This post about groundwater sapping was written by Steve Grimmer, Little River’s Artist Mechanic.

I spend a fair bit of time walking in the Shawnee National Forest here in southern Illinois, and frequently spot geomorphological features and processes that our Emriver models illustrate so well. The latest example is on a short Rail-to-Trail path near my home that starts on public property and then goes through private land. The railbed follows the contour line of the hilly land and has numerous culverts allowing surface water to drain under it. The landowner closest to the trailhead is quite friendly and welcomes hikers and rock climbers on his property. In an effort to make the area more accessible and welcoming, ‘Sam’ recently brought in a bulldozer to push back overgrown vegetation from the rail bed and to clear new paths to bouldering areas on the property. For the most part, he did a good job.

Figure 1. Former low area, excavated to make a pool.

What is Groundwater Sapping? 

Before I continue with our story in the Shawnee National Forest, let’s talk about groundwater sapping. This is a phenomenon wherein groundwater outflow from a specific point transports finer sediment downstream, causing weathering or erosion. This in turn causes headward migration of the channel and the creation of a gulley or even a valley. Interestingly, sapping is also a military term for digging a tunnel towards a besieged fortification, often using excavated soil to form the roof of the tunnel. We see evidence of groundwater sapping all over the world and there are even valleys on Mars that appear to have been formed via groundwater sapping.

Back to the Shawnee National Forest

Adjacent to one part of the trail, there was once a low area drained by a plastic culvert that went under a concrete drive. In the spring and early summer, the area was a bit marshy, but it was dry in summer and fall. The landowner decided to excavate that spot to make a little pool that is filled by groundwater and surface runoff. In Figure 1, we can see several problems starting. The culvert under the concrete drive is now clogged with dirt and debris, so the pool will likely overflow its banks this spring when we get big rains. On the far side, we can see the exposed rocky soil where the hillside was excavated to enlarge the pool. It is already eroding back under the thin layer of topsoil, and will soon undermine the adjacent trees which will fall across the trail.

Figure 2. Water from the pool is making its way around the plugged culvert and undermining the concrete drive.

Water in the pool is now going around the culvert and is undermining a concrete drive that is occasionally used by the landowner. In Figure 2, we can see the retaining wall has failed and the soil under the driveway is beginning to wash away. What is most fascinating to me, however, is the Sapping that is occurring on the hiking trail itself. 

Figure 3. Groundwater sapping causing an incision across the hiking trail.

The now-constant pressure of groundwater from the new pond and plugged culvert is forcing groundwater to flow through the old rail bed, which is made from packed gravel and cinders. The sapping is causing the erosion to migrate across the trail towards the new pool. There are only about ten feet remaining between the incision and the pool, and when the two meet, the pool will likely drain quickly, with predictable results. 

Groundwater System Example

Where have I seen this before? Let’s look at stills from footage of experiments done recently in our Emriver Em3 stream table with our Groundwater System running. In Figure 4, we see a groundwater-fed pool on the left (sans Canis Familiaris) with the same undercutting of the uphill bank exhibited in Figure 1. On the right, groundwater sapping is causing headward migration of the gaining stream. Our Memphis modeling media has just the right cohesion to mimic what we see in the field. Note the little meander forming spontaneously in the zone of seepage.

Figure 4. Groundwater Sapping in the Emriver Em3 stream table.

In Figure 5, we see that the headward cutting has progressed all the way to the pool. Note the characteristic theatre-shaped headwall. Our Memphis media is also modeling a steep cut bank and point bar formation while maintaining a downstream U-shaped channel. Figure 6 is as expected! Perhaps the next time I see ‘Sam’ down on the trail, I’ll see if he’s amenable to opening up that culvert and sacrificing the pool in favor of the hiking trail.

Figure 5. The headward cut has reached the pool in the Emriver Em3 stream table

Figure 6. What can we expect this spring with the return of 4-inch rains?

You can watch the Groundwater System video where I pulled Figures 4-6 from here.