Short- and Long-Term Risks of Back Beach Development along the California Coast

California is widely known for its beaches, and they draw millions of visitors annually. Of California’s 1760 km (1100 mile) of coastline, however, only about 500 km (312 miles), or 28%, can be characterized as low relief, relatively flat, and fronted by wide sandy beaches (Griggs, 2010). In their natural, predevelopment state, these beaches, like beaches everywhere, would narrow under the impact of winter storm waves and then widen again the following spring and summer as sand was pushed back onshore by the less energetic waves.

When the California coast experienced extended periods of relatively calmer weather, and there was a large natural supply of sand, in addition to a favorable coastline orientation and/or natural barriers to littoral transport, beaches could grow wider such that dunes might even develop on the dry back beach. Between the mid-1940s and 1978, the coast of California experienced a cooler and generally calmer Pacific Decadal Oscillation (PDO) cycle (Figure 1). El Niño events were generally milder and less frequent, although all El Niño events are not created equal, and the duration and magnitude of these events, and thus their impacts on the shoreline, can vary considerably. This was also the period following World War II when California’s population grew rapidly from 9.3 million in 1945 to 22.8 million in 1978, a nearly 2.5-fold increase.

Erosional cycles coincide with the warm phases of the PDO, which are periods characterized overall by more frequent and more intense El Niño events, greater wave energy, and storms generally approaching the coastline from the W or SW. These conditions were responsible for much of the beach and coastline erosion and also coastal storm damage along the California coast between 1978 and 1998 (Storlazzi and Griggs, 1998, 2000). Beach widening, or accretion, on the other hand, generally coincides with the cool phase of the PDO (La Niña events characterized by fewer strong coastal storms, less wave energy, and, as a result, less shoreline erosion), which extended from 1947 to 1978 and from about 1998 to 2013. There are typically some lag effects, however, so that the beaches don’t respond immediately to changing climatic conditions.

The incentives and opportunities for new housing during this relatively calm climatic period led to the subdivision and development of homes on coastal bluffs, dunes, and even on some of the beaches of the state in southern and central California. Beaches were wide and appeared to be stable, and, at some time in prior years, earlier entrepreneurs had laid claim to these backshore areas, followed by land divisions and house construction. Today, state ownership of all shoreline area below the mean high tide (MHT) line is the law, but this was not the case during the 1800s. The location of the MHT now is highly bureaucratic and surveyor-driven (Lester, 2021). The MHT elevation is recalculated every 20–25 years over an 18.6 year celestial cycle, or so-called tidal epoch. The location at which this line or elevation intersects the shoreline will change over time as the condition of the beach changes and will also increase in elevation as sea levels continue to rise.

These “tidelands”—the area below MHT—became public property when California became a state in 1850 (Lester, 2021). There were still a number of unresolved questions and disputes about land ownership at this time, however, and a key piece of legislation was the 1851 Land Claims Act. One of the complications was that the United States recognized some prior land claims under Spanish and Mexican law (but not necessarily fairly or evenly). Some Native land claims were also recognized, but this was also very inconsistent, biased, and uneven. The Land Claims Act was intended to clear all of this up with a 2 year statute of limitations. By 1853, all the land that was deemed “unclaimed” went into the public domain and became therefore open for the Homestead Act. (This is when a lot of Native Californians lost their land claims and rights, and they did not even necessarily know this had happened.) As a result, many back beach areas, whether or not they were actually inland of or higher than MHT, became private property. Lots were created, sold, and subsequently developed during the period extending from the mid-1940s to the mid-1970s.

About 1978, California’s coastal climate transitioned from a calmer, cooler PDO interval to a warmer, stormier period dominated by a El Niño–Southern Oscillation (ENSO) climatic pattern, and the California coast was hit by some of the most damaging storms in several decades (Figures 1 and 2). During the El Niño winters of 1978, 1982–83, 1987–88, and 1997–98, considerable shoreline development was damaged and destroyed by large waves arriving at times of high tides, all occurring during periods of El Niño–elevated sea levels.

During the 1978 winter, many oceanfront residents, planners, engineers, and public officials were surprised as private and public oceanfront losses reached $86 million (in 2023 US dollars [USD]). Five years later, high tides, elevated sea levels, and storm waves during the 1982–83 El Niño winter inflicted about $317 million (in 2023 USD) in damage to California’s shoreline (Griggs, 2010). During the months of January, February, and March of 1983, eight major storms struck the coast accompanied by large waves (Seymour et al., 1984, Walker et al., 1984). The arrival of large waves unfortunately tended to coincide with high tides, which further exacerbated the damage. Losses were widespread from N to S and not restricted to broken windows and flooding of low-lying areas; 33 oceanfront homes were totally destroyed, and 3000 homes and businesses were damaged. The elevated sea levels and large waves damaged breakwaters, piers, park facilities, seawalls, coastal infrastructure, and public and private structures (Griggs, Patsch, and Savoy, 2005). This event was a wakeup call for all of the state’s coastal communities and made it clear that the California coast wasn’t the endless summer we had been lulled into believing it was.

