Methodology for Identification of Suitable Sites for ...€¦ · CREP Wetland GIS Methodology August 31, 2009 1 Methodology for Identification of Suitable Sites for Constructed Wetlands

CREP Wetland GIS Methodology August 31, 2009

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Methodology for Identification of Suitable Sites for Constructed Wetlands

to Remove Nitrate in Indiana

Documentation of Methods for the Indiana State Department of Agriculture

Margaret McCahon, Graduate Research Assistant, Purdue University

Dr. Jane Frankenberger, Agricultural and Biological Engineering, Purdue University Dr. Indrajeet Chaubey, Agricultural and Biological Engineering, Purdue University

Dr. Eileen Kladivko, Agronomy, Purdue University


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Contents Notes ......................................................................................................................................................... 3

Downloading and Preparing Data Layers ...................................................................................................... 4

Roads data ................................................................................................................................................ 4

Cropland data ............................................................................................................................................ 7

Soils data ................................................................................................................................................. 12

Hydrology data ........................................................................................................................................ 18

Elevation data ......................................................................................................................................... 20

Regulated Drains data ............................................................................................................................. 27

Maps of Data Layers................................................................................................................................ 28

Wetland Locating Analysis .......................................................................................................................... 31

1. Target locations where the sufficient contributing area approaches a stream. ............................ 31

2. Define areas on which wetlands may be created. .......................................................................... 32

3. Manually select potential locations using topography. .................................................................. 34

Recommended steps in manual locating analysis .................................................................................. 38

Preliminary Wetland Design ....................................................................................................................... 40

1. Create “dams” at the most suitable locations. ............................................................................... 40

2. Create wetland designs. .................................................................................................................. 41

3. Delineating each site’s Watershed. ................................................................................................ 43

4. Determine suitability of each design. ............................................................................................. 44


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Notes An ArcInfo license is required for most steps in this method. ArcHydro is helpful but perhaps note

required, as similar tools exist in the ArcToolbox.

We are bringing together data from many sources, and eventually they will all be used together in the

analysis. Therefore, it is important to create all rasters to have the same cell size and extent, and to

have all data in the same coordinate system. Data preparation usually takes place within an ArcMap

document, so one of the easiest ways to keep data in the same projected coordinate system is to import

it into a document that already had the correct coordinate system. You can do this by pulling data with

the correct coordinate system into an empty ArcMap document first.

Many tools from ArcToolbox are used in this methodology. If you do not know where a tool is, you can

use the Index tab and type in the name of the tool, then select the appropriate tool and click “locate.”

Gathering and preparing data for the analysis is the most difficult and time-consuming task. After this,

the analysis is fairly straight forward and simple. Also, most steps can be performed multiple ways, so

do not hesitate to use tools you are more familiar with.


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Downloading and Preparing Data Layers

Roads data Download the Tiger2008 census data from: ftp://ftp2.census.gov/geo/tiger/TIGER2008/

Select the state of Indiana, then each county, one-by-one, and download the file called “edges.” Unzip

each to an appropriate folder.

Preparing roads data

1. In ArcCatalog, right click on a geodatabase and create a new feature class named Roads.

Select line features as type, click next. Import a geographic coordinate system from one

of the county’s “edge” files (GCS_North_American_1983), click next twice. Create a new

field name called “Road_Name”, with data type Text. Finish.

ftp://ftp2.census.gov/geo/tiger/TIGER2008/


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2. Right-click on the new Roads feature class, click load, load data. Browse for each

county’s edge file and click “add.” When all counties are added, click next twice. Match

the Road_Name target field to FULLNAME (string), which contains the name of each

road. This will help you identify locations on the map by address. Click next. Select

“load only the features that satisfy a query”, open Query Builder, and create the

following query: "ROADFLG" = 'Y'. This selects only roads from the county edge classes.

Click OK, next, then finish.


