Analysis – Depression-Free Flow

This analysis shows how water flows without taking the active depressions into account. In other words, all the depressions are treated as if they have been filled. Therefore, no water can be stored in the depressions, and the water will keep flowing until it enters the sea. This type of analysis is useful when creating an overview of surface runoff in a larger area, like e.g. an entire neighbourhood or a national reserve. With Depression-free Flow, you are able to view the flow paths equivalent to the maximum topographical watershed. Select Flow Accumulation or Watersheds to use the analysis.

Flow Accumulation

Flow Accumulation illustrates the directions of flow paths and show how the surface water accumulates into the sea. Contrary to flow accumulations in the flash flood analysis, this flow accumulation always visualizes the flow paths to the sea and does not take any retention in depressions into account. The direction of the flow paths can viewed by zooming in on the map (see picture below).

Slider: Adjusting the flow detail:

The flow detail slider enables you to adjust the minimum size of the flow paths. This can be beneficial if you want to eliminate very small flows, and focus on the more significant flow paths.

Point query: Querying the analysis at a point

Point query will tell you the exact watershed area for a given point on the map.

Profile: Seeing the profile of a flow path

The profile tool does not interact directly with the depression-free flow analysis, but it can be helpful to investigate the elevation across a flow path. Enable an elevation model for this to work.

Watershed tool: See the watershed of the flow paths

When using the watershed tool, you are able to view from where the surface runoff gathers in the flow path.

1. Choose to see the watershed analysis Depression-free flow
2. Select the flow path of which you want to see the flow path.
   

Now, you can see how the paths connected to the chosen path are all part of the same watershed.

Download

You can download flow paths as line segments in the same resolution as the elevation model. This makes the data set relatively big. In order to filter which data to download, use the slider to determine the minimum size of the flow paths you want. See how to use the download tool in toolbar section.

Watersheds

Watersheds based on a depression-free flow analysis corresponds to the maximum topographic watershed of a river mouth or surface flow path leading to the ocean.

What are watersheds

The watershed analysis visualizes the topographic watersheds all the way to an outlet in the sea. Each colored sections illustrates a different watershed. The colors are randomly selected, and their only function is to separate the watersheds.

NOTE: if you want to know more about watersheds related to specific rain events where depressions are not assumed to be filled, see watershed (flash flood mapping).

How to use Watershed analysis

Enable ‘watersheds’ in the analysis section of the dock under Depression-free flow.

Download

Watersheds can be downloaded as a raster where each cell contains the ID of its watersheds. It can also be downloaded as a vector where the watersheds will be represented as polygons.