Nature-based solutions are becoming an increasingly important component of sustainable coastal risk management. For particularly destructive hazards like tsunamis, natural elements like vegetation are often combined with designed elements like seawalls or dams to augment the protective benefits of each component. One example of this kind of hybrid approach is the so-called tsunami mitigation park, which combines a designed hillscape with vegetation. Despite the increasing popularity of tsunami mitigation parks, the protective benefits they provide are poorly understood and incompletely quantified. As a consequence of this lack of understanding, current designs might not maximize the protective benefits of tsunami mitigation parks. Here, we numerically model the interactions between a single row of hills with an incoming tsunami to identify the mechanisms through which the park protects the coast. We initialize the tsunami as an N wave that propagates to shore and impacts the coast directly. We find that partial reflection of the incoming wave is the most important mechanism by which hills reduce the kinetic energy that propagates onshore. The protective benefit of tsunami mitigation parks is thus comparable to that of a small wall, at least for tsunamis with amplitudes that are comparable to the hill height. We also show that hills could elevate potential damage in the immediate vicinity of the hills where flow speeds increase compared to a planar beach, suggesting the need to include a buffer zone behind the hills into a strategic park design.

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A retreat critique: Deliberations on design and ethics in the flood zone Many millions of Americans live in coastal areas threatened by sea level rise; in all but the very lowest sea level rise projections, retreat will become an unavoidable option in some areas of the U.S. coastline. 1 Introduction: Designing retreat?Climate change is reorganizing our planet: locations of water, drought, heat and cold no longer inhabit familiar zones. Adaptation will require reorganizing the way we live: the locations of our homes, cities, infrastructures and even nations. Managed retreat, or simply 'retreat', is a climate adaptation strategy geared towards moving human settlements away from zones vulnerable to disasters. Most commonly associated with flooding and sea level rise, retreat is increasingly proposed as a mechanism to deal with wildfires, mudslides, tsunamis and even drought.The devastating toll of recent hurricanes inspired a spectrum of proposals in academic and popular media ranging from structural flood mitigation to the relocation of households out of floodplains. As put forth in the US 2018 National Climate Assessment, retreat 2 _the movement of populations and infrastructure away from hazard zones_is likely to become a necessary component of adaptation toolkits as climate change makes coasts, riverbanks, wildland-urban interfaces and other zones increasingly dangerous to inhabit. Most proposals for retreat, however, fail to query the equity of retreat actions, leaving vulnerable populations to carry the burdens of migration away from hazardous areas, while wealthy inhabitants are allowed to stay. What are the broader political, cultural, ecological and urbanistic implications? What are the impacts on receiving communities?

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