USGS

Map of Coastal San Mateo County1982-83 El Niño Coastal Erosion: San Mateo County, California

Kenneth R. Lajoie and Scott A. Mathieson


Navigation table:

INTRODUCTORY TEXT

SAN MATEO CTY. COASTAL STABILITY MAP

SAN MATEO CTY. CRITICAL COASTAL EROSION MAP

COASTAL EROSION MAPS

COASTAL GEOLOGY MAPS

INTRODUCTION

Heavy surf and rains severely eroded beaches and fragile sea cliffs along the coast of San Mateo County, California during the severe winter storms of 1982-83, the last El Niño climatic event. During that storm season, the main coastal highway was damaged and closed for a considerable period of time by landslides, and many buildings were damaged or threatened by sea-cliff retreat and slope failure.

In the Spring of 1983, the USGS photographed the San Mateo County coast from the air and on the ground to document the cliff erosion and structural damage caused by the severe winter storms. Much of the resultant data set was published in a general article on coastal erosion in San Mateo County (Lajoie and Mathieson, 1985), but the complete data set was never published as a separate document.

The purpose of this report is to present the complete data set in a useful format prior to the potentially damaging storms of the predicted 1997-98 El Niño storm season. This information should prove useful to coastal residents, and to local emergency-response agencies, as a guide to potential erosion hazards along the San Mateo County Coast.

COASTAL STABILITY MAP

Segments of the San Mateo County coast are classified as stable and unstable based on the inherent resistance of the exposed rocks to wave erosion and slope failure (see coastal stability map). The stable coastal segments in the northern part of the county are backed by resistant granitic rocks, which form high bluffs (maps 5 and 6) to low cliffs (map 7). The stable coastal segments in the southern part of the county (maps 20 to 24) are backed by resistent sedimentary rocks that form low cliffs. Coastal segments of intermediate stability are backed by moderately resistent sedimentary rocks that form high bluffs (maps 4 and 16) and low-to-high cliffs (maps 7 to 9, 12 to 18, 25, and 26).

Unstable coastal segments are backed by weakly indurated or highly fractured sedimentary rocks that form high bluffs (maps 1 to 2, and 8), weakly indurated terrace deposits that form low cliffs (maps 3, 4, 6, 7, 9 to 12, and 23 to 25), and loose beach and dune sand that forms spits and low cliffs (maps 4, 9, 10, 19, and 23 to 25). Along much of the San Mateo County coast, fragile terrace and loose dune deposits overlie resistent to moderately resistent bedrock. In those areas the base of the cliff is stable and the upper part is unstable. If the cliff is sufficiently high that wave surge does not erode the terrace deposits exposed in the upper part of the cliff face, that coastal segment is classified as stable (maps 7, and 18 to 23).

CRITICAL EROSION MAP

Critical erosion is here defined as any coastal erosion that threatens or damages man-made structures. Cliff erosion that does not threaten or damage man-made structures is not classified as critical, even though it might be severe. Most critical coastal erosion occurs in the northern part of San Mateo County where development is much denser than in the southern part (see critical erosion map).

COASTAL EROSION MAPS

Notes describing the moderate to severe coastal erosion in San Mateo County during the 1982-83 El Niño are compiled on twenty-six page-sized maps at a scale of 1:12,000 (see coastal erosion index map). The bases for these maps are enlarged portions of the 7 USGS 7.5' topographic quadrangles that cover the coastal area of San Mateo County (see coastal erosion and geology index maps). Notes describing critical erosion are colored red, and those describing noncritical erosion are colored black. The areas of critical erosion are summarized separately on a smaller-scale map (see critical erosion map).

COASTAL GEOLOGY MAPS

The erosion information presented on the coastal erosion maps is placed in a geologic context on twenty-six coastal geology maps using the same bases as the coastal erosion maps (see coastal geology index map). The geology maps generalize coastal morphology and geology by means of schematic cross sections. Natural coastal processes such as cliff erosion, slope failure, wave erosion, landslides, block falls, debris slides, and cave collapse are illustrated diagrammatically on the cross sections. Ultimately, wave erosion is the primary or initiating erosive process along the San Mateo County coast. Waves erode the base of the sea cliffs or coastal bluffs, thus oversteepening and destabilizing the slopes above. Landslides, debris slides, and block falls are secondary erosive agents that move loosened rock material down the slopes to the beaches below, where the material is eventually eroded away by wave activity. Along most of the San Mateo County coast groundwater that seeps from the cliff face softens and loosens even otherwise resistent bedrock material, thus contributing to the erosional process.

