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Watershed development and drainage assessments
This document summarizes how GIS data and software can be used to accurately assess drainage districts and generate watersheds. Key details include generating watersheds from contour, parcel and soils data layers, accounting for sinks and peaks in elevation data, attributing computer-generated watersheds to match existing administrative boundaries, and using tools like ArcGIS and Draincalc to calculate equitable assessments for individual parcels based on watershed membership and overlay analysis. The process allows non-GIS users like local government officials to benefit from automated watershed and assessment generation to improve efficiency and accuracy over previous hand-drawn methods.
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Watershed development and drainage assessments
- 1. GIS for DrainageAssessment Andrew Harrison, GISP Business Development Manager The Schneider Corporation (317) 826-7393 [email_address]
- 2. Overview Generating watershedfrom GIS data Use of calculation software to assess drainage districts with Parcel, Watershed & Soils Layers
- 3. Accuracy… Fancy softwaredoes not equal accurate data Additional data layers will only be as accurate as basemap Parcels must be maintained and up-to-date through a known date almost all GIS layers will refer back to ownership information (in local government)
- 4. Watersheds of thePast?
- 5. Software ESRI ArcGIS/ArcInfoSpatial Analyst Hydrology Modeling Extension
- 6. Contours – 2’Seamless
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- 13. Assumptions and Obstacles: 3 things to know about drainage (assumptions) Water runs downhill Water drains into streams Payday is Friday The boss is a jerk! Obstacles “ Sinks” and “Peaks” Culverts
- 14. “ Fill” Processfor Sinks and Peaks
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- 16. Zoomed Stream ElevationRaster
- 17. “ Burning” inthe Streams
- 18. Watershed Generation ProcessHydrology Modeling extension is used to “burn in streams” and build watersheds using raster images created from contour data Computer generated watersheds are merged/attributed to match County’s existing tiered administrative watershed system
- 19. Threshold = 500Watersheds
- 20. Threshold = 5000Watersheds
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- 23. Tiered Watersheds SubWatersheds
- 24. Tiered Watersheds Subsof Sub Watersheds
- 25. Tiered Watersheds Subsof Subs of Sub Watersheds
- 26. Tiered Watersheds Subsof Subs of Subs of Subs Watersheds
- 27. What can Ido with this watershed data? Provide greater accuracy to what was originally a poor data source Hand drawn maps On-screen digitized More equitable assessment for drainage Geogear draincalc
- 28. Draincalc Overview Atool to utilize watershed data Allows drainage officials to quickly, accurately create the assessment Utilizes the geoprocessing functionality of ArcGIS Meant to allow non-GIS people to benefit from GIS
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- 34. Range of WatershedAssessment Options Flat rate per benefited acre Percent of total (benefited acres) Setting minimum assessment amount(s) Additional assessment options Soils Parcel class/landuse Distance from hydrological features Any customization you come up with
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- 37. Proportion of total- values
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- 39. Apply Assessment RuleSelections
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- 43. Summary How isthis useful in the local government office? Using GIS we were able to work from only contours and parcels Generate watersheds Calculate assessments for those watersheds Print hearing notices Mail merge to labels (by watershed)
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- 46. Benefits What benefitsare gained from using GIS for these tasks? More accurate watersheds More equitable assessment Speed and efficiency
- 47. Thank you! Presenter: Andrew Harrison, GISP Business Development Manager The Schneider Corporation (317) 826-7393 [email_address]
Editor's Notes
- #4 Fancy software does not equal accurate data – main point (your software doesn’t dictate your accuracy) Additional data layers will only be as accurate as basemap – layers built on top of orthos, plss (in this part of the country) and parcels will only be as good as the basemap
- #5 This county held an attribute in their parcel database that showed which watershed it was (mostly) in In reality they probably looked something like his.
- #6 What is needed? We utilized ArcGIS, spatial analyst and a hydrology modeling extension downloaded from ESRI scripts website.
- #7 2 foot contours used as the base data for elevation
- #8 Linear stream features were tabulated from local, regional and state sources
- #9 A raster was created from the two foot contours. Lighter areas are higher elevation QC process: local knowledge (water in this area flows to the NW) There’s a break in the SE.