Again, in 1997–98, another major El Niño winter seriously impacted the state’s coastline; more property was lost, more homes were damaged or destroyed, and some damaged structures and pieces of infrastructure were removed. Although most oceanographic and meteorologic indicators suggested that the 1997–98 ENSO disturbance was more intense than the 1982–83 event, the state’s coastline suffered far less damage than in the earlier event. In this latter event, the largest waves from the two biggest storms hit during lower periods in the monthly tidal cycles, significantly reducing the impact of the waves on the shoreline. Another important factor contributing to the disproportionate damage between the two winters was the higher percentage of shoreline that had been armored during the intervening 15 years. By the time the 1997–98 ENSO event arrived, most of the areas seriously damaged in the 1982–83 winter had been armored by substantial seawalls or revetments.

About 1998–99, the PDO again transitioned to a cooler, calmer interval with fewer and less energetic ENSO events (Figures 1 and 2). Nonetheless, 2015–16 was a major El Niño year, which was determined to be one of the strongest of the last 145 years, resulting in anomalously high wave energy along the U.S. West Coast and record erosion for many California beaches (Smith and Barnard, 2021). The 2009–10 El Niño event was brief but also intense (Kim et al., 2011) in terms of cumulative coastal erosion in parts of California (see Pacifica, below).

In early January 2023, conditions nearly identical to those 40 years earlier in January 1983 impacted the California coast. A wave buoy off of Monterey Bay on the central coast recorded peak significant wave heights of 8.5 m (28 ft), which arrived at the shoreline coincident with an extreme high tide (2 m [7 ft]) and strong onshore wind (Figure 3). Damage was extensive to both private beachfront property and public infrastructure (Figure 4). In December of 2023 and early January of 2024, nearly identical conditions again hit the coast and caused additional damage, although this was an El Niño year.

Four individual communities (Figure 5) provide important perspectives on the short- and long-term stability of back beach development in coastal California. While there are many other examples, these four provide a clear history of shoreline impacts by extreme events on both private development and public infrastructure.

The 9-km-long (5.6-mile-long) City of Pacifica coastline between Mussel Rock and Point San Pedro, south of San Francisco on the central California coast (Figure 5), consists of a low, narrow coastal plain in its northern half fronted by high steep bluffs and a lower coastal terrace to the south. In northern Pacifica, this coastal terrace slopes gently southward from an elevation of about 49 m (160 ft) near Mussel Rock to sea level at Laguna Salada, a natural lagoon north of Mori Point, over a distance of 5.3 km (3.3 miles) (Figure 6). The nearly straight shoreline along the seaward margin of this narrow plain consists of a narrow sandy beach backed by low vertical bluffs eroded into moderately consolidated sands and gravel. The littoral sand forms a thin layer over a range of geologically young sedimentary rocks (often referred to as hardpan; Figure 7; Greene et al., 2014).

Pacifica is located at the south end of the San Francisco Littoral Cell (BCDC, undated; ESA et al., 2016). Sand transport is dominated by exchange between the ocean and San Francisco Bay (Battalio, 2014; Battalio and Trivedi 1996). There appears to be little beach-sized sand supply at present except that supplied by bluff erosion and possibly littoral transport from Marin County north of San Francisco (Barnard et al., 2013). Offshore transport of finer sands appears to be the primary loss to the overall sand budget, although these finer sands were historically transported inland by strong onshore winds, forming large dune fields, particularly in San Francisco, and littoral deposits on the north San Francisco shore (Battalio, 2014).

The Pacifica shoreline in its natural predevelopment configuration had a beach and dunes indicative of an abundance of littoral sand. The oldest aerial photographs of the San Mateo coastline were taken in 1928 as part of the plan to build a highway from San Francisco south to Santa Cruz (Figure 8). This was 96 years ago, and the coastline was strikingly different. Mori Point headland served as a natural barrier that impounded southward-moving sand to form a beach that extended nearly 5 km (3 miles) up the coast to the north (Figures 6 and 8).

