A need to better monitor the effects of coastal defence measures on coastal socio-ecological systems to improve future adaptation solutions

Abstract

Decision-making in a coastal socio-ecological system involves managing a site-specific complexity arising not only from the interaction between hydrodynamic and morphological conditions, but also from the interactions between human structures and ecological systems, as well as the conflicting needs and interests of local actors. In addition, the projections of climate change impacts on coastal systems have a degree of uncertainty, which increases the general unpredictability of coastal dynamic behaviour, and adds a layer of complexity to the decision-making process. Scientific knowledge can help reduce some of the inherent uncertainties, and is essential when it comes to making sound decisions on the choice of appropriate coastal defence measures (CDMs) that are adapted to specific coastal environments and improve the resilience of coastal communities. This paper is based on a meta-analysis of 355 CDMs case studies drawn from 301 publications. From these published case studies, the objectives were to analyze the geographical and physical contexts in which CDMs monitoring was carried out and, based on the findings, to recommend areas of improvement necessary to help make sound decisions in any type of coastal environment. The meta-analysis showed that study sites are not evenly distributed around the world. Most originate from Europe (n = 106), the USA (n = 151) and Australia (n = 30), while few studies have been carried out in Africa and Asia where dense population resides in high-risk zones. Also noticeable is the absence of sites in high latitude climates where ice plays a major role in the erosion process. Five basic variables (coastal type, sediment type, wave characteristics, tidal range, and currents or sediment transport characteristics) are used in publications to characterize study sites according to their physical components. However, only 13 of the 355 sites included a complete characterization using the 5 variables, of which coastal and sediment types are the most frequently identified (77.2% and 72.7% respectively). In general, CDMs are studied in the context of unconsolidated low shore (59.4%) and in sandy environments (74.6%). Information on tidal range, wave climate, and currents, or sediment transport characteristics, is much scarcer. Since 1990, 3 of the 10 CDMs identified in the studies have received more attention than the others; these are beach nourishments, seawalls and breakwaters, with respective cumulative study sites of 164, 67 and 50. The geomorphological effects of CDMs are the most studied (55.1%), followed by ecological (31.2%), hydrodynamic (9.1%), and social (4.6%). Overall, this meta-analysis helped identify knowledge gaps regarding geographical and physical contexts in which CDMs monitoring was held. It also gave an indication of the kind of improvement necessary for global-scale adaptation planning, and for a better decision-making process to reduce coastal risks in the most vulnerable coastal communities. Finally, the analysis shows that 4.4% of the studies on defence measures include monitoring of their effects on the coastal zone. A conceptual scheme is proposed for the evaluation of adaptation solutions based on the global monitoring of coastal zones to measure coastal change trajectories in the context of climate change.