Marine Sanctuaries Conservation Series MSD-05-3 

U.S. Department of Commerce February 2005 

National Oceanic and Atmospheric Administration 

National Ocean Service 

Office of Ocean and Coastal Resource Management 

Marine Sanctuaries Division 

The Impacts of Coastal Protection Structures 

in California’s 

Monterey Bay National Marine Sanctuary
About the Marine Sanctuaries Conservation Series 

The National Oceanic and Atmospheric Administration’s Marine Sanctuary Division (MSD) 

administers the National Marine Sanctuary Program. Its mission is to identify, designate, 

protect and manage the ecological, recreational, research, educational, historical, and aesthetic 

resources and qualities of nationally significant coastal and marine areas. The existing marine 

sanctuaries differ widely in their natural and historical resources and include nearshore and 

open ocean areas ranging in size from less than one to over 5,000 square miles. Protected 

habitats include rocky coasts, kelp forests, coral reefs, sea grass beds, estuarine habitats, hard 

and soft bottom habitats, segments of whale migration routes, and shipwrecks. 

Because of considerable differences in settings, resources, and threats, each marine sanctuary 

has a tailored management plan. Conservation, education, research, monitoring and 

enforcement programs vary accordingly. The integration of these programs is fundamental to 

marine protected area management. The Marine Sanctuaries Conservation Series reflects and 

supports this integration by providing a forum for publication and discussion of the complex 

issues currently facing the National Marine Sanctuary Program. Topics of published reports 

vary substantially and may include descriptions of educational programs, discussions on 

resource management issues, and results of scientific research and monitoring projects. The 

series facilitates integration of natural sciences, socioeconomic and cultural sciences, 

education, and policy development to accomplish the diverse needs of NOAA’s resource 

protection mandate. 
The Impacts of Coastal Protection Structures in California’s 

Monterey Bay National Marine Sanctuary

Rebecca Stamski 

Monterey Bay National Marine Sanctuary 

Monterey, CA 

 

Silver Spring, Maryland 

February 2005 

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

National Oceanic and Atmospheric Administration 

VADM Conrad C. Lautenbacher, Jr. (USN-ret.) 

Under Secretary of Commerce for Oceans and Atmosphere 

National Ocean Service 

Richard W. Spinrad, Ph.D., Assistant Administrator 

Marine Sanctuaries Division 

Daniel J. Basta, Director 
DISCLAIMER 

Report content does not necessarily reflect the views and policies of the National Marine 

Sanctuary Program or the National Oceanic and Atmospheric Administration, nor does the 

mention of trade names or commercial products constitute endorsement or recommendation for 

use. 

REPORT AVAILABILITY 

Electronic copies of this report may be downloaded from the National Marine Sanctuaries 

Program web site at www.sanctuaries.nos.noaa.gov. Hard copies may be available from the 

following address: 

 National Oceanic and Atmospheric Administration

 Marine Sanctuaries Division

 SSMC4, N/ORM62 

 1305 East-West Highway 

 Silver Spring, MD 20910 

COVER 

Upper Left: Coastal armoring in Capitola, CA. MBNMS 

Center Left: Erosion warning sign, Santa Cruz, CA. MBNMS 

Lower Left: Riprap along Westcliff Drive, Santa Cruz, CA. MBNMS 

Upper Right: Riprap along Westcliff Drive, Santa Cruz, CA. MBNMS 

Center Right: Seawall in Capitola, CA. MBNMS 

Lower Right: Seawall in Santa Cruz, CA. MBNMS 

SUGGESTED CITATION

Stamski, Rebecca. 2005. The impacts of coastal protection structures in California’s Monterey 

Bay National Marine Sanctuary. Marine Sanctuaries Conservation Series MSD-05-3. U.S. 

Department of Commerce, National Oceanic and Atmospheric Administration, Marine 

Sanctuaries Division, Silver Spring, MD. 18 pp. 

CONTACT 

Rebecca Stamski 

California Sea Grant Fellow 

Monterey Bay National Marine Sanctuary 

299 Foam Street 

Monterey, CA 93940 

(831) 647-4228 

Becky.Stamski@noaa.gov
i

ABSTRACT 

 This report outlines the potential impacts of coastal protection structures on the resources of 

the Monterey Bay National Marine Sanctuary. At least 15 miles of the Sanctuary’s 300-mile 

shoreline are currently armored with seawalls and riprap revetments. Most of these coastal 

protection structures are placed above the mean high tide line, the official boundary of the 

Sanctuary, yet some influences of armoring impinge on the marine realm and on recreational use. 