Historical shoreline data between 1853 and 1998 indicate the shore was much wider in the 1940s (Figures 8 and 9) (ESA and PWA, 2011; derived from Hapke et al., 2006). From 1946 to 1998, the shoreline fronting Beach Boulevard retreated at a rate of about 0.5 m (1.7 ft) per year, with greater erosion (about 1.2 m [4.1 ft] per year) farther south near Mori Point (ESA et al., 2016). The corresponding volume of eroded sand is about 1.3 million cubic meters (1.7 million cubic yards). A review of historical human intervention indicates that the wide beach that existed in the 1930s to 1940s may have resulted from an increased sand supply from the north. Between the late 1800s and early 1900s, the San Francisco shore (Ocean Beach, about 8 km [5 miles] to the north) was filled seaward 61 to 91 m (200 to 300 ft) with dune sands (Battalio and Trivedi, 1996; Olmsted and Olmsted, 1979), indicating a volume of between 2.3 and 8.4 million cubic meters (3 and 11 million cubic yards). To counter the ensuing erosion of the artificial shore, 1.8 million cubic meters (2.4 million cubic yards) of sand were placed between 1915 and 1935. Hence, it is possible that Pacifica’s wide beaches in the 1930s–40s resulted from a pulse of sand migrating south from San Francisco, which eroded rapidly when the artificial supply diminished.

Beaches north of the sand fillet (where fillet refers to a sand deposit and widened beach updrift of a groin, headland, or other coastal feature) caused by the Mori Point headland have narrowed to an average width of less than 15 m (50 ft), and waves regularly impinge upon the bluffs and armoring structures constructed to resist bluff erosion (ESA et al., 2018). Average bluff erosion rates were about 0.7 m (2.4 ft) per year (1930 to 1998; Hapke and Reid, 2007) but have increased to over 1.5 m (5 ft) per year between 2004 and 2017 (CSA, 2019).

At about the same time that historical bluff erosion began to increase, shoreline development began in earnest (1950s and early 1960s) and gradually encroached further onto the low-relief shoreline. This led to beach narrowing and, over time, the loss of the beach as the low bluff was armored with riprap initially, followed subsequently by a concrete seawall at its southern end.

Many of today’s streets in the Sharp Park area were already in place in 1928 so that comparisons with the present-day shoreline can be made. An evident feature in the 1928 aerial photograph (Figure 8) and also in a circa 1900 ground photo (Figure 10) is that the beach was backed by extensive sand dunes that extended inland tens of meters (hundreds of feet). The beach at that time looks to have been over 30 m (100 ft) wide at the northern end near Paloma Avenue, and it then widened progressively to the south to over 92 m (300 ft) at Clarendon Road, with active dunes extending inland from the beach. There were just a few scattered buildings on the low terrace at the time, and there was no Beach Boulevard along the edge of the coastal bluff, although most of the other roads were in place.

In recent years, the seawall along Beach Boulevard has frequently been overtopped and also damaged under conditions of very large waves at times of extreme high tides. These conditions occurred during the severe El Niño winters of 1982–83 and 1997–98, leading to erosion of the high steep bluffs at the northern end of this stretch of shoreline, and ultimately, the demolition of 10 bluff-top homes in April 1998. Extensive erosion occurred again in the 2009–10 winter, resulting in the demolition of two homes and three large apartment buildings in 2016–18 after attempts to armor the bluffs failed (Figure 11). Immediately north, a new seawall was constructed following the 2009–10 El Niño and was severely damaged during the 2015–17 winters and was subsequently renovated and expanded. As a factor of interest, prior to construction of this seawall, the bluff was excavated so that the structure could be set back as far landward as practicable without undermining existing apartments, and the bluff sediments were placed on the beach to be sorted by waves. This approach of structure setback, negotiated with the California Coastal Commission, likely contributed to the structure surviving the extreme 2015–17 conditions.

A feature that was evident then, as it is today, is that Mori Point forms a natural barrier to southerly littoral sand transport. The beaches existing in 1928 were wider adjacent to Mori Point but narrowed to the north (Figure 6). This trend has persisted to the present due to the southerly littoral drift trapping effect of Mori Point, and it is most evident along the Beach Boulevard shoreline. When the beach is exposed at lower tides, it is considerably wider south of the fishing pier than to the north, such that waves break closer to the seawall north of the pier and have a significantly greater impact on the seawall.

Aerial photographs from 1972 show only a few areas where rock had been placed at the base of the eroding bluffs (Figure 12), but a narrow beach existed along the entire length of Beach Boulevard. By the time of the 1987 photograph and following bluff erosion during the 1983 El Niño, a concrete seawall fronted by a rock revetment had been built along 427 m (1400 ft) of shoreline north of the fishing pier (Figure 13). There is no significant beach present in the September 2017 aerial image (which is when the beaches would normally be widest). At its greatest width, the beach adjacent to Mori Point is about 61 m (200 ft) wide; near Clarendon Road, it is about 46 m (150 ft) wide, and at Paloma Avenue, there is no beach (Figure 13). This difference in beach width clearly has had and will continue to have potentially significant impacts on the bluff erosion and any coastal armoring along Beach Boulevard.

