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HomeMy WebLinkAbout; Coastal Erosion Hazards San Diego County; Coastal Erosion Hazards San Diego County; 1997-10-23COASTAL EROSION HAZARDS IN SANTA CRUZ AND SAN DIEGO COUNTIES, CALIFORNIA Laura Moore, Ben Benumof, Gary Griggs Institute of Marine Sciences Department of Earth Sciences University of California, Santa Cruz EXECUTIVE SUMMARY California has 1100 miles of coastline, 86% of which is actively eroding. The golden state is also the most populous in the country, one of the most rapidly growing and 80% of California's 32 million people now live within 50 km of the shoreline. The state's population is projected to reach 50 million by the year 2020. The conflict between the desire to live on the shoreline and the inherent geological instability of the coastal bluffs, dunes and beaches of the state has produced a risk of significant proportion. As part of the Federal Emergency Management Agency's program to assess the feasibility and economics of adding ocean front property prone to erosion to the federal flood insurance program, erosion hazard studies were carried out in most coastal states. The objectives were to determine long term shoreline erosion rates, to project these 60 years into the future, and to then determine the extent of development which would be lost if this erosion proceeded. Two California counties were analyzed in this study: San Diego, a heavily urbanized southern California county with ~ 122 km of primarily cliffed shoreline; and Santa Cruz, a county in the central state characterized by both urban and agricultural/open spaces uses with -67 km of cliffed shoreline. Due to ongoing cliff and bluff erosion, approximately 30 percent of both counties has now been armored. A state-of-the-art softcopy photogrammetry system was used to determine shoreline erosion rates using aerial photographs spanning 41 years in Santa Cruz County and 42 to 62 years in San Diego County. The position of Moore, Benumof and Griggs Page 1 10/23/97 the erosion reference feature, typically the top of the cliff or bluff, or the seaward edge of the dune or vegetation were used for historical comparisons. Areas of wide sandy beaches in both counties have advanced and retreated as a function of storm frequency and nourishment. Sea cliffs however, depending upon the rock type and its inherent weaknesses, the wave exposure and human impacts, have been eroding at long term average rates of a few up to 60 cm/year. These "average" erosion rates determined have been affected by shoreline armoring and will be lower than pre- armoring erosion rates and higher than post-armoring rates. INTRODUCTION Coastal hazards are a function of the presence of human beings and their activities as they interact with naturally occurring coastal processes. California has 1100 miles of coastline; 86 percent of it is actively eroding. With the exception of changes due to coastal erosion, the coastline has the same general configuration as it did in 1850 when the estimated population of California was 93,000. The state population grew steadily for the next 100 years, but following World War II, it virtually exploded; between 1950 and 1970 it nearly doubled, growing from 10.6 to 20 million (Figure 1). The 1996 population of nearly 32 million represents a tripling since 1950 and 80 percent of these 32 million Californians live within 50 km of the shoreline. The state now has over 17,000 residents for each kilometer of shoreline, or 17 people per meter. The population of California is projected to reach 50 million by the year 2020. The conflict between accelerating oceanfront development and the inherent geological instability of the shoreline is becoming a dilemma of increasing magnitude. The coastline is caught between rising sea level and ocean storm waves from the front and the increasing waves of humanity from the rear. The ongoing natural hazards of cliff retreat, storm inundation, and beach erosion have either been unrecognized, unappreciated, poorly understood, or ignored by many coastal builders, developers, and home buyers in the past. Much of California's oceanfront development took place from the mid-1940s to the mid-1970s, a period characterized by below average Moore, Benumof and Griggs Page 2 10/23/97 rainfall and storm frequency. The wave impact and shoreline erosion accompanying the storms of the past two decades, however, have been responsible for over $150 million in damage. High tides and storm waves during the winter of 1983 inflicted over $100 million in damage to oceanfront property; thirty-three oceanfront homes were completely destroyed and 3000 homes and 900 businesses were damaged. All oceanographic indications as of October 1997 are that the El Nino condition, which has developed in the tropical Pacific, is as significant as any within this century. This suggests that a stormy winter with elevated sea levels and large storm waves is highly probable. Hazardous Coastal Environments Coastal geologic hazards in California occur most frequently in the form of shoreline erosion (both seacliff and beach) and coastal flooding (both wave impact and inundation). Human interference with coastal processes (littoral drift and sand supply) and coastal bluff stability (altering the stability of coastal bluffs materials) can exacerbate hazard conditions. Along the California coast, three geomorphic environments have been widely developed and are potentially hazardous for any structure. These include: 1) eroding cliffs or bluffs, 2) beaches, and 3) active coastal dunes. Along the urbanized seacliffs of southern California geologic instability has been increased through the addition of large volumes of irrigation water required to maintain lawns and non-native vegetation in the yards of cliff-top homes. Landscape irrigation alone is estimated to add the equivalent of 125 to 150 cm of additional rainfall each year to garden and lawn areas which naturally might receive only 25 to 40 cm of rainfall. This excess water has led to a slow, steady rise in the water table that has progressively weakened cliff material and lubricated joint surfaces in the rocks along which slides and blocks falls are initiated. In addition, surface runoff discharged through culverts or storm drains at the top or along the face of bluffs leads to gullying or failure of weakened surficial materials. A 1971 regional inventory of the California shoreline classified only 14.2 percent of the coast as non-eroding. Of the remaining 