Devil's Slide Improvement Project

Route 1 from Half Moon Bay Airport to Linda Mar Boulevard in Pacifica,

San Mateo County, California





Robert Gross, Chief
Office of Environmental Planning, South
Caltrans District 4
P.O. Box 23660
Oakland, CA 94623-0660


Project Purpose and Need

The purpose of this project is to provide a safe, dependable and stable State highway route to bypass the geologically unstable area of Route 1 at Devil's Slide in San Mateo County. Since the construction of the existing Devil's Slide portion of Route 1 in 1937, several geologic factors have contributed to landslides, falling rocks, and grade subsidence resulting in diminished roadway width. Despite drainage improvements, pavement reinforcement and rock anchors, Route 1 continues to experience difficulties and closures due to landslides and roadway subsidence.

Alternatives Considered

Tunnel (preferred alternative)

The preferred tunnel alternative is a 1,219 m (4,000-foot) long, double bore facility with one lane in each direction. The north approach road is about 457 m (1,500 feet) long, and the south approach road is about 305 m (1,000 feet) long. Proceeding south from Pacifica, the alignment departs from existing Route 1 (KP 64.1/PM 39.8), along a 7% uphill grade, crosses the valley at Shamrock Ranch, passes through a small ravine, enters the tunnel beneath San Pedro Mountain, and exits the tunnel just south of the Devil’s Slide area where it rejoins the existing highway (KP 61.0/PM 37.9).

Two tunnel design variations, one using 9.1 m (30-foot) wide bores (variation A), and one using 11.0 m (36-foot) wide bores (variation B), are analyzed in the draft SEIS/R. The total project costs of tunnel variations A and B are estimated to be $134,900,000 and $137,550,000, respectively. Tunnel variation B provides pedestrian and bicycle access inside the tunnel, while variation A provides a pedestrian/bicycle path outside the tunnel.

Martini Creek Alignment

The Martini Creek alignment alternative was the previous preferred alternative identified in the 1986 FEIS, and was selected in the Federal Highway Administration (FHWA) 1986 Record of Decision. In September of 1986, approximately $52 million in special Federal emergency relief (ER) and other funds were obligated for the Martini Creek alignment alternative when the Federal government approved the plans, specifications and estimates (PS&E) for constructing the Martini Creek alignment. Recently revised cost estimates for the Martini Creek alignment have increased to $112 million ($85.9 million for construction plus 30% for engineering) due to inflation and various design changes.

Certain aspects of the project design have been modified since the original proposal in 1986 to specifically avoid or minimize impacts to recently discovered sensitive habitat and/or plant and animal species in the project area. Longer bridge structures have been incorporated to clear-span valuable riparian and wetland habitat at Martini Creek, Green Valley Creek, the Shamrock Ranch ponds, and San Pedro Creek. A new structure has been included in lieu of a culvert near station 446+00. In addition, modified cut and fill slopes would reduce the disturbance to vegetation, thereby reducing impacts from siltation both during construction and afterwards during heavy winter rains. These same design changes will lessen impacts to anadromous fish populations in San Pedro Creek.

The passage of Measure T in November 1996 initiated the process to amend the San Mateo County certified Local Coastal Program (LCP) to provide a tunnel as the preferred alternative, and to delete references to the Martini Creek alignment alternative. The Measure also requires voter approval of any other alternative to the tunnel, except repair of the existing highway. On January 9, 1997, the California Coastal Commission voted to certify the LCP amendment as submitted by the County.

No Project Alternative

The no project alternative, as described in the 1986 FEIS, consists of continuing with the general maintenance and repair of the existing alignment. The periodic slides and slipouts, resulting in road closures of various lengths of time, can be expected to continue. There is a high probability that a seismic event of sufficient magnitude will eventually occur and result in permanent closure of the existing roadway.

Alternative Considered but Withdrawn

Repair/Maintain Existing Alignment (Slide Dewatering)

Based on an independent draft report by Dr. John Hovland (Hovland, 1998), a local geologist, it has been proposed that the existing highway could be preserved by an extensive effort to drain the active slide and the adjacent landmass. Dewatering slides can be an effective means of increasing the stability of a slide mass when geological and hydro-geological conditions are conducive to point drainage technology. However, the success of subsurface drainage efforts, whether they be passive horizontal drains, well points employing pumps, or a combination of these techniques, are dependent upon site specific conditions such as: the permeability of the geologic formations; the connectivity and continuity of groundwater reservoirs and the ability to intercept the groundwater with wells or drains.