Years of observations of coastal armoring, whether seawalls or rock revetments, have shown that, while these structures can provide at least short- to intermediate-term protection of the backshore/bluff/cliff area if well engineered and constructed, they also have well-documented impacts (Griggs, 2005). The issue of passive erosion is particularly relevant to the Beach Boulevard shoreline. A continuing rise in sea level, as recorded for the past ∼18,000 years, and that is now occurring at an accelerated rate, will cause any shoreline and the beach to gradually move inland or landward. Where a backshore area consists of erodible material (such as the low bluffs historically fronting Beach Boulevard), the bluffs can erode, and a beach will remain and continue to move landward. However, where the backshore is armored, as it is along about 9% of the entire San Mateo County coastline today (Griggs and Patsch, 2019), as well as along the entire length of Beach Boulevard, the bluffs cannot migrate inland, and the beach will gradually narrow and finally will completely disappear or be flooded. This is known as passive erosion, and it has already taken place along the northern end of Beach Boulevard (Figure 14).

There is a well-documented history of storm damage and armoring along Beach Boulevard that extends back to 1955 (Table 1; Fulton-Bennett and Griggs, 1986). In 1984, as a result of continuing bluff erosion along Beach Boulevard during the 1982–83 El Niño event, a concrete seawall with a toe revetment (Figure 15) was constructed along the ∼427 m (∼1400 ft) of bluff from the northern end of Beach Boulevard (just north of Paloma Avenue) to the Pacifica Municipal Fishing Pier. In 1987, the south wall and toe revetment were constructed, which extended about 366 m (1200 ft) from the pier to the south end of Beach Boulevard. Linking the two seawalls was a sheet pile seawall system at the shoreward end of the fishing pier.

With the beach narrowing, wave impact to the seawall increased, resulting in riprap collapsing and moving seaward as waves overtopped the wall and scoured soil from behind the wall and under the walkway. The wall failed in January 2001 (Figure 16).

Dangerous high-velocity water flows caused by waves and wave runup overtopping the Beach Boulevard seawall have also impacted pedestrians:

Since December 2015, the city saw the dramatic failure of the seawall along Beach Boulevard near the intersection of Santa Maria Avenue as well as damage to the Pacifica Pier. Private properties located on Esplanade Avenue and Palmetto Avenue to the north had also been affected. A second section of the Beach Boulevard Promenade collapsed near the intersection with Santa Maria Avenue in early February 2016, leaving a large void (Figure 16).

During the large storm waves and high tides of late December and early January of the 2023–24 El Niño winter, storm surge led to waves overtopping the Beach Boulevard seawall again. Overtopping water propagated inland and south along the streets, following the slope of the land (Figure 17). This carried large volumes of coarse sand and pebbles from the shoreline over the wall, across the sidewalk and Beach Boulevard, and onto residence’s driveways, a distance of about 60 ft (Figure 18). The sand overwhelmed the storm drains, and the water extended inland two blocks into a low-elevation area that was historically part of Laguna Salada. The wave overtopping is dangerous to pedestrians and vehicles and has damaged multiple homes.

This overtopping and sediment deposition formed what is essentially a perched beach over the area that was a beach, dunes, and low bluff prior to coastal development (Figures 18 and 19). With projected sea-level rise as well as the recurring El Niño events, with their elevated water levels, combined with large waves and very high tides, we can expect coastal processes to continue to overtop and damage the existing development and reclaim the former beach, forming a perched beach.

The city of Capitola on the shoreline of northern Monterey Bay (Figure 5) began as a fishing village with a pier for shipping cattle hides and tallow in the early to mid-1800s. It was called Soquel Landing at the time as Soquel Creek discharged into Monterey Bay at that location. Camp Capitola was initially developed in 1869 on the low alluvial plain at the mouth of the creek as a summer camp with tents, cabins, and small cottages. As the summer popularity of this area increased, lots were sold, homes and businesses were built, and even a grand hotel was constructed. A comparison of photographs of the oceanfront Esplanade from 1910 and 2006 illustrates how development encroached well onto the beach over time (Figures 20 and 21). What was sand and a beach playground for children became parking and entertainment and dining establishments. On the west side of the beach, a resort consisting of rental and for sale units was built literally on the sand in 1923. The Venetian Court, now a century old, is generally recognized as the oldest condominium complex on the California coast.

