Sequoia makes laser diffraction and acoustic backscatter sensors for this application.
On-line monitoring of silt in hydro-power plants is a reality using Sequoia’s LISST-Infinite or LISST-Hydro instruments, designed specifically for this purpose. The LISST-Infinite and LISST-Hydro measures the concentration as well as the size distribution of silt passing through the turbines, and warns the operator if the concentration approaches a level that would be dangerous and cause abrasion of the turbines. Read the entire article, published in Water Power & Dam Construction’s newsletter in June 2012.
In July-2015, we have introduced a less accurate but significantly less expensive instrument, the LISST-ABS. It offers a single estimate of suspended sediments. The measurement is heavily weighted for the dangerous coarse grains. See here for more details.