Streams and Flow – Understanding

In this section, we discuss the fundamentals of interacting with stream networks in SCALGO Live. The principles in this section apply to the national stream network analysis as well as the ones in workspaces.

The screenshot below shows a user accessing a typical stream network setup. It looks and works the same regardless of whether or not it is based on a section of a stream or on an arbitrary flow path in the terrain. For simplicity of the disposition, we will refer to them both as stream networks.

Layers

Network

The network layer shows the center line of the network. It is modelled as a network of nodes and edges with cross sections. Each edge is annotated with a cross section, and this is assumed to be constant over the length of the edge. In SCALGO Live, these cross sections are sampled directly from the elevation model. The stream network itself is rendered as a sequence of edges displayed in alternating shades of blue. An edge goes between two green nodes.

The cross section used to model the section of the stream covered by the edge is rendered as a thin purple line. The position, orientation and length of the line represent the measured part of the cross section. If you select an edge using point query, you can see the full length of the cross section used in the computations.

Subwatersheds

Subwatersheds show you how surface water is routed to the stream network. Read more about this in the section below called tracking subwatersheds along streams.

Flow Rate

Flow Rate is an analysis that shows the extent of a flood based on a user-chosen variable flow rate setting. Read more about this in the section below called Flow
Rate analysis.

Overflow Paths

The overflow paths shows indirect flood risk from the boundary of the area marked as flooded in the analysis. Read more in computations.

Water Level Rise

The water level analysis shows the flood extent based on a uniform increase in the water level of the stream. Read more about this in the section below called Water Level Rise analysis.

Screenshot of the stream network showing a tributary (bottom right) merging into a larger stream. Nodes are shown as green circles, cross sections are shown as purple lines where they were sampled from. The node where the two streams merge is shown in orange.

Flood risk screening

Stream networks are born with two types of computations and more are possible in workspaces. In the Flow Rate analysis, you can use a slider to select the flow in the stream and immediately see the effect of the chosen flow rate on the water level in the stream and onthe terrain. In the Water Level Rise analysis, you directly control the elevation of the water level relative to the elevation model (the bottom ofthe stream).

Flow Rate analysis

As mentioned above, the flow rate computation lets you pick an arbitrary flowrate. When the analysis is enabled, a slider appears that allows you to choose a flow rate and see the watershed of the stream. The default setting is in liters per second per square kilometer watershed. When running thecomputation, this is converted into an actual discharge value by multiplying the watershed area of the nodes in the stream network by the watershed size of that point. In this way, any given slider setting makes sense both for small and big streams. If you want to see the absolute flow value used at a location, you can use the length profile.

Screenshot of the stream network showing the flow rate analysis with a flow of 21.83 liters per second per square kilometer of the upstream area. In this area, it corresponds to an absolute flow of about 4 cubic meters per second on the main stream and 100 liters per second on the tributary to the right.

Water level rise analysis

In the water level rise analysis, you select how much of an increase in water level you want to model. The results show the increased water level along the stream network by the amount chosen. The water level is specified relative to the value of the elevation model along the stream. This is also useful for modeling an increase in lake and basin water levels.

Screenshot of the stream network showing the Water Level Rise analysis modeling an increase in the water level of 1.3 meter. The effect is shown on the lake as well as the stream leaving the in the middle of the picture.

Exploring the stream network and analysis

There are a number of ways you can explore the stream network and the analyses. This includes length profiles, cross sections profiles and stream watersheds.

Point query

You can use point query to select any edge in the stream network by clicking on them on the map (remember to enable Network in the dock first). Once selected, the full extent of the cross section used in the computation is shown on the map and point info with further options appears. Point info lets you view the length and cross section profiles.

The point query tool has been used to select an edge of the terrain. The dialog in the lower right gives extra options for this stream network.

Cross section profiles

If you select cross section profile in point info, the cross section for that section of the network is displayed in the profile window. The bottom of the stream is marked with a green node.

Note that the cross section is sampled from an elevation, so if you want to change the cross section you can simply edit the elevation model using the standard workspace editing tools in a workspace.

Showing the cross section (gray line) in the profile window.

Length profiles

The length profile tool allows you to look at length profiles along a stretch in the stream network. Click on profile from node in point info, and then, click on another edge of the network downstream or upstream from the original point. After clicking on the second point, the length profile window will show. The length profile illustrates the elevation at the bottom of the stream and highlights the profiled section on the map. You can also enable any of the other profile-able layers in the dock and see that in the profile. For instance, if you enable an analysis, you will see the water level from the computation in the profile.

If you have selected a flow-based computation, you will also have the option to show the flow (the Q values) in the profile. Simply use the checkbox at the top of the profile window, as shown below.


Tracking subwatersheds along streams

Subwatersheds shows you how surface water is routed to the stream network. It shows the subwatershed of each node in the stream network. The subwatershed is defined as the area of the terrain that has a flow path to the edge between two consecutive nodes along the stream. Large flow lines are also highlighted to make it easier to get an overview of larger watersheds. When a water level in the stream is modeled, it is transferred onto the terrain by going backwards up through the flow paths leading into the stream. Thus, it is also useful when trying to understand the outcome of a particular flooding scenario. For more information about subwatersheds, please read computations.