- #10 Flow direction is created from elevation raster, tells that direction that water will flow. Uses 20’ cells. The GIS calculates what direction the majority of water will from the cell. Water accumulates at the center where it all flows NW (noted by lines of pixels side by side)
- #11 This raster shows where water will accumulate as it flows, lighter areas are “deeper”
- #12 Raster stream layers is made from the vector streams layer. This is done so that the rasters can be calculated against each other. Lighter is higher elevation
- #13 Rasters match up with each other, trends (NW flow and SE break) evident in each.
- #14 Joke from an Indiana surveyor, what he heard his first day on the job
- #15 Image from ArcGIS help – it’s good for something! GIS is only as intelligent as you make it. In calculating the watershed little inconsistencies in the elevation data (peaks & sinks) would create an obstacle and we use this process to “fill” them. This process forces water to flow “somewhere” off the map (an assumption)
- #16 Another obstacle, “culverts”. The source contours show this at a an elevation consistent with the road even though a culvert was put in place to allow for water flow. We must account for this culvert.
- #17 How do we take care of that? Look at the raster value for this elevation (892 feet).
- #18 We can “burn in the stream” or force water to flow to the stream by calculating an exaggerated amount (negative 1000 feet). Not “real” but gets the job done.
- #20 Next we tell the extension a threshold for how many watersheds we want, in this case 500.
- #21 We can up that to 5000, which uses the same model to define more watersheds (in effect sub-watersheds), ironically it takes less time to create more. So its easier to start with a big number and scale back.
- #22 Next we matched the watersheds to their administrative watersheds (remember these)
- #23 Starts to take shape, administrative/computer generated watersheds
- #24 Sub watersheds – they recognize some legally but not all from the 5000
- #25 And more…
- #26 And more…
- #27 Yep, more…
- #36 Shows all watersheds in the county. Inside each watershed you can create one or more “projects” that you may assess for.
- #37 Opening a project, you can define how it will be assessed. In this case, it was a reconstruction of the a ditch. The allocation mode chosen was “proportion of total” so it will figure out the number of benefited acres for each property owned in the watershed and what proportion of the total watershed acres that is, it will then calculate what the corresponding percentage of the project cost ($150,000). Other options would be to just assess based on a rate per acre ($3/acre). The last option is rate per point (which allows you to ramp the cost up and down based on criteria). All are based originally on benefited acres.
- #38 Each parcel in the watershed is represented. Data pulled from various sources (CAMA, GIS, etc). Watersheds acres is “benefited acres”. “ $ total” is what that land owner will be assessed. Dollar/$Acre goes back to “rate per acre” method. Totals to 13,169+ acres, $150,000 (see earlier slide)
- #39 Influence the assessment using “assessment methods” The combination of four parts: transform method , operation, target and aggregator. Transform method determines how the raw input value will be turned into a point value. options include: look up table (assign a weight in points to some or all the values in a data source field), fixed value (designates a value that will overrule any other factors), identity (the value in the data source is literally the point value), linear (applies a common assessing formula y=(M*x)+B where y is the output, x is the input, M is the scale and B is the offset) and inverse (applies an inverse assessing formula y=(M/X)+B). The operation setting establishes what will be done with the point value once it has been determined. Operation setting options include: sum (output of the transform method added to the running total of points), factor (output of the transform method multiplied by the running total of points), and minimum (running total of points can be no less than the output of the transform method ). The target setting determines whether the output of the transform method and operation is applied to the running point total (score) or the actual dollar value (dollars) of the assessment. The aggregator setting determines how the rule will handle cases where multiple data values exist for single parcels (i.e. multiple soil types with different values are covered by one parcel). Aggregator options include: none (one value is expected, no aggregation needed), average (computes the straight average of all the scores), area weighted average (gives more weight to the point scores of larger areas), sum (adds all point values together), sum of acreages (adds up acreages and uses that sum as the point total).
- #40 Apply them here, use one, use many. Different weights can be established.
- #41 After the assessment is calculated you can utilize crystal reports to create reports