Introduction to Basic Ground-Water Flow

By the earthDr!

Analogy of the Flow of Rainfall Runoff to Ground-Water Flow

Ground-water flow is a little more difficult to understand than is the flow of water as sheet flow during or just after a rainfall.  Let's start with flow of water over the surface of  hilly terrain to begin to understand ground-water flow. The figure below-left depicts hilly terrain in a 3-dimensional perspective.  Even on hilly terrain there are numerous points that share the same elevation Select  any  point on hilly terrain and then, a line can be drawn connecting  all  contiguous points, and their adjacent points,  that share this same elevation.  Usually  a  curved  line  is needed to join all points of similar elevation   However, on a flat surface a straight  line will join equal elevations.   These  lines are called contour  lines.  Typically, some  multiple of 5, 10, or 20 is  selected to construct a line connecting  points of equal elevation on a topographic map which is used to describe topographic relief (relief = elevation change).  If contour  lines are drawn at 750, 760, and 770 feet  then, multiples of 10 were used to construct contours of  equal  elevation.  Other lines could be constructed from a greater or lesser common multiple.

Sometimes multiples of a 1-foot  contour is done. Since the grade at the surface of  the water table is often so flat (limited grade or hydraulic gradient - not steep), a contour of the water table is often constructed in some multiple of 0.01 foot (1 foot rise in 100 foot run) or sometimes 0.001 (1 foot rise in 1,000 foot run) for a very flat water table.  However, contours of the water table in multiples of 5 and 10 feet can be practible where there is significant relief (elevation change) such as in more mountainous terrain.  Contouring in multiples of 0.01 foot would not be practicable for mountainous terrain to describe either surface relief or the surface of the water table since this would involve an inordinate number of contours being constructed for the steep slopes.

In the first figure, contour lines are constructed in multiples of ten.  The contour lines run from a 60-foot to 100-foot elevation above mean sea level. Remember that the contour line join points of equal elevation. Therefore, any point along the 70-foot contour line should be at an elevation of 70 foot. A 3-dimensional plot of contour lines, while it illustrates the concept of a line joining points of equal elevation, is not as useful a tool as a topographic map where contour lines are depicted in plan view as in the figure to the left. In a 3-dimensional plot, higher elevations in the foreground can obscure contour lines in the background, thus, limiting its use. This figure in plan view is just the view from above of the first figure, which is displayed in 3 dimensions. Plan view (a view from above) is just the most convenient perspective to view the topographic relief as illustrated in the 3-dimensional view above. It won't take you long to get accustomed to conceptualizing a plan view, which is in 2 dimensions, in 3 dimensions.

The last figure to the left is just a slight modification of the previous figure. Arrows have been drawn at right angles to some of the contour lines. The arrows have been drawn to illustrate the direction of flow of water across hilly terrain during a heavy rainfall. Excess water will flow across the land surface as a sheet of water. Typically, the water will flow down the steepest slope. The steepest slope is always going to be at a right angle to the contour of the land joining points of equal elevation. The controlling factor for direction of sheet flow over hilly terrain is where surface elevations change the most as indicated by the close spacing of the constructed contour lines. Change in elevation determines the direction of sheet flow of water.