Building on the back beach, however, as has been discovered in a number of other California shoreline locations, has clear risks. Accounts from historic newspapers document a few of the impacts that high tides and storm waves have had along the developed back beach of Capitola, including the Venetian Court units (Table 2).

The impacts of the January 2023 storm waves and high tides in Capitola were nearly identical to those 40 years earlier in January to March of 1983. Logs and debris washed into the restaurants along the Esplanade, and the street was covered with beach sand (Figure 22). The Venetian Court was again flooded with considerable damage. The hazards of building on the back beach again became evident as beach sand and debris were deposited inland of the oceanfront restaurants, an area that was historically a beach (Figure 23).

The shoreline at Rio del Mar along northern Monterey Bay consists of an approximately 90-m-wide (300-ft-wide in summer) beach backed by 30–45-m-high (100–150-ft-high) steep coastal bluffs (Figure 24). By 1929, a road about 1.6 km (1 mile) long, which would become Beach Drive in the future, had been constructed along the back beach, lots had been sold, and several homes were beginning to be built on the beach (Figure 25). At the upcoast or northwestern end, the road ran along the base of the bluff, and lots were created about 24 m (80 ft) seaward of the bluff out on the beach on the ocean side of the road. Over time, 27 homes were built on this section of back beach, which came to be known as The Island. Along Beach Drive, the road and homes had encroached 24 to 30 m (80 to 100 ft) seaward of the base of the bluff, essentially moving the shoreline seaward (Figure 26). This distance or alignment presumably was the MHT line established at some earlier period when this back beach area was claimed by private interests.

Thirty years later, most of the lots both landward and seaward of Beach Drive had been developed, and there were 60 additional homes on the inland side of the roadway and 32 more homes on the seaward side. Because the road and homes were built nearly at beach level (4.2 m [14 ft] above mean sea level [MSL]), this stretch of shoreline has been repeatedly flooded and damaged over the past century. A detailed look at the history of damage and protection provides some perspective on the impacts and costs of building on the back beach, using data compiled from state and local agency files by Fulton-Bennett (1984) and summarized here in Table 3.

Waves during winter storms of 1981 destroyed an older seawall protecting Beach Drive as well as the sidewalk, one lane of the road, and a portion of a sewer line beneath the roadway (Griggs and Johnson, 1983). Some homes were subject to broken windows and flooding. An unsuccessful attempt to halt additional erosion by utilizing 189 L (50 gallon) steel drums cabled together (which quickly failed) was followed by the placement of emergency riprap. Following this effort, a joint private/state/federally funded $4.6 million (in 2023 USD) seawall was built along the central portion of Beach Drive. This structure consisted of steel H piles with 15 × 30 cm (6 × 12 in.) timbers placed within the flanges of the piles used as lagging. The wall was topped with a concrete cap and a steel railing.

In January 1983, the combination of elevated sea levels from a large El Niño event, very high tides, and large storm waves overtopped the new seawall and washed sand across Beach Drive and into carports (Figure 27). Damage to homes on the inland side of Beach Drive was relatively minor, but it was clear that many of the homeowners had experienced such events before and had a combination of concrete walls with drop-in planks fronting their homes, plywood panels and brackets for additional protection, and sandbags ready (Figure 28).

Further south, however, homes had been built out on the beach on the seaward side of Beach Drive. They were protected by a variety of structures, including timber bulkheads, large concrete panels, a concrete bulkhead, and riprap. Several days of 4 to 6 m (13 to 20 ft) waves and over 1.8 m (6 ft) tides damaged or destroyed virtually every protective seawall and bulkhead (Figure 29). The lack of a uniform integrated structure took its toll; weaker walls were overtopped or battered down, and the waves then washed out the sand fill behind the walls, which led to collapse. By the end of January, with the protection gone and sand levels lowered up to 2.4 m (8 ft), piers and pilings were exposed and undermined. Several of these back-beach houses collapsed onto the sand, and many others were seriously damaged (Figure 30).

