|Coastal Erosion and Armoring in Southern Monterey Bay |
Eighty-six percent of California's 1,770-kilometer (1,100-mile) coastline is eroding, yet now more than ever people want an ocean view. Coastal erosion has been occurring for the past 18,000
years, when the last glacial period ended and sea level began to
rise. Extreme variability in the rates and severity of coastal erosion,
particularly in relation to El Niño storm patterns and local geo
logic conditions, complicates property protection decisions.
The Monterey Bay National Marine Sanctuary, which includes
444 kilometers (275 miles) of coastline, has recognized that human responses to coastal erosion may impact sanctuary resources
and has set up a working group of regional scientists, planners
and consultants to determine the most effective way to minimize those impacts.
|Figure 1. Variability of coastal erosion rates along southern Monterey Bay
An 18-kilometer stretch of relatively undeveloped, dune-backed coast in southern Monterey Bay was selected as the pilot region to begin a comprehensive analysis of erosion rates and management alternatives. By compiling data from scientific studies and consulting reports, the working group estimated that coastal erosion in this region varies from 28 to as much as 244 centimeters per year. (See Figure 1.) In general, erosion rates are highest near Fort Ord and decrease to the north and south, a pattern that is likely dictated by variability in the concentration and incident angle of wave energy.
Coastal erosion is a natural process that becomes problematic
for people when buildings or infrastructure are threatened by wave action that erodes cliffs and other back-beach landforms. There are numerous responses to deal with coastal erosion, however these are not always acceptable alternatives, since they have varying levels of impact to the environment, economic costs and effectiveness
in combating erosion; and they may conflict with policies and
regulations of permitting agencies. The most logical alternative is avoiding the hazard in the first place by establishing setbacks for new development -- based on local erosion rates -- to ensure that structures will not be threatened within their projected lifespan. When existing development is endangered by a retreating shoreline, property owners can sometimes relocate their structures landward or demolish them. (See Figure 2, p. 7.) Other ways sometimes
considered to slow erosion include increasing the sand supply to beaches by importing sand, referred to as beach nourishment;
constructing groin fields, a series of linear, shore-perpendicular barriers; or removing coastal dams, which can trap sand upstream. These methods seek to widen beaches and thereby reduce wave attack on coastal cliffs and dunes. Alternatively, structures such as artificial reefs can be constructed offshore to decrease wave energy reaching the coastline. These erosion mitigation measures are generally very costly, and because the long-term benefits and impacts will vary greatly depending on local conditions, they should be evaluated on a case-by-case basis.
(a) August 2003 photograph of Stillwell Hall on Fort Ord in Marina
(b) October 2004 photograph of the same site, after the building and riprap were removed because attempts to save the structure from collapsing into the ocean proved to be too costly and ineffective. (Photo Kenneth and Gabrielle Adelman, http://cacoast.org)
By far the most common method of protecting coastal property and mitigating erosion in California has been the construction of coastal protection structures, such as riprap revetments or seawalls (collectively referred to as "armoring"). More than 24 kilometers
of the sanctuary's coastline have been armored, and this figure continues to grow. Various physical and biological impacts of coastal armoring may affect sanctuary resources both directly and indirectly. For example, armoring can restrict vertical and lateral access to beaches, cover up a significant portion of recreational beach area and depreciate the coast's aesthetic value. Armoring cliffs with cement or rock piles may reduce the amount of sand on local beaches, because a portion of the natural sand supply comes from the breakdown of cliff and dune material by erosion. In addition, there are potential impacts to biological communities through smothering or changes in benthic habitat.
Another potential effect from coastal armoring in an actively eroding area, such as southern Monterey Bay, is the narrowing of beaches. When armor is placed in front of a building to halt erosion, the shoreline is essentially fixed at that location. Adjacent landforms will continue to retreat landward due to coastal erosion, creating
an artificial headland out of the armored segment of coast. The beach will remain the same width on either side of the armored
area but will narrow or disappear in front of the armor; this is
illustrated in the 'before' image of Stillwell Hall. (See Figure
2a.) A little more than one year after Stillwell Hall and the riprap were removed, wave action started to erase the artificial headland, and the beach returned. (See Figure 2b.)
The lack of development along much of southern Monterey
Bay provides the sanctuary and its partners with an ideal, and increasingly rare, opportunity to be proactive in terms of coastal development. Combining scientific knowledge of the region's dynamic coastline with sound management will undoubtedly help to conserve the natural beauty and value of this resource.
Monterey Bay National Marine Sanctuary