Exercise Objectives In this exercise you will prepare your lidar bare earth DEM for further analysis by channel busting “false dams” such as culverts, evaluating it for sinks, depressions and defects and fixing them using a few tools from the Arc Hydro extension. Prerequisites Arc Hydro (free with no license needed) - installed and activated in ArcMap - 9.3.1 ArcHydro1.4.zip - 10.0 ArcHydro2.0.zip - 10.1 ArcHydro2.0.zip - 10.2 ArcHydro2.0.zip **For Arc Hydro Extension download and install instructions click here.

Exercise 2:

Prepare Lidar Bare Earth DEM for Hydro Modeling

Exercise 2: Prepare Lidar Bare Earth DEM for Hydro Modeling

Exercise 2 Page 1

Contents 1. Lidar Bare Earth DEM Hydro-Enforcing. .................................................................................................... 2

2. Burn Culverts Into DEM............................................................................................................................. 4

3. Fill Sinks in CulvDEM ................................................................................................................................. 6

Exercise 2: Prepare Lidar Bare Earth DEM for Hydro Modeling

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1. Lidar Bare Earth DEM hydro-enforcing When you receive your data from the lidar vendor, it may or may not come as a hydro-enforced DEM. When performing any hydrological analyses, it is critical that the DEM surface be properly prepared prior to beginning work. You will begin by examining the DEM for defects (aka “false dams” and whether it has been hydro-enforced) and then perform some steps in Part 2 to fix defects you find.

a) Start ArcMap

1.If not already started, from the Start menu, navigate to All Programs > ArcGIS > ArcMap

10. Set Workspace

2.When running the Arc Hydro processes, a folder named “Layers” is created in your

Workspace. This folder contains the Grid outputs of your processes.

b) Add clipped Storrie Fire data

1.In ArcMap, click the Add Data button

2.Navigate to the Course Data folder (wherever you extracted the data to) and locate the

following datasets and add them:

Be_storrie_clip_int.img

Be_hillshade_clip.img

Culverts.shp

NF_FeatherRiver.shp

Note: The next series of processes will be run on the subset of the lidar bare

earth DEM.

3.If you have the Image Server activated, please access it and add the following: Imagery |

1 meter | Region 5 | California_NAIP_2010.

For help with adding the Image Server please refer to this link:

http://fsweb.geotraining.fs.fed.us/www/index.php?lessons_ID=996 click on

Quick Tip Document and find the section on Adding the Image Server).

166.2.126.235 is the image service “Name” that you need to add.

4.After installation, activate the Arc Hydro toolbar by right-clicking in any open space in

the ArcMap toolbar area at the top of the screen and selecting Arc Hydro tools from the

dropdown menu. Dock the toolbar in a location of your choosing or leave free standing.

Exercise 2: Prepare Lidar Bare Earth DEM for Hydro Modeling

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TERRAIN PREPROCESSING: Terrain preprocessing is one of the most important steps in data preparation for water resource analyses using Arc Hydro Tools. The role of terrain preprocessing is twofold:

1. Develop hydro-enforced (e.g. corrected) DEM and its derivatives, primarily the flow direction and flow

accumulation grids.

2. Develop a series of inter-related layers that improve the performance of Arc Hydro tools related to

watershed delineation and stream network design. You want to ask yourself this question before you do any DEM reconditioning or hydro-enforcing. If you have a Deranged network, please see Appendix 3 for additional steps you may want to consider with your own datasets.

c) Burn in Major Linear River from NHD – “Agree DEM”

In order to impose the flow through the large, polygon-like stream, we will use

the NHD version of the stream and “burn” it into our DEM.

1.From Arc Hydro: Terrain preprocessing > DEM manipulation > DEM reconditioning.

2.Select be_storrie_clip_int.img as the DEM

3.Select stream layer to burn in (e.g. North Fork Feather River)

4.Stream buffers: set to 50 cells and drop/raise value of 10 and 50.

5.Output DEM:AgreeDEM

Exercise 2: Prepare Lidar Bare Earth DEM for Hydro Modeling

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NOTE: Change the Symbology of the AgreeDEM to Calculate Statistics “From Current Display Extent” and then zoom in tight to the NF of the Feather River; you will see the reconditioning that has taken place on the DEM.

d) Create/obtain a point shapefile of all culverts and drainage structures

Use GIS data, road logs, or utility database, etc. to determine culvert locations. If

you can locate a database with the point locations, that is preferable.

