Riparian Wetland Restoration Site Selection Using GIS i Preface This bulletin is intended as a guide for managers, planners, and policy-makers involved in wetland restoration projects. This step-by-step process is meant to be a first step in site selection and will not replace necessary fieldwork to determine if a particular site is suitable for a specific project. While this method is designed for restoration site selection of riparian wetlands, with some slight modifications the process can be applied to other wetland types as well. This is not intended as a tutorial for using GIS software. A basic understanding of ArcGIS 8.3, including menu item options, projections and formatting is a prerequisite for completing these analyses.
Riparian Wetland Restoration Site Selection Using GIS ii Contents Introduction...1 Materials...2 Identifying Suitable Restoration Areas...3 Advanced Processes...11 Conclusions...14 References...15
Riparian Wetland Restoration Site Selection Using GIS 1 Introduction Recognition of wetland values has stimulated interest in protecting these ecosystems from further losses. These ecosystems provide services including flood abatement, improving water quality, supporting biodiversity and recharging aquifers (Zedler, 2003). In addition to protecting our remaining wetlands, it has become evident that further steps are necessary to enhance our wetland resources. Beginning in the early 1990's, the U. S. Fish and Wildlife Service and the U. S. Department of Agriculture s Natural Resources Conservation Service began an effort to reverse the tide of wetland losses by establishing wetland restoration programs. These programs, and others like them at the state and local level, are an increasingly popular strategy for stemming the loss of wetland functions as land is developed or converted for agricultural uses. Choosing the right wetland mitigation site is important because the conditions needed to form wetlands are quite complex, and failure to meet wetland conditions is common. Project failures may result in low-quality wetlands and high reconstruction costs. Site selection can be a long and tedious process. Candidate sites are evaluated by examining the hydrology, landscape setting, soil, and plant types (Mitsch, 2000). To try and minimize the time and expense required for selecting candidate sites, geographic information systems (GIS) can be used to help select and prioritize candidate restoration sites. There are many different kinds of wetlands and many functions that wetlands perform. The processes described in this bulletin are specific to riparian wetlands that are being restored to minimize the impact of nutrient loading in the watershed. The case study used as an example is in the lower Neuse River watershed in eastern North Carolina. The Neuse River is classified as a nutrient sensitive watershed by the Water Quality Committee of the state Environmental Management Commission. As part of this classification, wetland regulations are strictly enforced and actions taken to improve water quality are strongly encouraged (NC DENR-DWQ, 2001).
Riparian Wetland Restoration Site Selection Using GIS 2 Materials The following data layers are needed to complete the process. 1. Soils data: This dataset should contain the hydric status of the soils. An A indicates primary hydric classification by the NRCS while a B denotes secondary hydric status. CGIA: Soils, Detailed County Surveys - North Carolina Center for Geographic Information & Analysis. Var dates. 2. Hydrography: Riparian areas require a nearby channel. CGIA: Hydrography (1:24,000) - North Carolina Center for Geographic Information & Analysis. 1999. 3. Hydrologic Units: We will need to delineate the watershed we are interested in. For maximum flexibility, this dataset should include watershed delineations at HUC-8, HUC-11 and HUC-14 levels. CGIA: Hydrologic Units: Subbasins - North Carolina Center for Geographic Information & Analysis. 1994. 4. Land use/ Landcover (LULC): Restoration projects take place on impacted landscapes. We will identify potential project areas by looking at how land is utilized within the watershed. EPA BASINS: Landuse/Landcover Spatial Data of CONUS - U.S. Environmental Protection Agency. 1998.
Riparian Wetland Restoration Site Selection Using GIS 3 Identifying Suitable Restoration Areas Follow these steps to determine the possible restoration sites in your study area: 1. Select Area To begin with, you need to know the area in which your search will be confined. In this instance, we will start at a large scale in the lower Neuse watershed and get more specific as the process continues (Figure 1). If you know the specific catchment in which you plan to restore a wetland, it may be easier to begin at that level. 2. Clip Layers to Area If the GIS data layers you start with are larger than your study area, you should clip your data to the extent of your study area. You do not need to waste time performing analyses outside your area of interest. Clip these layers to the borders of your study area in ArcMap by selecting the Geoprocessing Wizard from the Tools menu (Figure 2). Your input layer is the soils, hydrography or LULC layer you want to clip and the clip layer is boundary of your study area from step 1. Add your clipped hydrography shapefile to your active map display (Figure 3).