In addition, continued sea level rise and accompanying coastal retreat will force many of these 

structures below the high tide line over time. The Monterey Bay National Marine Sanctuary 

staff has recognized the significance of coastal armoring, identifying it as a critical issue in the 

Coastal Armoring Action Plan of the draft Joint Management Plan. 

 This summary is intended to provide general background information for Sanctuary policies 

on coastal armoring. The impacts discussed include: aesthetic depreciation, beach loss due to 

placement, access restriction, loss of sand supply from eroding cliffs, passive erosion, and active 

erosion. In addition, the potential biological impacts are explored. Finally, an appraisal of how 

differing armor types compare in relation to impacts, expense and engineering is presented. 

While the literature cited in this report focus predominantly on the California coast, the 

framework for this discussion could have implications for other actively eroding coastlines. 

KEY WORDS 

Coastal erosion, armoring, coastal protection structures, seawall, riprap revetment, geology, 

sanctuary, California, Monterey Bay 
ii

TABLE OF CONTENTS 

Topic Page

Abstract................................................................................................................... i 

Table of Contents.................................................................................................... ii 

List of Figures......................................................................................................... iii 

Introduction............................................................................................................. 1 

Impacts of Coastal Armoring Structures................................................................. 4 

 Visual Impacts ................................................................................................... 4 

 Placement Loss .................................................................................................. 6 

 Access Issues ..................................................................................................... 7 

 Reduction of Sand Supply from Armoring of Cliffs ......................................... 8 

 Passive Erosion .................................................................................................. 9 

 Active Erosion ................................................................................................... 10 

 Biological Impacts ............................................................................................. 11 

Engineering of Coastal Armoring Structures.......................................................... 13 

 Riprap................................................................................................................. 13 

 Seawalls ............................................................................................................. 14 

Conclusions............................................................................................................. 15 

Literature Cited ....................................................................................................... 16 

Acknowledgements................................................................................................. 18 
iii

LIST OF FIGURES 

Figure Number and Title Page

Figure 1a: Oblique aerial photographs of Stillwell Hall in 2002............................ 1 

Figure 1b: Oblique aerial photographs of former Stillwell Hall site in 2004 ......... 2 

Figure 2: Concrete blocks used as riprap................................................................ 4 

Figure 3: An unplanned assemblage of coastal armoring....................................... 5 

Figure 4: Cliff stabilization built to fill a seacave................................................... 6 

Figure 5: Riprap covering up significant amount of beach area............................. 7 

Figure 6: Recent bluff stabilization project ............................................................ 8 

Figure 7: Aerial-perspective, schematic diagram depicting beach loss over time 

 due to fixing the coastline with armor ..................................................... 9 

Figure 8: Algae growing on riprap.......................................................................... 12 

Figure 9: Riprap boulders settled into sand ............................................................ 14
i
1

INTRODUCTION 

 Eighty-six percent of California’s 1,100 miles of coastline is eroding (Griggs 1999), yet now 

more than ever, people want to live at that retreating boundary between land and sea. Erosion of 

the coast has been occurring for the past 18,000 years, when the last glacial period ended and sea 

level began to rise. Coastal cliffs are retreating at an average rate of 10 to 30 centimeters per year 

across the state (Griggs and Patsch 2004a), although erosion rates can be as high as 4.5 meters 

per year (Griggs and Patsch 2004b). Extreme variability in the rates and severity of coastal 

erosion, particularly in relation to El Niño storm patterns and local geomorphic conditions, 

complicates property protection decisions. In addition, the politics of public and private land 

ownership in the coastal zone make coastal armoring a contentious issue for Californians. 

 

 Several alternatives have been recognized to deal with the confluence of coastal development 

and eroding shorelines, none of which are a panacea. In the extreme case, property owners can 

demolish the building or relocate it landward, either on the same parcel or on an entirely 

different inland location. Neither of these choices is ideal for property owners, though they 

should be considered as serious, long-term solutions to this inevitable dilemma, given the rising 

costs of other alternatives (Griggs 1986). For example, Stillwell Hall, built in the 1940’s as the 

Fort Ord soldier’s club in Seaside, California, was torn down in March 2004 because the cost of 

both coastal armoring and relocating were too high (Figures 1a and b). 

Figure 1a: Oblique aerial photograph of Stillwell Hall in 2002 in Marina, CA. Coastal protection structures placed 

at the base of the aged building were neither effective nor efficient enough keep up with intense coastal erosion of 

adjacent bluffs. Photograph copyright © 2002 Kenneth & Gabrielle Adelman (http://www.californiacoastline.org/). 
2

Figure 1b: Oblique aerial photograph of the former Stillwell Hall location after the building and associated coastal 

protection structures were removed in March 2004. Photograph copyright © 2002 Kenneth & Gabrielle Adelman 

(http://www.californiacoastline.org/). 

