The river network computations use the so-called Manning formula to compute the water height at each of the nodes in the open channels of the river network (model information). The computation proceeds upstream from the outlets of the river network. At each step the water height is computed based on the previously computed (downstream) water height, the slope of the edge connecting the two nodes as well as the cross section information, the flow values and the Manning number. In closed cross sections the Manning formula is used unless the entire cross section is filled with water, in that case a different formula based on the so-called Darcyâ€“Weisbach formula is used.

The water flows on the surface of the terrain and the flow is modelled in a steady-state where all flow passing into an edge in the network also passes out of it, there is no water storage in the network. As such, the model is stationary and does not provide a time-dependent estimate of the flood risk.

Illustration showing the conceptual flow of the computation. The water flows from left to right (from upstream to downstream) and the computation proceeds from downstream upstream. The figure shows the variables involved in computing the water height at the upstream node.

A river network can have multiple computations available for analysis and comparison. They are represented as a list of layers in the river network box. The order of layers corresponds to the creation times of the computations. Read more on studying individual computations later in this document.

A computation is defined from a set of initial conditions which include:

- Flow rate: This number specifies how the flow at a node is calculated in the absence of node-specific information (defined below). The flow rate is in litres per second per square kilometer and is multiplied by the watershed area to get the flow at the node.
- Ocean level: Concurrently with the river network computation a sea-level rise computation is carried out for all cells that are not upstream of any river network cell, using the sea-level set in this field. This can be used to simulate a sea-level rise uniformly on the coast at the same time as the effect of the sea-level rise change is included in the river network computation.
- Node information: You can fix the flow value and water height at certain
nodes in the river network, typically these values come directly from
hydrometric stations - or through a statistical analysis for specific return
periods. The water height is absolute (
*not*relative to the elevation model). You can also set some more advanced flow controls (Δ*Q*and*Q*_{max}), but the computation of those is outside the scope of this documentation.

Given the initial conditions listed above we compute the flow (*Q*) at every
node in the network, this *Q* depends on the initial conditions and the watershed
sizes of the nodes in the network. For each node *N* we find the first
*upstream* node that has a *Q* value set in in the initial conditions.
If no such node exists, the *Q* value at *N* is set by multiplying the size of the
watershed of *N* by the flow rate. Otherwise, if a node *W* is found upsteam of *N*
where a flow value *Q*' was specified in the initial conditions, then *Q*
value of *N* is set to that of *Q*' but reduced by the difference in watershed size
between *N* and *W* (if *W*'s watershed was twice the size of *N*'s then the flow at *N*
will be set to half of *Q*'). In the figure below, nodes on the purple paths get
their flow values through this later method, whereas the nodes in the orange section
get their flow values through the flow rate multiplied by their watershed size.

Distribution of flow values in a river network. The user has specified Q values
at three points (*Q*_{1}, *Q*_{2} and *Q*_{3}) in the river network, and has specified a water
height at the
outlet.
In the orange section, the flow rate is used to compute the flow, in the pink sections the nearest upstream node. Scaled by watershed size. The example also shows a split in the rivernetwork and how the Q is divided in that case

Each computation has a map layer that shows the water column height of the flooding on the terrain. By using the "Water Height" slider that appears, you can set a threshold for the minimum water level of interest. Using the point query tool you can examine the exact water height of a particular point in the terrain. If you want to see the full extent of the flooding, set the value to 0. See the demonstration video below for an example of working with river network computations.

To examine the initial conditions of the computation, hover the mouse cursor above the name of the computation and select the gear icon that appears to the right, then select the "Network Analysis" tab in the dialog (see screenshot below).

Examine an existing computation by clicking the gear icon to the right of the computation name and select the "Network Analysis" tab.

In the information dialog you can rename the computation and change its description by hitting the pencil icons. You can also see the initial conditions used for that particular river network. For each of the nodes that have initial conditions you can click on the "Z" to zoom to the location of the node.

To create a new computation use the "Create Computation" action in the river network menu. This will bring up a dialog in which you can specify the initial conditions. It is frequently desirable to start with initial conditions that were already used in a previous computation, so the system allows for you to import one or more sets of initial conditions. The screenshot below shows the dialog.

*Note*: You should always set a water height at the nodes at the outlet of your river(s) to appropriately seed the computation. This is also where you set a higher water level if you want to similate an increase in e.g. ocean levels simultaneously with your flow event.

Creating a computation through the "Create Computation" action in the river network menu.

To add a node with initial condition information, click the "Add Node" button and then click on the node in the network. The node will now appear in the table and you can edit the values. Use the "D" button to delete the node and the "Z" button to zoom to its location.

The video below demonstrates how to work with computations. It also shows how you can query the water height at a particular point in the terrain, and along a length profile on the river. The length profile also allows you to see the *Q* values used along a stretch of the river.

Examining an existing river network and creating a new computation. This video demonstrates enabling a computation and querying the height of the water column in a point on the terrain, and along a length profile. It also demonstrates examining the initial conditions for a computation and how to create a new computation with a different set of initial conditions.