For a laboratory / field test, I'm looking for an easy solution to measure the soil surface elevation. The test runs a flow on a bed. When the erosion takes place the sediment concentration goes high in the water and makes the water unclear. Therefore, the usual laser, infrared, or ultrasonic depth measurement does not doe to high sediment contents. The range of distance between where a device could be mounted and the bed is 15-30 cm. What kind of device can measure a short distance through sedimented water?
The industrial sensor for sensing the level of sludge in a process tank is an "interface level analyzer". I worked on such a device many years ago, and it did indeed track a muddy layer of sludge 10-20 feet down in a murky sewage settling basin.
You can always do a physical test. Use a servo which periodically lowers a test object. Measure the object's position directly. Since object will be raised out of the water most of the time, it should not affect your test much.