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Catchment Delineation Workflow

Catchment Delineation WorkflowFor MS4 Compliance

PREPARED FORCape Cod Stormwater Managers Group3225 Main StreetBarnstable, MA 02630

PREPARED BY

101 Walnut StreetPO Box 9151Watertown, MA 02471617.924.1770

June 18, 2019


Table of Contents

1 Introduction...............................................................................1

Software Requirements...............................................................................1Input Datasets.............................................................................................1

2 Workflow...................................................................................3

Step 1: DEM Pre-Processing........................................................................3Step 2: DEM Enhancement..........................................................................6Step 3: Creating Catchment Areas..............................................................7Step 4: Creating Outfall Catchments.........................................................11

Buffer Pipe Features.......................................................................11Dissolve Pipe Buffer.......................................................................12Creating Pipe System ID Field........................................................13Spatial Join of Pipe Buffers and Pipes.............................................14Spatial Join of Pipe Network and Drainage Structures....................15Spatial Join of Catch Basin and Catchment Layers.........................16Associate Catchments with their Outfall IDs..................................17

i Table of Contents


1IntroductionThis document describes a methodology for delineating outfall catchment areas in ArcGIS for Massachusetts municipalities. The delineated catchment areas can be used for MS4 permit compliance analyses. The following procedure was developed using an existing methodology and ArcGIS tools developed by the Metropolitan Area Planning Commission (MAPC). The MAPC “Outfall Catchment Mapping and Ranking” document is currently located here: https://www.mapc.org/resource-library/ms4-outfall-catchment-calculator/ This document was prepared by VHB for the Cape Cod Stormwater Managers Group as part of a MassDEP 604b grant-funded project.

Workflow OverviewThe catchment delineation workflow defined in Section 2 consists of 4 steps. These steps are summarized below:

Step 1: DEM Pre-Processing – This step assists in combining multiple Digital Elevation Model (DEM) LiDAR tiles into one file that can be used for the remainder of the workflow.

Step 2: DEM Enhancement – In this step, the LiDAR is enhanced by using additional reference layers to make select features more pronounced. This will allow the tool to better detect flow paths and lead to a more accurate catchment delineation.

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https://www.mapc.org/resource-library/ms4-outfall-catchment-calculator/%20


Step 3: Creating Catchment Areas – In this step, the tool uses the enhanced DEM and mapped catch basins or outfalls to develop catchments.

Step 4: Creating Outfall Catchments – This step finished the workflow by combining all catchments created in step 3. The end result of this step is one catchment per outfall, tagged with the associated outfall ID.

The outfall catchment areas created using this workflow will likely require additional cleanup, the methods for which will vary, but may end up being a manual process, in many cases. Where topography is relatively flat, as is common on Cape Cod, this tool may have difficulty identifying catchments to the MS4 system.The quality of stormwater infrastructure data can also cause the tool to be inaccurate. This can include: catch basins or outfalls in the wrong or unexpected location (high point), catch basins with no associated pipe networks, and interconnected pipe networks. This may result in several catchments not being associated with an outfall. Work done to clean up drainage infrastructure data prior to run this workflow will lead to better results.

Software RequirementsThis workflow requires ArcGIS 10.2 or higher, and the Spatial Analyst extension.

Input Datasets Digital Elevation Model (DEM): High Resolution (1m cell size) Raster

DEMs derived from LiDAR Terrain data are available for download from NOAA Data Access Viewer. https://coast.noaa.gov/dataviewer/#/lidar/search/. The following NOAA help document describes how to download the elevation data: https://coast.noaa.gov/data/docs/dav/dataviewer-help.pdf.

o *If available use local, high resolution LiDAR data. Public Rights of Way: MAPC Massachusetts Land Parcel Database.

Statewide or regional datasets can be downloaded here: https://www.mapc.org/learn/data/

Water Features: MassDEP Hydrography available at http://maps.massgis.state.ma.us/map_ol/oliver.php. It is recommended to clip this dataset to your municipality.