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3. Clip the roads to the watershed, either by using the Clip tool or using Select by Location

to select all roads that intersect the watershed.


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Cropland data Download the NLCD 2001 landcover data from http://www.mrlc.gov/nlcd_multizone_map.php

Determine which zones you need (Indiana is in zones 8 and 11), click the zones on the map and

download “Land Cover zip file.” Unzip files to an appropriate folder.

Preparing cropland data

1. Pull the landcover raster into an ArcMap document

2. Clip the raster to the watershed. You can use Raster Calculator under Spatial Analyst,

setting the 8-digit watershed as a mask under Options and extent and cell size to the same

as the original landcover raster. Input a function like the one below:

Landcover = [landcover11_3k_022007.img]

3. Reclassify the raster to two groups: cropland and non-cropland. The landcover class for

cropland is Cultivated crops, which has a value of 82. Under Spatial Analyst, select

Reclassify. Make sure your options are set so that the extent and cell size will be the same

as the input landcover raster. In the Reclassify tool, set all values except 82 to 0, and set 82

to 1. Now cells with the value 1 are cropland, and those with value 0 are non-cropland.

http://www.mrlc.gov/nlcd_multizone_map.php


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4. Make sure the cropland raster has the same extent and cell size as the elevation data. First

you need to project the raster into UTM, the same projection as the elevation data, and this

should be the same projection as your map. You can do this by right-clicking on the

cropland raster, go to export data, and make sure to select the spatial reference of the data

frame (current) and the output raster to be square (see image below).


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5. Next you match the extent and cell size of the cropland raster to the elevation raster. This is

actually not critical for the following method, but it is a good idea if you want to experiment

with combining the two datasets at some point. This is quite simple. Go to Spatial Analyst,

Options. Set the extent and cell size to the elevation data layer (DEM_watershed). On the

general tab under analysis coordinate system, select “analysis output will be saved in the

same coordinate system as the active data frame”. Now open Raster Calculator, and input:

Cropland_UTM = [DEM_watershed]. Now your cropland raster should have 10 x 10 m cells,

perfectly aligned with the DEM.

6. Finally, I have found that the cropland raster does not always recognize roads as non-

cropland. This is a problem when you want to define a particular field as a suitable site for

wetland placement. To make sure fields break at roads, you must erase roads from the

cropland raster. First, you must create a raster version of the roads data. Under Spatial

Analyst, Options, make sure the Extent and Cell size are set to the Cropland_UTM raster.

Then go to Spatial Analyst, Convert, Features to Raster to convert the roads feature to raster

(see window below).


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However, the new roads raster only has values for road cells, and if we were to combine this

raster with the cropland raster, the analysis is limited to only these locations. So we must

reclassify the roads raster to have values over the entire extent of analysis. We can do this

by going to Spatial Analyst, Reclassify. To make the next step simpler, reclassify road cells

(which have a value) to 0, and Nodata cells to 1 (see below). Name the output

Reclass_Roads.

Finally, we must “erase” the roads from the cropland raster. We can again use Raster

Calculator, inputting the function:

Crops_No_Road=[Cropland_UTM2]*[Reclass_Roads]


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Now you should have a cropland dataset where fields are broken by roads.

7. The last step is to convert the cropland raster to feature, for ease in later analysis. Go to

Spatial Analyst, Convert, Raster to Features. It’s probably better not to simplify polygons.

Name the raster crops_no_road.shp. The new polygons have gridcode 0 or 1, but you only

want the ones with gridcode 1, which means they are crop fields (gridcode 0 means

anything but croplands). Select by attribute the crop polygons with gridcode 1, and then

export the selected data into a new feature class called feature_cropland.


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Soils data This methodology assumes that the user already has access to a SSURGO soils dataset. Soils polygons

should have the following attribute fields: hydclprs and drclasswet.

