Watershed Analysis and A Look Ahead

Transcription

1 Watershed Analysis and A Look Ahead 1

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3 Specific Storm Flow to Grate What data do you need? Watershed boundaries for each storm sewer Net flow generated from each point across the landscape

4 Elevation Fill Sinks Flow Direction Flow Accumulation Flow Paths Snap Pour Points Watersheds 4

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7 Tools/steps: See ArcGIS help partially fill DEM to remove error sinks - there are real sinks, so don t overfill calculate flow direction, flow accumulation, flow lines, watersheds. Note you will have to move some of the grates/pourpoints to flow lines because of DEM/image mismatch subtract canopy interception from rainfall (raster calculator) calculate soil/surface absorption for walking areas (0 for impervious, from function for soils) weighted flow accumulation aggregation along connected flow lines repeat for different storms, surface conditions

8 Hydrologic Processing Fill (perhaps iterate); start with a small z-limit (0.15 m or so) Flow Direction Sink Flow Accumulation Threshold, to identify flow lines Review, modify pour points to move near flow lines Snap Pour Points Watershed Review, and iterate steps as needed 8

9 Fill Results After applying fill, subtract original from fill (raster calculator) display and recolor to identify filled areas (classified symbology, no color to 0, red to all others) 9

10 Flow Direction Flow Accumulation 10

11 Flow Accumulation Before reclassification Flow Accumulation After reclassification 11

12 Flow Accumulation - symbology classification Reclassify symbology (not to layer), to show flow paths.no color to lowest flow accumulation category No Color 12

13 Snap Pour Points Set small snap distance, 1 to 2 meters - if doesn t work, manually edit input pour point layer, and re-run snap pour points Creates new pour point layer, raster here 13

14 Watershed Boundaries Only worry about watersheds that drain to a sewer grate - ignore the large sinks 14

15 How do I do my analysis?

16 Products Runoff Estimate for Each Pour Point Volumes in cubic meters at each grate Watersheds for each grate 4295 Students, Table for 1, and 2-inch storms (convert to metric) 5295 Students, also add 1/4 and 4-inch storms to output tables (convert to metric) Modified Layers Under Flow Mitigation 1. New layers for each mitigated rainfall level: Canopy, impervious surface modification, green roofs, underground storage under each rain layer 2. Tables of accumulated flow to support mitigation Turn in both 1 and 2 2

17 Approach What data do you need (do you have it all)? What operations do you need (first compile a list, which will grow as you develop the process) What is the order of application? e.g., what is the sequence of operations, what operations, to which data, when?

18 Data Tree Canopy - get canopy outline from image, LiDAR data Surface absorption - combination of soils data and impervious surface from your walkable areas layer (0 absorption for roads, sidewalks) Grate locations and watersheds Buildings - some drain directly to stormsewer lines, some flow to adjacent surface. Modify surface absorption layer

19 Storm sewer grates Vegetation canopy Landcover Buildings Elevation Rainfall layer Others? Data

20 Build a Cartographic Model

21 Calculations in Raster, or Vector? Tool for calculations in Raster?

22 Calculations in Raster, or Vector? Tool for calculations in Raster? 22

23 Raster/Vector Conversion Conversion Tools

24 Weighted Flow Accumulation - sums a weight surface across a flow direction - our weights can be surplus water 24

25 Once Watersheds Accurate, Complete, Do a Weighted Flow Accumulation on Surplus Water Raster Calculator for Cell-based Surplus Water Rainfall minus Canopy Interception minus soil absorption Variable canopy interception (5295) or fixed (4295) Soil absorption a percentage based on texture class 25

26 Points to Remember Convert your criteria to general actions (e.g., distance, slope, density). Identify the layers you want to use as sources (generally one per criterion) Identify and test each operation you hope to use Draw a flowchart (road map) of your analysis Apply it, and react. 20