published in Pacifica Tribune November 6, 2019
|Harbor built with Roman Cement|
Imagine a seawall surviving 2000 years! Most modern seawalls have been built using Portland cement reinforced with metal rods. Counter-intuitively the metal reacts with seawater causing the concrete to corrode and crack within decades. But Romans developed a different kind of cement. Roman cement needed no reinforcement and has lasted 2000 years. Where no natural harbors existed, Romans created breakwaters and protective artificial harbors. By analyzing the composition of Roman cement, material scientists believe we can build similar long-lasting seawalls and breakwaters, and thus prevent coastal erosion that threatens human communities. Furthermore, scientists who worry about carbon dioxide emissions are likewise promoting Roman cement because it dramatically reduces emissions compared to the manufacture of Portland cement.
Currently, California’s Coastal Commission resists constructing sea walls. They argue seawalls prevent local erosion that is essential to providing sediments needed to maintain beaches. However, locally eroded sediments are often quickly removed to the deep ocean. It is the larger supply of sediments entering the ocean via rivers and streams that primarily determines the fate of a beach. The mining of San Francisco Bay sediments reduces that nourishing sediment supply. Furthermore, most California rivers and streams are dammed. That also reduces our sediment supply and blocks migrating salmon. Because California has historically suffered from natural 100-year droughts, some argue we need more dams to store water. But if there is no rain, there will be no water to store. But there is a promising solution - desalinization.
Developing technology has greatly reduced the cost of converting ocean water into fresh water. If desalinization supplies enough freshwater there would be no need for more dams and our communities would be resilient against inevitable natural droughts. And with an abundant supply of reliable freshwater, we would not need to divert water from critical ecosystems. And possibly, we could remove a few dams that now block sediments and fish.
One criticism of desalinization had been that the process releases a plume of extremely salty water back into the ocean, which can have detrimental effects on local marine organisms. But we can diffuse and dilute that salty release. More importantly, concentrated sea salt can be harvested and sold at competitive prices. That not only reduces the salty outflow but reduces the cost of desalinization. Harvested salt could also replace supplies from salt ponds, hastening restoration of San Francisco Bay’s tidal marshes.
Of course, desalinization plants must be constructed on the coast. To protect their buildings, long-lasting seawalls are required. Currently property owners located on fragile bluffs are dropping rip-rap boulders at the foot of eroding cliffs. They armor at their own expense, but not all owners can afford to do so. With a helter-skelter of armoring, waves get re-directed and focused onto unprotected sections, which accelerates local erosion. But coastal property owners are the first-line of defense against further erosion that could eventually undermine the entire community’s roads and infrastructure. The only reasonable solution is to build seawalls that protect coastal communities. Of course, property owners who enjoy their private coastal views should pay a greater share of that construction, but the community must also contribute their share to protect threatened community infrastructure.
Unfortunately, some people are urging coastal residents to “retreat” and “abandon the coast”. They’d rather spend billions moving homes and infrastructure inland. They’re petrified by the most highly-unlikely speculations that suggests a 10-foot sea level rise if Antarctica slides into the ocean. But research reveals Antarctica’s glaciers had retreated far more between 4000 and 6000 years ago. Temperatures were warmer then and still Antarctica remained stable. More importantly, during those warmer times the estimated rise in sea level was only 0.02 inches a year. More recently, research finds increasing snowfall is offsetting ice lost by Antarctica’s glaciers vulnerable to underwater melting.
Armoring fragile coastal bluffs, protects human habitat without harming local ecosystems. Beaches will survive if we maintain the natural sediment supply from local streams and rivers. The O’Shaughnessy seawall protecting San Francisco’s Golden Gate Park is a 90-year testament that beaches can still grow in front of seawalls. Creating breakwaters as the Romans had, can further protect beaches and prevent currents from removing beach sand. Secured ocean coastlines can support construction of desalinization plants and secure our freshwater requirements, reduce the need for dams, and reduce water diversions that would otherwise benefit wetland ecosystems. Desalinization can reduce groundwater extraction that’s causing coastal towns to subside below sea level. The sky is not falling. The sky is the limit for possible win-win solutions for both people and the environment.
O’Shaughnessy seawall on San Francisco's Ocean Beach, Built in 1929
Jim Steele is director emeritus of the Sierra Nevada Field Campus, SFSU and authored Landscapes and Cycles: An Environmentalist’s Journey to Climate Skepticism.