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Grassed waterways are installed to reduce the risk of concentrated flow (gully) erosion. This practice may be effective in preventing gully erosion for three reasons. First, the growing grasses can reduce mean velocity of runoff, which discourages soil detachment. Second, grass vegetation subjected to high water velocity may be pushed to lie flat on the surface, and the flattened grass may then provide a physical barrier to prevent gully formation. Third, the fibrous root systems of grasses lead to increased soil strength, which can limit detachment of soil particles that otherwise may be prone to occur with seepage from the soil surface under saturated conditions. Although grassed waterways are among the most common of conservation practices, they remain under-utilized in many of the country’s steeper farmed landscapes, and their capacity to reduce erosion under saturation excess runoff (seepage) conditions may be under-appreciated. Grassed waterways have not been the most frequently evaluated practice in recent conservation-effectiveness research, but several papers by Fiener and Auerswald (2003; 2006) provide a good starting point to learn more. We emphasize that grassed waterways are designed to convey runoff, and are not meant to trap sediment.
The tool interface requires that the user define either a standard deviation threshold (between 2 and 5 standard deviations above the mean SPI value), or a specific SPI value. SPI values that are greater than the value specified will be selected as locations suitable for grassed waterways. SPI values above the selected threshold are first recoded to a value of 1, then smoothed using a majority filter. Values of 1 are expanded by 1 cell to increase overall connectivity between cells, then thinned to a maximum width of 1 cell. Regions are then converted to an output polyline layer. The input stream reach polyline is converted to a raster and serves to remove grid cells corresponding to the stream network. The output is clipped to agricultural field (excluding pasture) as identified by an “isAG” value of “1”. Finally, grassed waterways less than 50 meters in length are excluded from the output.
Fiener, P., and K. Auerswald. 2003. Effectiveness of grassed waterways in reducing runoff and sediment delivery from agricultural watersheds. Journal of Environmental Quality. 32(3):927-936.
Both the stream reach and catchments feature classesare generated using the TauDEM software program. The stream reach will contain numerous fields in the attribute table (Table 3), several of which are required for use in later tools in the ACPF toolbox. Each reach (junction to junction) in the output stream reach will have a corresponding catchment, or drainage area, to that section of stream. The “LINKNO” field in the attribute table of the output stream reach will equal the “gridcode” field in the attribute table of its associated catchment.
Both the stream reach and catchments feature classesare generated using the TauDEM software program. The stream reach will contain numerous fields in the attribute table (Table 3), several of which are required for use in later tools in the ACPF toolbox. Each reach (junction to junction) in the output stream reach will have a corresponding catchment, or drainage area, to that section of stream. The “LINKNO” field in the attribute table of the output stream reach will equal the “gridcode” field in the attribute table of its associated catchment.
Both the stream reach and catchments feature classesare generated using the TauDEM software program. The stream reach will contain numerous fields in the attribute table (Table 3), several of which are required for use in later tools in the ACPF toolbox. Each reach (junction to junction) in the output stream reach will have a corresponding catchment, or drainage area, to that section of stream. The “LINKNO” field in the attribute table of the output stream reach will equal the “gridcode” field in the attribute table of its associated catchment.
Description taken from TauDEM (Tarboton, 2004):
“With this method, the DEM is first smoothed by a kernel with weights at the center, sides, and diagonals. The Peuker and Douglas (1975) method (also explained in Band, 1986), is then used to identify upwardly curving grid cells. This technique flags the entire grid, then examines in a single pass each quadrant of 4 grid cells, and unflags the highest. The remaining flagged cells are deemed 'upwardly curved', and when viewed, resemble a channel network. This proto-channel network sometimes lacks connectivity, and/or requires thinning, issues that were discussed in detail by Band (1986). The thinning and connecting of these grid cells is achieved here by computing the D8 contributing area using only these upwardly curving cells. An accumulation threshold on the number of these cells is then determined via drop analysis.”
The area-threshold method is recommended for watersheds formed in more recent (Wisconsinan age) glacial landscapes, where classic stream formation and geomorphic land dissection processes have had little influence in the development of the landscape. That is, watersheds with dense networks of drainage ditches may be better suited to the area-threshold approach to begin the process of identifying streams.
The “correction” or hydro-modification of a DEM is conducted by the user to ensure flowpaths throughout the watershed are accurately represented. It is most important to make these corrections along major flow paths; for instance, it is more important to correct for flow beneath every highway bridge identified as a false impoundment than it is to correct for flow beneath every roadside driveway that has (or may not have) a culvert. The extent of the flow path network is reviewed in combination with the depth raster to identify candidate locations for DEM editing in the next tool (Manual cutter and dam builder). A value between 1 and 10 acres of contributing area is suggested for the area threshold. The smaller the area threshold selected, the more extensive will be the set of candidate locations for editing. This acreage threshold (rounded down to the nearest integer) is embedded in the name of the output, (i.e. a 2.5 acreage threshold will be recorded as "2ac" in the name of the output file.