973 2016 11 18 Hydrologically Conditioning DEMs.pdf

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Hydrologically Conditioning

DEMs for Urban Stormwater

John Miller, Water Resources Planner

Matt Breen, P.E., CFM, Water Resources Engineer

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Geospatial Data for Water Resources

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Topography Hydrography

Refers to relief or terrain and the 3-dimensional quality of the earth’s surface

Hydrography is the map

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Geospatial Data for Water Resources

Topography Hydrography

Elevation Derivatives:

Slope

Upslope area

Flow length

Shaded Relief

Surface water features:

Rivers and streams

Lakes and ponds

Watersheds

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Geospatial Data for Water Resources

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Hydrologically Conditioning a DEM

Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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• 3D representation of a terrain’s surface

oBare earth, void of vegetation and manmade features

oRaster data

Digital Elevation Model (DEM)

30 40 50 30

27 25 40 25

15 20 30 20

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Where to get a DEM

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DEM Resolution

10 3-meter 6-meter 10-meter

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

1980’s 90m

1990’s 30m

2000’s 10-20m

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Elevation Data: National Elevation Dataset (NED)

• Primary elevation data product of the USGS

• Publicly available data

• Industry standard for environmental applications

• 3 resolutions:

1. 3 meters (1/9 arc-second) 2. 10 meters (1/3 arc-second) 3. 30 meters (1 arc-second)

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The State of Maryland LiDAR initiative provides high-resolution statewide elevation data.

Elevation products offered include:

 1 – meter DEM

 2 – meter DEM

Elevation derivatives offered:

 Aspect

 Shaded relief

 Hillshade

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Topography via ArcGIS Online (AGOL)

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Topography Hydrography

Hydrography is the map

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Hydrologically Conditioning a DEM

Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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National Hydrography Dataset (NHD) High Resolution

• Nationwide, contiguous geometric network

• Packaged in a geodatabase with 6 layers, relevant ones include:

Flowlines - Industry standard

stream network with unique reaches

Waterbody – lakes/ponds

Point Event – gaging station, dam point, and live links to USGS gaging stations

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Why topography and hydrography are not enough

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To Surface Waters

City of Mississauga, Ontario

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Existing

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Lacks

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Parallel

Streams

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Common Issues with Stormwater

Networks

?

Hydrologic Loops

Multiple

potential outlets Parallel Streams

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Common Issues with Stormwater

Networks

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Hydrologic Loops

Multiple

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Hydrologically

Connected

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Building a Contiguous Hydrologic Network

Connect stormwater infrastructure to the stream network.

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Contiguous Urban Hydrologic Network

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Hydrologic

connection between manmade drainage and jurisdictional waters.

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Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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To Surface Waters

City of Mississauga, Ontario

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Stream Burning

Burn

Depth

Original stream depth

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Tips Before Getting Started

• These should all match:

Cell size

Projection

Processing Extent

Snap Raster

• Can be changed in Environment Settings

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Burning Hydrologic Network Workflow

Merge Tool

• Merge the stormwater network with the NHD flowlines.

Polyline to Raster Tool

• Add a new field with a value for each feature. Convert the merged hydrologic network to a raster.

Reclassify Tool

• Reclassify the value of hydrologic network cells to -1000 and NoData cells to 0.

Raster Calculator Tool

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Hydrologic Network Burning Process

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Burn Hydrologic Network

Merge Tool

Reclassify Tool

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Hydrologic Network Burning Process

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Merge Tool

Reclassify Tool

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Partitioning the

Landscape with the

Reclassify Tool

Hydrologic

Network =

-1000

Non-hydrologic

network cells = 0

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Hydrologic Network Burning Process

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Merge Tool

Reclassify Tool

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Hydrologically Conditioning a DEM

Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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What is a sink or pit?

