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Digital Elevation Model Based Watershed and Stream Network Delineation

Digital Elevation Model Based Watershed and Stream Network Delineation. Reading. How to use. Understanding. Key Spatial Analysis Concepts from Exercise 3 . Contours and Hillshade to visualize topography. Zonal Average of Raster over Subwatershed. Join.

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Digital Elevation Model Based Watershed and Stream Network Delineation

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  1. Digital Elevation Model Based Watershed and Stream Network Delineation Reading How to use Understanding

  2. Key Spatial Analysis Concepts from Exercise 3 • Contours and Hillshade to visualize topography

  3. Zonal Average of Raster over Subwatershed Join

  4. Subwatershed Precipitation by Thiessen Polygons • Thiessen Polygons • Intersect with Subwatersheds • Evaluate A*P Product • Summarize by subwatershed

  5. What Is Involved? Add Field

  6. Field Calculator to Multiply A and P

  7. Summarize (Sum) for unique subwatersheds (by HydroID)

  8. Now use field calculator to take the ratio

  9. Subwatershed Precipitation by Interpolation • Kriging (on Precip field) • Zonal Statistics (Mean) • Join • Export

  10. Runoff Coefficients • Interpolated precip for each subwatershed • Convert to volume, P • Sum over upstream subwatersheds in Excel • Runoff volume, Q • Ratio of Q/P

  11. The Terrain Flow Information Model • Pit removal • Flow direction field derivation • Flow Accumulation • Channels and Watersheds • Raster to Vector Connection

  12. The terrain flow information model for deriving channels, watersheds, and flow related terrain information. Pit Removal (Filling) Raw DEM Channels, Watersheds, Flow Related Terrain Information Flow Field Watersheds are the most basic hydrologic landscape elements

  13. DEM Elevations 720 720 Contours 740 720 700 680 740 720 700 680

  14. The Pit Removal Problem • DEM creation results in artificial pits in the landscape • A pit is a set of one or more cells which has no downstream cells around it • Unless these pits are removed they become sinks and isolate portions of the watershed • Pit removal is first thing done with a DEM

  15. Pit Filling Increase elevation to the pour point elevation until the pit drains to a neighbor

  16. Pit Filling Original DEM Pits Filled Pour Points Pits The lowest grid cell adjacent to a pit Grid cells or zones completely surrounded by higher terrain

  17. 80 80 74 74 63 63 69 69 67 67 56 56 60 60 52 52 48 48 Hydrologic Slope - Direction of Steepest Descent 30 30 Slope:

  18. 2 2 4 4 8 1 2 4 8 4 32 64 128 4 1 2 4 8 16 1 2 4 4 4 4 8 4 2 1 2 1 4 16 Eight Direction (D8) Flow Model

  19. 32 64 128 16 1 8 4 2 Flow Direction Grid

  20. Grid Network

  21. Flow Accumulation Grid. Area draining in to a grid cell 0 0 0 0 0 0 0 0 0 0 0 2 2 2 0 2 2 0 0 2 0 1 0 0 10 0 0 1 0 10 1 0 0 0 14 0 1 0 0 14 1 0 4 1 19 4 1 0 1 19 Link to Grid calculator ArcHydro Page 72

  22. Stream Network for 10 cell Threshold Drainage Area Flow Accumulation > 10 Cell Threshold 0 0 0 0 0 0 0 0 0 0 2 2 0 0 2 0 2 2 2 0 0 0 1 0 0 1 10 0 0 10 0 1 0 0 1 0 0 14 0 14 4 1 0 1 1 0 4 1 19 19

  23. 1 1 1 1 1 1 3 3 3 1 1 2 1 1 11 2 1 1 1 15 2 1 5 2 20 TauDEM contributing area convention. 1 1 1 1 1 3 3 1 1 3 1 1 2 1 11 1 2 1 1 15 5 2 1 2 25 The area draining each grid cell includes the grid cell itself.

  24. Streams with 200 cell Threshold(>18 hectares or 13.5 acres drainage area)

  25. Watershed Draining to Outlet

  26. Watershed andDrainage PathsDelineated from 30m DEM Automated method is more consistent than hand delineation

  27. Stream Segments 201 172 202 203 206 204 Each link has a unique identifying number 209 ArcHydro Page 74

  28. Vectorized Streams Linked Using Grid Code to Cell Equivalents Vector Streams Grid Streams ArcHydro Page 75

  29. DrainageLines are drawn through the centers of cells on the stream links. DrainagePoints are located at the centers of the outlet cells of the catchments ArcHydro Page 75

  30. Catchments • For every stream segment, there is a corresponding catchment • Catchments are a tessellation of the landscape through a set of physical rules

  31. Catchment GridID DEM GridCode 4 3 5 Vector Polygons Raster Zones Raster Zones and Vector Polygons One to one connection

  32. Catchments, DrainageLines and DrainagePoints of the San Marcos basin ArcHydro Page 75

  33. Catchment, Watershed, Subwatershed. Subwatersheds Catchments Watershed Watershed outlet points may lie within the interior of a catchment, e.g. at a USGS stream-gaging site. ArcHydro Page 76

  34. Advanced Considerations • Using vector stream information (DEM reconditioning) • Enhanced pit removal • Channelization threshold selection • Computational considerations

  35. “Burning In” the Streams  Take a mapped stream network and a DEM Make a grid of the streams Raise the off-stream DEM cells by an arbitrary elevation increment  Produces "burned in" DEM streams = mapped streams = +

  36. AGREE Elevation Grid Modification Methodology – DEM Reconditioning

  37. Carving Lower elevation of neighbor along a predefined drainage path until the pit drains to the outlet point

  38. Carving Original DEM Carved DEM Carve outlets Pits

  39. Filling Minimizing Alterations Carving

  40. Minimizing DEM Alterations Original DEM Optimally adjusted Carved Pits Filled

  41. AREA 2 3 AREA 1 12 How to decide on stream delineation threshold ? Why is it important?

  42. Delineation of Channel Networks and Catchments 500 cell theshold 1000 cell theshold

  43. Hydrologic processes are different on hillslopes and in channels. It is important to recognize this and account for this in models. Drainage area can be concentrated or dispersed (specific catchment area) representing concentrated or dispersed flow.

  44. Examples of differently textured topography Badlands in Death Valley.from Easterbrook, 1993, p 140. Coos Bay, Oregon Coast Range. from W. E. Dietrich

  45. Gently Sloping Convex Landscape From W. E. Dietrich

  46. Topographic Texture and Drainage Density Same scale, 20 m contour interval Driftwood, PA Sunland, CA

  47. “landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape.” (Montgomery and Dietrich, 1992, Science, vol. 255 p. 826.) Lets look at some geomorphology. • Drainage Density • Horton’s Laws • Slope – Area scaling • Stream Drops Suggestion:One contributing area threshold does not fit all watersheds.

  48. Drainage Density • Dd = L/A • Hillslope length  1/2Dd B B Hillslope length = B A = 2B L Dd = L/A = 1/2B  B= 1/2Dd L

  49. Drainage Density for Different Support Area Thresholds EPA Reach Files 100 grid cell threshold 1000 grid cell threshold

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