85.8 percent, 128 km (7.3%) were classified as undergoing critical erosion (defined as areas where structures and/or utilities were threatened), with the remainder Moore, Benumof and Griggs Page 3 10/23/97 designated as experiencing non-critical erosion (Corps of Engineers, 1971). A subsequent investigation by the California Department of Navigation and Ocean Development (Habel and Armstrong, 1978) defined the erosion problem somewhat differently. Approximately 160 km (10.9%) of coast were delineated as eroding with existing development threatened, and an additional 480 km (39.4%) were classified as eroding at a rate fast enough that future development would eventually be threatened. Thus a total of 640 km (39.4%) of the California shoreline was considered threatened due to high erosion rates. The most recent inventory of hazardous coastal environments expands the scale of the problem areas again. In 1985, sixteen coastal geologists participated in the preparation of a statewide inventory of coastline conditions, classifying 500 km (28.6%) as high risk, and an additional 650 km (36.8%) as requiring caution (Griggs and Savoy, 1985). These data indicate that two-thirds of the California shoreline is hazardous. Several factors combine to complicate shoreline erosion measurements and the determinations of long term shoreline erosion rates in California. 1] The process of cliff or bluff erosion tends to be an episodic one with the major erosion events taking place during the simultaneous occurrence of high tides and storm waves. Thus while we tend to utilize "average" annual erosion rate values (based on analysis of historic aerial photographs and/or maps) to describe the erosion risk or hazard at a particular location, the failure of coastal cliffs typically takes place in large incremental events during storm conditions. 2] The process of cliff retreat or erosion is site specific and due to a combination of a) wave attack, b) rainfall and runoff (which produce gullying, mass movements, and piping) and c) seismic shaking. These processes operate infrequently and over an unpredictable time scale. 3] By 1985,12 percent of the entire coastline of California, approximately 210 km, had been armored or protected. Some of these protective structures were emplaced over half a century ago, and others have been built within the past decade. Because most coastal protection structures lead to at least a short term halt in coastal retreat, coastal erosion rate measurements in areas which have been armored need to be clearly explained and understood. Moore, Benumof and Griggs Page 4 10/23/97 In spite of a growing body of scientific information on the location and nature of coastal geologic hazards and their associated risks, oceanfront development continues. In some instances, coastal property that was designated as hazardous a decade ago is now being declared as buildable due to a combination of political pressure and increased property value. Undeveloped oceanfront lots along the Malibu coast, for example, typically have price tags in the $1 to $3 million range. Despite knowledge of the inherent hazards of building on the bluff or beach in Malibu, however, permitting agencies, such as the California Coastal Commission, tend to approve permits with elaborate mitigating measures intended to reduce the risks. One might assume that the Coastal Act and the Coastal Commission would have provided the state with a set of policies and an institutional system for dealing with coastal hazard issues. For several reasons this has not been the case. At the time the Coastal initiative was written and approved by the voters (1976), the principal issues were environmental concerns, beach access and wetland preservation. Issues of coastal storm damage, shoreline retreat, setback distances, and coastal protection structures were not as obvious and pressing as they are today. As a result, the Coastal Act policy statements were notably deficient in these areas. In addition, although much is known about coastal hazards in California, local political climates and high coastal property values have hindered the translation of this knowledge into policy and planning guidelines at all levels of government. Policies and practices regulating oceanfront property and its development vary widely throughout the state. Some communities have articulated a set of policies that encourage community or state purchase of undeveloped oceanfront property. These communities have often also set forth guidelines and requirements for new development or protection plans. Other communities are openly encouraging shoreline development adjacent to areas of documented high erosion rates. A combination of local politics and economics, rather than the history of shoreline erosion and storm inundation, appears to be the most important factor controlling the policies and practices established for a particular region. Moore, Benumof and Griggs Page 5 10/23/97 Study Sites Two counties were selected for the FEMA Erosion Hazard Mapping Study. San Diego County, the southernmost of California's coastal counties, is representative of the heavily populated southern California region. Santa Cruz County, located in central California, south of San Francisco, is representative of less populated central and northern California. Santa Cruz County, California Santa Cruz County shoreline (Figure 2) consists of 67 km of shoreline and has a population of 243,000. This region contains a mixture of urbanized clifftop and oceanfront (beach and dune) development as well as agricultural and open space uses. The northern portion of Santa Cruz County consists primarily of open space and agricultural land. The cliffs in this region are eroded into uplifted marine terraces 10 to 35 meters in height, consisting of Miocene/Early Pliocene Santa Cruz Mudstone and are fairly resistant to erosion. In addition, cliff retreat in the northern section of Santa Cruz County poses little threat since no structures are located on the clifftop. For this reason, we chose to concentrate the FEMA Erosion Hazard Study on the central and southern portions of Santa Cruz County where structures are more likely to be threatened by coastal erosion. The cliffs of the central portion of Santa Cruz County consist of sandstones and siltstones of the Pliocene Purisima Formation. These cliffs are retreating and significant damage to private homes, apartments, parks and public infrastructure has occurred over the last two decades. The