Riparian Wetland Restoration Site Selection Using GIS 4 3. Create Buffer Create a 200 foot buffer around the clipped hydrography layer in ArcMap by selecting the Buffer Wizard from the Tools menu. Depending on the extent of hydrology and size of your project area, this process may take some time.
Riparian Wetland Restoration Site Selection Using GIS 5 4. Create Hydric Soils Layer In ArcCatalog, make two copies of your soils dataset. Name one pri_hyd.shp and one sec_hyd.shp. Add both of these to your active map display. In ArcMap choose Select By Attributes from the Selection menu (Figure 4). Choose pri_hyd.shp from the layer pulldown menu and choose Hydric from the list of fields. Click the equal sign and then select from the unique fields dialog. Click the OR button and then choose the Hydric field and click the equal sign again. Now choose the B from the unique fields dialog box. Your selection statement should read: "HYDRIC" = ' ' OR "HYDRIC" = 'B'. Click the Apply button. After the areas are selected, close the Select By Attributes window and choose Editor, Start Editing (Figure 5). Select the soil layers and choose the Modify Features task with pri_hyd selected as the target. Right click on the selection in the Display window and choose Delete. Save your edits. The pri_hyd layer now only has soils with A as the hydric status. Repeat the above steps for sec_hyd, choosing the soils with B as the hydric status. 5. Create Impacted Land Use Layer Using the same methods described in the previous step, create a shapefile from the original LULC shapefile that includes only impacted (agriculture, urban, etc.) land uses. 6. Select LULC that Intersect Buffer In ArcMap choose Select By Location from the Selection menu. Select features from the impacted LULC layer created in the previous step that intersect features in the 200 foot buffer created in step 3 (Figure 6). Once these features are selected, right-click on the impacted LULC layer and select Open Attribute Table (Figure 7a). Click the Options button at the bottom of the new window and select Switch Selection (Figure 7b). Choose Start Editing from the Editor menu and select the impacted LULC layer as your target. Delete the selected items (impacted land use areas that do not have a connection within 200 feet of a channel). Save your edits and stop editing.
Riparian Wetland Restoration Site Selection Using GIS 6 7. Convert Shapefiles to Rasterfiles Now we need to combine the impacted LULC layer with the each of the hydric soils layers. The easiest way to do this is with the Spatial Analyst tool. The files we are currently using are not compatible with this type of analysis, so first we will need to convert our features to raster data. From the Spatial Analyst menu, select Convert Features to Raster (Figure 8a). For the soils layers, make sure that the Field is indicating the Hydric field and change the output cell size to 10. For the LULC layer the field should be pointing to the field that indicated the LULC code and the cell size should also be 10 (Figure 8b).
Riparian Wetland Restoration Site Selection Using GIS 7 8. Classify Data Layers For our calculations we will want all of the files to have either a one or a zero at every point. The soils layers should be classified this way now, but the LULC layer will have multiple values. To change all of these values to ones, select Reclasify from the Spatial Analyst menu. Click on each of the new values and change each of them to one. Enter an output file and click OK. 9. Combine Layers To combine the LULC layer with the soil layers, choose Raster
Riparian Wetland Restoration Site Selection Using GIS 8 Calcualtor from the Spatial Analyst menu. Choose the LULC layer, click on the * and then choose one of the soil layers (Figure 9). Click the Evaluate button. This will select locations where there are both impacted LULC with a connection to a channel and hydric soils. This will return a calculation layer. Right-click on the calculation layer and select Make Permanent. Add the permanent file to your map and delete the calculation layer. Repeat this step for the other soil layer. 10. Convert Rasterfiles Back to Shapefiles From the Spatial Analyst menu, select Convert Raster to Features. Select one of the input raster files, make sure that the Output geometry type pulldown is pointed to Polygon and save your output.
Riparian Wetland Restoration Site Selection Using GIS 9 11. Calculate Area Right-click on one of the combined LULC/ hydric soil layers and select Open Attribute Table. Click the Options button at the bottom of the new window and select Add Field. Name the new field Area and select Float as the Type. Right-click the field heading for Area. Click Calculate Values. Check Advanced (Figure 10). Type the following VBA statement in the first text box: Dim dblarea as double Dim parea as IArea Set parea = [shape] dblarea = parea.area Type the variable dblarea in the text box directly under the area field name. Click OK. 12. Convert to Acres Right-click on the combined LULC/ hydric soil layer and select Open Attribute Table. Click the Options button at the bottom of the new window and select Add Field. Name the new field Acres and select Float as the Type. Right-click the field heading for Acres. Click Calculate Values. Double-click on Area
Riparian Wetland Restoration Site Selection Using GIS 10 and then type /4046.86867 in the text box to convert square meters to acres (if your projection is not in square meters, calculate the correct conversion factor). Click OK. 13. Delete Areas Less Than 10 Acres Right-click on the combined LULC/ hydric soil layer and select Open Attribute Table. Right-click the field heading for Acres and choose Sort Ascending. Select all the rows that have less than ten acres. Choose Start Editing from the Editor menu and select the combined LULC/ hydric soil layer as your target. Delete the selected items (combined LULC/ hydric soils less than 10 acres). Save your edits and stop editing. Repeat steps 11, 12 and 13 for the other combined LULC/ hydric soil layer. You now have potential restoration locations that are ten or more acres. You can look at these at a higher resolution at the HUC-14 level (Figure 11).
