Highway 84 Streambank Mitigation

La Honda, CA

Caltrans
Summer 2000

Storms in the winter of 1997/98 resulted in significant erosion and damage to Highway 84 along San Gregorio Creek, in San Mateo County. To repair this erosion, Caltrans contracted the construction of a gabion structure. Riparian habitat lost during this construction was mitigated by a contract with Hanford ARC to construct a rootwad structure adjacent to the gabion wall.

San Gregorio Creek is habitat to migrant fish populations and other sensitive species, requiring extensive protection during construction. Furthermore, it was necessary for Hanford ARC to access the streambed during construction. To accommodate these issues, an Environmentally Sensitive Area (ESA) barrier wall was constructed around the gravel bar and project area within which equipment would work. This wall consisted of keyed plywood, covered in plastic, with straw bales and orange fence lining the inside. Fabric was laid on the gravel bar to separate excavated material from the streambed gravel. A sediment trap with a silt bag was installed at the downstream end of the project to filter key trench water. Also, a tank was on site to be used in the event the sediment trap became inadequate.

The installation included five rootwads in two separate structures. Three of the rootwads were placed in an excavated bank, secured by header and footer logs, anchors, rock and backfill. The second set of two was installed on the slope surface, bolted to a crossed log arrangement, between two existing redwood trees. This structure was also bolted into drilled rock. The embankment was planted with willow stakes and containerized riparian trees.

Following completion of the rootwad structure, Caltrans retained Hanford ARC to install toe protection downstream of the gabion wall. This work included Ajacks toe protection and minor bank regrading. Again, a barrier wall was constructed and work was performed on a gravel bar within this wall. All work in the channel was completed by hand, and equipment worked from above.

Challenges of this project included construction on the shoulder of Highway 84, work in a channel with stringent water quality standards, and close proximity to sensitive species.