Complex, Coupled Human and Natural Systems: Our Inability to Predict the Future of Developed Beaches

Numerical models of coastal processes are commonly used to predict the impacts of human activities on developed beaches. Examples abound of our failures in this regard, and are often excused as artifacts of unusual storms or problems with numerical models. Recent work on complex natural systems coupled with human systems highlights the extreme difficulty of predicting even decadal-term outcomes, much less long-term futures. These coupled systems possess many properties and non-linear effects that are not embedded in our relatively simple models for nearshore sand movement or for shoreline behavior as a result of sea-level rise. Examples from Saco Bay, Maine, a large, river-fed pocket beach system with 300 years of human impact will demonstrate the futility of modeling the system. Legacy effects of structures are thought to decay exponentially over a decade, but evidence of erosion and accretion in different parts of the bay after 150 years is observed. Added complexity occurs when new structures are prematurely built to modify the presumed equilibrium effect of earlier ones. Thus, people built on the collapsed remnants of a tidal delta that accreted to a beach near a jetty, and then began adding new protective structures as the new land eroded and caused distant erosion. Surprise effects like a law that precluded seawalls following a damaging storm spawned a political outcry for large, new breakwaters that may lead to more distant erosion. It is difficult to understand how numerical models or longshore transport of the Bruun Rule, employed at any stage of this region's history, would have adequately predicted how people and nature would combine to reform the developed beach.