 Beach nourishment has been highlighted recently as a solution to coastal erosion; proponents 

of this alternative maintain that increasing beach width by physically adding sand to a beach 

buffers wave energy and slows retreat rates. Federal, state and local government agencies are 

pursuing this method as a way to protect property from erosion damage, but the costs are 

generally very high and the net, long-term benefits of beach nourishment will vary greatly 

depending on local conditions (Leonard et al. 1990). 

 A survey of west coast beach nourishment projects determined that only 27% survived more 

than 5 years and 18% lasted less than 1 year (Leonard et al. 1990). In some cases, sand that was 

placed on a beach during summer was completely washed away the following winter. This is not 

unexpected given the high littoral drift rates that characterize most of the coast of California. A 

cost-benefit analysis of proposed nourishment projects should include site-specific evaluations of 

littoral budgets. In addition, the availability of an appropriate, cost-effective sand source and 

potential interference between beach users and sand transportation equipment should be 

addressed. As an alternative, beach width might be increased by removing dams that impound 

significant amounts of sand in coastal streams (Willis and Griggs 2003). 

 By far the most popular option to manage shoreline retreat in California has been the 

construction of coastal protection structures (also referred to as coastal armoring). 

Approximately 10% of California’s coastline is currently armored (Griggs in press-a). The 

complexity and significance of coastal armoring in California are evidenced by numerous 

scientific studies, involvement of non-profit organizations, such as the Sierra Club and the 

Surfrider Foundation, and media coverage of the issue, such as KQED’s Coastal Clash

documentary (http://www.kqed.org/w/coastalclash/home.html). The costs of armoring can be 

significant; millions of federal, state and private dollars have been expended annually on shore 
3

protection, which can cost anywhere from $1800 to $7600 per linear foot of coast (Griggs in 

press-a). 

 Armoring varies widely in type of material, degree of engineering, and relative success in 

preventing coastal erosion and providing property protection (Griggs and Fulton-Bennett 1988). 

Riprap and seawalls are the most common armoring structures used in central California. Riprap 

can be defined as any large (1 to 6 ton) rocks used for coastal protection, with varying degrees of 

engineering; seawalls are continuous, rigid structures with vertical or concave faces. To clarify a 

common misconception, it is important to note that armoring is emplaced to protect buildings 

and infrastructure, not beaches (Kraus and McDougal 1996). Groin fields and beach 

nourishment are the common methods by which beaches are expanded, both of which are 

fundamentally different than armoring, which is constructed to halt erosion of cliffs, bluffs or 

dunes that have buildings behind or on top of them, or to protect a building built on the 

backshore (above the high tide line). 

 Public concern over use and regulation of the California coastline was aired in the early 

1970s, the result of which was the establishment of the California Coastal Act in 1976. This act 

created the California Coastal Commission (Commission), which is tasked with regulating 

development in the coastal zone. Under the Coastal Act, new armor is only allowed to protect 

existing structures and new buildings must be setback enough to ensure that erosion will not 

threaten the house within its projected lifespan. The Commission, in coordination with local 

coastal planning agencies, provides permits for the emplacement and maintenance of coastal 

protection structures. Unfortunately, ambiguous language within the Coastal Act has been 

exploited and there is a need to design more specific long-range plans to limit the extent of 

coastal armoring (Griggs et al. 1992). 

 The Monterey Bay National Marine Sanctuary (Sanctuary), which encompasses 276 miles 

(444.2 km) of coastline from Marin to Cambria, was designated in 1992 to protect the resources 

of this unique marine environment. As of 1998, 15.1 miles (24.3 km) of the Sanctuary’s 

coastline have been armored (Griggs et al. in press-b). Various physical and biological impacts 

of coastal armoring may affect the resources of the Sanctuary both directly and indirectly. Most 

coastal protection structures are placed above the high tide line, the official boundary of the 

Sanctuary; yet some influences of armoring may impinge on the marine realm, and continued sea 

level rise and the accompanying coastal retreat will force many of these structures below the 

high tide line over time. The Sanctuary recognized the significance of protection structures on 

the shoreline and has identified it as a critical issue in the Coastal Armoring Action Plan of the 

Joint Management Plan. 

