What Were They Thinking?

What

Were Hilton They Ocean Beach (North) Thinking?

Field 37°45'0"N Trip Ocean Beach (South) Guide

AAG Thornton Beach Vista San Lynvale Court

Francisco Northridge Landslide 2016 Mussel Rock, Westline Dr 37°40'0"N dunes Ocean View Apartments Jerry Davis* Leonhard Blesius* Milagra Creek Bob Battalio** Pacifica Pier . *Geography & Environment Mori Point . San Francisco State University geog.sfsu.edu San Pedro Creek .

**Chief Engineer, VP Pedro Point Trailhead . 0123450.5 Environmental Kilometers 01230.5 37°35'0"N Science Associates Miles Devils Slide Trail www.esassoc.com National Geographic, Esri, DeLorme, HERE, UNEP-WCMC, USGS, NASA, ESA, METI, NRCAN, GEBCO, NOAA, increment P Corp.

122°30'0"W 122°25'0"W

Stop 1. Ocean Beach, San Francisco  Beach profiles by Geog 101 (Our Physical Environment) classes.  Guest speakers: Bob Battalio, ESA

Ocean Beach has demanded the attention of engineers in San Francisco since the 19th century, with seawalls a common tactic (Battalio & Trivedi 1996), and visible along the drive south from the Cliff House. Sediment dynamics favoring the north end of the beach appear to relate to intersecting refracting waves resulting from offshore bars, creating large breaking waves (Fig. 1) and high wave energy. Many studies have been published about Ocean Beach’s situation, including an analysis of past erosion rates (Battalio and Trivedi 1996), predictions of the effect of sea-level rise (Revell et al., 2011), and wave-driven surf-zone dynamics (Jones et al. 2010). The challenge is part physical, part political and part economic.

Our main stop at south Ocean Beach is in a zone separating northward and southward sand migration, but we'll start with a brief stop at north Ocean Beach, the source of sand for a nourishment project at the south. At south end, the beach and dune system ends with a transformation to an uplifted terrace of Plio-Pleistocene sedimentary rocks. Sand transport on the dune system just north of this location creates a maintenance problem as sands blow onto Great Highway. The parking lot here is progressively losing space, with futile attempts at armoring. A 2011 (13 July) decision by the California Coastal Commission demanded a long- term fortification plan, and denied approval for riprap armoring that has been used in the past (Zito 2011).

Fig. 1. Waves breaking over the SF bar at -36' mllw depth. Wave intersection patterns from focusing of refractions. Biggest waves and widest surf zone in middle of Pacheco to Ulloa St. Right figure shows dominant sand transport.

Fig. 2. Ocean Beach at Sloat 1972 (upper) and 2010 (lower), showing many apparent changes, including changes in management practices. Note the tunnel entrance in 1972, now covered by dunes. Copyright (c) 2002-2010 Kenneth & Gabrielle Adelman, California Coastal Records Project, www.californiacoastline.org.

Fig. 3. Beach profile at Ocean Beach, as surveyed by students of GEOG 101 at San Francisco State University over the course of 12 years. Notice the generally higher profiles in the fall (green profiles), reflecting the lower wave energy in the summer. Winter storms erode the beach, which is evident by the lower profiles in the fall (red profiles). A particularly drastic example of strong winter storms eroding the beach occurred in the winter of 2009/2010. The accumulation of magnetite was much thicker than in other years. By October 2011 the dune face had retreated considerably.

Proposed Restoration and Protection of the Lake Merced Tunnel

One key goal of restoration work at the south end of Ocean Beach is protecting the Lake Merced Tunnel, which carries combined sewer and storm drainage from Fort Funston under Great Highway to the Westside Pump Station. The preferred design by SPUR (2015) is to preserve the tunnel, protecting it with a secant wall with cap, maintaining dunes enhanced by dredged sands, and replacing the highway with a bike trail. The wall is a similar concept to what was constructed at Taraval, four blocks to the north, in the 1940's, that has survived occasional exposure during El Niño events in 1982-83, 1997-98, and 2009-2010.

An equilibrium profile and sand budget approach was used for the Ocean Beach Master Plan, based on a shore profile presumed to be a response to wave dissipation, with the profile shifting according to changes in sand supply and sea level. A key to the success of this proposal is application of the Bruun Rule, which predicts that shore recession is approx- imated by sea level rise divided by the slope of the shore face. Shore face slopes at this site have been estimated by ESA PWA as 1:50 (SPUR 2015). Adequate sands are assumed to be available from dredging of the nearly Golden Gate shipping channel (SPUR 2015).

Fig. 4. Proposed dune restoration, replacement of highway with bike trail, and protection of the Lake Merced Tunnel with a secant wall and cap (SPUR 2015).

