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
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.