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Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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Flow Direction

• Calculated from DEM

• ArcGIS uses D8 Algorithm

Grid slope is calculated as a focal operation (3 x 3 cell window)

Flow is routed to the steepest downslope neighboring cell

• Describes a network of flow path

DEM Elevation

Flow Direction Grid Symbolic Flow Direction Flow Direction Code

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Flow Direction

DEM Elevation

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Flow Direction

DEM Elevation

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Flow Direction

DEM Elevation

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Flow Direction

DEM Elevation

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Flow Direction

DEM Elevation

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Hydrologically Conditioning a DEM

Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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0 0 0 0 0 0 0 1 1 2 2 0 0 3 7 5 4 0 0 0 0 20 0 1 0 0 0 1 24 0 0 2 4 7 35 1

Flow Accumulation

• Relies on the flow direction raster to route flow across the DEM

• Flow direction grid describes a network of flow paths

• Each cell in the raster represents the number of upstream cells

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Upslope Accumulated Area (UAA)

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Upslope accumulated area (UAA)

Amount of area draining to a point on the landscape

Source: Brian McGlynn Duke University

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Advanced Flow Accumulation Methods

•

Flow accumulation grids

can be weighted according

to landscape variables:



Nonpoint source

pollution



Soil types

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Case Study Application: Riparian Buffers

•

Why do they matter?



Filter pollutants



Trap sediment



Dissipate stream energy



Provide wildlife habitat

Source: EPA

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• For each cell in the watershed, calculate the number of riparian zone cells between that cell and the stream.

• Non-riparian cells are weighted as 1, riparian cells weighted as 0.

• Numbers represent the count of buffer cells a source must pass

through to reach the stream.

Case Study Application: Riparian Buffers

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• For each cell in the watershed, calculate the number of riparian zone cells between that cell and the stream.

• Mean value of source cells:

6 ∗ 2 + 9 ∗ 1 + (1 ∗ 0)

( 6 + 9 + 1) = 1.31

• Higher the value, greater the buffering potential of watershed.

Case Study Application: Riparian Buffers

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Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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Snap Pour Point Tool

• Pour points are outlets for drainage areas.

• Pour points could be outfalls, stormwater facilities outlets, stream confluences, stream gauges, etc.

• Searches within a user-defined distance around the specified pour points for the cell of

highest accumulated flow.

• Converts vector data to raster.

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Hydrologically Conditioning a DEM

Obtain DEM Build Contiguous Hydrologic Network Burn Hydrologic Network Fill Sinks Flow Direction Flow Accumulation

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Watershed Tool

• A watershed is the area of land where all of the water that falls in it and

drains off of it goes to a common outlet as concentrated drainage.

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Watershed Tool

• Watersheds can be delineated from a DEM using flow direction.

• Output is a raster of watersheds,

which can be converted to polygons via the Raster to Polygon tool.

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Watershed Tool

• Other GIS layers can be overlayed with these polygons to determine drainage characteristics:

Impervious surfaces

Land use and land cover

Existing TMDLs or other regulations

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Urban “Watersheds”

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Hydrologic

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• Cooperative Effort between SHA and MDE

• SHA to use for bridge and highway design

• MDE has used as the basis for hydrologic updates to flood studies

• Optimal balance between preserving environmental quality and hydraulic performance of drainage structures

• Deterministic vs. Statistical

• Hydrologic DEMs allow for rapid processing of drainage areas, the input variable that most closely correlates with discharge

• In addition to DA value, the spatial catchment is delineated to allow for analysis of additional parameters

(limestone, forested, impervious, soils, slope)

• Scripting for batch mode

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Computing Peak Discharges

• Limitations

• Peak flow only

• Gage density/input parameter variability

• USGS Streamstats

• No gage weighting

• Lag in edition

• No customization (LiDAR, storm sewer, etc.)

• www.mdfloodmaps.com

• Discharges computed for 10-, 50-, 100-, and 500-year events

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Computing Peak Discharges

• GISHydro

• WinTR-20

• Calibration to plus one standard error of

prediction

• Rainfall-Runoff Modeling

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Summary

• Hydrologic Conditioning supports:

• Rapid watershed delineation • Pour points

• Outfalls

• Infrastructure (curb inlets, catch basins, BMPs)

• Incorporation of urban stormwater

features into drainage network/surface model

• Weighted Raster analyses

• Peak Discharge Computation • Continuity throughout the State

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Questions?

[email protected]

www.mdfloodmaps.com