Stop 2. Thornton Beach Overlook, Daly City.

We are on a terrace created during the last interglacial (Sangamon) around 100,000 years ago, and now uplifted up to 150 m above sea level. Stratigraphic units include the Plio- Pleistocene (3 to 0.45 Ma) Merced Formation of marine sedimentary rocks and the marine, estuarine and terrestrial Colma Formations deposited during that interglacial (Konigsmark 1998, Brabb et al. 1998, Clifton et al. 1988). The Merced includes lenses of ~600 ka Rockland ash from the Lassen area. The Colma is less consolidated than the Merced, and is apparent as a more horizontally bedded unit, due to its more recent age, and the combination produces an unstable cliff face as a result of its 150 m of relief and often >100% slopes.

At this site, we have a great view to the south of a sequence of large landslides below housing developments built by Henry Doelger in the 1950’s (made famous by Malvina Reynolds’s 1962 song “Little Boxes”). This overlook was built a few years ago, following many years of neglect since the road to Thornton Beach collapsed in a 1982 landslide. When visited in 1985, we could still see picnic tables remaining from this developed state beach. The displays at this overlook present a simple interpretation of the landslide as well a bit of the history of this problematic site. One interesting piece of its history is the Ocean Shore Railroad (1905-1921), built from San Francisco along the cliffs, with its goal of reaching Santa Cruz unrealized as a result of the high cost of maintenance along this landslide-prone coastline.

From this overlook, we’ll try to explore down the social trails to get a look from below the failed cliffs. In one climb-down, we’ll be able to use metal handholds that you suddenly realize are pieces of rebar from some structure that moved down with the collapsing cliff.

During the following two stops, we’ll see other parts of this problematic terrace as we go south towards the San Andreas Fault at Mussel Rock. Along the way are many locations where homes once stood, homes whose owners received little compensation from the city whose planning department didn’t stop this poorly-considered development plan.

Units north of San Andreas Fault:

Northridge landslide

Mussel Rock

dunes Apartments

1998 El Niño

Units south of San Andreas Fault:

Pier

Mori Point

San Pedro Creek

Devils Slide

Units south of Pilarcitos Fault (at San Pedro Creek):

Figure 5. Geologic map along Daly City and Pacifica, California. Modified from sections of : Brabb, E.E., R.W. Graymer, and D.L. Jones (1998). Geology of the Onshore Part of San Mateo County: Derived from the Digital Database Open-File 98-137., US Geological Survey. 6

Stop 3. Northridge Landslide. December 2003.

Photos: J. Davis (2003)

Photo: J. Davis (2016)

Figure 6. The massive Northridge Landslide. In December 2003, 500,000 cubic yards of failed material created a temporary peninsula out into the Pacific (Cotton, Shires & Associates, Inc. 2004). The church and its parking lot were being built at the time. Subsequent failures have removed part of the lot..

Stop 4. Mussel Rock (Westline Drive and Mussel Rock) Mussel Rock is where the San Andreas Fault goes out to sea. This site, at one of the largest coastal landslides in California, is within a few kilometers of the epicenter of the 1906 San Francisco Earthquake. It is also the site of many foolish ideas, starting in 1874, when Tobin’s tunnel was built to allow carriages to travel down the beach from San Francisco to Salada Beach, now in Pacifica. Tobin lived at Rockaway Beach. We have already discussed the Ocean Shore Railroad (1905-1921) and the Henry Doelger homes built in the 1950s. In 1957, during construction of these homes, 11 homes had to be removed as a 5.3-magnitude earthquake with its epicenter here. Five homes were demolished on Lynvale Court. A total of 21 homes have been demolished on Westline Drive, which until 2000 used to continue past the dead-end where we’ll be stopping.

Figure 7. Block diagram of the Mussel Rock Landslide. Note contact between Franciscan Complex and Merced Formations, and San Andreas Fault trace from 1906. From Smelser (1987).

Stop 5: Bluff-top dunes.

Figure 8. Bluff-top Holocene dunes. See geologic map (fig. 5) to realize that these dunes are simply reactivated dunes, now uplifted 60 m. View to the south is of the Dollaradio Radio Transmitter (1920s), now a private residence, and the apartments failing to the south on Esplanade in Pacifica. 9

Stop 6. Ocean View Apartments and The Bluffs, Esplanade, Pacifica.

We are still on Holocene dune sands, estuarine deposits and other coastal sediments, but here uplifted only 50 m. Major bluff erosion during 2009/10 winter. Several apartments are red- tagged. “The Bluffs” named appropriately, but somewhat better financed.

