Discussion of: Griggs, G., 2024. Beach Nourishment: A Critical Look. Journal of Coastal Research, 40(5), 962–971. Free

Gary Griggs’s article, “Beach Nourishment: A Critical Look,” published in the Journal of Coastal Research, Volume 40, Issue No. 5 (2024), contains abundantly rich information on the artificial beach nourishment projects performed over the last 100 years in the United States, starting with the pioneering one at Coney Island in 1923. The article draws primarily on data published by influential researchers and government agencies, expanding on these findings to examine them within a broader context and with consideration of climate change perspectives.

The article examines sediment sources, volumes deposited, public costs incurred, and benefits gained by coastal communities, ultimately concluding that beach nourishment is a short- to medium-term strategy that cannot ensure the permanence of many coastal settlements in light of rising sea levels. Most of the scientific community agrees that strategic retreat is the more economically sustainable solution when taking a long-term perspective and considering future generations, at least for areas not densely urbanized. This strategy was demonstrated by applying the Dynamic Interactive Vulnerability Assessment model to several countries (e.g.,Brown et al., 2021; Hinkel et al., 2014; Jonkman et al., 2013).

Meanwhile, the World Bank has estimated that despite the projected US$18.3 trillion to be spent on coastal defenses against sea-level rise by 2100, two-thirds of the world’s coasts will not be protected (Nicholls et al., 2019). The same document projects that although 3% of coastlines are currently artificially nourished, this figure could rise to 18%–33%, demonstrating that beach nourishment is nevertheless considered a suitable strategy to counteract the advancing sea, even though it will pose serious problems because, considering the volumes already extracted annually, sand is a scarcely renewable resource (UNEP, 2019).

It is valuable to note that sea-level rise contributes not only to coastal erosion, but it also hinders the discharge of river floods into the sea, thereby increasing the risk of flooding in coastal plains, and raises the groundwater table, which is connected to sea level, leading to the submersion of areas not directly reached by seawater. It will not be possible to live along low coastlines solely by defending against the waves, and the adaptation or relocation of coastal settlements will become a pressing necessity.

With this in mind, we believe that the negative assessments of artificial nourishment’s effectiveness are skewed by insufficient attention to the sedimentological aspects of the issue. Reading Griggs’s paper, we notice the lack of consideration that most of the cited authors dedicated to assessing the effectiveness of beach nourishment on sedimentological aspects. In the data provided in the Griggs article, and by most of the authors cited therein, grain sizes of native vs. borrow sediments are not addressed. In only two places does Griggs indirectly reference this topic, listing among the limitations of beach nourishment the “compatibility of added sand to the beach being nourished” and citing Parkinson and Ogurcak (2018), who contest Houston’s work (2017) advocating nourishment as a viable climate change adaptation strategy, partly due to “compatibility of sand sources with native beach material.”

Even in the accurate analysis of beach replenishments conducted in the United States in the 20th century by Finkl, Benedet, and Campbell (2006), an entire paragraph was dedicated to the methods for determining the stability of artificial nourishment providing the volumes for the described projects, but the sizes of the sediments used were not addressed. However, the same authors, state that the use of methods based on granulometric parameters has declined and that “the present design approaches to beach nourishment instead favour the use of equilibrium profile considerations and combination of coastal analysis” (Finkl, Benedet, and Campbell, 2006). Dean’s profile equation (Dean, 1991), however, has mean sediment size as the determining variable, so we are back to sedimentology but relying on only one textural parameter.

Pure granulometric stability indicators describe aggregates with mean size and sorting (critical ratio, R_Φcrit, by Krumbein and James [1965]; overfill factor, R_A, by James [1975]; renourishing factor, Rj, by Hobson [1977]), giving significance to the tails of sediment distribution, finer (first to be lost) and coarser (which remain longer as lag deposit). The complete grain-size distribution is used to calculate the stability index (Is) by Pranzini, Anfuso, and Munoz-Perez (2018), which gives a value even when the native-borrow combination cannot be analyzed with other parameters or provides a general result of one.

The stability index can be used independently from sediment log-normality (necessary if mean size and sorting enter the equation) with Inman (1952) and Folk and Ward (1957) and ensures that it is possible to assess how much the new beach will differ granulometrically from the original one, allowing the comparison among aggregates that are coarser than the native one.

The option for coarser aggregates introduces a critical question. Is beach nourishment primarily intended to restore a surface for tourism and recreational use, or is it predominantly a coastal defense measure? And can be both!

In the literature, however, limited data are available on the durability of beach nourishment concerning the size of native vs. borrowed sand. Grouping nourishment projects that used finer sand with those that used coarser sand prevents an accurate assessment of the actual effectiveness of this coastal erosion defense strategy (USACE, 2002).

Regarding economic activities, it is relatively straightforward to assess direct costs and benefits, as seen in a seminal study that is continuously updated (Houston, 1995, 2022). Assessment is more complex, involving many more factors and a much longer timeframe. For assessment, the history of most nourishment projects to date offers little guidance; sand, often finer than native sediments such as that which settles in low-energy areas or material dredged from the continental shelf, is almost always used, whereas coarser sediments, such as granules, gravel, and pebbles, have rarely been used.

It can be argued that nourishment with fine sand is poor advertising for soft coastal defense strategies, as shown in many Italian cases. Following an unsuccessful nourishment project using improperly sized sand, stakeholders have demanded hard defenses, thus contributing to the proliferation of groins and detached breakwaters that characterize the coastline of this country. In some cases, public authorities responsible for the project were warned that the sand would be lost within just a few months, but, pressed by the stakeholders’ demand for a strong beach expansion despite having limited financial resources, they completed the project. Regarding gravel nourishment, only a few monitored examples are found in the literature that refer to the point where the loss of volume has been caused by the abrasion of the grains rather than by their longshore or offshore loss (Nordstrom et al., 2008), but all demonstrate a high longevity (e.g., Bujak et al., 2023; Cinelli et al., 2021; Kumada et al., 2010; Liu et al., 2020; Onaka et al., 2017).

Gravel beaches created in Italy along coastal stretches where the beach had long disappeared—whether they are open or protected by emerged or submerged structures (i.e. groins, reefs, or artificial islands)—have proven resilient against wave attacks, even storms with return periods exceeding a decade (Pranzini, 2009) (Appendix Figures 1A-3A).

Perhaps these case studies have not been presented with sufficient emphasis to the international community, but before abandoning artificial nourishment as a coastal settlement defence strategy, some evaluation in this regard is warranted.

That said, as we have noted, it is not solely by defending against waves that we can adapt to sea-level rise, and strategic retreat needs to be the predominant solution; in this, we agree with Griggs.