 The impacts of coastal protection structures are of great concern to local governments, private 

property owners, and the public. The most commonly recognized impacts (as outlined in Griggs 

(in press-a)) are: visual effects, placement loss, access issues, loss of sand supply from eroding 

cliffs, passive erosion, and active erosion. In addition, there are potential impacts to the 

biological communities that utilize the coastal zone. The following is a discussion of these 

impacts, including an appraisal of how differing armor types compare in relation to impacts, 

expense and engineering. This report is intended to provide general background information for 

regional coastal armoring plans for the Sanctuary. 
4

IMPACTS OF COASTAL ARMORING STRUCTURES 

Visual Impacts 

 Public outcry over armoring is largely the result of visual impacts, because coastal protection 

structures often look unnatural or unsightly and can negatively affect recreational beach 

experiences. In the past, emergency armor has been emplaced with no regard to aesthetics or 

long-term engineering standards. Slabs of concrete have been dumped at the bases of cliffs, as is 

the case near Lighthouse Point in Santa Cruz (Figure 2), and several different kinds of structures 

have been haphazardly used on the same stretch of coast, such as is the case at Opal Cliffs in 

Capitola (Figure 3). 

Figure 2: Concrete blocks used as riprap near Lighthouse Point in Santa Cruz, California. 
5

Figure 3: An unplanned assemblage of coastal armoring at Opal Cliffs in Capitola, California can have a visual 

impact on the beach user. 

 

 There are ways to mitigate visual impacts. New seawall engineering, with faces crafted of 

gunite or shotcrete, can be sculpted and colored to resemble the surrounding cliff or bluff face. 

Such walls are already used extensively in cliff stabilization on roadways and are gaining further 

use in the coastal zone, including a new, natural-looking wall protecting the cliff behind 

Cowell’s Beach in Santa Cruz (Figure 4). 
6

Figure 4: Photograph of cliff stabilization built to fill a seacave at Cowell’s Beach in Santa Cruz. The 2004 gunite 

wall, outlined by the black dashed line, mimics the shape and color of the natural cliff. 

 

 The Commission, in compliance with section 30253 of the Coastal Act (2005), requires that a 

licensed civil engineer, who has experience in coastal processes, design new armoring structures. 

This has helped to reduce the number of shoddy, unregulated structures. One solution to the 

problem of unattractive, incongruent armoring being considered by the Commission is to plan a 

continuous, uniform coastal protection structure that spans across several parcels to ameliorate a 

regional erosion problem (Griggs and Fulton-Bennett 1988). 

Placement Loss 

 Coastal armoring structures will inevitably cover up some portion of the beach in front of the 

structure they are built to protect; this impact is referred to as impoundment or placement loss. 

As with visual impacts, this is a tenuous issue with the public who lose beach area in order to 

benefit private property owners. The amount of beach that will be lost from construction of new 

armor can be easily calculated for inclusion in engineering plans and environmental impact 

statements. 

 In terms of impoundment, seawalls are almost always more favorable than riprap structures. 

Most seawalls are only a few feet (~6’) thick and are built flush with the cliff or bluff, 

impounding a minimal amount of backshore area. Engineered riprap, on the other hand, should 

have a width (cross-beach) to height ratio of at least 1.5:1 or 2:1 to maintain structural integrity 

during strong storms (Griggs and Fulton-Bennett 1988). Thus, riprap that is designed to be 20 

feet high could cover up to 40 feet of beach (in the shore-perpendicular direction); multiplying 
7

by the shore-parallel length of the riprap yields the square feet of beach lost by impoundment. In 

this case, if a 20-foot stretch of coast needed to be armored, an average seawall would impound 

120 ft2

 of beach (6 ft wide x 20 ft long), while riprap would cover 800 ft2 of beach (40 ft wide x 

20 ft long). Westcliff Drive in Santa Cruz, for example, has had extensive beach loss because of 

riprap emplacement (Figure 5). 

Figure 5: Riprap covering up significant amount of beach area in Santa Cruz, California, near the intersection of 

Westcliff Drive and San Jose Avenue. 

Access Issues 

 In conjunction with impoundment, access to beaches can be lost when armor is installed. 

Lateral access is restricted when armoring bisects the shore-parallel continuity of a beach; the 

time frame of access-loss will depend on tidal and seasonal changes in beach width. As with the 

issue of impoundment, riprap tends to cover up a wider section of beach than a seawall, and 

would therefore be more likely to cut off lateral access. Yet in instances when the beach is 

narrow, especially during high tides and/or winter months, seawalls can also impair lateral 

access. 

 Vertical access, the ability to get to a beach from behind or above, can also be affected when 

armoring is placed on the coast. Riprap and seawalls can cover up paths or trails that lead to the 

beach. However, in most cases, a protection structure is placed against a near-vertical cliff or 

bluff face where vertical access was most likely difficult prior to armoring. As a mitigation 

measure, stairs or paths can be integrated within coastal protection structures to facilitate vertical 
8

access to the beach, though maintenance of these paths can become a safety issue over time if the 

armoring is structurally compromised (Figure 6). 