Roadway Center Lines: MassDOT Road Inventory available at https://geo-massdot.opendata.arcgis.com/datasets/road-inventory-2018

Catch Basin Layer: a point layer to be provided by the municipality (if available – not all towns will have this*). This layer will need an integer

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https://geo-massdot.opendata.arcgis.com/datasets/road-inventory-2018

http://maps.massgis.state.ma.us/map_ol/oliver.php

https://www.mapc.org/learn/data/

https://coast.noaa.gov/data/docs/dav/dataviewer-help.pdf

https://coast.noaa.gov/dataviewer/%23/lidar/search/


field to be used as a unique identifier (Note: The Object ID cannot be used).

Stormwater Pipe Network Layer: a line layer to be provided by the municipality (if available – not all towns will have this*)

Outfall Layer: a point layer representing to be provided by the municipality (required to be mapped as part of the 2003 MS4 Permit). This layer will require an integer field to be used as a unique identifier (Note: The Object ID cannot be used).

If the catch basin or outfall layer does not have a unique identifier field:1. Add a field to the layer called ‘CB_ID’ for catch basins or ‘OutfallID’ for

outfalls, field type: integer2. Use “Calculate Field” to populate the new CB ID or OutfallID with the

Outfall ID values from the original layer.*If drainage structure and pipe data is unavailable, workflow can be completed with outfall layer. This process is less accurate and will require additional review.

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2WorkflowStep 1: DEM Pre-ProcessingThis step is only necessary to begin if the DEM is broken into more than one file. This step will combine multiple files into one raster. If using NOAA data, this step may be skipped.

After downloading the raster DEM tiles for a municipality, the tiles need to be mosaicked together into a single raster DEM. MAPC developed an ArcGIS script tool for this process. The “Lidar Mosaic” is one of three tools in the “MAPC Stormwater Catchment” toolbox. To add the toolbox to your Toolbox window in ArcGIS, right-click in the white space of the toolbox window and select ‘Add Toolbox’ from the pop-up menu.

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1. Click on the ‘Connect to Folder’ icon and copy and paste the file path for where you have the MAPC toolbox saved on your computer.

2. Select your new folder connection in the ‘Look in:’ drop-down at the top of the ‘Add Toolbox’ window, then select the ‘MAPC Stormwater Catchment.tbx’ and click ‘Open’.

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3. The ‘MAPC Stormwater Catchment’ toolbox will be added to your Toolbox window and contains three script tools as shown below:

4. Before the ‘Lidar Mosaic’ tool can be run, the source python script must be assigned to the tool. To do this, right-click on the ‘Lidar Mosaic’ script tool and select ‘Properties’ from the dropdown menu. In the ‘Properties’ window, select the ‘Source’ tab then browse to the location the script is saved on your local computer: Select the ‘Lidar_Mosaic’ python file and click ‘Open’. Then click ‘OK’ in the tool properties window.

5. In order to run the ‘Lidar Mosaic’ tool, all of the DEM tiles will need to be saved in a single folder, which will be the ‘Workspace’ parameter in the tool dialog window. Note: the output filename cannot have more than 13 characters.

Step 2: DEM EnhancementEnhancing the mosaiced DEM by “burning” features such as existing streams, curbs, and road crowns helps capture more realistic drainage patterns in the catchment areas. The MAPC ‘Create Burn Raster’ tool creates burn rasters from vector features such as road centerline data, and the lidar mosaic. Refer to the following table from MAPC’s “Outfall Catchment Mapping and Ranking” document for details about burn features and the recommended burn values:

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1. Convert the road ROW polygon from the parcel dataset to polylines using the ‘Feature to Line’ tool.

2. Assign the source python script for the ‘Create Burn Raster’ using the same process described under Step 1.4.

3. The ‘Create Burn Raster’ tool can be run for one vector feature at a time or in batch mode to create several burn rasters at once. To run in batch mode, right-click on the tool and select ‘Batch’ (recommended for efficiency).