Optionally, create a point file of these locations, being as precise with their

placement as you can. (This may require field time, using a GPS to record the

culvert locations).

1. Open the Attribute Table of the Culverts.shp

2.Create an “ELEV” field and set the values to an elevation that is lower than the lowest

elevation in your DEM.

3.Table Options > Add Field

4.Name it “ELEV” and make it a Short Integer

5.Right click the new ELEV column header and choose Field Calculator.

6.In the expression box, type 0 and click OK. All fields should be populated with the value

0. Close the Attribute Table.

2. Burn Culverts Into DEM a) Convert culvert points to a raster ESRI Grid.

1. From ArcToolbox: Conversion Tools > To Raster > Point to Raster

2. Choose ELEV field as value

3. Name it Culv_Ras (be sure to leave the file extension “blank” so that an ESRI Grid will be

created).

4. Set cell size to match AgreeDEM (e.g. 1 m)

b) Enlarge raster culverts to be able to cover roads (Spatial Analyst extension must be activated)

1. From ArcToolbox: Spatial Analyst > Neighborhood > Focal Statistics

Exercise 2: Prepare Lidar Bare Earth DEM for Hydro Modeling

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2. Name it CulvRasFocal (be sure to leave the file extension “blank” so that an ESRI Grid

will be created).

3. Select rectangle as shape; input height and width (e.g. 30 by 30)

NOTE: This step will likely take some trial and error to get the right size

neighborhood by which to “grow” the culverts by to ensure they will

cross the road feature. See graphics below: the one on the left used 10

by 10 Focal neighborhood and the one on the right used a 30 by 30. The

10 by 10 neighborhood was not sufficient in crossing the road.

4. Calculate MAXIMUM (creates integer dataset, not floating).

c) Moasic the two raster datasets to “merge” them together (i.e. culvert dataset and AgreeDEM)>

1. From Arctoolbox: Data Management Tools > Raster > Raster Dataset > Mosaic to New Raster

2. Add CulvRasFocal and AgreeDEM

3. Pixel type for dataset is 32 bit Signed

4. Mosaic method needs to be set to allow for the culvert raster to get priority in assigning

10 by 10 Focal Neighborhood

30 by 30 Focal Neighborhood

Exercise 2: Prepare Lidar Bare Earth DEM for Hydro Modeling

Exercise 2 Page 6

elevation to new raster cells (e.g. CulvRasFocal should be on the top of the table of contents and

has the minimum value, so you could choose FIRST as the Mosaic operator so the first raster

(CulvRasFocal) will be used).

5. Create new raster dataset: CulvDEM.

NOTE: The “culverts” have been burned into the DEM to allow for

proper flow across the roads. The image on the right is a zoomed in

version with a transparency (30%) on the DEM and the NAIP 2010

imagery in the background.

6. Once the new raster is created zoom in to the newly burned in culverts and explore the

values at the culvert locations and around it. Your culvert locations should be lower than

all the cells around it. We just created sinks! Next we need to fill them. This process just

ensured we didn’t have a local maximum at the culvert locations.

3. Fill Sinks in CulvDEM a) Create a Sinks raster dataset.

1. From Arc Hydro: Terrain Preprocessing > DEM Manipulation > Fill Sinks

2. DEM is the CulvDEM

3. Deranged Polygon is Null

4. Hydro DEM: name it Fil

5. Fill Method: Fill All

b) Inspect your Fill raster

1. Add the Fil raster to your ArcMap document-click the Add Data button and navigate to

the location of your raster outputs.

2. Zoom into the location of some of the culverts (e.g. scale of 1:3,000).

3. Using the Identify tool inspect the values in and around the locations of the burned in

culverts.

You should notice the sinks and culvert values have been “raised” to the

minimal values of nearby cells to allow for preferred flow through those

sinks. For example, the value at the yellow star in the Fill Sinks raster is

606, and in the CulvDEM it was 0.

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Note: If you are working on your own dataset, and don’t have a culvert shapefile, you could first run the stream creation process (see next exercise) and if there are parts of the terrain where the streams are running along the road or aren’t following the true stream location, this may be a location to place a “culvert”. You could create a new shapefile and either digitize a line or points in the location of the needed culvert. Also, if you have large lakes that are real sinks, please see Appendix 3 for how to handle these. This is the end of this exercise. Please return to the tutorial for the next step in the process and to access Exercise 4.