Performance of Short-Term Beach Erosion Mitigation at Capostrano Bay Community

Capistrano Bay Community Service District Dana Point, California

By Craig Wright

Introduction

Due to the effects of extreme weather events and climate change, many beachfront developments in Southern California are experiencing rapid beach erosion and shoreline retreat. Rising sea levels and increasing wave action are causing many deleterious effects to California’s coastline, including loss of sensitive habitats, recreational assets, public and private properties, and civil infrastructure. To mitigate these effects, numerous short-term beach erosion mitigation strategies and products have been employed across the Southern California coastline, having a wide range of results. The purpose of this case study is to discuss the performance of various beach erosion countermeasures as short-term beach erosion mitigation solutions at Capistrano Bay Community, a Special Government District located in the City of Dana Point, California.

Background

The Capistrano Bay Community Service District (“The District”) is located along a thin strip of beach between the Pacific Coast Highway and the Pacific Ocean, at the southeast end of Dana Point. The District was formed in 1959, when the Orange County Board of Supervisors voted to approve the request of Beach Road property owners to convert the community into a Special Government District. 

The District receives a percentage of the property taxes generated from the community as well as a District User Fee to fund its operations. To fund large-scale capital improvements, the District formed an Assessment District (AD99-1) in 2003, which generated funding for replacement of a roadway, curbs, gutters, and storm drain systems.

The District has suffered notable beach damage from high-energy storm events during high tides on several occasions over the past several decades. Significant storms in 2005 and 2011 resulted in the loss of large volumes of sand. On November 30, 2018, high surf and high tides destroyed a wooden walkway, seawall, and basketball court. The District was most recently affected by a beach erosion event on July 4, 2020, when unusually high tides combined with large swells resulted in local flooding and beach loss. These events have resulted in significant beach erosion and shoreline retreat along the coastline within The District (Figure 1). 

Sea level rise resulting from the effects of climate change is expected to continue in the coming decades. The City of Dana Point Sea Level Rise Vulnerability Assessment reported a 66 percent probability that sea level rise will range between 0.7 and 1.2 feet by 2050, and 1.8 to 3.6 feet by 2100 . For coastal homeowners at The District, mitigating the growing prospect of erosion has become an issue of paramount importance. 

Short-Term Mitigation Solutions

Since 2005, residents of The District have been armoring their beachfront to protect it against an advancing ocean. Coastal protection strategies and products vary in cost, size, effectiveness, and life span. Coastal homeowners at The District have used retaining walls, sandbags, riprap, gabion baskets, beach contouring, and permeation grouting as some of the means to temporarily mitigate the risk of further erosion events affecting their properties. 

Sandbags and Gabion Baskets

Large sandbags constructed of woven polypropylene fabric, and having a capacity of approximately 1 cubic yard each, have been employed in front of several homes in The District (Figure 2) and along Capistrano Beach to the north. They are cost effective and when partially buried and stacked with a batter, are capable of withstand heavy storm surge. They are filled with available beach sand and gravel, and can be installed quickly with a small excavator, backhoe, or loader.

Gabion baskets are strong, stone-filled cages that are stacked and tied together to form a wall. Gabion baskets observed at The District were made from geogrid reinforcement, filled with beach gravels and cobbles (Figure 2), and fastened together with metal ties. Similar to sandbags, they are cost effective, very versatile, relatively easy to install, and capable of withstanding heavy storm surge. 

During our reconnaissance, we observed that temporary sandbags deployed for emergency protection, appeared to be well past their temporary lifespans and exhibited evidence of degradation. The degradation of these materials may become a source of ocean pollution, as plastic debris was observed strewn along the beach. Likewise, we observed strips of geogrid from stone-filled gabion baskets littering the beach. Another downfall of the gabion basket is when the basket is partially or totally buried, the sharp metal ties can become a hazard under foot.

Retaining Wall Systems

During our site reconnaissance, we observed modular-block gravity and wood cantilever retaining walls that were either structurally compromised or completely failed. A gravity wall depends on its own mass to resist lateral pressures from behind the wall. Figure 3 shows an example where the energy of the waves overcame the mass of the gravity wall, resulting in the wall toppling over. The wood retaining wall (Figure 3) appeared to be constructed of shallow wood piles with wood lagging bolted to the face. This type of wall relies on passive resistance from soil in front of the piles to overcome the lateral pressures from behind the wall. In this example, the lateral pressures were greater than the resisting forces and the wall rotated forward. 

We observed some retaining walls with exposed foundation elements (Figure 1), where beach erosion had removed material from in front of the wall, increasing their exposed heights, and decreasing passive resistance. While these walls were still functioning, we assume that the capacity of the walls are less than their original design. As the sand continues to be eroded from the beach, we expect retaining wall systems within the influence of erosion to continue to be undermined. 

Permeation Grouting and Riprap

Permeation grouting is a type of engineered ground improvement where flowable cement-grout is injected into loose granular soil through high-pressure injection pipes. The grout flows into the interconnected pore space of the soil, and when cured, binds the particles together to strengthen the soil mass. At The District, grout injection locked beach sand, gravel, and cobble together to form a block of improved ground in front of the property (Figure 4). This countermeasure is relatively expensive and requires mobilization of pumps, grout plants, and specialty equipment. If beach erosion later exposes the improved ground, it leaves a rough, hard surface with a less desirable aesthetic, and the exposed injection pipes become a tripping hazard. Although the improved ground becomes more resistant to vertical scour, it does very little to dissipate wave energy from affecting property behind it.

