Mitigating Tsunamis with Seawalls

Note: The summarizing introduction in italics, below, appears at the beginning of three separate pages, each of which reconsiders an example of human efforts to mitigate a natural hazard. Once you’ve read that intro once, you can skip directly to the main text on subsequent pages.

Understanding that ecosystems are complex systems, rather than mechanistic ones operating via linear cause-and-effect, dramatically changes how we perceive Western culture’s strategies to mitigate natural disasters. McPhee’s Laws provide a powerful low-jargon way to think about the situations described on these pages. Resilience ecology offers conceptual language — robustness-fragility trade-off (RFTO) — that often seems more rigorous and legitimate to people in Western culture. It also adds a human-impact component that McPhee addressed more in terms of conflicting stakeholder demands. A brief summary of RFTO principles regarding system fragility and human vulnerability heads each section, to facilitate the process of application.

 Seawalls and other concrete barriers failed to stop the Tōhoku earthquake-generated tsunami of 2011 that claimed nearly 18,000 lives on Japan’s northeast coast (Solomon 2011). Japan has responded by rebuilding the seawalls higher to make the system more robust: 11 foot seawalls are being replaced with ones the height of a four-story building (Kyung-hoon 2018).

(1) By becoming robust to a particular set of disturbances, complex systems necessarily become more sensitive to disturbances outside that set. Fragility that emerges from RFTO is dangerous because its presence is usually not visible until it’s revealed through unexpected failures (Ishtiaque et al 2017).

Japan’s new and much larger seawalls make the coastline more robust with regard to preventing tsunamis. Because they are so much bigger, they must have deeper footings to support them, and to prevent erosional undercutting by the normal processes of onshore and offshore currents that move water, sediments and nutrients — movements the seawalls necessarily inhibit. Local Japanese who’ve spent their lives on the sea and make their living from it worry the wall will cause decline or destruction of oyster beds and fishing grounds (Kyung-hoon 2018). Their fears do not seem misplaced, though extensive research on “coastal armor” structures has only recently begun to collect data. However, it’s clear that construction of seawalls and dikes has significant impact on coastal areas worldwide (Ritter et al 2008:554). Seawalls degrade coastal marine and estuarine ecosystems in several ways. They block the movement of sediments and nutrients from land into nearshore environments where they’re critical to marine communities. They trap toxic chemical pollutants and effluvia in estuaries that cannot drain. And they create turbidity that suspends sediments in tidal zones and estuaries. These changes impact marine resources “through single, cumulative, or synergistic processes [and] . . . each of the main drivers of change create a set of pressures; in turn, each pressure creates a set of state changes in the natural environment, which leads to impacts on the human system” that depends on marine resources (Borja et al 2010:1250).

Obviously, these changes would impact the local ecosystem in many ways, but what might not be obvious is how it could create ecosystem fragility that makes tsunamis worse instead of preventing them. As one small example of how these factors connect, it is common knowledge in geology and oceanography that the height of waves on a coast, and the point at which they “break” or curl over, depends in large part on the depth of the water. The nature of the substrate beneath the water the wave energy is moving through makes a difference, too. For instance, big objects such as rocks or broken chunks of concrete can create turbulence in the incoming wave, which impacts the sort of work it can do on the things it encounters. Water depth also affects how the water in the waves moves in a more general way, and that motion also influences the kind of work the wave can do. When more energy moves through the water after an earthquake, the effects of any changes in the system that have made it possible for waves to be taller, to break close to structures onshore (such as seawalls), and/or to apply more energy to the structures they encounter will be magnified. So it’s entirely possible, just based on well-known basic science, to see that 40-foot seawalls with comparably deep and massive footings that alter coastal sediment flow and substrate topography can change waves and make their movement more rotational. Those are the kinds of changes that could make a tsunami more capable of breaching a wall that was designed specifically to stop a wave produced by a nearshore topography that the seawall itself changes. There are many other potential scenarios by which a more massive seawall could very well change the physical conditions of the coast in a way that made tsunamis bigger and more powerful, and all are reasonable conjectures based on basic science of coastal systems. But we cannot predict any of them in a complex system. Prediction can only really be done in a linear cause-and-effect system. We won’t actually know what’s happening until the fragility emerges and becomes visible as a massive tsunami that overtops or breaches the seawall. Then we’ll be able to do a post-mortem on what it was that happened, and why. That’s the point at which, once again, people will face the choice between seeing that unexpected failure as “a one-time glitch we can fix this time” so that “from now on” the seawall works — or as evidence that human control of nature was always an illusion.

(2) Structural measures that successfully reduce exposure to disturbances eventually result in increased vulnerability, because reduced exposure causes the loss of risk awareness in societal memory, thereby encouraging increased development in risk prone areas.

Seawalls dramatically increased urban development and population density on Japan’s coastal plains after 1960 (Fackler 2011). As time went by, local people relied on the walls for protection more and more completely. They stopped teaching their children the tsunami protocols that could have saved many lives in the hour between the major earthquake and the tsunami that followed it (Fumio Yamashita, quoted in Fackler 2011). They also stopped paying attention to the ancient tsunami stones their ancestors had left to warn them — stones such as the 600 years old marker near the coastal city of Kesennuma that warns: “Always be prepared for unexpected tsunamis. Choose life over your possessions and valuables” (Bressan 2021). Many of the people killed in the tsunami died because, instead of heading at once to high ground after the earthquake, they went to their homes to inspect the damage to their property (Bressan 2018). They weren’t worried about a potential tsunami because they thought the seawall would protect them. “There was this sense that modern construction and concrete wave walls could protect the towns from tsunamis. And these all proved completely ineffectual.” (Solomon 2011).

Sotaro Usui, head of a tuna fishery on the Japanese coast where the 40-foot wall is going up, points out that the higher seawall has another impact that’s much bigger and far more insidious. “Everyone here has lived with the sea, through generations,” he observed. “The wall keeps us apart — and that’s unbearable” (Kyung-hoon 2018). So the seawall more completely separates humans who were still connected to the natural world from that world. What we seeing here is live, “caught in the act” propagation of Western culture’s maladaptive behavior spreading through what’s essentially a process of forced acculturation.

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