Streams and Flow – Workspaces
Workspaces provide a powerful way to use stream networks in SCALGO Live. Once created, they can be configured in a number of ways to get the behavior you want. In this section, we first discuss how to create a workspace based on the national analysis. Then, we elaborate on how to create a stream network on any flow path in the terrain. Last, we discuss how to work with the workspace.
If you want an introduction to the basics on streams in SCALGO Live, please read Introduction.
Creating a stream network workspace from the national analysis
To create a workspace based on a stretch of stream in the national analysis, click on workspace in the toolbar and select the streams. You can now draw a rectangle or a generic polygon to select the area you want to include in your workspace.
To see what areas are included in the national analysis, use the stream network layer.
If your stream is not included in the national analysis, you can use the following method to create your stream network based on a flow path on the terrain.
Creating a stream network terrain flow path
You can instantiate a stream network on any flow path in your workspace. This lets you model flow along streets, in small channels and in streams that are not included in the national analysis.
First, create a normal Flash Flood Map workspace and run the analysis. You can now see the flow paths, and those are key to creating your stream network.
You can enable the stream network functionality like this:
- Select the toolbox icon beside the workspace and workspace tools appears.
- Select the Configure... option.
- Enable the streams option.
- Go back to the workspace tools dialog.
Now, you have extra functionality available, namely configure events and configure network. Configure events is used for setting up specific flow events, but it is not required initially. Configure network allows you to define the actual stream network, this is required and is explained in the next section.
Specifying the stream network
In order to use the stream functionality in your stream network, there must be a stream network. The stream network can come from two sources:
- The national computation center line: If you have created your workspace from the national computation, the center line is automatically included in your workspace, and you do not have to do anything further. However, if you desire, you can still use the method described next to add more segments to your stream network.
- Flow paths: You can use so-called fingertip and outlet points to define a subset of flow paths and thereby specify the stream network. Every flow path between a fingertip point and and outlet defines a stream network line, as shown in the figure below.
There are two important concepts when specifying a stream network: fingertip and outlet points.
To define a stream network, click on configure network in the workspace tools. The following tools are now available:
Select an existing network point to move, rename or delete it.
Create a new stream outlet point. Place this where you want the stream to end
Create a new fingertip point. Place this upstream of an outlet point to define the upstream starting point of a stream.
See the following screenshot for an example of use:
Workspaces use a fixed Manning number, configurable in the configure network dialog. You can also configure the spacing between the sampled cross sections from this dialog.
Running the analysis
Once you have created your workspace and you have your stream network, you can recompute using Run analysis in the workspace tools. Even though you can create custom event specifications, these are not required initially. The flow rate and water level rise analyses do not require an up-front specification of flow and are therefore very well-suited for workspaces, for which they are always enabled when working with streams. They allow you to quickly get started using the stream functionality in workspaces without having to worry about the relative complexity of custom events.
If the flow rate and water level analyses are insufficient for your case, you can define events in more detail. This is explained below.
You can get fine-grained control of the flow along your stream network using configure events in workspace tools. A computation is defined from a set of initial conditions which comes from specifying the flow value and water height at certain nodes in the stream network. These values typically come directly from hydrometric stations or through a statistical analysis for specific return periods. For each of these event points you can specify the following:
- Water level: 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 stream network computation. The water height is absolute (not relative to the elevation model). Water heights are used only at the event points where they are defined.
- Flow: This is used to set the exact flow in volume per time (e.g. m3/s) through that point and affects all points in the upstream until the next event point.
Given the initial conditions listed above, we define 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.
To find the flow for a node in the network, we go downstream in the network until we find the first node or event point that defines a flow value. Then, we use this flow value, but here it is scaled relative to the watershed size, implying e.g. that when a stream splits in two upstream paths, the flow is also split according to the relative watershed sizes of the two upstream paths. Thus, if a tributary enters a river, and the tributary has a watershed size of 20% of that of the main river, then we scale the flow at the place they come together by 20% for the tributary and by 80% for the main stretch. The figure below shows an example.
Normally, workspaces are fully independent of the surrounding terrain, in particular flow across the workspace boundary is not taken into account. However for stream networks, especially those based on larger streams, it is essential for the upstream areas to be correct to compute good flow values. Therefore, stream networks take the full watersheds of flow paths entering the workspace into account. However, boundary dynamics are not taken into account. For instance, if a flow path enters a workspace, leaves it again and then re-enters in a different place on the boundary, and you change the watershed size of the exit point, this change is not taken into account when the flow path enters the workspace again.
We use initial conditions based on depression-free flow. This might cause issues if you create your workspace in a depression due to flat routing of the national analysis. Therefore, you should make the workspace large enough that its boundary is not intersecting any large depressions in the model. You can use the depression map to verify where the depressions are.