We observed riprap (large rock boulders) commonly employed in the southern stretches of The District. Riprap was placed at the north end of The District’s coastline in 2005 and again in 2011. At The District, riprap has been placed on top of the sand as a rock armor or rock revetment to dissipate wave energy and protect coastal homes from storm inundation. Riprap is flexible, versatile, and when embedded in the sand, may be the most robust temporary countermeasure observed at The District. Although, cheaper than an engineered solution, importing riprap is relatively expensive and requires large equipment to place the boulders. As scour begins to affect the toe of the riprap revetment, boulders eventually roll and topple seaward, and need to be maintained by restacking and redistributing the boulders.

While pressure grouting and riprap are robust solutions, both systems are still considered short-term and come with a relatively high cost. Permeation grouting resists further scour in front of the property, but does very little to stop wave action from hitting property behind the improved ground. Riprap reflects and dissipates only some of the wave energy, and must be maintained regularly. In addition, both the City of Dana Point and the California Coastal Commission discourage further use of riprap.

Muscle Wall Barriers

Muscle Wall is a linear low-density polyethylene (LDPE) portable barrier product used for flood control, sediment control, stormwater control, coastal erosion, and material containment (Figure 5). The approximately 6-foot-long barrier pieces are interlocked, secured to each other using ratchet tie down straps and filled with readily available water to produce a stable barrier system. 

When testing the Muscle Wall product as a flood barrier, the United States Army Corps of Engineers (USACE) found it remained stable under various loading conditions, stating, “The barrier was undamaged by waves, overtopping, debris impact, or riverine current.”  

In addition, the USACE concluded that compared to a typical sandbag barrier of the same height, the Muscle Wall barrier took 27 times fewer person-hours to construct, had 4 times less seepage, and withstood all tests without damage. For reference, one 4-foot-tall Muscle Wall barrier is equivalent to approximately 468 typical sand bags. 

In 2016, the first Muscle Wall barriers were installed at individual residential properties within The District as a temporary revetment to protect beach sand and property structures/improvements from wave action and erosion. They perform best when installed partially buried, to protect the toe of the wall from scour, and set in a tiered-wall configuration (Figure 6). The Muscle Wall barriers can be disassembled after the storm passes, or left in place for multiple seasons. Property owners within The District also employ Muscle Wall barriers as temporary flood protection along decks and patios to divert water away from the home. The system may be rapidly deployed by the homeowner and taken down and stored after the storm surge subsides.

Numerous Muscle Wall barrier applications have been deployed at The District to preserve beach sand behind the wall, while coarse gravel and cobble collected in front of the wall. Several of the walls had become buried due to the natural, dynamic action of the beach. While permeation grouting and gabion baskets present a potential beach hazard when partially or completely buried, the polyethylene Muscle Wall barriers, with their contoured corners and relatively soft shells, did not appear to present the same hazard when buried (Figure 7). 

In contrast to the observed degradation of sandbags and gabion baskets, we did not observe evidence of damage or degradation of the Muscle Wall product deployments, either in adjacent beach reaches or in ocean waters.

Discussion

During our site visit, we observed that unprotected properties adjacent to a property utilizing one of the mitigation measures discussed above, often exhibited greater effects of scour than properties adjacent to other unprotected property. This is likely the effects of flanking erosion (also known as end scour). Flanking erosion occurs at the ends of a beach erosion mitigation system or product, where wave energy is reflected laterally along the shore, causing unprotected properties to erode faster. Any beach erosion solution that hardens a portion of the coastline will induce flanking erosion to adjacent properties. 

We acknowledge that there are many other mitigation measures and products available for temporary mitigation of beach erosion; however, the purpose of this study was to discuss the main mitigation measures observed within The District. Table 1, below, presents a summary matrix showing relative effectiveness of the different countermeasures discussed in this paper with regard to the following considerations.

• Longevity – The relative ability of the mitigation measure to provide the intended mitigation through multiple events/seasons.

• Durability – The relative ability of the mitigation measure to maintain its shape/position.

• Resistance to Degradation – The likeliness of the mitigation measure to bio- or photo-degrade resulting in loss of erosion protection and/or pollution generation.

• Initial Value to Cost (V/C) Ratio – The relative value of the initial installation of a mitigation measure divided by the cost of the measure.

• On-Going V/C Ratio – the relative value of a mitigation measure divided by the cost of on-going maintenance of the measure.

Conclusion

Many beachfront developments in Southern California are experiencing rapid beach erosion and shoreline regression. Coastline and adjacent improvements within the Capistrano Bay Community Service District have suffered notable beach damage in recent decades. As storm surges, sea level rise, and increasingly destructive wave action continue, more erosion events are anticipated. 

Without a permanent, community-wide engineered solution employed, property owners within The District have deployed a range of products and systems to mitigate beach erosion. As the California Coastal Commission discourage further use of riprap, property owners within The District have sought other temporary solutions to protect their homes.

Of the beach erosion mitigation measures observed within The District, the short-term performance, versatility, speed of deployment, longevity, and cost effectiveness of the Muscle Wall product stands out in the Erosion Control Matrix (Table 1). Muscle Wall has proven to be an agile, resilient, and durable shoreline erosion product. Muscle Wall can be rapidly deployed before a storm, and can outperform other short-term beach erosion mitigation solutions in terms of their longevity and resistance to degradation and beach pollution. Barriers can be taken down and reused, or remain in place for years at a time with little to no maintenance. Due to its durable, corrosion-resistant LDPE material, we expect the Muscle Wall barriers to withstand continued use for many years to come or until a more permanent engineered system can be employed.

Craig Wright is an Engineering Geologist CMEngNZ (PEngGeol). Craig’s experience in New Zealand and the United States involves performing geotechnical consultation, project management, field mapping, fault and landslide investigations, geotechnical hazard analysis, stormwater monitoring, and geotechnical instrumentation installation and monitoring. He provides feasibility studies to advise clients of potential geotechnical hazards and construction constraints of their property.