Computing Flow Vectors Based on a Grid Model

RockWorks | ModOps | Grid | Directional | Flow Table

This program reads a grid (surface) model and computes flow from high to low nodes. It stores the flow vector line segments in the RockWorks datasheet.

(This program uses the same "engine" as the 2D Flow Map and the 3D Flow Diagram. All of the modeling options are identical. The main difference is that the flow vectors are stored as line segments within the datasheet.)

! This program requires that a RockWorks surface model (.RwGrd file) already exist. See Importing Grid Files if you need to import an external surface model first.

Menu Options
Step-by-Step Summary

Menu Options

• Input Model
  • Input (Surface/Terrain) Grid Model: Click to browse for the name of the existing grid model for which the flow vectors are to be computed.
  • Fill Sinks: This option will fill any closed depressions such as sinkholes and sampling/gridding artifacts before generating the drainage net. The following example depicts drainage nets for the same grid model with and without enabling the Fill Sinks option.

  

• Flow Parameters
  • Smooth Flowlines: If checked, this option will remove the angularity from the drainage net segments. Unfortunately, too much smoothing may result in crossed flowlines.
    • Smoothing Passes: A setting of "1" means that the stream segment vertices will be averaged once (i.e. a given vertex is equal to the average of itself, the preceding vertex, and the next vertex). A setting of "2" will average the polyline vertices again, and so on. Typically, a setting of 1 will suffice.

    

    
    
  • Gradient Filter: This option is used to filter out stream polyline segments in order to highlight areas in which the stream gradient falls within a user specified range (e.g. very flat versus very steep).
    ! Note that stream gradients are typically very shallow (e.g. between zero and 0.2 degrees).
    • Minimum Gradient: This is the minimum gradient stream segment that will be represented within the drainage net. If you wish to include all low-gradient stream segments (e.g. effectively flat stream segments - ponds and lakes), set the minimum gradient to -1 (negative one). Conversely, if you want to include just areas that may contain rapids, set the minimum gradient to something greater than one degree.
    • Maximum Gradient: This is the maximum gradient stream segment that will be shown with the drainage net. If you wish to remove all high-gradient stream segments (e.g. waterfalls, rapids), set the maximum gradient to 2.0.

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  • Minimum Drainage Area Filter: Use this option to exclude portions of the flowlines that are negligible, thereby highlighting the major drainages.
    The Flow Vectors program generates flowlines for every node within the input grid. Starting at a given node, the program will move downhill until it can't move any farther (e.g. a map edge or a sinkhole). In the process, a record is kept of the number of upgradient nodes that have been traversed in order to reach each node along the flowpath. In this way, it is possible to keep track of the upgradient area that is draining into each point along a flowline. The minimum area filter setting can therefore be used to exclude portions of the flowlines that are negligible, thereby highlighting the major drainages.
    • Minimum Drainage Area: Use this setting to omit portions of flowlines that drain smaller regions. For example, let's say that; (1) your map units represent meters, (2) you've set the minimum drainage area to "1,000" and (3) area units to "Map Units" (see next item). In such a case, the flowlines would represent only the streams that drain upgradient regions greater than 1,000 square meters.
    • Units: This defines the units that are used to specify the minimum drainage area:
      • Grid Cells: When set to "Grid Cells", the minimum drainage area simply means that X cells must drain into a given stream vertex in order for that portion of the stream to be visible.
      • Map Units: Let's say that your map units represent kilometers. A setting of 100 would correspond to 100 square kilometers.
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  • Isolate Watershed: The following sub-options provide a means for showing a particular watershed (drainage basin).
    • Automatic: This option will identify the grid node that has the largest upgradient surface area that drains into that node. This node then serves as the starting point for the drainage net.

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    • Manual: This option allows you to specify the coordinates for a point within the project area. The program will then identify all of the upgradient nodes that drain into that node and draw the corresponding drainage net.

    

  • Filter by Stratigraphy: This option lets you selectively extract the stream segments that flow through a specified stratigraphic unit.
    • Stratigraphic Unit: Clicking on this option will present you with a list of the stratigraphic units that are defined within the Borehole Manager.
    • Surface Stratigraphy Grid: Enter the name for a grid model that contains the stratigraphic surface grid. A stratigraphic surface grid is essentially a geologic map (i.e. the nodes are defined by the intersections of the stratigraphic surfaces and the ground surface). These models are typically created by using the Borehole Manager / Stratigraphy / Surface Map program.
    • See a detailed example.
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  • Gradient Sign Convention: Most of the RockWorks programs employ a convention in inclinations below horizontal are negative. This option provides a means to override this convention if so desired.
    • Gradients < Horizontal = Negative: Gradients that are below the horizontal (downward pointing segments) will be expressed as negative numbers. Conversely, gradients that are greater than the horizontal (upward pointing segments) will use positive numbers.
    • Gradients < Horizontal = Positive: Gradients that are below the horizontal (downward pointing segments) will be expressed as positive numbers. Conversely, gradients that are greater than the horizontal (upward pointing segments) will use negative numbers.
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• Output Options
  • Decimal Places: Expand this heading to define the number of decimal places that are to be represented in the output vectors.
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Step-by-Step Summary

1. Be sure you have a RockWorks grid model (.RwGrd file) already created, for input into this program.
2. Select the Grid | Directional Analyses | Grid -> Flow Vector Table menu option.
3. Enter the requested menu settings, described above.
4. Click the Process button to continue.

The program will read the input grid model, compute slope and aspect for each node, and compute the vectors representing the up-to-down flow using the settings you've selected. The line segments for the flow vectors will be stored in the RockWorks datasheet. In addition, the program will record the segment's gradient and drainage area.

Here's an example of additional analyses that can be performed: creating a rose diagram based on stream directions and creating a histogram of stream gradients.

Accidental Science: The preceding examples were based on DEM data for the Colorado Plateau (see http://en.wikipedia.org/wiki/Colorado_Plateau). It is interesting to note that the drainage directions depicted within the rose diagram above show that the primary drainage directions are to the northwest and southwest. Note the total absence of drainage segments trending towards the east. This suggest that the portion of the Colorado Plateau that includes the Colorado and San Juan rivers is tilted to the west or the plateau was tilted first to the northwest and later on the the southwest (or vice-versa).

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