southern portion of the county shoreline lies within Monterey Bay. The cliffs in this area, also made up of sandstone and siltstone, are fronted by wide sandy beaches such that cliff erosion occurs primarily due to terrestrial processes and infrequent seismic shaking (Plant and Griggs, 1990). The southernmost stretch of Santa Cruz County is fronted by the active Pajaro Dunes which are subject to erosion during severe storms. There is considerable back beach development in southern Santa Cruz County and wave inundation commonly affects these areas. In 1983 the county sustained $8.2 million in coastal storm damage. Moore, Benumof and Griggs Page 6 10/23/97 San Diego County, California San Diego County has a population of approximately 2 million people living along 122 km of shoreline. The San Diego County shoreline (Figure 3), from San Mateo Point to the Mexican International Border, is an erosional coastline consisting primarily of narrow beaches backed by steep seacliffs which have been extensively urbanized. The seacliffs of San Diego are cut into raised coastal marine terraces, range from 5 to 115 meters in height, and are primarily composed of consolidated sedimentary rocks overlain by unconsolidated terrace deposits. Bluff and cliff erosion is an ongoing concern. During the severe winter storms of 1983 and 1988, this county sustained $17.5 million in public and private coastal property and infrastructure damage. Kuhn and Osborne (1987) have shown that much of the coastal erosion occurring in San Diego over the past 45 years has been a result of subaerial mass-wasting during above average rainfall events which rapidly saturate the seacliffs, providing optimal failure conditions. The majority of the rocks exposed in the San Diego County seacliffs are Eocene siltstones, mudstones, shales, sandstones, and conglomerates capped by Pleistocene marine terrace deposits (Kennedy, 1975). Late Cretaceous sandstones, shales, and conglomerates are also present and are exposed in the seacliffs from the Point Loma Peninsula to La Jolla (Kennedy, 1975). In general, the seacliffs composed of older Cretaceous material are more resistant to erosion than those composed of Eocene material and as a result, account for the occurrence of headlands at both Point Loma and Point La Jolla. The San Diego County shoreline can be divided into three littoral cells including the Oceanside, the Mission Bay, and the Silver Strand cells. Under natural conditions, sediment is supplied to San Diego beaches by rivers, streams, and seacliff erosion. In addition, large volumes of sand-sized material are artificially supplied to the beaches via public and private beach nourishment projects. Everts (1991) has determined that the sediment supplied to San Diego County beaches may serve as an effective buffer against wave induced erosion and that the amount of sand supplied, both naturally and artificially, often determines the erosional susceptibility of the coastline. Moore, Benumof and Griggs Page 7 10/23/97 RESEARCH METHODOLOGY Shoreline Erosion Reference Features In California, upon certification of "Local Coastal Plans (LCP's)" by the California Coastal Commission, individual local jurisdictions have the power to regulate shoreline land-use decisions. The majority of local coastal "setback" regulations refer to the shoreline reference feature as the landwardmost edge of the bluff-top or dune. In the case of an overhanging or oversteepened cliff edge, development setbacks may be based on a 30 degree line projected from the base of the cliff to the surface of the proposed development site. For the purposes of the FEMA Erosion Hazards Study, and in order to obtain meaningful and accurate erosion rates, four different erosion reference features were mapped depending on the character of the shoreline and whether or not it had been altered by the presence of protection devices. Consistent with California state policies, the landwardmost edge of the bluff top or cliff top (Figures 4 and 5) served as the primary erosion reference feature for both Santa Cruz and San Diego Counties. In areas which are extensively developed and armored such that the cliff-top or bluff-top is not a feasible erosion reference feature, the landward edge of existing shoreline protection structures and development served as an alternate erosion reference feature. In other areas characterized by low-lying, unconsolidated dune and beach deposits such as the Pajaro Dunes area in Santa Cruz County and the Silver Strand and Imperial Beach areas in San Diego County, the erosion reference feature was the seaward edge of dune vegetation. Finally, for the stretch of coast directly north of the Pajaro Dunes in Santa Cruz County, a rounded cliff edge prevented the use of the cliff -top as erosion reference features. In this location, the erosion reference feature was the base of the bluff. As a result of the episodic nature of coastal erosion, and because the shoreline is influenced by both marine and terrestrial processes which may operate on different time scales, erosion rate data determined using different erosion reference features should not be directly compared. The same caution applies when examining erosion rates which have been determined using sets of photographs which span different time periods. Moore, Benumof and Griggs Page 8 10/23/97 Calculating Erosion Rates Photography flown for the National Oceanic and Atmospheric Administration in 1994 at a scale of 1:24,000 served as base imagery for both Santa Cruz and San Diego Counties. This flight provided the only existing continuous coverage within the time frame required by FEMA. Aerial photography taken in 1953 at a scale of 1:12,000 was the historical source for Santa Cruz County while a combination of 1932,1949,1952, and 1956 imagery at scales of 1:9600,1:20,000,1:12,000, and 1:12,000 respectively, provided historical shoreline data for San Diego County. Although the use of four sets of photographs was necessary, the majority of the San Diego coastline was covered by the 1932 and 1952 imagery. To generate shoreline erosion rates for Santa Cruz and San Diego County, softcopy photogrammetry and geographic information system technology were employed. The steps involved in the application of softcopy photogrammetry to aerial photographs are summarized in Figure 6. This process begins with conversion of all imagery to digital format by scanning at a resolution of 600 dots per inch (~ 42 microns) using an Agfa Horizon Plus scanner. Photos