2020, when unusually high tides combined with large swells resulted in local flooding and beach loss. These events have resulted in signifi - cant 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 beach - front 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, sand - bags, riprap, gabion baskets, beach contouring, and permeation grout - ing 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 hav - ing a capacity of approximately 1 cubic yard each, have been employed in front of several homes in The District (Figure 2) and along Capist - rano 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 in - stall, and capable of withstanding heavy storm surge.
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 grav - ity 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 grav - ity 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.
Figure 3: Failed retaining walls
Failed gravity wall
Failing wood retaining wall
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 spe - cialty 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 im - proved 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
Figure 2: Sand Bags and Gabion Baskets
Gabion Baskets filled with locally sourced beach gravel and cobble
During our reconnaissance, we observed that temporary sandbags de - ployed for emergency protection, appeared to be well past their tempo - rary 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
June 2022 csengineermag.com
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