It is widely accepted in the coastal science community that for any given stretch of beach, a wider beach fronting adjacent coastal bluffs will provide those bluffs more protection from wave attack than will a narrower beach. In other words, the wider the beach, the less chance for wave run-up, and the more protection afforded to the base of the coastal bluff (which is also interchangeably referred to as “seacliff” in the published literature).
Thus, whereas structural means of beach retention were common 30 to 50 years ago, beach nourishment has become a preferred method in recent decades because beach nourishment represents a “soft” method of shoreline stabilization, in contrast to “hard” alternatives such as groins (e.g., the shore perpendicular groins along the majority of Newport Beach's shoreline). So long as the grain size and angularity of the imported sand has similar characteristics to the sand grains of the natural beach and can withstand the local wave energy/environment, soft stabilization alternatives, such as sand or cobble beach fills, mimic nature and are intended to be dynamic, responding to changes in wave and current conditions.
Importantly, introducing new sand onto the beach can compensate for a reduced sediment supply once delivered by rivers and streams. In this way, beach nourishment represents a means of restoring a more natural system. Wider beaches, in turn, reduce the need for hard structures by providing the base of coastal bluffs with protection from wave attack and incremental sea level rise while simultaneously increasing recreational opportunities.
With respect to newly proposed oceanfront bluff-top development projects being processed for permitting along the Encinitas, Solana Beach, and Del Mar shorelines in north San Diego County, CA, and any postulated, non-speculative impact of future sea level rise on bluff-top erosion within the economic lifetime of these projects, it is important for all interested parties to meaningfully recognize that a substantial 50-year federal beach nourishment project has been approved and funded for the Encinitas and Solana Beach coastlines, with replenishment set to take place every five-to-ten years of the project life. The U.S. Army Corps of Engineers estimates the beach nourishment project will widen the Encinitas, Solana Beach, (and therefore) northern Del Mar beaches by approximately 50 feet via the process of littoral sand transport (predominantly, north-to-south in San Diego County) and routine nourishments over the 50-year period. This long-term beach nourishment project, which has been widely reported in the media, will undoubtedly provide Encinitas, Solana Beach and Del Mar beaches with significant added protection from wave attack at the base of the seacliff due to the presence of a wider beach that will help combat future bluff retreat due to sea level rise.
Interestingly, beach nourishment aside, with respect to any postulated assumptions about the influence of varying degrees of future sea level rise on coastal bluff erosion in San Diego County, the fact of the matter is that sea level has been rising linearly at the mean rate of 2.15 mm/yr (+/- 0.19 mm/yr) since 1906 as monitored by monthly mean sea level data collected at Scripps Institution of Oceanography in La Jolla, CA.
Whereas various future sea level scenarios have been reasonably projected and publicized in areas where sea level rise is modeled across low-lying beach profiles absent of uplifted, rocky coastal seacliffs – for example, the low-lying “Dog Beach” at the rivermouth of the San Dieguito River (where coastal cliffs do not exist due to the local geology and the beach profile is controlled by wave and tidal influences) – the quantitative impact of rising sea level and its effect on the rate of erosion of dense, rocky coastal cliffs (such as those in Encinitas, Solana Beach and Del Mar, where the Torrey Sandstone and Del Mar Formations are the dominant cemented materials resisting wave attack, as opposed to unconsolidated beaches, dunes or weakly lithified terrace deposits that are much more susceptible to wave attack), is subject to numerous assumptions and speculation as to seacliff erodibility as reported in the published literature addressing coastal bluff retreat considering sea level rise. (See A.P. Young et al (2014), “Estimating Cliff Retreat in Southern California Considering Sea Level Rise Using A Sand Balance Approach”, Marine Geology (2014) 15-26; and M. J. Bray and J.M. Hooke (1997), “Prediction of Soft Cliff Retreat With Accelerating Sea Level Rise”, Journal of Coastal Research, Vol. 13, No. 2, pp. 453-467 [concluding that “[c]hanges on cliffed coasts are not easily predicted because recession is the cumulative result of numerous interacting variables].)
At the very least, it is reasonable to conclude that the substantial public benefit that will result from the erosion protection (i.e., increased beach buffer) that will be provided by the aforementioned 50-year federal beach nourishment project aimed at increasing beach with by 50-feet in Encinitas and Solana Beach (and, incidentally, in Del Mar) will (a) substantially offset the impact of any future sea level rise along these North County coastal bluffs, even under the most drastic modeled scenarios, and (b) via the processes of natural littoral drift, will certainly reinforce already dense/strong bedrock formations (i.e., the Torrey Sandstone and Del Mar Formations) that already form a robust and resistant face along the base of the seacliffs in Encinitas, Solana Beach and north Del Mar (i.e., to the north of the San Dieguito Rivermouth).