are then imported to ERDAS Imagine Production ®. The next step is orthorectification of recent photography using ERDAS Imagine Production and the OrthoMAX® add-on module by Vision International®. Orthorectification (the removal of distortions and relief displacements from aerial photography) of recent photographs begins with interior orientation. In this process ground control points surveyed in the field and processed at UC Santa Cruz are selected on the aerial photographs. Fiducial marks are also digitized and camera calibration data are entered. The program uses this information to run a triangulation and calculate camera parameters at the time of exposure for each photograph in the set. Once a triangulation has been accepted as adequate, the program calculates transformation equations for each photograph. After equations have been generated for each photograph, digital stereo pairs are created. These stereo pairs are then used to generate digital elevation models which are edited by adding lines to delineate breaks in slope, adding numerous control points, and by removing poorly placed points. Once editing is complete, the digital elevation models and the triangulation equations are combined with the digital images to generate orthophotographs for the 1994 imagery. Moore, Benumof and Griggs Page 9 10/23/97 Rectification of historical imagery is achieved using the Ground Control Editor in Imagine Production. This tool allows the user to select points from a mosaic of the 1994 orthophotographs and match them with points on the historical photography. From these points a transformation is calculated and upon resampling, the historical image is georeferenced to the projection and coordinate system (UTM, WGS84 Spheroid) of the 1994 photographs. Once the historical photography is rectified, the Imagine Production Vector Module is used to digitize the position of the shoreline erosion reference feature on both the recent and historical imagery. In some locations, especially in Santa Cruz County, there are gaps in the shoreline erosion rate data where the erosion reference feature does not exist, as in the case of river mouths or pocket beaches, or where the erosion reference cannot be digitized with confidence. Two reasons exist for the later case: 1) overhanging vegetation sometimes makes it impossible to recognize and therefore delineate the cliff edge, or 2) the cliff edge is rounded, not visible or smeared on the orthophotograph and cannot be accurately located. After digitizing, the recent and historical shoreline erosion references, the vector coverages are imported to Arclnfo®. An Arclnfo AML called Shorelinegrid written by Bill Duffy of Northern Geomantics, Inc. is then used to calculate erosion rates. The Shorelinegrid Program coverts both shoreline erosion reference feature coverages to grids with a user-specified spacing of 1 m. In these grids, a value of 1 is assigned to a cell if the erosion reference feature passes through it. The program then calculates the shortest distance between cells with a value of 1 in the recent grid and cells with a value of 1 in the historical grid. Rates are determined by dividing this distance by the number of years between shoreline erosion reference features. Based on these rates and under the simplifying assumption that erosion will continue at the same rate and in the same direction, this AML also projects the position of the shoreline erosion reference sixty years into the future. In the results section, Santa Cruz and San Diego Counties are broken down into regions and the range of average erosion rates for each region are reported. These rates are average erosion rates because they have been determined using two end-points: the historical position of the erosion reference feature and the recent position of the erosion reference feature. Thus, rates presented are averaged over time, not space. Due to the episodic Moore, Benumof and Griggs Page 10 10/23/97 nature of erosion, these rates do not accurately reflect the amount of erosion that can be expected in any given year. Finally, the A and V flood zones on the FEMA Flood Insurance Rate Maps were digitized in ARCINFO. Using the rates generated by Shorelinegrid, these zones were projected 60 years into the future under the assumption that the flood boundaries will retreat at the same rate as the shoreline. Maps at a scale of 1:5,000 were then produced using the Map Composer module of ERDAS Imagine Production. The 1994 ortho- photographs serve as the backdrop for these maps. Plotted are the 1994 erosion reference feature, the projected 2054 erosion reference feature, most recent A and V zone landward boundaries, 2054 projected A and V zone landward boundaries, gutter lines, projected gutter lines, and base flood elevations when available Shoreline Armoring Approximately 30% of Santa Cruz County has been armored. Typical shoreline armoring consists of riprap at the base of cliffs and seawalls, riprap and revetments protecting back-beach development. Some of these protection structures have lasted ~30 years while others have not survived a single winter. Because the time of emplacement, extent and nature of shoreline armoring varies greatly from location to location, the presence of shoreline armoring was not taken into account when determining shoreline erosion rates. For this reason, erosion rates in areas which have been protected between the time of historical and recent photography (1953 and 1994) may be misleading. These rates will be lower than pre-emplacement rates and higher than post-emplacement rates because they have been calculated over a time period which encompasses change prior to and following emplacement. Like Santa Cruz County, approximately 30% of the San Diego County shoreline has been armored. In areas such as Mission Beach, La Jolla Shores, and Oceanside, either concrete seawalls or riprap protect long, continuous segments of coastline and have a significantly decreased shoreline erosion rate over the time period studied. In these areas, the edge of shoreline armoring served as the shoreline erosion reference feature. Erosion rates calculated in these areas have been affected by the emplacement of shoreline armoring and must be used with caution. Segments of shoreline that contain Moore, Benumof and Griggs Page 11 10/23/97 light or sparse armoring or areas which have been armored for a relatively short amount of time were mapped with either the Top of Cliff or The seaward edge of dune vegetation' erosion