Forty years later, on the early morning of 5 January 2023, very large waves (with significant wave heights recorded at an offshore buoy at 8.5 m [28 ft]; Coastal Data Information Program [CDIP] Buoy 156, Monterey Canyon Outer; see Figure 3) arrived simultaneously with extreme high tides (∼2 m [∼7 ft] at the National Oceanic and Atmospheric Administration [NOAA] Monterey tide gauge) and strong onshore winds. Waves overtopped the rock revetment along The Island and brought sand, large logs, and debris onto the patios and up against the homes (Figure 31). Down the coast to the south, the low seawall along Beach Drive, which had been in place for 40 years, survived this event with no significant damage, but the waves overtopped the wall (which is at just 4.5 m [15 ft] mean lower low water) again and carried sand and debris onto and across Beach Drive and into carports and garages on the inland side of the roadway (Figure 32). Many of these homeowners are aware of this frequent occurrence and have their own barriers with slots for boards that can be inserted at times of storm surge and wave overtopping (see Figure 28), as well as brackets for plywood or wood panels to protect the ocean-facing windows of their homes under these conditions.

The early January 2023 tide and wave conditions were identical to those that occurred 40 years earlier in January of 1983 and left what was essentially a perched beach with sand and debris along Beach Drive, which was the back beach under predevelopment or natural conditions. A similar perched beach formed in early 1983 as the ocean was reclaiming the original natural shoreline.

Southern California’s steep Santa Monica Mountains have restricted growth along the Malibu coast to a narrow ribbon along the Pacific Coast Highway (Figure 33). While most of the land between Point Mugu and Santa Monica long remained unincorporated, mostly in large private ranches, which originally developed from portions of Mexican land grants, the completion of what is now the Pacific Coast Highway between 1906 and 1929 began to open the way for corridor development.

Mugu Submarine Canyon heads very close to the shoreline about 4 km (2.5 miles) west of Point Mugu (Figures 33 and 34) and drains off most of the littoral drift from the upcoast Santa Barbara Littoral Cell (approximately 765,000 m³ [1 million cubic yards] per year on average; Griggs and Patsch, 2018). Historically, a small amount of that littoral sand (∼38,225 m³ or 50,000 cubic yards per year) bypassed the canyon head and was carried eastward along the shoreline towards Point Dume, Malibu, and then continued to Santa Monica Bay by the prevailing waves from the west (Knur, 2000; Moffatt and Nichol, 2009). Beaches along this stretch of coast are generally intermittent and narrow and form where either rocky outcrops or headlands retain sand or where the shoreline orientation is favorable for sand accumulation. The most significant barrier to littoral sand transport along this coast has been Point Dume, a large volcanic headland about 26 km (16 miles) east of Point Mugu (Figure 33).

The oldest aerial photograph of the Point Dume area from 1924 shows an undeveloped coastline and a wide sandy beach extending west or upcoast that had accumulated against the headland (Figure 35). The trapping of littoral sand from the west impounded over 1.5 million cubic meters (2 million cubic yards) of sand and built a beach nearly 6 km (4 miles) long. This continuous beach has historically been narrowest at the west end near Lechuza Point, wider along much of Broad Beach, and then reached its greatest width along Zuma Beach next to Point Dume. However, over time, a significant portion of the landward part of Zuma Beach was covered by the access road along the back beach, as well as extensive beach parking lots, and the Pacific Coast Highway.

For well over a century, and perhaps much longer, a several tens-of-meters-wide (several hundred-foot-wide) beach existed along what became known as Trancas or Broad Beach (Figure 36). This approximately 1.6-km-long (1-mile-long) beach was wide enough that dunes had formed on the back beach, which was above MHT for a sufficiently long time period to have been claimed private rather than state land. The backshore was subsequently subdivided, and small beach cottages were constructed, initially in the 1930s and 1940s. By 1944, a number of additional homes had been built on the back beach, and by 1964, most of the parcels had been developed with homes (Figure 37). Broad Beach became significantly narrower when the original lots were subdivided, and home construction occurred.

In recent decades, most of the old cottages were gradually demolished, lots were combined, and the small cottages were replaced by much larger structures that became homes for a number of people in the movie industry. This also led to frequent publicity about the progressive narrowing of Broad Beach. These newer homes were much larger and encroached farther seaward on the shoreline, further reducing the width of the original beach and dune buffer zone (Figure 38). The beach width is important because this is the area used by each home for their on-site septic tank and leach field.

While the historic back beach and dune areas were created under one set of climatic, wave, and sand supply conditions, they may not be stable under a different set of conditions, for example, sand supply reduction, change in wave climate, or during the simultaneous occurrence of elevated sea levels and severe wave attack, as well as a continuing rise in sea level.

Previous studies of the long-term changes in the beaches along the stretch of shoreline from Lechuza Point to Point Dume indicated that beach widths have fluctuated over time, primarily in response to changing storm and wave conditions associated with different phases of the PDO (Orme et al., 2011; Zoulas and Orme, 2007), as explained earlier in this paper.

