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Watershed Analysis and

A Look Ahead

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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

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Elevation

Fill Sinks

Flow Direction

Flow Accumulation

Flow Paths

Snap Pour Points

Watersheds

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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

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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

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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)

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Flow Direction Flow Accumulation

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Flow Accumulation Flow AccumulationBefore reclassification

After reclassification

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Flow Accumulation - symbology classificationReclassify symbology (not to layer), to show flow paths….no color to lowest flow accumulation category

No Color

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Snap Pour PointsSet 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

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Watershed Boundaries

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

How do I do my analysis?

ProductsRunoff 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 22

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?

DataTree 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

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

Build a Cartographic Model

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

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

Raster?

Raster/Vector Conversion

Conversion Tools

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

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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

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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.