reference feature. RESULTS AND DISCUSSION Santa Cruz Erosion Rate Results San Lorenzo River Mouth to Soquel Point (Refer to Figure 2 for Locations) This stretch of coastline is characterized by low (6-8 m) cliffs of weak siltstone and sandstone fronted by sandy beaches. The erosion reference feature is the top of the cliff. The cliffs backing Seabright Beach in the northernmost section of this stretch, were eroding rapidly until construction of the jetties of Santa Cruz Small Craft Harbor in 1963 (Griggs and Johnson, 1976). A beach 200 m wide developed upcoast of the west jetty which has permanently protected the cliffs of Seabright Beach from direct wave attack (Griggs and Savoy, 1985). For this reason, average erosion rates for the time period from 1953 to 1994 are low and range from 0-7 cm/year. A significant portion of the remainder of this stretch (from the Santa Cruz Harbor to Soquel Point) is armored with riprap along the face and base of the cliff. Most of this armor was emplaced in the years immediately following jetty construction when littoral drift was being impounded. Shortly after harbor completion, however, annual dredging was initiated such that the annual longshore transport of -230,000 m3 has been again added to the nearshore system. Erosion rates for this area (corresponds to map sections 1-6) vary considerably and range from 0 to 39 cm/year. Opal Cliffs to Capitola Cliffs of relatively weak and pervasively jointed sandstone and siltstone ranging in height from 7 to 20 m typify this stretch of coast which was also affected by littoral drift reduction and therefore increased cliff retreat in the years immediately following construction of the Santa Cruz Harbor jetties (Griggs and Johnson, 1976). A wide sandy beach which had always been present in front of the small village of Capitola also disappeared following harbor construction exposing the back beach area to wave attack (Griggs and Savoy, 1985). Through construction of a groin and artificial nourishment, the beach has since returned and provides protection from wave attack except Moore, Benumof and Griggs Page 12 10/23/97 during severe storms (Griggs, 1990). The cliffs along this reach are very erodible, heavily urbanized, and discontinuously armored with riprap and a variety of seawalls. Many of these homes in this area are directly threatened by eroding seacliffs. The cliff top served as the erosion reference feature and average erosion rates range from 0 to 46 cm/yr (corresponds to map sections 7-11). Capitola to New Brighton_ This stretch of coast consists of 25 m high cliffs of sandstone and siltstone which have been heavily urbanized. Local bedrock jointing patterns have produced structural weaknesses which make this area particularly susceptible to block falls in which large masses of cliff fail in a single event. In addition, this stretch lacks a permanent protective beach and the height of the cliffs and block failure makes shoreline armoring infeasible. Weak erodible stratigraphic units at the base of the bluff combined with the lack of a protective beach, and the extensively jointed, failure-prone bedrock have produced regular failure of these bluffs which has been documented for over a century. At least 6 apartment units and two homes on the bluff top in this area have been either demolished or relocated in the past two decades and the public street along the cliff edge has now been either abandoned or has collapsed to the beach below (Griggs, 1986; Figure 7). The cliff top served as the erosion reference feature in this region (corresponds to map sections 12- 14) and average erosion rates range from 0 to 39 cm/year. New Brighton to Aptos Seascape An uplifted marine terrace is the dominant coastal landform in this area and a steep cliff, approximately 30 m high, forms the seaward edge of this terrace (Griggs and Savoy, 1985). Although a portion of this area has remained open space, the back-beach at the northern edge and the back-beach along the entire southern half of this stretch has been extensively developed. This stretch of coast lies within the protected inner portion of Monterey Bay and wide sandy beaches protect the seacliffs (but not the homes) almost permanently. For this reason, although the seacliffs are composed of the same sandstone and siltstone found in the cliffs to the north, they are vegetated and more stable. Cliff failure in this area results from gullying caused by surface runoff, or slumping induced by heavy rainfall (Griggs and Moore, Benumof and Griggs Page 13 10/23/97 Savoy, 1985) or seismic shaking (Plant and Griggs, 1990). Back-beach development in this area is more at risk than cliff-top development, since it is subject to cliff failure from behind and wave inundation from the front. Seacliff State Beach facilities and homes built on the back beach have repeatedly been damaged or destroyed during major storms (Figure 8; Griggs and Fulton-Bennett, 1988; Griggs and Savoy, 1985). One extensive back-beach development at the southern end of this area is protected by rip-rap and another by a massive seawall (Aptos Seascape; Figure 9). Except for the area in front of the seawall, the erosion reference feature for this stretch remains the top of the seacliff as consistent with California policy. In the development protected by the seawall, the seawall itself serves as the erosion reference feature. Average erosion rates for the cliffs along this stretch (corresponds to map sections 15-20) range from 0 to 22 cm/yr. Manresa State Beach This region just south of the Aptos Seascape seawall, consists of high bluffs (30-40 m) of poorly consolidated sand and silt which are subject to gullying or rapid erosion when either vegetation is removed or the protective beach in front is eroded (Griggs and Savoy, 1985). This area supports a mixture of bluff-face condominiums (Figure 10), bluff top homes and park and agricultural uses. The top of the cliff serves as the erosion reference feature for this stretch. Average erosion rates are variable ranging from 0 to 39 cm/yr for most of the stretch (corresponds to map section 21-25 and 27-29). However the highest rate in the county, 63 cm, was found along the bluffs directly south of Manresa Beach above the condominiums which experienced major failure during the 1989 Loma Prieta earthquake (Plant and Griggs, 1990) (corresponds to Map section 26). Sunset State Beach/Pajaro Dunes, The extreme northern portion of this stretch is dominated by high, rounded coastal bluffs underlain