The generally wide beach along Broad Beach began to change around 2000, however, starting at the upcoast or west end as the beach adjacent to Lechuza Point started to narrow, and by 2002, it had disappeared altogether (Griggs and Patsch, 2018; see Figure 39). Two years later (2004), there was no dry beach exposed for the first 200 m (650 ft) down the coast from Lechuza Point. While there are some year-to-year fluctuations, by 2006, the first dry beach was about 400 m (1300 ft) down the coast to the east. By August 2008, the month when beach width should normally be the greatest, there was no dry beach for fully half of the length of Broad Beach, and the beach was narrowing continuing to the east. As of September 2013, there was no dry beach fronting the entire 1.6 km (1 mile) of Broad Beach (see Figure 38).

Most of the homes at the narrow west end of Broad Beach were either already protected by seawalls or supported on concrete caissons, but for the remainder of the beach-level homes, in response to the continuing loss of sand and progressive narrowing of the beach, large temporary sandbags or riprap were installed following the 2007–08 winter to protect the homes (Figure 40). The septic tanks and leach fields buried beneath the beach in front of these homes became an additional issue. Waves from the winter of 2008 damaged or destroyed the sandbags, however, which led to the placement of a “temporary” rock revetment by the next winter that extended virtually the entire 1.6 km (1 mile) length of Broad Beach (see Figure 38).

After well over a century of a relatively permanent and stable broad beach, why did this stretch of shoreline change so quickly and the beach narrow or erode to the point where a 1.6-km-long (1-mile-long) rock revetment was required to save the homes and protect their beach-level septic systems from being exposed? The revetment is considered a “temporary” solution by the California Coastal Commission, the coastal agency with authority for essentially all coastal land-use decisions in the state. Broad Beach homeowners hired consultants to answer the question of why this beach loss occurred and provide recommendations as to how they are going to protect their properties. Most of the Broad Beach homes are in $10 to $20+ million range but were built on the back beach when conditions were different and more favorable.

Historically, leakage of sand from the upcoast Santa Barbara Littoral Cell, across the head of Mugu Submarine Canyon, provided a significant source of beach sand to the Zuma Littoral Cell (see Figure 33; Griggs and Patsch, 2018). About 1995, however, the headward growth of Mugu Submarine Canyon towards the shoreline led to the interception of essentially all of the littoral transport (see Figure 34; Moffat and Nichol, 2009). An erosion wave has slowly migrated through the Zuma Littoral Cell and has now led to significant narrowing of Broad Beach and threats to the 109 homes and their sewage-disposal systems built on the back beach. There is no solution underway at present, but a consultant’s proposal is to import a large volume of sand (230,000–460,000 m³ or 300,000–600,000 cubic yards) from an inland quarry to nourish the beach and then continue to renourish the beach on a regular basis.

The back beach or the landward portion of a number of California’s once-wide beaches was developed decades ago with private homes, commercial establishments, and also public infrastructure during times when these beaches were wide and appeared permanent. Much of this construction and development activity occurred during a cooler and relatively calm PDO interval, which also coincided with the period following World War II when the state’s population boomed. In recent decades, however, this development has been repeatedly impacted by short-term extreme events, typically very large waves arriving simultaneously with extreme high tides, often during major El Niño events, which further elevate water levels. Reductions of sand supplies and fluctuations and changes in the wave climate have also been factors in these impacts to shoreline development.

Over the long term, rising sea levels will increasingly add to the shoreline challenges facing both private development and public infrastructure. Realistic solutions or responses are limited, however, and include armor and repeated beach nourishment. These are expensive and will only be effective over a few decades at best. Climate change is real, it’s now, and it’s everywhere. While homeowners understandably are not interested in managed retreat, if not managed, then it will be unmanaged. Each of the state’s oceanfront communities where back beach development has been damaged or destroyed, or is being threatened, needs to identify their most vulnerable assets or development and, using California’s most up-to-date assessment of future sea levels (Adusumilli et al., 2024) and short-term extreme events, plan for the future when maintaining or protecting these areas will no longer be feasible. A planning process has begun in most of the state’s coastal cities and counties under the recommendations from the California Coastal Commission, but the timing, mechanics, and economics of such a process are still evolving (Lester et al., 2022). Moving back from the coastline is not a new process, however (Griggs, 2015).