Figure 9. The northern stretch of Esplanade in Pacifica, 2002 (upper) and 2010 (lower). Copyright (c) 2002-2010 Kenneth & Gabrielle Adelman, California Coastal Records Project, www.californiacoastline.org

Figure 10. View from the beach 50 m below the Ocean View Apartments. Note failed attempt at stabilizing the upper bluff with cemented materials anchored by pins inserted into the sand.

Figure 11. From Collins & Sitar (2008). “(a) Profile panorama of study area. Bluff height ranges from 30 m in the north to 14 m in the south. Bluff S2 is stabilized with riprap and was not part of this investigation. (b) Geologic profile map of study area (after Collins, 2004). Individual Qw and Qp sub-units are grouped by their degree of cementation, and can be differentiated in the field as individual units.” Used with permission (Elsevier).

Figure 12. Short-term erosion rates predicted for Manor (left) and Sharp Park (right). From Battalio (2011).

Drive-by (no stop). Esplanade 1998 El Niño site. After a February 1998 El Niño storm, the cliffs eroded back here more than 10 m in two weeks. Ten houses severely damaged and red-tagged for demolition. Seven were demolished; one fell into the ocean.

Figure 14. Home collapsing down the cliff in 1993. From SF Chronicle.

Stop 7. Milagra Creek Outfall

Milagra Creek drains 186 ha (460 acres) with its headwaters Milagra Ridge in the Golden Gate National Recreation Area. The creek is culverted in its lower reaches across highway 1 and in the 1980's the canyon near the mouth was culverted and filled to create an RV park. That RV park is losing spaces.

Figure 15. To the south of Milagra Creek outfall are some prime examples of destruction porn, with expensive homes undercut in January 2016, and one (with chimney in top image) that was burned (under investigation) on Feb. 12, shortly after it was yellow-tagged. 15

A. 1972

B. 2016

Fig. 16 . Milagra Creek mouth area in 1972 and 2016. In 1972, the creek flows out of a canyon, which was culverted and filled in the 1980's to create an RV Park. That RV Park is now losing ground. In C & D, compare the house with the chimney in 1972 and 2016. The house was yellow-tagged in 2016, and burned on Feb. 12, 2016; the cause of the fire has not been published. Photos from California Records Project ((c) 1972-2010 C. 1972 D. 2016 Kenneth & Gabrielle Adelman) and NOAA (2016).

Stop 8. Pacifica Pier.

Popular for crabbing and fishing (no license required), this pier was originally built as a sewage outfall. The nearby old sewage treatment plant now houses the City Council Chambers, and the current plant is located just south of Mori Point, and is a model tertiary treatment plant that uses UV light to kill pathogens, and releases clean water to establish the Calera Creek restoration draining to Rockaway Beach. The armored area near the pier frequently is undermined by wave action. To the south is San Francisco’s Sharp Park Golf Course, built on Laguna Salada and protected by a berm. There is a recent movement to close the golf course to incorporate it into the Golden Gate National Recreation Area. Very political.

Stop 9. Mori Point.

One of the newest parts of the Golden Gate National Recreation Area, this site was the home of motocross racing in the 1960s, and has experienced significant erosion as a result. This was also the site of one of the last scenes in Harold and Maude when it appears that Harold has driven his hearse off the cliff.

Figure 17. Mori Point. 1970-1978: Coastside Motorcycle Club race. Today: GGNRA.

Stop 10. San Pedro Creek Watershed. San Pedro Creek, aligned along the Pilarcitos fault, divides Mesozoic Franciscan assemblage sandstones to the north, and Paleocene sandstones and Salinian granitics of 600-m Montara Mtn to the south. As the first asistencia for the San Francisco mission, grazing impacts date to the 18th century. The lower reaches of the creek and wetland were ditched in the 19th and 20th centuries for agriculture, followed by suburbanization 1940s – 1970s. Approximately 1/3 developed, the watershed still supports a creek with steelhead. Restoration projects to improve habitat and remove migration barriers have included an Army Corps of Engineers flood control project near the mouth. See the San Pedro Creek Watershed Coalition web-site at http://pedrocreek.org for more information on other projects.

Stop 11. Devil's Slide Trail

While the actual Devil's Slide is a fault plane exposed along the hike, this site is known as the deep-seated landslide that create ongoing maintenance problems for the Ocean Shore Railroad and then Highway 1 which used the same grade through Saddle Cut here. After frequent road closures and a local political effort called "Think Tunnel," a 1314-m (4312 ft) tunnel, California's second longest, was built through the Paleocene sedimentary and granitic rocks of San Pedro Mountain, in order to bypass the problematic section.

