Channel liner -- Most of the various types of channel liner have proven effective for erosion protection. Some types of channel liner include low quality concrete (lightly reinforced), rock, soil retention blankets, articulated concrete blocks, and revetment mattresses. Channel liners, when used as an outlet velocity protection measure, should be applied to the channel area immediately downstream of the culvert outlet for some distance, possibly to the right of way line and beyond (with appropriate easement). (See Chapter 6 for types and guidelines.) These liners, however, are viewed as creating environment problems, including decreased habitat and increased water temperature. They also are viewed to increase impervious cover, decrease time of concentration, and change the hydrograph timing downstream. In many instances, the liner may stabilize the area in question, only to have the problem shift downstream to where the channel is not lined.

Pre-formed outlets - Pre-formed outlets approximate a natural scour hole but protect the stream bed while dissipating energy. These have been shown to be effective protection in areas threatened by excessive outlet velocities. Such appurtenances should be lined with some type of riprap. (A velocity appurtenance for a culvert may be classified broadly as either a protection device or a control device.)

Channel recovery reach -- Similar to a pre-formed outlet, a channel recovery reach provides a means for the flow to return to an equilibrium state with the natural, unconstricted stream flow. The recovery reach should be well protected against the threat of scour or other damage.