Preparing soils data

1. In ArcCatalog, right click on the Soils geodatabase and create a new feature class called

“Soils”. Under “Type” choose “Polygon Features.” Click Next. Import the coordinate

system from one of the county soil layers. Click Next three times. Add two fields, name

them something like “Hydric” and “Drainage,” and give them data type “text.” The “Hydric”

field is the keeper of information about whether or not a particular soil polygon is hydric,

and the “Drainage” field shows whether the polygon is poorly drained, very poorly drained,

etc. Click Finish.

2. In ArcCatalog, look at the table of one of the soils feature classes. Note the field names that

correspond to “Hydric” and “Drainage”. In this case, mine are called “hydclprs” and

“drclasswet”.

3. Right click on the new Soils feature class and go to load, load data. One-by-one, add each

county’s soil feature class created in the previous section (see figure below). Note: if you

have trouble loading all the data at once, you can perform these steps on each soils feature

class separately. Click Next twice.


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In the following window, you need to match up the “Hydric” and “Drainage” fields with

“hydclprs” and “drclasswet” (see figure below). Click Next twice, and Finish.

4. Open ArcMap, and drag in your new Soils feature class. Bring in your watershed or area of

interest, and overlay the two to make sure you have successfully brought in all Soils

polygons.

5. At this point you may clip the Soils feature class to the Watershed using the “Clip” function

in ArcToolbox, although it is not necessary if you’re following this method.


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Preparing Hydric soils 1. Next, create a new feature class that has only those soil polygons which are categorized as

hydric. You can do this a number of ways, either in ArcMap of ArcCatalog. I chose to use

ArcCatalog.

2. Right-click on the Soils geodatabase and go to New, New feature class. Name this feature

class “HydricSoils”, choose polygon features, click Next. Import the same coordinate system

as the Soils feature class. Click Next three times. Add a field called “Hydric” with “text” data

type. Click Finish.

3. Right-click on Hydric Soils feature class and go to Load, Load data. For input data, choose

the Soils feature class we just created. Click Add, then click Next twice. Make sure the

“Hydric” field matches with the “Hydric” field for the Soils feature class. Click Next. In the

following window, choose “load only features that satisfy a query.” Click Query Builder,

and choose to only include data where the “Hydric” field is “All hydric” or “Partially hydric”

(see figure below). Click Next. Click Finish.

4. Now pull the HydricSoils feature class into the ArcMap document to view the result. Right

click HydricSoils in the Display tab, go to Properties, Symbology, Categories, Unique values.

Choose “Hydric” for the Value Field, click Add Values, click Complete List. Choose to

symbolize “Partially Hydric” and “All Hydric” soils with different colors.

You can see my result in the figure below, where blue is “All Hydric” and green is “Partially

Hydric”. Only two counties, Fountain and Boone, appear to have “Partially Hydric” soils, and


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these soils dominate the landscape. We don’t want to have this discrepancy in the soils

layers, so I chose to create a new Hydric Soils feature class that has only “All Hydric” soils.

5. Finally, for later analysis the Hydric soils layer should be a raster dataset. To convert the

feature to raster, first go to Spatial Analyst, Options. Set the Workspace to the Soils

geodatabase, the Mask to the Watershed (or to the DEM), and the Extent and Cell size to

the final DEM (this DEM layer is what we base the extent and cell size on for all raster

datasets). Now click Spatial Analyst, Convert, Feature to Raster. Set the Input Features to

HydricSoils, the Field to 1, NoData to 0, and save the output raster as HydricSoils in the Soils

geodatabase.

Note: if the conversion to raster fails, try setting the Field to SHAPE_length, and after the

conversion, reclassify the raster so that all values are set to 1 and NoData is set to 0.


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Preparing tile-drained estimate

1. Follow the same steps as above (1-3), creating a feature class called “PoorlyDrainedSoils.”

Instead of adding the “Hydric” field, add the “Drainage” field. When loading the data,

change the query to include data where “Drainage” field is “very poorly drained”, “poorly

drained”, or “somewhat poorly drained.” See image below.

