Rock Cage (Gabion) Dams
Contour Trenching: A woven galvanized wire basket sometimes lined with geotextiles and filled with rock, stacked or placed to form an erosion resistant structure.
Purpose: Also known as rock fence check dams, these structures are used in intermittent or small perennial channels to replace large woody debris that may have been burned out during a wildfire. The rock cage dams provide a degree of grade stability and reduce flow velocities long enough to trap coarse sediments.
Relative Effectiveness: Not enough interviewees rated this treatment. Comments by some individuals indicated favorable results. On mild gradients these structures work well. Some failures occurred on steeper slopes when high velocity flows are greater than 3 ft/sec (1 m/sec). This is a common theme for all channel treatments. Most of the failures occur where treatments are imposed on steep gradient sections of ephemeral or first to second order perennial channels. Rock cage dams often last long enough and trap enough fine sediments to provide microsites for woody riparian vegetation to get reestablished. Rock cage dams on the Wenatchee National Forest were very successful, trapping 2000 to 10,000 yd3 (1500 to 7600 m3 ) of material after just one storm.
Implementation and Environmental Factors: Like most other ESR channel treatments, proper dam design and installation by experienced crews are crucial to success. The rock cage dams must be properly placed, keyed in, and anchored to stay in place during runoff events. Downslope energy dissipators are recommended because they reduce the risk of the rock cage dams being undercut.
Other Factors: Construction of these structures is dependent on the availability of adequate amounts and sizes of rocks. Rock cage dams need to be cleaned out periodically if they are to maintain their effectiveness.