Hydrology tools
Effective management of drainage at any project site holds significant importance. The hydrology tools assist users in comprehensively addressing key considerations associated with drainage. These tools, grouped under the umbrella of hydrology, comprise five distinct functionalities:
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Watersheds: computes independent flow catchment areas of the TIN model and creates corresponding features.
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Drainage to a point or area: given a point, a feature or a list of features, calculates the area that drains towards each selected point or feature.
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Streams and ponds: computes the expected location of streams in the TIN model, as well as locations where water would pool, represented as ponds.
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Flow paths from point or area: given a point, a feature or a list of features, calculates all outward flow paths from each selected point or feature.
Regular grids
Within the Terrain module, while most surface functions treat the terrain as a Triangulated Irregular Network (TIN), the hydrology tools take a different approach. Instead of dealing directly with triangles, these tools view the terrain as a grid of rectangular cells. The level of detail in this grid depends on the cell size chosen: larger cells mean less detailed representation, while smaller cells offer finer detail.
Below are visual representations illustrating this difference, showing the terrain as a grid with a 1m. cell size, a 10 m. cell size, and as a TIN:
Notice how much more detail is visible in the fine grid as opposed to the course grid. While the fine grid may seem like the preferred choice for hydrology analysis, using the finer grid increases computation time. Furthermore, if the grid spacing is higher resolution than the original surface data, the quality of the solution will not be improved beyond the limits of the original data, but computation time will be dramatically increased.
Cell size
The cell size used for generating the regular grid.
Hydraulic enforcement
Water typically flows from an area of high elevation to an adjacent area of lowest elevation. This behaviour can be overridden by adding hydraulically enforced features.
When a feature is marked with the property "Hydro-Enforcement", all hydrology calculations will force water to flow through it, from its starting point to its endpoint. This is done independently of the terrain around it. This can be useful for representing culverts or channels.
Depression filling
If we were to run the hydrology tools without filling depressions, the flow calculations would terminate at every low point regardless of the depressions size or validity. In practice there are two common conditions that make filling depressions essential:
1. Actual depressions that will fill with water and once full, flow will continue from their outlet,
2. Artificial depressions that are the product of imperfections from surface modeling approximations
Filling these imperfections is often referred to as hydro-flattening. To better illustrate the concept, consider the surface and stream feature illustrated below:
In the figure above, the flow (blue polyline) terminating (left image) is not desirable. In this example, the low point where flow terminates without hydro-flattening is a product of undesirable triangles in our TIN. Rather than identifying and fixing every undesirable triangle that may interrupt flow, we can automatically fill these depressions provided the depression is less than an assigned depth capacity. With depression fill depths assigned, the resulting behaviour provides results similar to the image on the right.
Depression fill depth
By setting this parameter, all depressions deeper than this value will be considered ponds, i.e., outlets towards which flow is directed, while shallower depressions will be filled automatically, i.e., flattened. Higher values of this parameter will lead to fewer ponds, and more filled depressions.
Other general parameters
Target TIN surface
This option allows choosing a file or select the current terrain TIN as a target surface.
Allow flow to outside
If checked, the operations will treat the outer boundaries of your TIN model as outlets, and therefore flow will be allowed to flow outside it. Otherwise, if unchecked, the operations will consider the TIN boundary as a barrier, and will not allow flow outside it. In practice, the latter will result in the formation of ponds along the TIN boundary, where water will collect, and which will serve as outlets.
Remove existing features
If checked, existing features such as watersheds, drainage areas, or streams and ponds will be deleted prior to running each operation.
General recommendation
It is recommended to use the same settings such as cell size and depression depth on all hydrology tools within a project, to help ensure the results complement one another. Otherwise, drainage areas and watersheds may conflict with streams generated using other settings.