2. Convert the PoorlyDrainedSoils feature to raster, as in previous step 5, reclassifying so that

all very, somewhat and poorly drained soils have the value 1, and all other soils in the

watershed have the value 0.

3. Take the intersection of the two rasters – cropland and poorlydrainedsoils – to determine an

estimate of land that is tile-drained. You can do this using raster calculator, inputting the

following equation:

TileDrained = [crops_no_road] * [PoorlyDrainedSoil.img]

In your new raster, TileDrained, cells with value 1 are estimated to be tile-drained soils, and

those with value 0 are not. In reality, a field that is mostly poorly drained is most likely all

tiled, and a field that is well drained would not be tiled in one poorly drained spot. But this

is just an estimate that will be used only when many fields are aggregated together.


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4. Convert raster TileDrained to feature to use in later analysis. You can use the Raster to

Polygon tool in ArcToolbox. I called this new polygon feature TileDrainedEstimate.


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Hydrology data

Download high resolution streams and watershed data from the National Hydrography Dataset for the

watershed (HUA) of interest. You can download the data from this site:

http://nhdgeo.usgs.gov/viewer.htm.

Zoom in, select Subbasins, click “Polygon Extract”, click inside the subbasin of choice, and a window

pops up for you to download the data. Use the High Resolution dataset, which recognizes more open

drains as streams. This layer will be used to narrow the sites for the analysis to those that do not

intercept open drainage.

http://nhdgeo.usgs.gov/viewer.htm


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Preparing Streams and Subbasin data

After downloading and unzipping the data, open the geodatabase and draw the following into ArcMap:

NHDFlowline under Hydrography, and Subbasin under Hydrologic Units.

1. To prepare Subbasin data, select the subbasin of interest and export the selected data into a

new feature class called “Watershed”.

2. To prepare Streams data, “select by attributes” those flowlines that relate to surface flow –

in this case I selected connectors, canals/ditches, streams/rivers, and artificial paths (shown

below; in this case, all of the flowlines are streams, but there may be a situation where

other FTypes must be eliminated). Export the selected data into a new feature class called

“Streams”.


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Elevation data

Download National Elevation Data (NED) from http://seamless.usgs.gov/index.php.

You can download by rectangle or coordinates, as shown for the 8-digit watershed below:

http://seamless.usgs.gov/index.php


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Make sure to select the appropriate resolution – in this case, I used 1/3 arc second. You can also choose

to download data in larger chunks if you click “modify data request”, scroll to the bottom of the page,

and under “delivery options” choose the maximum size (250 MB). Download and unzip the data to an

appropriate location, then pull all DEMs into an ArcMap document.

Preparing final DEM

1. If the files exceeded 250 MB, and you have multiple rasters, you will need to merge the DEMs to

one dataset. There are a few ways you can do this. I chose to use Raster Calculator, which is in

the Spatial Analyst toolbar (you need to first select the Spatial Analyst extention under Tools,

Extentions). Select Spatial Analyst, Options, and set Extent to “Union of Inputs.” Then under

Spatial Analyst, select Raster Calculator, and input the following:

DEM = merge(raster1, raster2, raster3...)

In my case this looked like:

DEM = merge([16912071],[48575743],[61257263],[96641403])

2. Now that all of the rasters are in one DEM, you want to clip the DEM to the watershed that is

your study area. One way to do this is to use the “clip” tool, or you can use raster calculator

provided you’ve set the watershed as “mask” under spatial analyst, options. In raster calculator,

evaluate:

DEM_watershed = DEM

3. At this point, the DEM does not have a projected coordinate system. You can pull this DEM into

a new ArcMap document, but first you want to draw another dataset into the document that

has the desired coordinate system. In this case I used UTM Zone 16N.

Now you can use Raster Calculator to create a DEM in the correct coordinate system. Under

Spatial Analyst select options, and set the desired cell size (in this case I used 10 m), and under

the general tab select “analysis output will be saved in the same coordinate system as the active

data frame”.