Figure 6: In this bluff stabilization project at Pleasure Point in Santa Cruz, the protective gunite (outlined with dotted 

line) was colored and textured to match the existing bluff materials, and access steps were built into the structure 

(Photograph by Gary Griggs). 

Reduction of Sand Supply from Armoring of Cliffs 

 Comprehensive approaches to understanding coastal erosion include an assessment of 

anthropogenic reductions in sand supply. The breakdown of rocks and sediment in cliffs, bluffs 

and dunes creates sand that constitutes some fraction of the littoral budget. Armoring coastal 

landforms covers up those erosion-prone surfaces and may therefore reduce sand supply. These 

reductions can make downcoast beaches narrower, allowing more wave energy to erode cliffs, 

bluffs and dunes downcoast of the armored area. 

 The relative contribution of cliff erosion to sediment budgets will vary based on local geology 

and erosion rates. Thus, understanding the effects of coastal armoring on sand supply needs to 

be addressed on a littoral cell by littoral cell basis. For example, a recent study found that less 

than 1% of sand supply was contributed from seacliff erosion in the Santa Barbara littoral cell, 

while 12% of the sand in the Oceanside littoral cell originated from eroding cliffs (Runyan and 

Griggs 2003). 

 Mitigation measures for sand supply reduction from armoring include the replacement of 

sand, via beach nourishment, in equal volume to that which is lost by the emplacement of 
9

armoring structures. Yet, as noted above, the success and longevity of beach nourishment 

projects is debatable.

Passive Erosion 

 Passive erosion is perhaps the most significant and the most misunderstood impact of coastal 

armoring. Eighty-six percent of California’s coastline is eroding, the result of which is a 

landward retreat of beaches, cliffs and other coastal landforms. Yet when a structure, such as 

riprap or a seawall, is constructed in front of a building to halt erosion, the shoreline is 

essentially fixed at that location. Adjacent landforms (beaches, cliffs, etc.) will continue to 

retreat landward, creating an artificial headland out of the armored segment of coast. If armor is 

placed at the base of a cliff that has a beach in front of it, the beach will continue to migrate 

landward on either side of the armored area, but there will be no beach in front of the armor, as 

depicted in Figure 7. 

Figure 7: Aerial-perspective, schematic diagram depicting beach loss over time due to fixing the coastline with 

armor when coastal erosion rates are 20 cm/year, a realistic rate for California. Prior to its removal, Stillwell Hall in 

Marina, California, was a classic example of passive erosion (see Figure 1). 

 Passive erosion occurs regardless of the type of structure used; riprap and seawalls both fix 

the coastline and prevent the landward migration of beaches, cliffs, bluffs and dunes. One way 

to mitigate passive erosion is to nourish beaches with sand from other locations, though this is 
10

only a temporary solution to an incessant problem. As an alternative, removing dams from rivers 

will increase the sediment supply to beaches along much of the California coast (Farnsworth and 

Milliman 2003; Willis and Griggs 2003). 

Active Erosion 

 Localized, accelerated erosion that might occur because of interactions between armoring 

structures and waves is referred to as active erosion. This type of erosion includes scour at the 

base of a protection structure or on adjacent segments of shoreline, and changes in overall beach 

morphology. Many people feel that seawalls initiate active erosion and are therefore detrimental 

to coastal environments, yet recent investigations may suggest otherwise. 

 A summary of over 40 scientific studies on the interactions between beaches and coastal 

armoring structures (including seawalls and riprap) found that active erosion may not be as 

prolific a problem as was once thought (Kraus and McDougal 1996). The review determined 

that reflection of wave energy off of coastal armor (waves bouncing off perpendicular to a 

structure) generally does not cause changes in beach profiles or scour in front of the armor. In 

addition, they ascertained that beach profiles in front of armoring retained the same amount of 

sand as non-armored beaches during storm events. 

 In an eight-year study by Griggs et al. (1994; 1997), over 2000 beach profiles were collected 

and analyzed across armored and non-armored beaches around northern Monterey Bay. In this 

exhaustive investigation, scour was documented in front of an armoring structure only during 

extreme storm events and the imprint of that scour was ephemeral. The study did find that, as 

winter approached, the summertime beach berm migrated landward slightly faster in front of 

coastal protection structures when compared to beaches without armoring. However, once 

typical, narrow winter beaches were established, there was no significant alongshore difference 

in the shape of armored and non-armored beaches. In winter months, Griggs et al. (1994) did 

document some scour on the downcoast end of the structure, extending in an arc-shaped zone for 

as much as 50 to 150 m. Yet, as summer advanced, the beach width widened and there was no 

trace of scour or berm erosion caused by the armor. 

