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6— Leucadia South through Solana Beach
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Construction on Bluff Tops and Methods for Preventing Bluff Erosion

Beginning in about 1973 condominiums and apartments have been constructed along the bluff tops, many within ten feet of the edge. Numerous recent geologic studies done by consultants for real estate developers indicate negligible erosion of these bluffs since 1928, based on early aerial photographs. What was not readily apparent to the investigators is that bluff and cliff erosion is episodic, site-specific, and directly related to climate changes. The impact of urbanization on the cliffs was also not taken into account.

Examination of sequences of photographs of a specific site revealed not only that erosion occurs sand-grain by sand-grain but also that it is largely episodic in nature, often involving failure of 100 feet along the cliff at one time. In 1973 it became apparent that in the middle of a drought period, cliff collapse was caused at least as much by groundwater and the effects of man as it was by wave erosion.

The groundwater table began rising in 1973, commensurate with urbanization of the adjacent areas. This rise resulted largely from an excessive watering of lawns and the introduction of non-native vegetation to the region. Other contributing factors include agricultural irrigation, leach lines, and cesspools. Sorbon and Sherrod (1977) indicate that overirrigation of landscaping in residential subdivisions alone is equivalent to fifty to sixty inches of rain per year on each lawn. This extensive overwatering in coastal areas has at least three important effects: it has caused a slow but steady rise in the water table, even during the driest period preceding the rainy year of 1978; it adds


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Figure 32a
Views of landslide along the bluffs in south Del Mar, 1977. Note that the groundwater
caused movement of the landslide directly seaward from the new construction during
this drought year.
Photo:  G. Kuhn.

Figure 32b
February 1978 view of block fall and debris at H Street in Encinitas.
Photo:  G. Kuhn.


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weight to the cliff material and increases pore pressure; and this in turn contributes to landslides (fig. 32a ) and blockfalls (fig. 32b ).

At one location condominiums were constructed over an unrecognized ancient river channel fill (figs. 33a , 33b ). Erosion at the base of the cliff, at the southern end of the condominium site, amounted to ten feet between 1971 and 1978 in two separate five-foot episodes. The homeowners in the project have gone to great expense to stabilize the cliff base and bluff face. In April 1978 a concrete crib wall, designed to prevent further erosion, was near completion (fig. 34a ). However, during the first week of May the crib wall collapsed, severely eroding the bluff face (fig. 34b ). A concrete seawall now exists at the cliff base, and the upper bluff face has been terraced with concrete (fig. 34c ) at a cost of over $2 million to the homeowners.

Figure 33a
Oblique aerial view of a portion of south Solana Beach prior to the
bluff-top development, 1954.
Photo:  U. S. Navy.

Figure 33b
View of the same site as that in 33 a  following development of the bluff
top, 1974. Note that the bluff face began eroding during the construction.
Photo:  B and A Engineering.


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Figure 34a
April 1979 view of the concrete crib wall being installed on the bluff face at Solana Beach.
Photo:  G. Kuhn.

Figure 34b
May 1979 view of the same site as that in 34 a . Note that the crib wall collapsed
before completion.
Photo:  G. Kuhn.


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Figure 34c
View of the same site, 1980. The bluff face has been terraced with concrete, and the adjacent bluff face south of the terraces
has been planted.
Photo:  G. Kuhn.


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Figure 35a
View of a cave forming above sea level along a remnant of shear zone
in Leucadia, 1980. The cave was formed primarily as result of inland
groundwater exiting the cliff face.
Photo:  G. Kuhn, January 1980.

Figure 35b
View in the vicinity of 35a , showing the extensive cliff erosion that occurred during the storms of
January through February 1983.
Photo:  G. Kuhn.


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