Preparing contributing (watershed) area calculation

1. Add the ArcHydro toolbar. It may also be possible to do these calculations in ArcToolbox, under

Spatial Analyst Tools, Hydrology, though I have not tested this.

2. Under Terrain Preprocessing on the ArcHydro toolbar, select DEM Reconditioning (Agree). Use

the final DEM and Streams data.


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3. Under Terrain Preprocessing on the ArcHydro toolbar, select Fill Sinks. Use the Agree DEM from

the previous step.

4. Under Terrain Preprocessing on the ArcHydro toolbar, select Flow Direction. Use the DEM

created in the previous step. You may set the 8-digit Watershed as the Outer Wall Polygon.


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5. Under Terrain Preprocessing on the ArcHydro toolbar, select Flow Accumulation. Use the DEM

created in the previous step.

6. Next determine which locations have sufficient contributing area, which we define as 500 to

2000 acres. Each cell is 10x10 m, or 100 square meters, so this comes to 20,234-80,937 cells.

Right click on the Flow Accumulation grid (Fac_8dig), select Properties, Symbology, Classified.

Symbolize the grid manually in the following 3 classes: 0-20234 cells, 20234-80937 cells, and

above 80937 cells (see figure below). Click OK and OK.


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7. Go to Spatial Analyst, Options and make sure the mask is set to Watershed, and the extent and

cell size are set to the layer Fac_8dig. Go to Spatial Analyst, Reclassify, and reclassify the raster

so that cells with sufficient contributing area have the value 1, and all other cells have the value

0. I saved the raster as FlowAcc_8dig.


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8. These next steps are to limit the sufficient contributing area to those locations that also drain at

least 500 acres of tile-drained land. Make sure you have the TileDrained raster in the map.

Then go to Weighted Flow Accumulation under Terrain Preprocessing in the ArcHydro toolbar.

Using the same flow direction raster from above, and using the tile drained estimate as the

weight grid (0 = not tile drained, 1=tile drained), create a new flow accumulation grid (see image

below). The result is that only grid cells that are tile drained “count” in the flow accumulation

calculation. Now, perform steps 6 and 7 for the weighted flow accumulation grid, using the

threshold of 20234 cells (or 500 acres) of tile drained land.

9. Now we need to take the intersection of the two rasters created – the original, reclassified flow

accumulation raster, and the reclassified tile drained flow accumulation raster. One way to do

this is using Raster Calculator, by the following equation:


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SuffCA_8dig=[rTileFac_8dig] * [FlowAcc_8dig]

10. Under Spatial Analyst, select Convert, Raster to Feature. Input the SuffCA_8dig raster, select

geometry type Polyline. It’s best NOT to generalize lines for later parts of the analysis.

11. Locations that are in the stream are not suitable for wetland placement, so you need to

eliminate streams from the analysis. One way to do this is by buffering the streams layer and

“erasing” the buffered streams from the contributing area feature class. In the ArcToolbox, find

the buffer tool, and fill it out according to the image below, with a buffer of 50 m that is flat at

the ends of stream segments. Now under ArcToolbox find the erase tool, and erase the

buffered streams from the contributing area feature class (SffCA_8dig).


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Preparing contours calculation

1. Topographic contours of the land are needed to see where a wetland might be placed, and will

be used in creating the preliminary wetland shapes/designs. It is helpful to have contours in

feet rather than meters, so the first step is to convert the z units from meters to feet. You can

do this in raster calculator, by inputting the following function:

dem_z_in_ft=[dem_utm] * 3.2808399

2. Now find the Contour tool in ArcToolbox, and create contours from dem_z_in_ft with 1 foot

intervals. Alternatively, you can use the Contour tool in Spatial Analyst, Surface. If you are

working with a very large watershed, such as the 8-digit watershed I am using, you may run into

errors as you try to create contours. In this case, you may want to create contours in a small

portion of the watershed. I only created contours on the crops/fields that I found to be suitable

for wetland placement later in the analysis.