4. Once the burn rasters are created, they can be “burned” or added to the LiDAR mosaic using the ‘Raster Calculator’ Spatial Analyst Tool. Below is an example from the MAPC “Outfall Catchment Mapping and

Ranking” document:5. The final burned LiDAR raster will be used in Step 3. Name the file

‘LIDAR_Burn’

Step 3: Creating Catchment AreasThe MAPC ‘Complete Watershed Tool’ can be used to create catch basin watershed using the ’LIDAR_Burn’ raster and the catch basin layer. Note: This tool has not consistently run to completion. Step 3.8 and 3.9 describe a “successful” failure and the workflow to complete the Creating Catchment Area process to produce the same output as if the tool had run correctly.

1. In ArcCatalog, if you don’t already have one, add a folder connection to the C drive (C:\) on your computer. Running these tools on a network or cloud-based storage will cause the processing to take more time. For most efficient processing, run on your local machine.

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2. Add a new folder in the C:\ and name it ‘Workspace’. It is important that no part of the file path include spaces or special characters.

3. Add a new file geodatabase to the ‘Workspace’ folder you just created. Rename the new file geodatabase from the default ‘New File Geodatabase’ name, taking care not to use spaces in the name. The name ‘Complete_Watershed’ has been demonstrated to work.

4. Import the ’LIDAR_Burn’ raster and the catch basin point layer into the new geodatabase.

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5. Open the ‘Complete Watershed’ tool and select the parameters as described below:

a. Geodatabase: The new geodatabase in the ‘Workspace’ folder on your C:\

b. LiDAR Image: The ’LIDAR_Burn’ raster located in the new geodatabase in your ‘Workspace’ folder.

c. Pour Point Vector: The copy of the catch basin point feature class located in the new geodatabase in your ‘Workspace’ folder. If no catch basin data is available, the outfall locations can be used, but results could be less accurate and should be examined closely.

d. Pour Point Field: Select the ‘CB_ID’ field. e. Snap Distance: MAPC recommends a value between 5 and 10

(units are meters) – 10 worked best for MAPC with the Town of Milton. Lower values can result in “stringy” watersheds caused by discrepancies between the catch basin locations and the areas of high flow accumulation in the DEM. If using outfall locations rather than catch basins, err of the side of a larger snap distance, perhaps larger than 10.

f. Watershed Raster Output: Name the output ‘Watershed_Ras’.

g. Watershed Vector Output: Name the output

“Watershed_Vec’.

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6. Open the Environment Settings in the tool window and set the current and scratch workspaces to the geodatabase you created in Step 3.3 above.

7. Click ‘OK’ to run the tool. This should take a few minutes to run.8. The last step of the tool, converting the watershed raster to a vector

file, has been shown to not work consistently. A failed run will still create several files, including a watershed raster. (A “successful” failed run will still take a few minutes, if the tool fails immediately it is not set up properly, review the MAPC documentation and the chosen inputs.) Check the geoprocessing results window (add by opening the ‘Geoprocessing’ dropdown at the top of your ArcGIS application window and selecting ‘Results’). Here is an example of what the

geoprocessing results look like for a successful failed run:

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9. The last step of the tool, converting the watershed raster to a vector file, can be completed separately by using the ‘Raster to Polygon’ tool. The output watershed polygon can be named and stored however you like. Make sure to check the ‘Simplify polygons (optional)’ checkbox so that the output vector watersheds do not have an excessive number of vertices.

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Step 4: Creating Outfall CatchmentsThe following procedure describes how to link catch basin drainage areas to their associated outfalls. This step allows catch basin catchment areas that drain to the same outfall to be grouped. This procedure requires drainage network data by buffering the drainage line features (pipes and culverts) by a specified distance (i.e. pipe diameter), dissolves the buffers based on line connections, then uses a spatial join to add an ‘Pipesys_ID’ to the buffer. A series of additional spatial joins then adds this pipe system ID to drainage lines and catch basins. If your initial watershed delineation was completed using outfall locations rather than catch basins, this procedure is unnecessary.

