1.1 The Challenge: A History of Shoreline Armoring
For centuries, estuarine and coastal shorelines have been dramatically modified by humans for far-ranging and, at times, conflicting purposes, such as water access, commerce, aquaculture, and property protection. Legal principles governing these uses often tend to favor the interests of coastal property owners or societal rights to access and exploit natural resources. For example, in most coastal communities, shoreline armoring is allowed and accepted if property is deemed to be at risk. As a result, shorelines have been extensively armored globally and ecosystem function has diminished. The amount of shoreline armoring along a given coast varies depending on surrounding land use, with major coastal cities often having more than 50% of their shores hardened (e.g., Chapman and Bulleri 2003). However, shoreline armoring is not restricted to dense urban areas, as coastal areas in the process of change (e.g., agriculture to suburban) may experience the fastest rates of shoreline hardening (Isdell 2014). In sum, the United States has roughly 14% (22,000 km) of its extensive coastline armored (Gittman et al. 2015). In Europe, more than half of the >15,000 km of coastline that is eroding is artificially stabilized (EC 2004). Likewise, more than half of Mediterranean coastlines are armored and developed (EEA 1999). In Japan, approximately half of its coastline is reportedly eroding (15,900 km of the 34,500 km of total coast) and approximately 27% of that coastline has been hardened (Koike 1993). In Australia, the densely populated coastal cities typically have more than 50% of their coastlines armored (Chapman 2003). Pressures to abate erosion and secure shorelines in place will only continue, and likely increase, as the proportion of the global population living within 100 km of the coasts grows from one-third to an expected one-half by 2030 (Small and Nicholls 2003) and sea level continues to rise. This may be particularly problematic in areas where heavily urbanized landscapes intersect with higher-than-average rates of sea level rise; for example, the North American Gulf and mid-Atlantic coasts have the highest rates of rise in the United States (Boon and Mitchell 2015) and respectively have the third and fifth fastest-growing coastal populations in the continental United States (Crossett et al. 2004).
Closely associated with shoreline modification is the loss or alteration of intertidal and shallow subtidal habitats (e.g., wetlands, seagrasses) and ecosystem function (e.g., Bilkovic and Roggero 2008; Chapman and Bulleri 2003; Dethier et al. 2016; Dugan et al. 2011; Peterson and Lowe 2009 and references within). This has implications for ecosystem service provision to coastal communities including shore protection, fisheries production, and water quality benefits (e.g., Arkema et al. 2013; Bilkovic et al. 2016; Gedan et al. 2011; Scyphers et al. 2015). Growing concern about the cumulative effects of piecemeal alterations to the coastlines has reinforced the need for alternative shoreline management strategies.
While wetlands have been long recognized as providing some level of protection to coastal communities from wave-induced erosion, the intentional use of natural habitat elements to reduce shoreline erosion was first reported in the early 1970s (Garbisch and Garbisch 1994). Since that time, the understanding and practical application of nature-based techniques have grown tremendously. In recent years, nature-based approaches are being extensively promoted and practiced globally primarily because of (1) growing acknowledgment of the value of ecosystem services provided by coastal habitats (Barbier et al. 2011; Costanza et al. 1997) and the adverse effects of traditional armoring to coastal systems, (2) the extensive ongoing loss of many threatened coastal habitats (marsh, seagrasses) (Duarte 2009; Halpern et al. 2008; Waycott et al. 2009), and (3) the realization that dynamic erosion protection approaches that incorporate natural ecosystem elements (e.g., marsh, beach) may be mo...
