Dem based stream and watershed delineation
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DEM-Based Stream and Watershed Delineation. Francisco Olivera, Ph.D., P.E. Texas A&M University Department of Civil Engineering. Watershed Delineation. Watershed delineation is the process of identifying the drainage area of a point or set of points.

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DEM-Based Stream and Watershed Delineation

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Dem based stream and watershed delineation

DEM-Based Stream and Watershed Delineation

Francisco Olivera, Ph.D., P.E.

Texas A&M University

Department of Civil Engineering


Watershed delineation

Watershed Delineation

  • Watershed delineation is the process of identifying the drainage area of a point or set of points.

  • For many years, Civil Engineers have been using USGS paper maps for watershed delineation.

  • The maps are in Universal Transverse Mercator (UTM) coordinates and in 1:24,000 scale.


Watershed delineation1

Watershed Delineation

  • Water flows in the direction of the terrain steepest downhill slope.

  • Streamlines are orthogonal to the elevation contour lines.


Watershed delineation2

Watershed Delineation

  • Streamlines do not flow towards drainage divides, and do not intersect them.

  • Drainage divides are found along the highest points of the terrain.


Dem based stream and watershed delineation

DEMs

  • Digital Elevation Models (DEMs) are grids of elevation.

  • DEMs store the same type of information contour lines do, but with a different data structure.

  • Watershed delineation can be based on DEMs rather than contour lines.


30 meter dems

30-Meter DEMs

  • For the United States, 30-meter DEMs have been developed by USGS.

  • 30-meter DEMs are consistent with 1:24,000 paper maps.


30 meter dems1

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30-Meter DEMs


30 meter and 3 dems

30-Meter and 3" DEMs

~ 90m

30m


Hydrologic functions

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Hydrologic Functions

  • The eight-direction pour point algorithm (D-8) assigns a flow direction code to each cell, based on the steepest downhill slope as defined by the DEM.

  • The flow direction code indicates the cell – out of the eight neighbor cells – towards which the water flows.

  • Flow direction cannot be defined for cells within a terrain depression. DEMs have to be filled before determining flow directions.

Flow direction codes


Dem based stream and watershed delineation

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Hydrologic Functions

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Slope:


Hydrologic functions1

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Hydrologic Functions

Function:Flow direction

Argument:DEM

Flow network

(not a grid)

Flow direction grid

DEM


Flow direction

Flow Direction

  • Water flows to one of its neighbor cells according to the direction of the steepest descent.

  • Flow direction takes one out eight possible values.


Hydrologic functions2

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Hydrologic Functions

Function:Flow accumulation

Argument:Flow direction

Flow network

(not a grid)

Flow accumulation grid

DEM


Flow accumulation

Flow Accumulation

  • Flow accumulation is a measure of the drainage area in units of grid cells.

  • The cell itself is not included.


Hydrologic functions3

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Hydrologic Functions

Process:Stream delineation

Argument:Flow accumulation

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Flow accumulation

(> 2 cells)

DEM


Stream network

Stream Network

  • All grid cells draining more than a user-defined threshold value (blue streams) are part of the stream network.

  • All grid cells located downstream of user-defined cells (red streams) are also part of the stream network.


Hydrologic functions4

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Hydrologic Functions

Function:Stream links

Argument:Delineated streams

DEM

Streams links

Delineated streams

  • Stream links are assigned an identification number at random.


Stream links

Stream links

  • Stream links are the segments of a stream channel connecting two successive junctions, a junction and an outlet, or a headwater and a junction.


Hydrologic functions5

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Hydrologic Functions

Process:Outlets

Argument:Stream links

DEM

Outlets

Stream links

  • Outlets are assigned the identification number of their link.


Watershed outlets

Watershed Outlets

  • The most downstream cells of the stream segments (yellow/brown cells) are watershed outlets.

  • User-defined points (red dots) are also watershed outlets.


Modified stream links

Modified Stream Links

  • User-defined outlets modify the stream links by subdividing the segment in which they are located into two segments.


Hydrologic functions6

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Hydrologic Functions

Function:Watershed

Argument:Flow direction and Outlets

DEM

Watersheds

Outlets

  • Watersheds are assigned the identification number of their outlet.


Watershed delineation3

Watershed Delineation

  • The drainage area of each watershed outlet is delineated.


Raster to vector conversion

Raster to Vector Conversion

  • The grids of stream links and watersheds are vectorized to obtain stream line and watershed polygon datasets.

  • Both datasets include a field Grid-Code in their attribute tables, which stores the stream link and watershed identification number.

  • Since the identification number of the streams and their corresponding watershed is the same, the attribute tables can be related.


Dissolving spurious polygons

Dissolving Spurious Polygons

  • Cells connected to the main watershed polygon through a corner are defined as a separate polygon (spurious polygon).

  • These polygons are dissolved into the main polygon.


Merging watersheds

Merging Watersheds

  • Adjacent watershed polygons can be merged into a single polygon, if they share their outlet or one flows into the other.


Merging watersheds1

Merging Watersheds

  • The attribute tables are updated so that the watershed code (WshCode) of the streams is modified and the area of the new polygon is calculated.


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