In November 1997, the Caltrans HQ Engineering Service Center (ESC) Roadway Geotechnical Engineering - North Section initiated a dewatering feasibility study to determine if dewatering was possible and feasible as a long-term solution to the recurring slide movement within the most active area of Devil’s Slide. The major component of this effort was the installation of two test wells to potentially improve roadway stability prior to the anticipated wet "El NiZ o" winter, and to provide data and information needed regarding the feasibility of using deep wells to dewater and stabilize this area in order to support and maintain a stable and permanent highway.

The Dewatering Feasibility Study concluded that the subsurface groundwater regime within the study area is complex, and dewatering would be extremely difficult. The slide mass has a low to very low hydraulic conductivity and removing groundwater from the slide mass is expected to be difficult and have limited lateral impact on the groundwater table. This limited ability to remove groundwater from Devil’s Slide supports the conclusion that dewatering this slide area is not feasible and does not meet the purpose and need, and therefore is not considered to be a viable project alternative.

Environmental Setting

The project site is located in the Santa Cruz Mountains, which are part of the Coast Ranges of California. Devil's Slide is a name given to a steep, rocky coastal promontory located about midway between Montara and the Linda Mar District of Pacifica. The terrain is characterized by steep, eroded slopes with natural gradients ranging between thirty and seventy percent.

There are small coastal valleys throughout the length of the project along the major drainages within the Montara Mountain watershed. The soils in these valleys are deep and moderately well drained and have built up along the low terraces and alluvial fans along the stream channels.

The climate of the project area is Mediterranean with a strong maritime influence. The temperature range is narrow both seasonally and diurnally, while air moisture is relatively high.

Extremely dense northern coastal scrub covers most of the project vicinity, especially over San Pedro Mountain and along the steeper foot slopes of Montara Mountain. Small grassy openings and barren rocky areas are scattered throughout the scrub areas.

The proposed project corridor also traverses areas with other types of vegetation both native and introduced including, aquatic and coastal freshwater marshes/seeps, willow riparian scrub, coastal grassland, non-native forest, and pasture/ranch uses/non-native brushland.

Environmental and Socioeconomic Effects

Potential environmental and socioeconomic effects that could occur as a result of the proposed project are summarized in Table S-1.

TABLE S-1: Summary of Impacts





Martini Creek



AESTHETICS Visual contrast at portal areas, and bridge. Severe visual scars from cuts and fills. Revegetation will not fully mitigate. Vehicles and roadway visible from Park and surrounding area.
AIR QUALITY No impact No impact No impact
CONSISTENCY WITH LOCAL, REGIONAL AND STATE PLANS Consistent with County LCP. Certified by CA Coastal Commission.

Consistent with the current draft RTP.

Not fully funded.

Not in conformity with County LCP. Voter approval required to reverse existing Program. Not consistent with the current draft RTP. Not fully funded. No non-conformity or funding issues.
CONSTRUCTION short term dust, run-off and siltation impacts. short term dust, run-off and siltation impacts. Run-off and siltation impacts and repair periods.
CULTURAL RESOURCES No Impact No Impact No Impact
FARMLANDS No Impact Take of 4.2 ha (10.4 acres) of farmland. No impact
FOG Some visibility reduction at portal areas from June to September. Visibility reduced at higher elevations from June to September. Some visibility reduction from June to September.
GEOLOGY/SEISMOLOGY Minor deformations and architectural damage during major seismic event. Roadbed stable with some rockfalls/rockslides during major seismic event. Potential permanent road closure risk during major seismic event and/or landslide.
GROWTH INDUCEMENT No growth inducement impacts. No growth inducement impacts. No growth inducement impacts.
HYDROLOGY No significant floodplain encroachment or risk. No significant floodplain encroachment or risk. No significant floodplain encroachment or risk.
NATURAL ENVIRONMENT Temporary construction effects to peregrine falcon nesting activity and red-legged frog - mitigated by hacking program (falcon) and avoidance measures. Takes 1.41 ha (3.5 acres) of riparian habitat. Reduces home range for large mammals; creates migration barriers. Impacts to red-legged frog habitat. No impact
NOISE No noise receptors to be affected Adverse impacts in Park and proposed campgrounds

Some construction impacts from blasting.

Existing alignment generates more noise impacts along beach portion of Park.
PARKLAND No Impacts No Impacts (based on previous joint planning efforts) No impacts.
SOCIOECONOMIC Permanent acquisition of 74 acres required. 52.6 ha (130 acres) to be acquired. Some relocation of ranch property at south end. Continuous closures from slides will adversely affect businesses and residents south of Devil’s Slide
TRAFFIC No impacts. No impacts. Adverse impacts expected due to continual roadway failings.
WATER QUALITY Runoff/Sedimentation impacts expected during construction. Runoff/Sedimentation impacts expected during construction. No Impacts.

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