Editor’s note: This post about In-Channel Gravel Mining was written by Steve Grimmer, Artist Mechanic at Little River.

One of the things Emriver stream tables do best is a quick demonstration of the effects of channel disturbances on a stream reach that is in equilibrium. Because our media’s density is about 60% of lithic sand and gravel, it exhibits a very dynamic response to any perturbation in the system. Let’s look at a terrific demonstration that is appropriate for groups from middle school through college and for landowners and miners in river basins. It’s from Chapter 4 of our Lab Exercises Manual, included with every Emriver stream table, and free for anyone to download. 

4.1 In-channel Gravel Mining

Although outlawed or heavily regulated in many parts of the world, mining of sand and gravel from stream channels is common in some regions. The Emriver stream table is particularly useful for demonstrating the effects of gravel removal from channels, especially the offsite impacts that are usually unknown to laypeople. These include channel downcutting (also called incision) upstream and downstream of the site, increased lateral migration, and bank erosion upstream of the mined area.

Begin with a standard meandering channel with a sinuosity of about 1.3. Alternately, you can begin with a straight channel. In either case, route flow pulses of about 150 ml/sec through the model for at least 15 seconds to form channel features. In the straight channel, you should allow alternate point bars to form. Adjust flow discharge (Q) to 35 – 55 ml/sec. After decreasing the flow, make sure that the channel is metastable and sediment transport continuity is consistent throughout the channel.

Stream Table Setup

Here’s a super quick video from our teaching DVD (and our Youtube channel and our matching Youtube playlist) showing how to set the initial conditions in an Emriver Em2 with Alpha modeling media. I like to roughly level the media first, then draw my desired meanders with the triangular scraper before excavating the channel with a scoop. Then I pack the media with a wooden concrete float. Note that wet, packed media has much better cohesion than dry, loose media, leading to much more stable stream banks.

Stream Table Demonstrations

Once you have a relatively stable reach established (not too much bank erosion, and consistent sediment transport along the section), try some in-channel mining. With a scoop or your hands, remove 250ml (about 1 cup) of media from the channel. This video demonstrates an extreme example. From the video guide:

A very good example of channel incision up and downstream of a radical  in-channel gravel mining operation. The clip opens with a marginally braided self-formed channel flowing within sets of paired terraces. A scoop is used to create a large pit in the channel, causing a strong headcut to form. As this headcut moves past upstream cutbanks, they collapse. Note also another well-defined headcut forms after the sediment from this collapse is removed. We then see a very distinct incision downstream of the pit. Upstream, note that the sediment entering the frame at left appears to be much smaller than that entering the mined pit, and that this material is being eroded from the channel bed and banks.

Here’s another video example, with repeated removals. In this instance, we’re using a little wand on a wet/dry vacuum, but the principle is the same. The channel in both these videos is quite mature, complete with extensive terracing and point bars.  As our DVD companion manual explains:

This clip has good examples of bank instability caused by incision above an in-channel mine and also the rarely seen (at least in these models) distinct headcutting downstream of the mine pit. The clip opens with a low flow rate, which is increased as it progresses. The downstream headcutting, which is indicated by graphics at about 0:47, occurs after the pit is excavated a second time.

Figure 4.1 from our Lab Manual shows the expected response to mining in the channel. Note that the flow is from right to left in this image.

Figure 4.2 shows a nice schematic of the channel explaining how the mined area steals all the sediment from the stream. Since the flow leaving the mined area is starved of sediment, it will erode the downstream area to re-balance itself.

Places to watch for downstream erosion, in particular, are the outside banks of bends in the channel, which will slump and fail from toe scour and from the incision lowering the river overall. This exercise is a great illustration of the sensitivity of rivers and streams to in-channel mining, and it shows that great caution should be exercised before deciding to go in with heavy equipment.