Today I want to talk about an amazingly useful tool that I’ve used to great effect many times. Contour maps. A contour map is a way of visually representing a three dimensional environment on a two dimensional plane (a sheet of paper). In short, it shows the hills and valleys of the land area on the map.
The basic idea of contour mapping is that the lines are a constant elevation. So if you were to walk the line depicted on the map you would never go uphill or down. Each line represents a particular elevation, and the closer the lines are to each other, the steeper the grade. A circle, or any closed shape, represents a hilltop or sinkhole. When reading a contour map it’s always important to keep in mind, any sort of V shape means a ditch or channel, and it flows in the direction opposite of the way the V is pointed.
That description probably doesn’t make a whole lot of sense when written long form, but hopefully it will become more clear when you look at the map below. If you take nothing else from this post always remember, more lines mean more hills and less lines means flatter.
Contour maps are used for a lot of things. In hydrology they’re used for determining watershed boundaries and stream steepness. They’re indispensable to land developers, and anyone interested in building anything other than a small house, because they help determine the suitability of a plot of land for various uses. For example, the example map above has a lot of contour lines very close together indicating it’s very hilly. It’s obviously too steep to farm, and you wouldn’t want to try and put up any large buildings. A Walmart with a large flat parking area would be out of the question.
The primary use I want to discuss today is watershed delineation for bridge design. The watershed area is a primary component in determining how much water is flowing to a particular bridge. The sample map I’ve posted above shows the watershed of Crocker Spring Branch, which is a minor tributary to White’s Creek in northern Davidson County. I picked this area because it’s local to Nashville and it has lots of hilly topography, making it easier to see for people who aren’t familiar with reading a contour map.
The example is delineating a watershed for the bridge at the bottom right corner of the map. The overall watershed is in red with smaller interior watersheds drawn in blue. The general rule is that water flows perpendicular to the contour line. Determining watershed boundaries is a simple exercise in following the highest ridge. You start at the bridge location and move out following the highest points. I started to the left of the structure at the obvious ridge and followed it back from the road. It tends to get a bit tricky when there are lots of smaller creeks flowing into the mainline, and I drew in several of those for illustrative purposes. It’s a fairly simple process in a location with easily defined ridges like this, but it can be a bit difficult in flatter areas such as west Tennessee near the Mississippi River.
Here’s an overhead photo of the same area. It’s very difficult to see much without the contour lines.
I’m going to close with this map clipped from the same general area as Crocker Springs Branch. (I’ve placed red arrows beside a couple of the hills just to show how the contours close on each other.) You can see Whites Creek running through the center, with several offshoot tributaries. The general lack of contour lines around the main creek indicates a fairly wide floodplain and the general steepness of the tributaries indicates runoff is going to get to the main creek pretty quickly. So based on this map you can conclude that Whites Creek probably floods regularly and it can rise fairly quickly. You can also see how the roads are generally down in the flatter areas wedged in to one side or another of the floodplain at roughly the bottom of the hills. This is very common in this type of area because it’s much easier and cheaper to build a road on flattish ground, but you place it at the bottom of the rise so it’s less likely to flood.