We had something similar happen in a small town just outside of Lubbock Texas a few years back. A truck overturned carrying something like 4.8 million Denver-minted pennies overturned into a bar ditch during a downpour. The company I was working with at the time was housed in a building owned by my boss's father, and the father contracted with the government to clean and repackage the pennies, so a day or so after the accident three flat-bed semi's pulled up at one of our warehouses and started to offload something like 20 tons of dirt and pennies dug out of the ditch. It took almost 4 months to sort the coins.
The father tried 3 or 4 different methods for sorting the coins before he came up with a large sifter-screen with 1/4 inch wire mesh that he would dump a few bucket-loads of dirt onto and wash out with water, then sort out the rocks and plant-bits by hand, dry the coins and rebag them.