by inactive and vegetated sand dunes which are only partially developed. In this area, the base of the bluff served as an erosion reference feature (corresponds to map section 29) and average erosion rates range from 0-44 cm/yr. The remainder, and majority, of this stretch is characterized by low-lying, recent and active sand dunes which undergo periodic erosion during severe storms followed by subsequent rebuilding Moore, Benumof and Griggs Page 14 10/23/97 (Griggs and Savoy, 1985). Large numbers of condominiums and homes were built on these dunes and at beach level near the mouth of the Pajaro River (Figure 11) in the late 1960's and early 1970's during a relatively calm period characterized by few coastal storms. This development was also approved prior to the formation of the California Coastal Commission. This development is subject to undercutting and erosion during severe wave and tidal conditions. In 1983, storms eroded the front face of the dunes, cut the dune line back as much as 12-15 m and threatened numerous structures (Griggs and Johnson, 1983). Since then, several million dollars have been expended on a 1.5 km long revetment to protect the base of the dunes and the homes. The vegetation line at the base of the dune field (or revetment, where vegetation wasn't present) served as the erosion reference feature for this stretch. Because the dunes advance seaward and vegetation covers the dunes during periods of little storm activity, and because the development of Pajaro Dunes extends in many cases, seaward of the 1953 dune edge, much of this area appears to be accreting in our analysis. This is despite the fact the development of Pajaro Dunes is located in a high risk area. For areas that are eroding, average rates range from 0 - 22cm/yr. San Diego County Erosion Rate Results Oceanside Area (Refer to Figure 3 for Locations) The northernmost or Oceanside reach of San Diego county is characterized by a moderately wide sandy beach backed by city park facilities and dense beach development. In addition, buildings have been terraced into or constructed on top of 5 to 13 meter high cliffs. Since the construction of the Oceanside Harbor jetties in 1942, downcoast beach erosion has been a problem and has been mitigated by sand bypassing, dredging, and beach nourishment (Inman and Jenkins, 1985). Over the past 55 years approximately 12 million m of sand have been placed on Oceanside City Beach (Flick, 1994). This section of shoreline has been heavily armored by a combination of protective structures including concrete seawalls and riprap which serve as the shoreline erosion reference feature. Flooding and wave-overtopping of armoring occurred at many sites during the winter storms of 1941, 1978, 1980, and 1983 (Kuhn and Shepard, 1984). As a result of the extensive beach nourishment and armoring of the Oceanside area, shoreline erosion rates are Moore, Benumof and Griggs Page 15 10/23/97 minimal and average 0 to 3 cm/year for the majority of the reach over the 62- year period from 1932 to 1994. However, average erosion rates at Oceanside Harbor, where the historical dune vegetation has eroded, range from 2 to 21 cm/year Carlsbad Area The Carlsbad area may be divided into two sections consisting of Carlsbad State Beach and the area south of Carlsbad State Beach. The coastline at Carlsbad State Beach is characterized by a narrow, sand and cobble beach backed by 10 to 20 meter high cliffs composed of Eocene sandstone capped by Pleistocene terrace deposits. This section of coast has been armored with concrete seawalls and riprap, however most shoreline protection was not emplaced until the late 1980's (Figure 12). The Carlsbad seawall and promenade was constructed in 1988 to stabilize this portion of cliffs after it was severely eroded during the storms of the late 1970's and early 1980's (Flick, 1994). The shoreline erosion reference feature for this section is the landward edge of the cliff-top. Average erosion rates for the Carlsbad State Beach area range from 3 to 23 cm/year over the 62-year period from 1932 to 1994. The South Carlsbad State Beach area is characterized by a narrow cobble and sand beach backed by 3 to 20 meter high cliffs. The cliffs of this area are composed of Eocene sandstone that have been severely eroded by wave action and sub-aerial mass-wasting. The erosion reference feature for this section is the landward edge of the cliff top. Average erosion rates range from 3 to 58 cm/year over the 38-year period from 1956 to 1994. Encinitas Area The cliffs of the Encinitas area are composed of a number of Eocene- aged units capped by poorly consolidated Pleistocene terrace deposits. Both the Eocene bedrock units and the terrace deposits are generally susceptible to landsliding and human-induced erosion. The cliffs of the Encinitas area are also subject to wave erosion during above average high tides and storm periods as the beaches are generally very narrow. Shoreline protection in the Encinitas area is not continuous and varies widely in type of construction. The shoreline erosion reference feature for this section of coastline is the Moore, Benumof and Griggs Page 16 10/23/97 landward edge of the clifftop. Average erosion rates for this section range from 2 to 29 cm/year over the 62-year period from 1932 to 1994. Solana Beach Area The Solana Beach stretch of coastline is characterized by a narrow, sandy beach backed by approximately intensively developed 20 meter high cliffs (Figure 13). The cliffs of this area are composed of Eocene sandstone overlain by unconsolidated Pleistocene terrace deposits. The Eocene material commonly fails along nearly vertical discontinuities as a result of cave collapse (Kuhn and Shepard, 1984). Shoreline armoring in this area is sparse but consists of concrete seawalls and rip-rap. The shoreline erosion reference feature for the Solana Beach area is the landward edge of the cliff-top. Average erosion rates for this section range from 3 to 31 cm/year over the 62- year period from 1932 to 1994. Del Mar Area The northern Del Mar area is characterized by a wide, low-lying, and popular sandy beach which offers protection to the dense residential development behind it. Several protective structures exist along this stretch including concrete seawalls, riprap, sheet-pile seawalls, and timber seawalls. The shoreline reference feature for the northern Del Mar area is the landward edge of shoreline armoring or the seaward edge of beach development as compared to the seaward margin