Regulated Drains data For this analysis, it is ideal to have a layer that shows were main tiles exist and which drains are open vs.

closed. Yet this layer is not easy to find, and only a few counties appear to have digitized regulated

drainage maps.


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Maps of Data Layers Below are some maps to show you what my data layers looked like after this preparation.


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Wetland Locating Analysis

1. Target locations where the sufficient contributing area approaches a

stream. These places are very likely near the intersection of closed and open drains, and are also a useful

upper limit for later analysis.

a. Create points at the endpoints of the segments of sufficient contributing area. Open the

tool Feature Vertices to Points, and choose the Point Type BOTH_ENDS.

b. For each set of points, select only the point of higher contributing area, which is near

the stream. Under Selection, choose Select by Location, and select from the new layer

points which are within 101 m from the stream. 101 m is used because all sufficient

contributing area segments were erased within 100 m of the stream (because the

contributing area did not always line up closely with the stream segments), and the

stream is not considered a suitable location for wetland placement.


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c. Right-click the suff_ca_endpoints feature class, go to data, export data, and export the

selected features to a new feature class called suff_ca_outlets.

2. Define areas on which wetlands may be created. Wetlands can be placed on crop fields. If the suff_ca_outlets are the most “downstream”

wetland outlet possible, then the wetlands will be placed somewhere within this point’s

watershed. Wetlands also cannot cross roads, and must be of a suitable size.

a. Find the Watershed tool in ArcToolbox, and input the flow direction raster (created on

the way to the contributing area raster), and the sufficient contributing area outlet

points from above. This tool will create watersheds above each of the outlet points.

After completed, browse through the map to make sure every outlet point has a

watershed. If you don’t have the right cell size or extent, or if you’re using a different

flow direction raster than the one used to create the sufficient contributing area, it’s

possible you will not have watersheds for each point. If this is a persistent problem, you

can manually create points in the correct locations to capture the entire watershed.


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b. Convert the watersheds from raster to feature, using Spatial Analyst, convert, raster to

feature. I didn’t simplify polygons, but it shouldn’t really matter.

c. Now we want to intersect these watersheds with the cropland fields. You can use the

Clip tool, with feature cropland as the input and feature watersheds as the clip features.

d. But not all fields in the watershed are suitable for wetland placement – only those fields

that also intersect a segment of sufficient contributing area. To identify these, select by

location those WatershedsOnCropland that intersect the sufficient contributing area

segments, and export the selected features into a new feature class called

SuitableCropFields.


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e. At this point you may decide to impose a size constraint, eliminating all fields/locations

that are not large enough to place a wetland. You can do this by using Select by

Attribute. You may decide on which lower limit to use. I might suggest at least 10-30

acres, because the wetland and buffer are placed based on topography and a very large

piece of land is required to find the appropriate topography. I did not impose such a

constraint, but went on to the manual search for suitable locations using topography.

3. Manually select potential locations using topography. The ideal wetland location has a particular topography. You can think of it as a sort of bowl-

shape, with an opening on one side of the bowl. The wetland itself should be relatively shallow

(no more than 25% of the wetland more than 3 feet deep; this is called “deep wetland”), to

allow for efficient nitrate removal. The buffer land around the wetland must be relatively steep

(no more than 4:1 ratio of buffer to wetland area), and must rise at least 4 feet above the

wetland elevation. Finally, you want to place a relatively small dam, so the contours will ideally

wrap around the wetland to form a small outlet.


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a. At this point you may want to print off a map of the outlets, contributing area segments,

and crop fields where a wetland may be placed. Then as you browse through the map

you can make note of any promising site on paper, and also keep track of where you

have already worked. See image below.

b. In the Arcmap document, turn on the following layers: SuitableCropFields, contours,

orthophotos, sufficient contributing areas, and suff_ca_outlets. Note: You may not

want to turn on orthophotos or contours until you’ve zoomed in on a particular field.