Buffer Pipe Features1. Open “Buffer” tool. For ‘Input Features’ select your pipe network layer.2. For ‘Output Feature Class’, navigate to your working folder and name

the layer ‘Pipes_Buffer’. 3. For ‘Distance’, choose a single linear unit to be applied to all pipes, or

specify a field in the pipe network layer that indicates pipe diameter. This will turn pipe line data into a polygon layer

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Dissolve Pipe Buffer1. Open the “Dissolve” tool. For ‘Input Features’ select ‘Pipes_Buffer’2. For ‘Output Feature Class’, name the layer ‘PipeBuffer_Dissolved’.3. Uncheck the ‘Create multipart features’ checkbox and run the tool.

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Creating Pipe System ID Field1. Open the attribute table for Pipes_buffer_dissolved2. Create a new field titled ‘pipesys_ID’ or a different unique ID name if

desired. Set the field type to be short integer.3. Select and then right-click on the header of the ‘pipesys_ID’ field and

select ‘Field Calculator’4. Enter the following formula: pipesys_ID = int([ObjectID]) and click ‘OK’.

Note: This assigns each pipe system an ID value based off the Object ID field. In the example below, the ObjectID field is named ‘FID’.

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Spatial Join of Pipe Buffers and Pipes1. Open “Spatial Join” tool. For ‘Target Feature’ select the original pipe

network layer. 2. For ‘Join Features’ select the pipes_buffer_dissolved layer (this layer

should have the Unique ID field created in the previous step). 3. For ‘Output Feature Class’, save the output as “pipes_ID_SJ’.4. For ‘Match Option’, select “INTERSECT”

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Spatial Join of Pipe Network and Drainage Structures1. Open “Spatial Join” tool. For ‘Target Feature’ select the catch basin

layer. 2. For ‘Join Features’ select the pipes_ID_SJ layer 3. For ‘Output Feature Class’, save the output as “CB_ID_SJ’.4. For ‘Match Option’, select “WITHIN_A_DISTANCE”5. You will need to set a ‘Search Radius’. Use your best judgement based

on the quality of the data in selecting your search radius. If catch basins and outfalls are generally snapped to pipes, use 0 or a short search radius. If you observe gaps between drainage point structures and pipes, use a larger search radius. Once search radius is entered click ‘OK’ to run the spatial join.

6. Repeat this step as outlined above, but with the outfall layer as the ‘Target Features’ and name the ‘Output Feature Class’ Outfalls_ID_SJ

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After completing this step, all pipes, outfalls, and catch basins that are interconnected will have the same pipesys_ID values.

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Spatial Join of Catch Basin and Catchment Layers1. Open “Spatial Join” tool. For ‘Target Feature’ select the catchment

layer created in Step 3. 2. For ‘Join Features’ select the CB_ID_SJ layer 3. For ‘Output Feature Class’, save the output as “Catchment_ID_SJ’.4. For ‘Match Option’, select “CLOSEST”5. You will need to set a ‘Search Radius’. Sometimes the catchment

delineation tool creates catchments that are directly next to their associated catch basins, but not intersecting them. Setting the ‘Match Option’ to ‘CLOSEST’ and selecting a search radius of 5 – 15 feet will allow the spatial join to register such points, even if they lie outside their associated catchment.

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Associate Catchments with their Outfall IDs1. Open “Add Join” tool. For ‘Layer Name’ select the catchment_ID_SJ layer

created in the previous step. 2. For ‘Join Table’ select the Outfall_ID_SJ layer 3. For both the input and output join field, select ‘pipesys_ID’4. The catchment_ID_sj layer will now contain all of the catchments with their

associated outfall IDs.

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