of the 1932 vegetation line. Average erosion rates for this stretch range from 2 to 13 cm/year over the 62-year period from 1932 to 1994. The southern Del Mar area consists of a narrow, sandy beach backed by nearly vertical, 15 to 30 meter high cliffs with a railroad bench cut into the face. The railroad was constructed in 1910 and has experienced numerous failures (Kuhn and Shepard, 1984). Pleistocene terrace deposits comprise the majority of the cliffs in this area, however the bedrock consists of an Eocene sandy claystone. The shoreline erosion reference feature for the southern Del Mar area is the landward edge of the railroad cut. Little shoreline armoring exists along this stretch and average erosion rates for the area range from 2 to 34 cm/year over the 62-year period from 1932 to 1994. Moore, Benumof and Griggs Page 17 10/23/97 Torrey Pines Area The Torrey Pines area is characterized by a narrow- to medium-width sandy beach backed by low, active dunes and very high, steep, eroding cliffs, many of which have been developed (Figure 14). Cliffs in this area exceed 90 meters in height and are primarily composed of Eocene sandstone and shale. Subaerial mass wasting is the dominant erosive mechanism in this area and many landslides have occurred. In 1982, a 175 meter long section of the Torrey Pines cliffs failed and approximately 1.8 million cubic yards of material was deposited on the beach (Vanderhurst et al., 1982). In addition, the Torrey Pines area is void of shoreline armoring. The erosion reference feature for this reach is the landward edge of the cliff-top which is generally marked by a landslide scarp. Average erosion rates for the Torrey Pines area range from 2 to 55 cm/year over the 42-year period from 1952 to 1994. La Jolla Area The majority of the La Jolla area is characterized by rocky, wave-cut platforms, 5 to 20 meter high vertical cliffs, and pocket beaches (Figure 15). The cliffs are composed of Cretaceous sandstone interbedded with shale and are capped by poorly consolidated Pleistocene material. Approximately 25 percent of the cliffs are fronted with various types of shoreline protective structures (Flick, 1994). The shoreline erosion reference feature for the La Jolla area is primarily the landward edge of the cliff-top except at La Jolla Shores where a sandy beach of variable width is backed by a low, armored cliff. The shoreline reference feature for the La Jolla Shores stretch is the continuous shoreline armoring occurring along this reach. Typical average erosion rates for the La Jolla area range from 0 to 17 centimeters/year over the 42-year period from 1952 to 1994 except for a short stretch near Bird Rock where erosion rates are as high as 26 cm/year. Pacific Beach / Mission Beach Area The northern section of the Pacific Beach shoreline is characterized by a moderately wide sandy beach backed by steep, 15 meter high, heavily- developed cliffs. The cliffs in this area are composed of Pliocene sandstone and conglomerate capped by Pleistocene material. The cliffs along this reach are largely unprotected and the erosion shoreline reference feature is the Moore, Benumof and Griggs Page 18 10/23/97 landward edge of the cliff-top. Average erosion rates range from 2 to 24 cm/year over the 42-year period from 1952 to 1994. The remainder of the Pacific Beach and Mission Beach shoreline is characterized by a low-lying beach of variable width backed by residential, public, and commercial development. This entire reach is protected by a concrete seawall which fronts a heavily utilized boardwalk and serves as the shoreline erosion reference feature. The concrete seawall was overtopped during the 1982, 1983, and 1988 storms (Armstrong and Flick, 1989), however no net shoreline erosion has occurred over the 42-year period from 1952 to 1994. Point Loma Area The Point Loma area is characterized by pocket beaches, wave-cut platforms, and high, steep cliffs. Many sea caves have formed in the cliffs along this reach as a result of undercutting by waves. The cliffs are composed of Cretaceous shale interbedded with sandstone and capped by poorly consolidated Pleistocene material. Many different types of shoreline protection structures occur along this stretch, however their occurrence is dis- continuous and site-specific. The shoreline reference feature for the Point Loma area is the landward edge of the cliff-top and average erosion rates range from 2 to 26 cm/year over the over the 42-year period from 1952 to 1994. Coronado / Imperial Beach Area The Coronado area is a section of coastline that has been highly altered by human efforts. The Coronado reach is relatively stable as a result of past beach nourishment projects and beach stabilization structures. The area is characterized by a wide, sandy beach backed by shoreline protective structures and is defined by two shoreline erosion reference features. At Sunset Park, the shoreline consists of low, active dunes and the seaward edge of dune vegetation serves as the erosion reference feature. A riprap revetment and associated development, serves as the shoreline erosion reference feature for the remainder of the Coronado reach. In 1904, the Coronado area was stabilized by the construction of the 2,200 meter long Zuniga Jetty to the north (Shaw, 1980). Between 1946 and 1990 approximately 35 million cubic meters of sand from San Diego harbor was deposited on the beaches of Coronado and the Silver Strand section to the south (Flick, 1994). As a result, no shoreline Moore, Benumof and Griggs Page 19 10/23/97 erosion has occurred; in fact, at Sunset Park the shoreline has accreted as much as 97 meters over the 45-year period from 1949 to 1994. The Imperial Beach area is characterized by a narrow sandy beach backed by dense residential and commercial development. The Imperial Beach area has been subject to beach erosion for many years (Flick, 1994), however like Coronado to the north, it has been somewhat stabilized by shoreline protective structures and beach nourishment. The shoreline erosion reference features for this reach are the shoreline protective structures and associated beach development in the City of Imperial Beach and the seaward edge of dune vegetation at Oneonta Slough to the south. Over the 45-year period from 1949 to 1994, no shoreline erosion has occurred along the Imperial Beach stretch as a result of shoreline armoring and beach nourishment. However, as much as 13 meters of shoreline retreat has occurred along the undeveloped reach at Oneonta Slough. SUMMARY/CONCLUSIONS 1) The coastlines of Santa Cruz and San Diego County are heavily developed in many areas. The population of California continues to rise as well as the numbers of Californian's living at the coast, suggesting that there will continue to be increasing development in coastal areas. 