 This study served to dispel, or at least bring into question, many common assumptions 

concerning interactions between armoring and beaches. First, seawalls are often perceived to 

cause more active erosion than other structures, such as riprap, because the former is less 

permeable than the latter. However, the 8-year study by Griggs et al. showed that there were no 

significant differences in beach profiles in front of a seawall versus riprap. Second, in contrast to 

the general sentiment that coastal armoring causes excessive erosion, there was no appreciable, 

long-term active erosion caused by seawalls or riprap on the Monterey beaches highlighted in the 

Griggs et al. study (1994; 1997). The real problem is passive erosion, which is an inevitable part 

of California coastal dynamics; beaches are eroding in front of armoring structures because the 

coastline is fixed at that spot, while the adjacent beaches are migrating landward. 
11

Biological Impacts 

 While a significant amount of attention has been given to the physical effects of placing hard 

armoring structures on the coast, relatively little consideration has been focused on potential 

biological impacts. Armoring can be emplaced both above and below the high tide line. It is 

important to recognize that this is an extremely dynamic environment; passive erosion, for 

example, ensures that much of the armor placed above the high tide line will end up below that 

line as the coast retreats. The habitats most impacted by armoring are the beach, above the 

highest high tide line, and the intertidal zone, between the highest high tide and lowest low tide 

lines. 

 Riprap placed against a cliff to slow erosion can cover up extensive portions of the beach, 

which serves as habitat for many species of invertebrates and birds. The endangered Snowy 

Plover (Charadrius alexandrinus), for example, nests and feeds on beaches in the Sanctuary 

(USFWS 2005). Armoring in the intertidal zone can also cover up vital, natural habitats for fish, 

algae and invertebrates (MBNMS 2004). Organisms can also be directly smothered or killed 

during the construction phase of these coastal protection devices. 

 In addition to physically removing or disturbing habitats, armoring also adds hard substrate 

that can have implications for populations in the area (McGuinness 1989; Osborn 2002). If 

riprap is placed on top of a beach, the new substrate may attract different algae species that 

would otherwise not have had an appropriate attachment medium at that location (Figure 8). 

Different species, which can capitalize on armoring surfaces, may compete with species that had 

previously occupied those regions. 

 A recent study in northern Monterey Bay found that rock type can have an impact on 

colonization and subsequent recruitment of other organisms (Osborn 2002). The rocks used for 

riprap revetments are usually foreign to the area in which they are emplaced; therefore physical 

properties of the riprap will vary from local conditions. In this study, community structure was 

documented on basaltic riprap and on sandstone, the natural cliff material. Significant 

differences in intertidal communities were found living on the two rock types. Rock properties, 

such as porosity and color, affect the dampness and heat of the substrate upon which intertidal 

organisms settle and may impact population dynamics. 
12

Figure 8: Algae growing on basaltic riprap in Santa Cruz, California. This riprap has covered up a beach, creating 

an artificial substrate for species recruitment. 

 In Sydney, Australia, a comparison between organisms living on rocky shores versus seawalls 

found that seawalls supported only half the diversity of mobile organisms when compared to 

nearby natural rocky substrates (Chapman 2003). The study found that the types of algae and 

sessile organisms were similar on both substrates, but that rocky shores were home to more rare 

species than seawalls. The paucity of mobile and rare organisms may have been due to the steep 

slopes and lack of rugosity, or microhabitats, used for conventional seawalls. Sculpting coastal 

armoring structures so that they mimic surrounding landforms may help avoid loss of community 

diversity. 

 Armoring may also have influences on large-scale biogeography. A review of intertidal 

systems in the northeast Pacific suggested that there is a substantial biogeographic break at 

Monterey Bay (Foster et al. 1991). This discontinuity in populations to the north and south of 

the Bay may be due to the extensive sandy shoreline that stretches from Aptos to Monterey. This 

area lacks hard substrate, upon which many intertidal organisms rely. If and when armoring is 

added to this area, it may provide settlement sites for species that otherwise could not live in the 

Monterey Bay intertidal zone. In this manner, the biogeographic break may be crossed and 

species from the north may be able to migrate south and vise versa. Similarly, armoring 

structures may provide habitat for species that are brought into harbors by boats, facilitating the 

spread of invasive species. 
13

ENGINEERING OF COASTAL ARMORING STRUCTURES 

 With the exception of passive erosion, all of the impacts described above can be influenced 

by the design and type of armoring used. Methods for constructing and repairing coastal 

protection structures are not standardized and poor engineering can lead to failure and additional 

costs (USACE 1981; Griggs and Fulton-Bennett 1988; Chenault 2000). To be sustainable and 

effective, armoring must survive the following forces: overtopping by large storm waves, 

undermining by scour at the base, outflanking by erosion on either side, and battering by waves, 

sediment and debris. The following is a brief description of some of the engineering 

considerations in building riprap and seawalls. 

