Hydrologic analysis
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Hydrologic Analysis. Francisco Olivera, Ph.D., P.E. Srikanth Koka Department of Civil Engineering Texas A&M University. Watershed Delineation. Watershed delineation is the process of identifying the drainage area of a point.

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Hydrologic Analysis

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Hydrologic analysis

Hydrologic Analysis

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

Srikanth Koka

Department of Civil Engineering

Texas A&M University


Watershed delineation

Watershed Delineation

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

  • 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 (blue arrows) are orthogonal to the contour lines.

  • Drainage divides (red line) are found along the highest points of the terrain.

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


Hydrologic analysis

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

720

720

740

720

700

680

720

700

740

680

30-Meter DEMs


30 meter dems2

700

680

30-Meter DEMs


30 meter and 3 dems

30-Meter and 3" DEMs

~ 90m

30m


Hydrologic functions

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


Hydrologic analysis

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Hydrologic Functions

1

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Slope:


Hydrologic functions1

Hydrologic Functions

Function:Flow direction

Argument:DEM

2

2

2

4

4

78

72

69

71

58

2

2

2

4

4

74

67

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49

46

1

1

2

4

8

69

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44

37

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128

128

1

2

4

64

58

55

22

31

128

128

1

1

4

68

61

47

21

16

Flow network

(not a grid)

Flow direction grid

DEM


Hydrologic analysis

DEM


Hydrologic analysis

Flow Direction


Hydrologic functions2

Hydrologic Functions

Function:Flow accumulation

Argument:Flow direction

0

0

0

0

0

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0

1

1

2

1

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0

3

8

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2

69

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0

1

1

20

0

64

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22

31

0

0

0

1

24

68

61

47

21

16

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

0

0

0

0

0

0

1

1

2

1

0

3

8

5

2

0

1

1

20

0

0

0

0

1

24

Hydrologic Functions

Process:Stream delineation

Argument:Flow accumulation

78

72

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64

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31

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21

16

Flow accumulation

(> 2 cells)

DEM


Hydrologic analysis

Stream Network

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

  • Red streams correspond to a lower threshold than the blue streams.


Hydrologic functions4

Hydrologic Functions

Function:Stream links

Argument:Delineated streams

78

72

69

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31

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Streams links

DEM

Delineated streams

  • Stream links are assigned an identification number randomly.


Hydrologic analysis

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

Hydrologic Functions

Process:Outlets

Argument:Stream links

78

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Streams links

DEM

Outlets

  • Outlets are assigned the identification number of their link.


Hydrologic analysis

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.


Hydrologic analysis

Modified Stream Links

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


Hydrologic functions6

Hydrologic Functions

Function:Watershed

Argument:Flow direction and Outlets

78

72

69

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37

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64

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21

16

DEM

Outlets

Watersheds

  • Watersheds are assigned the identification number of their outlet.


Hydrologic analysis

Watershed Delineation

  • The drainage area of each watershed outlet is delineated.


Hydrologic analysis

Raster to Vector Conversion

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

  • Both feature classes 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.


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.


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