copying and pasting.

Adding links to a model

There are two types of links available depending on the nodes you are connecting (refer to Figure 1).

Vertical links are used to connect most nodes. To add this link to a model, first refer to Figure 1 which defines the terminology. To create the link:

Horizontal links (or wetland links) are drawn between the Wetlands Hydraulic Connector node (source) and the Storage node (target) only. This process is similar to drawing a vertical link. Note that the node connectors appear on the left and right side, instead of above and below the nodes. Click and drag these connectors together as described for vertical links. Figure 2 shows an example of a horizontal link.

You can also drag the link vertically once it has been created by clicking on the red dot. This appears in the centre of the link when you click on the link. For more detail on the wetland link, refer to Wetland Link.

Dragging links

You can disconnect and reconnect a link between nodes rather than having to delete and re-add it using the Allow Link Dragged button in the Schematic Editor options toolbar. Note that not all links can be connected to all types of nodes, and specific nodes require certain links. Refer to Types of link routing for more detail.

Editing link parameters

To edit link parameters using a feature editor, refer to About feature editors.

Adding notes

You can include a text-based message, or note, for a storage routing link. Refer to Adding notes to nodes and links for details.

Link elevation

You can set the elevation for a link using the Location Control window (shown in Figure 3). Choose View » Location Control to open this window.

Figure 3. Location Control window

Types of link routing

Source supports three types of link routing. You can either use straight through routing, a lagged routing model or a storage routing model. To enable routing, right click on the link, choose Routing Type, then click on the required link routing.

You can check which routing models are in use in a scenario using the Project Hierarchy. The example in Figure 4 shows that both lagged flow and storage routing are in use. You are responsible for ensuring that you use the correct model for each link.

Refer to Types of links routing for more information.

Figure 4. Project Hierarchy (link models)

About dead storage

Dead storage refers to the capacity of a storage that is below the minimum operating level. At this water level, there is no outflow. The level of the reach with respect to dead storage at the beginning of the time-step affects its level in subsequent time-steps as follows:

• The reach is at or below dead storage and the fluxes during the time-step are insufficient to raise the level above dead storage; or
• The reach is above dead storage but fluxes during the time-step would lower the level in the reach to below dead storage; or
• The reach is above dead storage and remains above dead storage during the time-step.

To determine if the reach is at or below the dead storage level, Source:

• Computes an initial storage estimate by using inflows to fill the reach up to but not exceeding the dead storage level;
• Computes a revised storage estimate based on any remaining inflows and fluxes but ignoring outflows; or

If the revised storage estimate is above dead storage then outflows are computed, otherwise the initial storage estimate is used and outflows are set to zero.

The next section provides an overview of the parameters shown in Figure 5.

Initial conditions (flow or storage)

If necessary, one of these parameters may be used to seed a reach with either an initial flow or storage so that reach behaviour is fully defined from the first model time-step.

Reach length

The Reach Length is not used in computations and is only for documentation purposes.

Average regulated flow

This parameter is used to calculate travel time for orders in the ordering phase. It is not used in the flow distribution phase.

Number of Divisions

Conceptually, this parameter describes the number of times that a reach is replicated. The effective length of a reach is determined from its behaviour, which is controlled by the combination of the storage exponent m, the inflow bias x and the storage constant K. Specifying multiple reach divisions implies applying the same set of behavioural parameters multiple times. In other words, if the effective length of a single-division reach is 500 metres (as derived from its behavioural parameters), changing the # Divisions parameter to 2 implies a combined effective length of 1000 metres. If you want to sub-divide a 500 metre reach into two 250 metre sections, you must also change the behavioural parameters to achieve this.

Inflow bias

The weighting factor x is used to adjust the bias between inflow and outflow rate and allows for flow attenuation. A recommended starting value is 0.5.

Storage exponent

If m=1, linear (Muskingum) routing is implied, otherwise non-linear routing is implied. A recommended starting value for non-linear routing is m=0.8. Laurenson routing is obtained when m≠1 and x=0, in which case the storage routing equation simplifies to:

Equation 1

Storage constant

When using linear routing (m=1), the units of the storage constant K are in seconds. For models using daily time-steps, the recommended starting value is 86400 (the number of seconds in one day). When using non-linear routing (m≠1), the recommended starting value should be calculated as follows:

Equation 2

For example, if the reach length is 1000 metres, the time-step is one day, and m=0.8:

Equation 3

The parameters for storage flow routing are summarised in Table 1.