Riprap 

 Engineered riprap must have several elements to efficiently protect the coast, as outlined in a 

study of the effectiveness of riprap in central California (Griggs and Fulton-Bennett 1988): 

• Excavation of sand, if built on the beach 

• Filter cloth at base 

• A trench at the seaward edge of structure (referred to as a “toe trench”) below 

maximum scour depth, with large stone (4-6 tons) in toe trench 

• Stable slope angles of at least 1.5:1 (width across the beach: height), 2:1 is more 

successful 

• Small core stones placed down first on top of filter cloth 

• Large cap stone (3-5 tons) on top, positioned in an interlocking pattern 

• Height great enough to prevent overtopping 

 In addition to these engineering specifications, riprap generally requires regular maintenance 

by adding rocks every 5 to 10 years. These structures can cost over $1500 per linear foot (in 

2003 dollars), with annual maintenance costs of 2-15% of the initial cost. Riprap that is built on 

top of bedrock platforms generally lasts longer than those placed on beaches, because a 

significant amount of settling and burial can occur in active beach environments (Figure 9). 
14

Figure 9: Riprap boulders commonly settle into deep sandy beaches or get buried, as seen here near the intersection 

of Westcliff Drive and David Street in Santa Cruz, California. 

Seawalls 

 Seawalls have different engineering criteria. In general, due to their durability, concrete or 

gunite walls outlive timber seawalls and fare better than riprap, in terms of costs and benefits 

(Griggs and Fulton-Bennett 1988). Some recommended elements for seawall longevity include: 

• Concrete mix with a low water to cement ratio 

• Epoxy coated rebar with at least 3 inches of concrete between steel and seawater 

to prevent rusting of steel 

• Deep (8-15 ft below MLLW) interlocking sheet piles in sandy areas 

• At least 12” of thickness for the wall 

• Wide, seaward sloping apron shoreward of the wall to prevent overtopping, when 

appropriate 

• Holes drilled through entire width of the seawall (referred to as “weep holes”) to 

provide drainage and to prevent ponding behind the wall 

• Maintenance of weep holes to prevent loss of fill from behind, around or 

underneath the wall 

• Filter blanket (filter cloth and gravel) positioned behind seawall to reduce loss of 

fill 

 Seawalls, if constructed and maintained properly, can endure for decades. The 

O’Shaughnessy Seawall on San Francisco’s Ocean Beach, for example, has survived over 75 
15

years without major repair. Despite their durability, seawalls are often not used because they can 

be expensive. Recent extensions of the massive O’Shaughnessy structure have cost over $7,000 

per linear foot, while smaller-scale seawalls can cost upwards of $3,000 per linear foot, 

depending on their design and size (Griggs 1999). However, seawalls can fail, even with good 

engineering, because of severe, unpredicted storms, such as those that occurred during the 1982-

83 El Niño events. Thus, no seawall should ever be considered to provide permanent protection. 

CONCLUSIONS 

 Over 900 miles of actively eroding coastline, encroaching coastal development and crumbling 

bluffs pose an immense challenge to resource management agencies in California. The 

construction of coastal protection structures, such as riprap and seawalls, has been the most 

widespread solution to prevent loss of property, yet armoring has many impacts that reach far 

beyond the individual parcels they are designed to protect. Because armoring is built to save 

buildings, not to protect beaches, the public can be negatively affected by these coastal 

protection structures in terms of visual impacts, lateral and vertical access, and beach loss due to 

passive erosion. 

 Beach nourishment has been promoted as a solution to ameliorate the impacts of coastal 

armoring, yet the sustainability and cost-effectiveness of these projects has yet to be proven and 

is highly site-specific. Adding sand to beaches in an area with high littoral drift rates, as is true 

along most of the California coast, may be imprudent. Compensating for sand supply losses, by 

removing coastal dams, for example, may be a more logical, and more sustainable, solution for 

increasing beach widths. The State of California has recognized the need for a more holistic 

approach to sediment issues and is working to restore balance to littoral cells through the 

California Coastal Sediment Management Master Plan (CCSMW 2004). 

 There are alternatives that allow property owners to protect their assets, while mitigating 

some of the negative impacts. Regional-scale protection, for example, utilizes continuous, 

uniform armor to protect a stretch of coast that spans many parcels with similar erosion 

problems. These structures, such as gunite bluff stabilization, can mimic the natural cliff face 

and replace a mismatched array of protection devices. This type of large-scale structure may 

also improve structural integrity because weak points can be created at the junction between two 

different types of armoring. In addition, comprehensive armoring can include sufficient vertical 

public access and can ensure that the least amount of beach is covered up to improve coastal 

access. 