Zoom in to a suff_ca_outlet, and look at the one field where a wetland may be placed.

Search the image for a location with sufficient contributing area and topography that

appears to be promising. The image below shows a SuitableCropField (green) near the


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potential wetland outlet (purple dot), and you can see how the contours give a sense of

a potential wetland shape.

c. Create a new field in the SuitableCropFields feature class, and call this field Suitability,

leaving it as a short integer (see first image below). If you’d rather have this field be

text, that is OK too, as long as you use consistent text for the categories. In this field

you’ll make the suitability of various crop/fields for wetland placement, perhaps like

this: 5 (very promising), 4 (promising), 3 (possible), 2 (unlikely), 1(no way). To change

the value of this field for a particular cropland polygon, you can select the polygon, open

the SuitableCropFields attribute table, right click on the Suitability field, go to Field

Calculator, and type in the number that you’ve decided describes this field’s suitability

(see second image below). Make sure the field is only changed for the selected polygon.


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d. Complete these steps for all SuitableCropFields, browsing throughout the map, making

sure to record in some way those locations that you think are most promising.


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Recommended steps in manual locating analysis

1. Zoom in to the outlet, and look at orthophotos to make sure the sufficient contributing area is

not in an open ditch. It’s best to see that the outlet occurs at the interface between closed and

open drains.

Above: you can clearly see from the orthophotos that the ditch is open approximately below the

outlet point, and closed above. This is perfect, and now you can consider the sufficient contributing

area segment (red) just above the outlet point to see if this location has suitable topography.

2. Zoom out so that you can see the local topography. Visualize a dam near the outlet point, like

the black dotted line drawn below. Select a contour at least 5 feet above the lowest elevation

beneath the dam. This is an example of where a buffer may be placed. If this contour appears

reasonable, and does not cross roads, then select the contour four feet beneath this one,

creating your wetland. How do the sizes compare? Is the buffer huge, more than 4 times the

size of the wetland? The wetland should most likely have some portion greater than 3 feet

deep, if it is really large enough for its contributing area. Once you’ve done this a few times,

many sites will stand out more quickly as impossible or improbable, and you may not need to

actually select contours to see the shapes of these features.


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Above: the contours representing a hypothetical wetland and buffer are selected. You can see the

buffer is much too large, greater than 4 times the wetland size, and the wetland is much too small.

The dam (black dotted line) is a relatively short compared to the size of the wetland. There aren’t

really any other scenarios, because the road to the south intercepts higher contours. I might rate

this wetland a 2 in Suitability.

3. Rate the suitability by selecting the cropland polygon, right-clicking the Suitability field in the

SuitableCropFields attribute table, selecting Field Calculator, and inputting the appropriate

rating with 5 as most suitable and 1 as entirely unsuitable for wetland placement. Only one

critical failure is required to earn the rate of 1 – for instance, topography that makes wetland

placement impossible.


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Preliminary Wetland Design

1. Create “dams” at the most suitable locations. You will be using the contours to create polygon features for wetland, deep wetland, and buffer.

However, you need to decide where to cut off these features at the wetland outlet, so you will

create lines at hypothetical dam locations.

a. In ArcCatalog, you may want to create a new geodatabase called

“PreliminaryWetlandDesign.gdb”.

b. Right click this new geodatabase and add a new feature class. Call this feature class

“Dams”, select Line Features, click Next. Import a coordinate system from one of the

layers you’ve been using, such as the DEM in UTM. Click Next three times and Finish.

c. Add this new “Dams” feature class to your ArcMap document.

d. At each site, the first step of wetland design is to place the dam. You do this by using

the Editor toolbar. Select Editor, Start Editing. Choose to edit from the

“PreliminaryWetlandDesign.gdb” location. Choose “Task: Create New Feature” and

“Target: Dams”. With the sketch tool (pencil), click on one side of the dam and then

double click on the other. Save edits. You may move the dam around and change its

end points as you further decide where it should go.

e. A good place for a dam is a location where the contours narrow or come together, so

that the dam may be smaller. You don’t always have much choice, however, as the dam

location is determined by the location of wetland components. You would also prefer

the dam to be close to the interface between closed and open drains. Make sure your

dam is located on cropland.


