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Lab 2 – Topographic Maps, Watershed Delineation, and DEMs CE 321, Fall 2008
Objectives
- become familiar with USGS topographic maps
- learn how to visualize contour maps in three dimensions
- learn how to delineate watersheds using a topographic map
- learn how to delineate watersheds the modern way using a digital elevation model (DEM) and geographic information system (GIS) software
Background
A fundamental skill in environmental engineering is the ability to read a topographic map and accurately determine the drainage area (i.e. the watershed) for any point along a stream. This is the surrounding land area from which all runoff will flow to the point of interest. This skill is particularly useful because water quality at any location along a stream or river is related to land use (point and non-point sources of pollution) within the watershed draining to that location.
Topographic Maps
To be able to delineate a watershed, one first needs to know how to read a topographic map. Typically we work with a 7.5-Minute Series Topographic Map from the United States Geological Survey (USGS). These are named by “Quadrangle” - usually the largest town on the map. These maps have a scale of 1:24,000 or 1” = 2000’ and a contour interval of 20 feet. The contours connect points of equal elevation and are shown in brown. Heavy brown lines are the 100-ft contours. Other features shown are rivers, streams, municipal boundaries, towns, roads, schools, buildings, forested areas, etc.
The figure on the next page is part of the Easton, PA Quadrangle. It shows Easton, Lafayette College, and the lower reaches of Bushkill Creek winding around campus and emptying to the Delaware River.
Look carefully at the contour lines on the map and verify the following:
Based on the contours shown, elevations here at Lafayette College range from less than 300 ft at the stadium to above 360 ft (note the survey benchmark marked BM 350 near Pardee Hall)
By looking at the contours surrounding the campus area, one can determine that our campus sits on a hill – you also know this from first-hand experience
The ground surface slopes steeply downward (note the closely spaced contours) to the west and south (toward Bushkill Creek) and to the east (toward the Delaware River). To the north, the land dips and then rises up to a ridge at about 600 feet
Exercise 2. 3-D models from contours
For each of the following four topographic features, use Play-Doh to create a 3-D model of the contours shown
Exercise 3. Delineate a small watershed
Delineate the watershed of the small stream at the point shown (i.e. draw a line that encloses all the area from which runoff will flow to the point):
Tools: Pencil & eraser
General method (as they say, practice makes perfect):
Locate the “drainage divides”, the topographic features such as hills and ridges that divide flow between adjacent streams – rough sketch in these boundaries first so you know the general shape of the watershed
Sketch your way uphill from the point of interest, always perpendicular to contour lines
For hills and other contour loops or bends, draw the boundary such that it divides these features in half
Check that the watershed boundary is everywhere perpendicular to contour lines and does not cross any drainage divides
Finally, check that there are (1) no areas inside your delineated watershed where the contours indicate flow going away from the point of interest, (2) no areas outside your delineated watershed where contours indicate flow toward the point of interest
Near the bottom right of the image is the confluence of Bushkill Creek and the Delaware River. This is the outlet for the Bushkill Creek watershed.
- We now need to add a layer that has the outlet point. Click the yellow plus sign to Add Data, open the folder named outlet, and then select Outlet.shp (green icon) and Add
You should now see a small dot on the stream at the watershed outlet
Now before going to step 7, we need to be sure the watershed delineation will occur over the entire extent of the DEM:
First go to Tools > Options > Geoprocessing > Environments > General Settings > scroll down to Extent and select “same as layer Fill_ned_0791”.
Now go to Spatial Analyst > Options > Extent and select “same as layer Fill_ned_0791”
- Okay, we’re almost done – select Spatial Analyst Tools > Hydrology > Watershed
Under the “Input flow direction raster” box, click the down arrow and select FlowDir_fill Under “Input raster or feature pour point data”, click the down arrow and select Outlet, then OK
You will get new layer named Watersh_flow1 that is the shape of the watershed
- Convert this layer from a raster to a polygon so that it can be displayed with other data
Select Arc Toolbox > Conversion Tools > From Raster > Raster to Polygon
Under “Input raster” box, click the down arrow and select Watersh_flow1 and then OK
You will get a new layer named RasterT_watersh
Click the checkbox in the left panel to remove the previous layer (Watersh_flow1) from view.
Double-click on the box below the new layer RasterT_watersh1. Under Options – Fill color, select No Color so that the layer is transparent and you can see other layers behind it. Change the Outline Color to something that stands out (red for example).
Now if you display only the three layers Outlet, RasterT_watersh1, and Fill_ned_0791 (use the checkboxes) you will see the topography with the outlined watershed.
Whew that was a long process, but a big improvement over pencil and eraser!
Lab Assignment Due Sept 11. Use the pencil and eraser method to delineate the Mud Run watershed for the point shown (a color map is in the P:\CEE_Drive\CE321 folder).