The four California sites addressed in this paper (Beach Boulevard, Pacifica; The Esplanade, Capitola Village, Santa Cruz County; Beach Drive, Rio Del Mar, Santa Cruz County; Broad Beach, Malibu, Los Angeles County) are all intermittently exposed to high-energy wave action and potential damages. Our analysis indicates that the developers, homebuilders, and permitting agencies were likely not aware of this exposure, due to the wide beaches that existed after World War II, and the presumption that the beaches were stable and would not narrow over time. In hindsight and looking to the future, there are several findings to consider, as follows:

First, we now understand that sand is in transit, and a reduction of sand supply typically results in beaches narrowing downdrift. As beaches narrow, the waves impact the backshore, causing erosion of bluffs and dunes. This erosion dissipates wave energy and in many locations releases sand to the beaches. Hence, the backshore erodes, the shoreline migrates landward, and the beaches are maintained. Coastal armoring inhibits backshore erosion, sand supply, and shore migration. In this situation, beach development contributes to beach loss.

Second, we now realize that the climate fluctuates on time scales of 20 to 30 years (PDO) and 5 to 7 years (ENSO), and the amplitudes of these fluctuations and associated effects vary. These fluctuations affect local sea levels, storm tracks, incident waves, and coastal flooding and erosion. The calmer conditions during the 1950s–1960s were a blessing of sorts, but they were not permanent. Also, early colonization by Europeans and Eurasians (late 1800s and early 1900s) may have increased sand supply to beaches by placing sand on beaches (see earlier discussion on Beach Boulevard, Pacifica) or by watershed impacts (logging, grazing, grading, and hydraulic gold mining), and subsequently decreased coarse beach sand via sand mining (Thornton, 2016; Thornton et al., 2006) and dam construction (mid-1900s), thereby amplifying the climatic effects on beach erosion in the late 1900s and early 2000s.

Third, we understand that sea levels change, which result in shore changes (Adusumilli et al., 2024). Existing sea levels are high relative to historical elevations over the last 20,000 years, rising about 122 m (400 ft) before becoming nearly steady about 6000 years ago. Hence, for the last 6000 years, waves have eroded into the land, forming a platform upon which sand has accumulated. In unarmored parts of the California coast, the cliffs, bluffs, and dunes continue to erode, providing sand for local and downdrift beaches. Therefore, over time, erosion progresses, the coastal floodplain migrates, and exposure to coastal hazards also migrates landward, increasing the exposure of coastal development to coastal hazards. Sea levels are forecast to rise more quickly and result in greater coastal erosion, faster shore migration, and increased exposures.

Fourth, coastal armoring can be overwhelmed by progressively amplified wave impacts. As beaches seaward of armor narrow (passive erosion), larger waves impinge on the structures, increasing structural loads, which can result in failure of shoreline armoring. In addition, the larger waves impinging on the structures result in wave runup overtopping and flooding, which can damage development hundreds of feet inland. Recent studies have found that wave runup can increase by three to five times the amount of sea-level rise along armored shores (Battalio et al., 2016; Vandever et al., 2017).

Fifth, coastal flood maps provided by the U.S. Federal Emergency Management Agency, which are used to assess coastal property exposure and the need for flood insurance and appropriate structure design, are based on recent historic conditions, do not consider future erosion and sea-level rise, and hence underrepresent hazards and become progressively out of date (California Ocean Sciences Trust, 2015). Hence, developers, residents and their real estate agents, and the staff of planning and engineering departments typically do not have adequate technical guidance. This situation may also be exacerbated by those that would rather not be “mapped in the floodplain” due to concerns of impacts to property values.

The above discussion is not comforting for those who own or are otherwise responsible for the development at risk. Nor is it good news for their communities, both from financial and quality of life perspectives. Nor is it good news for those who enjoy the natural shore, be it for the views and sounds, access and recreation, or its geography, flora, and fauna. On the other hand, while the migrating coastal floodplain is obstructed, we also see the beach encroaching and building landward on top of development. The intense wave activity entrains sand and carries it over seawalls and deposits the sand on the roads, parking lots, yards, etc. If this process is allowed to continue, beaches will rebuild farther inland more often. Realignment of development and restoration of the backshore can facilitate these natural processes.

Communities might ask themselves: Would we develop this close to the shore today? Perhaps a balance of retreating in some areas and “holding the line” in others, at least over the near term, similar to the response of the natural shore, is a potential adaptation strategy? An example of this approach was developed for areas north of Pacifica as part of the San Francisco Littoral Cell Coastal Regional Sediment Management Plan (Figure 41). Publicly owned shore-protection devices were assumed to be maintained while sand could be placed elsewhere where erosion progressed. Areas of erosion would benefit from a managed retreat/realignment construct, which would include buy-out of threatened properties, progressive realignment of public infrastructure, and potential placement of sand to maintain beaches. Over time, the shore would become a series of pocket beaches between headlands, and the overall rate of erosion for the entire reach would be reduced. Whichever path is chosen, difficult decisions lie ahead. It is apparent that our historical practices are not sustainable.