this is an example of what insufficient erosion control will do to a hillslope. Somewhere i have a copy of a picture i took about five or six yrs ago of the same drain. There was much more hillslope under the sock. They put these 'socks', or dissipators, on the ends of cross-drain culverts where they drain onto steep slopes in an attempt to control erosion. Sometimes it doesnt work. This one is on wa state hwy 112 just west of West Twin River on the olympic peninsula.