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2. Create wetland designs. You will be using the contours to create polygon features for wetland, deep wetland, and buffer

at each site. If there are multiple wetland designs, create them all, and narrow the possibilities

later. Most likely there will be only one to three feasible designs at most sites. You want the

buffer elevation to be four feet above the wetland elevation, which is four feet above the deep

wetland (deep wetland is not required, so don’t worry if you don’t have any deep wetland in

many designs). The dam should cross all of these contours, and the dam and the contours

should create complete polygons.

a. Note: this process is made easier if your “Contours” shapefile is broken at the roads. I

chose to create contours only on crop fields that that intersected a sufficient

contributing area. The reason you want to do something like this is that you will be

selecting contours, and you would prefer the contours you select to only intersect the

particular dam you are working with, not the other dams in the map.

b. Use the Select tool in the Tools toolbar to select contours that represent wetland,

buffer, and deep wetland. Use Selection method: Add to current selection so that

multiple contours may be selected simultaneously. Make sure the buffer is the contour

4 feet above the wetland, and the deep wetland is the contour 3 feet below the wetland

surface.

c. Once all contours are selected, find the Feature to Polygon tool in ArcToolbox. Input

“Dams” and Contours, and click OK. If you are creating very many wetland designs, you

may use the default names and later pull all of these designs into a new feature class.

“Dam” (used to create the polygons)

“Deep wetland” (>3 feet deep)

“Wetland” (usual water level)

“Buffer” (4 feet above wetland)

1-foot contours

Orthophotos

Sufficient Contributing Area


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Result of Selecting Contours Result of Feature to Polygon tool

(colors added by symbology)


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3. Delineating each site’s Watershed. To know more about each site that we are considering for wetland placement, we really want to

be able to see its watershed. We already know that this site drains at least 500 acres of

cropland with poorly drained soils, so this is not necessary. But it is helpful for visual purposes

and to check your contributing area calculation to make sure it makes sense.

a. Use the Intersect tool in ArcToolbox to create intersection points where the “Dams”

cross the sufficient contributing area segments. These points are hypothetical wetland

outlets, so you can call this new point feature class WetlandOutlets. Make sure you

select Output Type: POINT. If you did NOT generalize lines when creating the

contributing area feature, then these points should be located exactly above the correct

cells in the Flow Direction raster.

b. Use the Watershed tool in ArcToolbox to delineate watersheds for this set of

WetlandOutlets points.


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c. Convert the raster data to polygon (feature) data. You can do this by using the Raster to

Polygon tool in ArcToolbox.

4. Determine suitability of each design. You want to be able to compare these designs based on the original specifications of the

comparative size of each wetland component.

a. For each wetland design, calculate and put in a table the following:

i. Watershed size in acres

ii. Wetland size in acres; Percent of watershed that is wetland (remember to add

the “deep wetland” to the “wetland” polygons for entire wetland)

iii. Deep wetland size in acres; percent of entire wetland is deep wetland

iv. Buffer size in acres; ratio of buffer to wetland


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b. These are the ranges you are aiming for:

i. Watershed size 500-2000 acres

ii. Wetland size0.5-2% of watershed size

iii. Deep wetland no more than 25% of wetland

iv. Buffer less than 4:1 ratio buffer to wetland

Documents

Methodology for Identification of Suitable Sites for ...€¦ · CREP Wetland GIS Methodology August 31, 2009 1 Methodology for Identification of Suitable Sites for Constructed Wetlands