Table 1. Link (Storage routing parameters)

Piecewise storage function

Link travel time can also be set using a piecewise linear editor. This describes a series of relationships between reach index flow rate q¯ versus travel time. The slope of the curve is the same as that for index flow rate versus storage so the rating curve can be derived using dead storage (zero flow) as the starting point. The data points can be entered manually or imported from a .CSV file, the format of which is shown in Table 2. Quadratic interpolation is used to find points in each defined segment on the curve (as in BigMod where x=1).

Table 2. Link (Storage routing travel time, data file format)

Piecewise routing allows you to specify how K varies with flow. If x=1, then K must always be less than or equal to the time-step. In BigMod routing, the highest value of K is found in the travel time relationship, and the reach should be sub-divided into sufficient divisions such that the highest value of K for each division is less than the half the time-step.

Link rating curve

Rating curves are used to describe the physical characteristics of the reach and convert a flow into a level, ie. they produce an output of level. They can be specified in one of two ways:

• Using the Cross Section Editor to specify a physical cross section, and then generating the rating curve; or
• Directly entering the rating curve - the piecewise linear editor allows you to define relationships with respect to water level, discharge rate, reach width and dead storage. You can define multiple rating curves for a reach, each scheduled to commence on a particular date.

To define a new rating curve directly:

• Right click Rating Curve and choose Add Rating Curve;
• Today’s date will automatically be entered for Start Date. To change this, click the calendar on the right side (see Working with date-pickers);
• Enter the water level, discharge rate, reach width and dead storage; and
• Enter an appropriate value for Overbank Flow Level.

You can also use the Import button to import a rating curve from a .CSV file the format of which is shown in Table 3.

Table 3. Link (Storage routing, Rating curve, data file format)

Where:  

level is the storage height in the reach in metres above dead storage

rate is the outflow from the reach in megalitres per day in the corresponding level

width is the surface width of the reach in metres at the corresponding level

storage is the dead storage in the reach in megalitres at the corresponding level.

There should be at least one row describing the maximum depth at which there is zero flow, and which quantifies the maximum amount of dead storage in the reach. Thereafter, the dead storage volume should remain constant. Table 4 shows an example of this. A depth of 0.5 metres defines the maximum amount of dead storage (100 megalitres), after which the dead storage remains constant. Note that if discharge is 0, then dead storage must be increasing, or it must be equal to the previous value of dead storage.

Table 4. Link (Storage routing, Rating curve, example)

To edit an exising rating curve, select the curve from the list of available curves under Rating Curve. Edit the data and click OK to close the editor. To delete a rating curve, right click the curve from the list and choose Delete.

You can also export rating curves to .CSV files by clicking the Export button. Table 3 shows the file format.

Link losses and gains

Choose Loss/Gain to specify flux as a function of flow using a piecewise linear editor.

By convention, losses are described using positive numbers whereas gains are specified using negative numbers. In other words, a gain is a negative loss.

Figure 5. Link (Storage routing, Loss/Gain)

You can enter the relationship manually, or import the data from a .CSV file, the format of which is shown in Table 5. This table shows the data file format for both evaporation and rainfall.

Table 5. Link (Evaporation.Rainfall, data file format)

Where:

time is the time of observation in "dd/mm/yyyy hh:mm:ss" format

value is the evaporation rate.rainfall in millimetres per time-step

Link evaporation

Choose Evaporation to specify the rate of evaporation per unit of surface area. Typically, this is done using a time series (loaded using Data Sources), the format of which is shown in Table 5. You can also specify the rate of evaporation as a single value, or as an expression using the Function Editor.

Figure 6. Link (Storage routing, Evaporation)

Rainfall on link surfaces

To specify the rate of precipitation per unit of surface area, choose Rainfall. Just like evaporation, this can be specified as a single value, as a time series (format shown in Table 5) or an expression. A time series can have multiple columns containing rainfall data.

Figure 7. Link (Storage routing, Rainfall)

Timeseries Flux

This allows the input of a time series of total water lost or gained on a link. Values can be positive or negative. A negative value denotes water returned to the link (a gain). See also Link losses and gains.

Figure 8. Link (Storage routing, Timeseries flux)

Constituents

Before you can configure constituents for a link, you must define them first for the scenario using Edit » Constituents. Refer to Links.

Ordering at links

Figure 9. Link (Ordering)

Ownership at links

Ownership must be enabled at the scenario-level (using Edit » Ownership) prior to configuring ownership at storage routing links. Refer to Ownership for details.