COASTAL HAZARDS

Basically, there are no natural hazards; there are just natural processes such as coastal erosion, slope failure, and flooding. Man creates hazards when ignoring the potential adverse impacts of these processes. Most of the erosion hazards along the coast of San Mateo County are of this nature. Buildings and roads were placed directly on exposed beaches (map 5), directly on active landslides (map 8), or too close to the edge of rapidly retreating sea cliffs (map 3) and landslide scarps (map 2).

Furthermore, man often creates hazards in previously stable areas by altering the natural environment in such a way that erosional processes are initiated or greatly accelerated. For example, at Thornton Beach (map 1) surface runoff from a broad slope was concentrated into small drains that failed during the heavy rains of 1982-83. As a consequence, a deep ravine was eroded below one drain, destroying part of a road. Nearby, a landslide was formed below a second failed drain. That landslide dropped 7 meters in the winter of 1982-83, cutting two streets and damaging several houses; some of the houses were eventually moved to prevent destruction. To the south in Pacifica (map 3), another inadequate drain failed, resulting in severe gully erosion that led to destruction of the southern end of a riprap seawall protecting a mobile-home park. The breach in the seawall focused wave energy behind the remaining part of the wall, which was then completely destroyed. The fragile sea cliff previously protected by the wall was cut back 10 to 15 meters, destroying several mobile-home pads. Farther south, just north of Devils Slide (map 5) and at Martini Creek (maps 5 and 6), failed drains beneath road fills completely destroyed sections of Highway 1, resulting in extended road closure. At Half Moon Bay (maps 9 and 10), a rubble-mound breakwater built in 1959-60 focussed wave energy at low, previously stable cliffs, resulting in sustained cliff erosion, which threatens Highway 1 to this day (Mathieson, Lajoie, and Hamilton, 1997). Surprisingly, the 1982-83 increment of erosion at this site, though significant, was not unusually large.

The coastal-erosion hazards and damage documented in this report could have been avoided or minimized by recognizing the nature of the active coastal processes, establishing their rates and planning set-backs and restrictive zones accordingly. Determining the rates of incremental processes (such as coastal erosion), however, is often difficult owing mainly to the short historical record. Also, rates of sea-cliff retreat usually vary drastically with time. A section of sea cliff might not erode for years or even decades and might therefore appear stable, but eventually a large storm might cut deeply into the cliff face. This sequence of events actually occurred on the north side of Mori Point (map 4), where a high beach berm built up over several decades protected low, fragile cliffs from wave attack. A building constructed close to the cliff edge survived many years unscathed, but heavy surf in the winter of 1982-83 stripped the protective beach and cut the cliff back about 20 meters, destroying the building. Unprotected, the cliff was again cut back 7 meters by just moderate surf in the winter of 1983-84. The beach berm at that site might again rebuild and provide protection for the cliff for another few decades. However, in the future the erosive phase of the cycle will probably be repeated.

SUMMARY

The erosional effects and resultant coastal hazards of the severe 1982-83 El Niño winter storms documented in this report should be a reasonable guide to what might be expected along the coast of San Mateo County in the coming winter, if the predicted El Niño event occurs at full intensity. Hopefully, the data set will prove useful to coastal residents and various county agencies in preparing for the coming storms.

REFERENCES

Lajoie, K. R. and Mathieson, S. A., 1985, [Coastal Erosion] San Francisco to Año Nuevo; in Griggs and Savoy, eds., Living with the California coast: Duke University Press, p. 140-177.

Mathieson, S. A., Lajoie, K. R., and Hamilton, J.C., 1997, Induced sea-cliff erosion southeast of the Pillar Point breakwater, Half Moon Bay, California: U.S. Geological Survey Open File Report 97-688, scale 1:200.


For further information, contact Ken Lajoie

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6-May-1998,
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