2) Although, cliff loss and wave attack/inundation pose a significant threat to developments on cliff-tops and in back-beach environments in both San Diego and Santa Cruz Counties, these hazards appear to have been largely ignored by government, builders and home owners in the past. 3) The Santa Cruz and San Diego County coastlines are similar in their geomorphology and the erosion rates determined for the two counties have similar ranges : 0-63 cm/yr for Santa Cruz County and 2-55 cm/yr for San Diego County. 4) Projections based on these erosion rates, suggest that numerous structures will be lost due to erosion of the coastal bluffs within the next sixty years. Moore, Benumof and Griggs Page 20 10/23/97 APPENDICES Appendix A: Shoreline Erosion Rate Maps provided under separate cover to FEMA Appendix B: Shoreline Erosion Rate Data for Santa Cruz County Appendix C: Shoreline Erosion Rate Data for Dan Diego County Moore, Benumof and Griggs Page 2! 10/23/97 REFERENCES Armstrong, G.A. and R.E. Flick, 1989. Storm Damage Assessment for the January 1988 Storm Along the Southern California Shoreline. Shore and Beach 57, No. 4, 18-23. Everts, C.H., 1991. Seacliff Retreat and Coarse Sediment Yields in Southern California. Coastal Sediments. 1586-1598. Flick, R.E. 1994. Shoreline Erosion Assessment and Atlas of the San Diego Region, California Department of Boating and Waterways and the San Diego Association of Governments, Sacramento, CA. Volume 1, 135pp. Griggs, G.B. and Johnson, R.E. 1976. The Effects of the Santa Cruz Small Craft Harbor on Coastal Processes in Northern Monterey Bay, California. Environmental Geology 1:229-312. Griggs, G.B. and Johnson, R.E. 1983. The Impact of the 1983 Storms on the Coastline of Northern Monterey Bay. California Geology 36:163-174. Griggs, G.B., and Savoy, L, ed., 1985. Living with the California Coast. Duke University Press, Durham, North Carolina, 393 p. Griggs, G.B., 1986. Reconstruction or Relocation: Viable Approaches for Structures in Areas of High Coastal Erosion. Shore and Beach 54:8-16. Griggs, G.B. and Fulton-Bennett, K.W., 1988. Failure of Coastal Protection at Seacliff State Beach, Santa Cruz County, California. Environmental Management 11:175-182. Griggs, G.B. 1990. Littoral Drift Impoundment and Beach Nourishment in Northern Monterey Bay, California. Jour, of Coastal Research, Special Issue on Beach Nourishment: 115-126. Inman, D.L. and Jenkins, S.A., 1985. Erosion and Accretion Waves from Oceanside Harbor. Conference record, Oceans 85: Ocean Engineering and the Environment. New York, New York: Institute of Electrical and Electronics Engineers. Kennedy, M.P., 1975. Geology of the San Diego Metropolitan Area, Western Area. Bulletin 200. Sacramento, California: California Division of Mines and Geology. Kuhn, G.G., and Osborne, R.H. 1987. Sea Cliff Erosion in San Diego County, California. Coastal Sediments, ASCE, Vol. 2, pp. 1839-1854. Moore, Benumof and Griggs Page 2 2 10/23/97 Kuhn, G.G. and Shepard, P.P. 1984. Sea Cliffs, Beaches, and Coastal Valleys of San Diego County. University of California Press. 193p. Plant, N. and Griggs, G.B., 1990. Coastal Landslides and the Loma Prieta Earthquake. Earth Sciences 43: 12-17. Shaw, M.J., 1980. Artificial Sediment Transport and Structures in Coastal Southern California. Ref. Series No. 80-41. La Jolla, California: Scripps Institution of Oceanography. Vanderhurst, M.L., R.J. McCarthy, and D.L. Hannan, 1982. Black's Beach Landslide. Geologic Studies in San Diego : 46-56. San Diego, California: San Diego Association of Geologists. Moore, Benumof and Griggs Page 23 10/23/97 40 i C/)zo 30 H O < 20 H 10H 1800 1900 2000 Figure 1. California's increasing population North Santa Cruz County t N Santa Cruz County California Capitola Opal Cliffs Aptos Seascape New Brighton Beach 2km Figure 2. Santa Cruz County Shoreline Map San Onofre ^.Nuclear Generating -=i. Station Oceanside Harbor •.Oceanside §» CarlsbadOCEANSIDE LITTORAL CELL •^Leucadia •Encinitas Splana Beach • Del Mar iTorrey Pines • La Jolla [\ Mission MISSION BAY LITTORAL CELL • Imperial Beach PacificOcean 10 mi Cliffed Area SILVER STRAND LITTORAL CELL Coronado \ CanyonV_^Tijuana River Mexico Figure 3. San Diego County Shoreline Map Figure 4. Typical cliff top, Santa Cruz County Figure 5. Typical cliff top, San Diego County BEST ORIGINAL Generalized Coastal Hazard Mapping Workflow Obtain Digital Imagery Gather GPS Control Obtain Camera Reports f Aerial \Triangulation Generate Stereo Pairs Generate/ Edit DEMs Orthophotographs f Digitze Shoreline Erosion Reference Features Run Shorelinegrid and Project FIRM Lines Generate Maps Figure 6. Generalized flowchart depicting the softcopy photogrammetry workflow applied in the FEMA Coastal Hazard Mapping Project. e>oroKCO LU DQ Figure 7. Undermining of piers for apartment complex in Capitola area of Santa Cruz County. This failure was induced by the Loma Prieta earthquake of 1989 and led to demolishing the six apartment units shown. Figure 8. Storm damage in 1983 in the Beach Drive/Rio del Mar area due to extensive beach scour and loss of support for pile foundation. rro COLU CO Figure 9. Aptos-Seascape in central Santa Cruz County where homes have been built on the beach, now protected by a large seawall. i; rr:O CO Hi CO Figure 10. Bluff face development in southern Santa Cruz County. This photo was taken directly after the Loma Prieta earthquake which produced bluff failure shown. Foundation damage to the home at the crest of the bluff led to demolishing the home. o or;o COLUCD Figure 11. Pelican Point condominium development at the mouth of the Pajaro River in southern Santa Cruz County. These units are only a few feet above sea level and susceptible to wave inundation during high tides and severe storm waves. Figure 12. Bluff top in the Cardiff area with nearly continuous condominium development. w2 gcro 0)LU CD Figure 13. Intensively developed cliff top in the Solana Beach area of San Diego County. •^ V^^M ' ffitfi Figure 14. Steep, high, unvegetated and developed bluffs in the Torrey Pines section of San Diego County. CD Of O CO M: CO Figure 15. La Jolla area of San Diego County with wave cut terraces and sparse shoreline armoring.