Stream Bounding Slopes

In progress…

Colette Breshears    MS Student in Geospatial Information Analysis, Boise State University

Stream bounding slopes, those adjacent to the channel, …………

If you have multiple USGS DEMs you’d like to use, but can’t get them to load properly (they show a medium grey color), see Mosaic Multiple DEMs lesson. Sometimes ArcMap applies a solid gray color to rasters as a default. Simply change Symbology from Stretched to Classified and back again. Grayscale color ramp should appear.

– DEM of study area
– ArcGIS 10.x w/ Spatial Analyst Extension

Part I: Create the Drainage Network
– Complete the Watershed lesson.

– Use the Raster to Polygon tool to convert all watershed pixels to a watershed-bounding polygon shapefile called ws_boundary (Conversion tools > From Raster > Raster to Polygon).

– Set the transparency 40% for ws_boundary and make sure it correctly aligns with the filled_dem DEM.

– Rename filled_dem raster to base_dem in ArcCatalog (Right-click on layer > Rename).

– Turn off all layers except for ws1_boundary and BaseDEM. Its a good idea to keep your Table of Contents organized from this point on.

– Clip base_dem with ws1_boundary (Data Management > Raster > Raster Processing > Clip tool. Name output ws1_DEM. Check the box next to “Use Input features for geometrical boundaries”.

– Turn off base_dem. You should now see the DEM for the watershed.

Part II: Delineate Reach Polygons
– Create a hillshade (ws1_hs) from ws_DEM (Spatial Analyst > Surface) to help visualize the drainage network and terrain.

– ……….Optional: To make sure that I was choosing a correct approximation for the width of my river valley, I created several points along my river (the deepest parts of the river valley) and calculated viewsheds from each in order to approximate the width of the actual river valley within the watershed. I picked 5 points – one on the mouth and the other four extending upriver, and gave them a height above the ground of 50 meters. I tried to pick terrain “bottlenecks” so that I wouldn’t be analyzing too much extended territory – only an estimate of the valley extent is necessary. Make sure you have the correct DEM selected in the Viewshed tool and in 3D Analyst toolbar.

– ……….Buffer your streamline (Geoprocessing > Buffer). Choose an appropriate buffer distance for your stream valley. The area inside this buffer will become your analysis corridor. Name output ws1_buff. In this example (…………), I used a ……….m buffer distance for the valley, which averaged……….m wide.

………..This step is accomplished in Watershed lesson: Using the Line tool (Draw toolbar), create an approximate representation of the main river along the river channel. Convert to a shapefile. This will become the boundary of the river valley buffer…………you need to select and isolate the trunk streamline as a separate shapefile………..

– Clip (Geoprocessing > Clip) the buffer shapefile using the ws1_boundary shapefile. Name final buffer ws1_buff_final.

– Clip (Data Management > Raster > Raster Processing > Clip) the ws_DEM raster with ws1_buff_final shapefile to create the stream-bounding slopes analysis corridor. Name output raster ws1_corridor.

– Create contours at an interval appropriate to the scale of the DEM and your watershed (Spatial Analyst > Surface > Contour). Name output shapefile ws1_contours.

………..Need to refine method of segmenting streamline and creating RR and RL polygons…….Use the Split Polygons tool on the Editor toolbar to draw polygons within the buffer shapefile. Polygon borders should somewhat follow the path of tributaries to the main river, encompassing slopes that face the river. After creating the cross-river polygons, trace the river to divide the buffer in half lengthwise……..Clear up this step….make intersections at tribs? or at contour-stream intersections? use COGO>Proportion tool to cut streamline into intervals? Convert polyline to points at a specified interval?Split Line at Point? Split Line at Vertices? Split tool in 3D Editor to automate RR and RL reach polygon creation? Does Hawth’s Tools extension do this better?……….

……………Smooth the clipped watershed raster with the Filter tool, set on Low.
Use the Slope tool to get a colored slope interpretation………….

…….The first, low-tech way to determine the average slope is to classify the slope raster into 5 classes using natural breaks, give each class a contrasting color and then to eyeball the dominant slope for each polygon.

………..The second way gives a more appealing map.

Part III: Slope Analysis
– ………Use the Zonal Statistics tool, with the reach polygons layer as the zonal input and the slope raster the input value raster. This will output a raster with the average slope per polygon, coded as a pixel value over the area of the overlying polygon.

– ………Go into the properties for this raster, and change the Symbology to Classified, then click Classify, select Natural Breaks (Jenks), and make sure there are as many classes as there are polygons in the polygon shape.

…….With all other layers off, you can use the Identify button on your toolbar to select pixels within different polygon areas to receive the calculated average slope for that polygon.

……..Why not do this instead of Z.S. step above (create the table in one step)?…..Use the Zonal Statistics to Table tool to create a summary table of slope statistics for each reach polygon. Use the Join tool on the polygons layer to add that table to it.

In Progress:
Q: I am seeking a method to create lines from points, where the both the line length and line orientation are controllable. Does Hawth’s tools do this?
– Skye Cooley

A: GME has the components that make that sort of thing possible, but you have to be clever about how you string them together. Sampling the points at equal intervals along the flow line is easy, and with sampleperppointsalonglines you can get those perpendicular end points in step 4. But then you have to convert those to lines, which requires a bit of cleverness with convert.pointstolines. Then for step 5 see geom.splitpolysbylines, but you may find it easier to do that bit in ArcGIS. – Hawthorne Beyer

Anderson et al. (2007), p. 413, Fig. 15 in Willett et al. (eds.), Tectonics, Climate & Landscape Evol., GSA Special Paper 398
Chang Intro to GIS, 6th edition

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