Riparian Wetland Restoration Site Selection Using GIS 11 Advanced Processes The above processes can help you quickly and easily identify potential restoration sites, by locating areas that have a hydrologic connection to a channel and a high likelihood for having hydric soils. What if that is not enough? With the LULC layers we already have and a model developed to predict nutrient loading in small watersheds we can predict how wetland restoration will affect nutrient levels and select the sites that have the most impact. The Simple Method was developed for the Center for Watershed Protection by Thomas Schueler to evaluate nutrient loading in a subcatchment. The calculation of annual pollutant loads (pounds/acres per time interval) uses the following formula: L = (R)(C)(2.72) or L = [(P)(Pj)(Rv)/12](C)(2.72)A Where: Rv = Mean runoff coefficient, expressing the fraction of rainfall converted into runoff Rv = 0.05 + 0.009(I) I = Percent of site imperviousness R = Runoff (acre-feet per time interval) R = [(P)(Pj)(Rv)/12](A) P = Rainfall depth over desired time interval (inches) Pj = Fraction of rainfall events that produce runoff (0.9 default value for this study) A = Area of the site (acres) L = Urban runoff load (pounds/acres per time interval) C = Flow-weighted mean concentration of the pollutant in urban runoff (mg/l or ppm) 12 = Conversion factor (inches/foot) 2.72 = Conversion factor (pounds/acre-foot-ppm) For the purposes of this study, where water quality data is not available for each subcatchment, the Simple Method provides a mechanism to differentiate between subcatchments using LULC data. The additional necessary inputs- precipitation, pollution concentration by land use type, and area are all available data, so that estimates of pollutant loading by subcatchment can be made based on land use types. The intent is to uniformly estimate pollutant loading across each subcatchment and use those estimates to compare and rank the subcatchments in a relative way.
Riparian Wetland Restoration Site Selection Using GIS 12 This nutrient model requires small land areas to be effective. By breaking up one of the HUC-14 units into subcatchments (Figure 12), we can look at which areas of the watershed have the worst nutrient problems and then examine which candidate sites might have the most impact in reducing nutrient loads. We have the LULC data that we started with at the beginning of these exercises. We can select the land uses in each of the subcatchments to calculate the total area in each land use in a given subcatchment. These values can be entered into spreadsheets developed for use with the Simple Method to calculate the nutrient level in each of the subcatchments (Figure 13)*. Once nutrient loads are calculated, land use values can be changed to alleviate pressures in the most heavily impacted subcatchments (Fig 14).
Riparian Wetland Restoration Site Selection Using GIS 13 * - The other coefficients such as precipitation and runoff coefficients are all available and their calculations are beyond the scope of this report.
Riparian Wetland Restoration Site Selection Using GIS 14 Conclusions Restoration is a complex process that requires planning, implementation, monitoring, and management. Site selection is a vital step in the early planning process. While the process outlined above does not eliminate the need for field data collection, it could greatly reduce the amount of time needed to complete these tasks, creating valuable savings in the planning process and saving project funds for the implementation and monitoring phases. While other more complex selection strategies take factors such as hill slope and elevation into account, this method provides the means for completing the analyses quickly and succinctly. The major limitation affecting the results will be the resolution of the data.
Riparian Wetland Restoration Site Selection Using GIS 15 References Mitsch, W.J. and J.G. Gosselink. 2000 Wetlands: Third Edition. John Wiley and Sons, Inc. New YorK, NY. North Carolina Department of Environment & Natural Resources, Division of Water Quality. 2001. Neuse River Basin Basinwide Assessment Report. Raleigh, NC. Zedler, J. B. 2003. Wetlands at your service: Reducing impacts of agriculture at the watershed scale. Frontiers in Ecology and Environment.