 A long-term, regional-scale erosion response and armoring plan could be evaluated for 

alternatives that may save costs over time. Under such a plan, prediction of erosion hotspots 

could alleviate many of the problems that occur from the construction of emergency structures, 

which are often less thorough in their impact analysis and mitigation. In addition, relocation of 

threatened structures could be considered a realistic and economically-viable option. Through 

the Coastal Armoring Action Plan, the Sanctuary is initiating a regional plan of this nature to 

protect the resources of the Monterey Bay and to help California’s coastal development be 

flexible and progressive in the face of an incessantly eroding shoreline. 
16

LITERATURE CITED 

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Chenault, C. D. (2000). Shore protections structures in Oregon: An analysis of demand and the 

implementation of engineering recommendations. Masters Thesis in Marine Resource 

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high coastal erosion." Shore and Beach 54(1): 8-16. 

Griggs, G. B. (1999). "The Protection of California's Coast: Past, Present and Future." Shore and 

Beach 67(1): 18-28. 

Griggs, G. B. (in press-a). "Impacts of Coastal Armoring." Shore and Beach. 

Griggs, G.B., K.B. Patsch, and L. Savoy, Eds. (in press-b). Living with the Changing California 

Coast. Berkeley, CA, University of California Press. 

Griggs, G. B. and K. W. Fulton-Bennett (1988). "Rip rap revetments and seawalls and their 

effectiveness along the central California coast." Shore and Beach 56: 3-11. 

Griggs, G. B. and K. Patsch (2004a). "Cliff erosion and bluff retreat along the California coast." 

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Griggs, G. B. and K. B. Patsch (2004b). "California's Coastal Cliffs and Bluffs." Formation, 

Evolution, and Stability of Coastal Cliffs-Status and Trends USGS Professional Paper 

1693: 53-64. 
17

Griggs, G. B., J. E. Pepper and M. E. Jordan (1992). "California's Coastal Hazards: A Critical 

Assessment of Existing Land-Use Policies and Practices." Special Publication of 

California Policy Seminar Program: 224. 

Griggs, G. B., J. F. Tait and W. Corona (1994). "The Interaction of Seawalls and Beaches: Seven 

years of monitoring, Monterey Bay, California." Shore and Beach 62: 21-28. 

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Interaction of Seawalls and Beaches: Eight years of field monitoring, Monterey Bay, 

California." U.S. Army Corps of Engineers, Waterways Experiment Station Contract 

Report CHL-97-1: 34. 

Kraus, N. C. and W. G. McDougal (1996). "The effects of seawalls on the beach: Part I, an 

updated literature review." Journal of Coastal Research 12(3): 691-701. 

Leonard, L., K. Dixon and O. Pilkey (1990). "A comparison of beach replenishment on the U.S. 

Atlantic, Pacific and Gulf coasts." Journal of Coastal Research Special Issue(6): 127-140. 

MBNMS (2004). "Monterey Bay National Marine Sanctuary Site Characterization." MBNMS. 

http://www.mbnms.nos.noaa.gov/sitechar/welcome.html. 

McGuinness, K. A. (1989). "Effects of some natural and artificial substrata on sessile marine 

organisms at Galeta Reef, Panama." Marine Ecology Progress Series 52(2): 201-208. 

Osborn, D. A. (2002). "The effects of geology on intertidal community ecology." Conference 

Proceedings: California and the World Ocean '02, Santa Barbara. American Society of 

Civil Engineers. 

Runyan, K. and G. B. Griggs (2003). "The effects of armoring seacliffs on the natural sand 

supply to beaches in California." Journal of Coastal Research 19(2): 336-347. 

USACE (1981). Seawalls - Their Applications and Limitations, CENT-III-8. Coastal 

Engineering Research Center, Fort Belvoir, VA. 7 p. 

USFWS (2005). "Snowy Plover Critical Habitat." Sacramento Fish and Wildlife Office. 

http://sacramento.fws.gov/ecosystems/coast/wsp_crit_hab_units.htm. 

Willis, C. M. and G. B. Griggs (2003). "Reductions in fluvial sediment discharge by coastal 

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167-182. 
18

ACKNOWLEDGEMENTS 

 I would like to thank Dr. Gary Griggs, Professor of Earth Sciences and Director of the 

Institute of Marine Sciences at the University of California Santa Cruz, for providing his 

expertise and review of this report. Dr. Andrew DeVogelaere and Brad Damitz of the Monterey 

Bay National Marine Sanctuary also provided thoughtful evaluation and advice. In addition, I 

would like to thank Dr. Steve Gittings and Kathy Dalton, of the National Marine Sanctuaries 

Program, for assistance with review and publication.