When increased velocity does result, the flood wave may move downstream faster and increase flood stages downstream from the cleared reach. In large rivers, the effect of drift removal on bank-full conveyance may be negligible. Channel modifications to prevent drift accumulation include channel straightening, channel enlargement, drift removal, and the elimination of islands, sloughs, and side channels. New forms of channel management are designed to remove as little drift as possible while still achieving management goals. Flow transporting drift distributes it to unblocked portions of the rack, which then become clogged.
Booms seem most effective where currents are slow, but net booms are effective even in faster flow.
171K GIF version of this photograph Figure 13. Trash rack and accumulated drift in Georges Creek near New Columbia, Illinois. The most serious problem in using trash racks or holding booms to protect highway bridges from drift delivery is the large amount of drift that can be transported through the relatively small channel cross section. This is much more than enough to completely block the channel. The existence of such accumulations indicates that a single flood in some rivers may transport more than enough drift to block the channel completely. . . .