That problematic section of road is now open for hiking and biking, and offers great views not only of the coast and distances but also a nice exposure of turbidites. These Paleocence rocks include shales, sandstones, and conglomerates deposited in turbidity flows in waters off the coast of southern California long before the San Andreas Fault initiated ~28 Ma as the Farallon Plate and East Pacific Rise was subducted. These sedimentary sequences have travelled northward with the granitics of the Salinian block. Those granitics can be seen in Montara Mountain, the Farallon Islands and Point Reyes.

We'll get a nice view at the crest of the trail, and should certainly see the site of the Ocean Shore Railroad tunnel, blasted closed by federal agents since the tunnel was used as a major storehouse of Canadian whiskey during Prohibition. We'll also clearly see the exposed turbidites in Pedro Point Rock. And if it's clear we may be able to make out the Farallon Islands to the west, which marks the edge of the continental shelf. It's likely that if you'd come here 15,000 years ago, you may have had view of herds of mammoths being hunted on the extensive coastal plains by early Americans.

The open space immediately to the north is also worth a visit, but we don't have time today. It's Pedro Point Headlands, recently transferred to San Mateo County Parks. The Pacifica Land Trust purchased this land 20 years ago, and has been working to restore it from decades of off- road motorcycle use. The area has even more impressive views than what we'll see on the hike today.

We'll hike the length of the trail (about ½ hour) and get picked up at the south end, and then return by bus to the Hilton. 19

References:

Andersen, D. W., A. M. Sarna-Wojcicki, R. L. Sedlock (2001). Geology and Natural History of the San Francisco Bay Area: A 2001 NAGT Field-Trip Guidebook. Battalio, R. T. (2011). Shore management adaptation to climate change. Presentation at Association of Environmental Professionals 2011 Annual Conference, Monterey, CA, 7 March 2011. California Shore & Beach Preservation Association. Battalio, R. T. (2014). Littoral processes along the Pacific and bay shores of San Francisco, California, USA Shore and Beach 82(1): 3-21. Battalio, R. T., and D. Trivedi (1996). Sediment transport processes at Ocean Beach, San Francisco, California. Coastal Engineering 1996. Proceedings of the 25th International Conference, Coastal Engineering Research Council of the American Society of Civil Engineers. Brabb, E.E., R.W. Graymer, and D.L. Jones (1998). Geology of the Onshore Part of San Mateo County: Derived from the Digital Database Open-File 98-137., US Geological Survey. Clifton, H.E. and R.E. Hunter 1987. "The Merced Formation and related beds: A mile-thick succession of late Cenozoic coastal and shelf deposits in the sea cliffs of San Francisco, California". In: Geological Society of America Centennial Field Guide— Cordilleran. Section. pp. 257-262. Clifton, H. E., Hunter, R.E., and Gardner, J.V. (1988). Analysis of eustatic, tectonic, and sedimentologic influences on transgressive and regressive cycles in the late Cenozoic Merced Formation. In: Cordilleran Section of Geological Society of America Centennial Field Guide. Volume 1. Paola, C., and Kleinspehn, K.L. Boulder, CO (Eds.): Geological Society of America. pp.27-262. Collins, B.D., N. Sitar (2008). Processes of coastal bluff erosion in weakly lithified sands, Pacifica, California, USA. Geomorphology 97 (2008) 483–50. Cotton, Shires & Associates, Inc. (2004). Northridge Bluff Landslide Engineering Geologic Investigation Preliminary Report Heiser, Shawn (2010). Living on the edge: Environmental history at Mussel Rock, Daly City, California. M.A. Thesis (Geography), San Francisco State University. Jones, I.S., T. T. Janssen, J. E. Hansen, P. Bernard (2010). Observations of wave-driven surf-zone dynamics on a high-energy beach, Ocean Beach, San Francisco. American Geophysical Union, Fall Meeting 2010, abstract #OS51B-1301. Konigsmark, T. (1998). Geologic Trips: San Francisco and the Bay Area. (Gualala, Calif.: Geopress). 174 pp. Revell, D. L., Battalio, R., Spear, B., Ruggiero, P., & Vandever, J. (2011) A methodology for predicting future coastal hazards due to sea-level rise on the California Coast. Climatic Change 109.1 (2011): 251-276. Smelser, M. 1987. Geology of Mussel Rock Landslide, San Mateo County. California Geology 40: March 1987, 59-66. SPUR (2015). Coastal Protection Measures & Management Strategy for South Ocean Beach. Ocean Beach Master Plan: Coastal Management Framework. Prepared for San Francisco Public Utilities Commission, April 24, 2015. Under contract to SPUR, Prepared by SPUR, ESA PWA, Moffatt & Nichol, McMillen Jacobs Associates, AGS Inc. 74 pp, with 6 appendices (94 pp.). Zito, K. (2011). “State panel rejects city's repairs to Ocean Beach”. San Francisco Chronicle Friday, July 15, 2011. 20