DEM-Based Stream and Watershed Delineation - PowerPoint PPT Presentation

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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

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.

• 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 Delineation

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

• Streamlines are orthogonal to the elevation contour lines.

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.

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

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

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

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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

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Hydrologic Functions

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Slope:

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Hydrologic Functions

Function:Flow direction

Argument:DEM

Flow network

(not a grid)

Flow direction grid

DEM

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.

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

• The cell itself is not included.

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Hydrologic Functions

Process:Stream delineation

Argument:Flow accumulation

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Flow accumulation

(> 2 cells)

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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.

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Hydrologic Functions

Argument:Delineated streams

DEM

Delineated streams

• Stream links are assigned an identification number at random.

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

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Hydrologic Functions

Process:Outlets

DEM

Outlets

• Outlets are assigned the identification number of their link.

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.

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

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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 Delineation

• The drainage area of each watershed outlet is delineated.

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

• 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

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

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.