Regulated Tileline & Ditch flowshed delineation: a comparison study

1. Regulated Tileline & Ditch flowshed delineation: a comparison study Larry Theller, Agricultural and Biological Engineering Dept., Purdue University Greg Lake, Howard County Surveyor’s Office

2. Goals: • Produce tile/ditch flowshed map for regulated drains • Compare ease-of-use and utility between traditional ArcINFO and new ArcHydro methodology • Produce and deliver training aids to county staff

3. ArcINFO • ESRI product. • Traditional script-based watershed delineation process. • User interface is difficult and obscure

4. ArcHydro • Interface and data model for ArcGIS 9 • Interface portion is used in this project • Data model is not used in this project – yet. • Interface is organized set of menus that step user through each phase of process in sequence.

5. ArcHydro Tools organizes the watershed tools into a sequential menu

6. Process: • Prepare water features (regulated and unregulated) • Create DEM • Prepare hydrologically correct DEM: • Water should flow through true path in DEM • All water should drain from map • Prepared DEM must extend beyond area of interest • Calculate flow direction and accumulation layers • Calculate drainage lines over the DEM • Use drainage and flow direction to create “catchments” • Subdivide “catchments” to match regulated portions

7. Existing water features, based on USGS layers, were not accurate enough Stream features not in bottom of valley

8. Preparation of water features includes digitizing of tiles, ditches, unregulated waters and “drainage points” where regulation changes.

9. Tiles and ditches were digitized by georeferencing scanned maps over aerial photos, and reconciling differences

10. Creation of DEM used model built for previous aerial photo projects

11. Traditional method to create hydrologically accurate DEM is to “burn” a slot into the DEM following the actual mapped flow of water

12. Major difference in two methods is this: ArcHydro uses a “valley” not a “slot”

13. Traditional ArcINFO method

14. ArcHydro default method

18. Catchments adjusted to legal drains

19. Problems:

20. Problems: Places where the DEM does not truly describe reality can require edits.

21. 4 Lane Highway Problems: Model says this water drains under highway High ground May require an onsite visit to check

22. Problems: Accuracy suffers if the mapped pathway of water is across the side of a hill rather than the bottom of the valley

23. Problems: Check drainage path versus actual tiles

24. Training • Excellent tutorial from UT • http://www.crwr.utexas.edu/gis/gishydro04/index.htm see “exercises” • Flash Movie based training from Purdue • http://pasture.ecn.purdue.edu/~caagis/PurdueGIS/video/howard/index • Book: Arc Hydro: GIS for Water Water Resources edited by Dr. Maidment.

25. Conclusions • ArcHydro adds excellent user direction to follow a complicated sequence. • Drainage patterns are represented in the DEM in a more realistic fashion. • For the future, a network-based approach: http://www.crwr.utexas.edu/gis/gishydro04/index.htm