BEACH REPLENISHMENT
Beach nourishment is considered a "soft" method of shoreline stabilization. Anecdotal evidence suggests that replenished sand is more easily erodible than natural sand deposits, the reason for this has not been explored. Therefore, all nourished material should be considered sacrificial in nature and will require periodic maintenance.
The characteristics of the supplied material should match the natural sand found on the receiver beach, most importantly in terms of grain size. If the sand source is too fine, it is not in balance with the available wave energy and will be rapidly washed away. Larger grain sizes may be more stable, however, they can create changes in the beach profile that were not present under natural conditions.
Color can also be important, particularly with respect to habitat. Of course, the sediment must also be free from contamination.
“Opportunistic" sand placement refers to an onshore sand source that is made available through projects not primarily intended for beach nourishment (stream clean-outs, harbor dredging, construction). The 2023 sand fill in Capistrano Beach, for example, was taken from the Santa Ana River for the purpose of channel clean-out. For most opportunistic projects, the sand is delivered to the beach via dump trucks, which is the most expensive method that can cause considerable environmental impacts due to traffic and air quality. Save Our Beaches is exploring the use of rail cars for delivery of opportunistic sand to our beaches (see “Gameplan.”)
"Deterministic" projects are undertaken for the primary purpose of beach restoration. In these cases, offshore sand deposits can be dredged specifically for beach replenishment. The 2024 federal dredging project at the Pier is considered a deterministic project.
What’s happening with the San Clemente project?
For the San Clemente dredging project, a suction-type dredger is used to mine sand from a specific borrow site; the sand is then stored within the ship’s hopper and transported to a nearshore position. The dredge then mixes the sand with water and discharges it through a moored pipeline onto the beach. At the beach, the slurry is discharged into a dewatering basin constructed to reduce the amount of resultant turbidity in the nearshore environment (decreasing environmental impacts to kelp for example). Bulldozers then spread the sand in the desired location.
At the beginning of the federal project in Dec/January, the proposed borrow site at Oceanside was found to contain mostly cobble and the project was paused. The borrow site for the Solana Beach Project (shown here) does have excellent sand deposits, and the current plan is to obtain a permit to utilize that site for San Clemente’s project, hopefully to re-start in March 2024.
Width of the replenished beach will decrease due to equilibration
In most dredging projects, as for San Clemente, sand is placed directly on the beach itself, immediately creating a wider beach. It should be noted that redistribution of this sand to a natural profile will result in a significant percentage of the placed sand moving offshore, resulting in a narrower beach under equilibrium.
Even though it may appear that the sand has “disappeared,” the sand is still providing an important function of creating an offshore sand bar and a gentler, shallower nearshore profile, both of which serve to attenuate wave energy and decrease coastal erosion.
Additionally, longshore drift will move sand downcoast, helping replenish and preserve additional beach segments.
Sand can also be placed nearshore, with mixed results
Alternatively, sand can be placed in the nearshore zone, generally by “bottom dumping” from the dredge ship. Initially, the dumped sand creates an offshore bar, which serves to attenuate wave energy and decrease coastal erosion. If strong onshore forces are operative in the area, the sand may also be moved onshore. This method may be less successful in areas with strong offshore transport directions.
For example, in northern Newport Beach, in 1992 a project was undertaken to place dredged material in the nearshore, just south of the Santa Ana River. About 1.25M CY of dredged material was placed offshore in water depths of 15-30 feet. Beach profiles over the next few years demonstrated that the sand moved onshore and the beach width locally increased by 100 feet. However, beaches to the south did not demonstrate a big impact.
Dredge spoils from Dana Point Harbor are regularly placed in the nearshore zone at Capistrano Beach. However, these materials are relatively fine-grained sand, and no benefit has been apparent from this program, although modest gains may have been achieved.
How have other local beach replenishments performed?
In northern Orange County, scheduled periodic nourishment at Surfside-Sunset Beach (undertaken by the U.S. Navy as mitigation for impacts from construction of the Seal Beach Naval Weapons Center) and nourishment with sand retention devices at Newport Beach have led to the placement of 18 million CY of sand between Anaheim Bay and Newport Harbor 1963-2002. The majority of this material has remained in the local sediment system and beach widths in this region have increased at an average rate exceeding four feet per year (Department Boating Waterways 2002).
No large-scale beach replenishment has been undertaken in south Orange County, except during the construction of Dana Point Harbor, completed in 1971, which placed 2 million CY of sand (from dredge spoils) in the nearshore. It is likely that this replenishment maintained our sandy beaches for the last 50 years.
In San Clemente, North Beach has had two opportunistic beach replenishment projects (with trucked-in sand). In 2005, 5,000 CY were placed, and in 2016 12,000 CY of sand were placed on the beach during the winter and lasted only a few weeks before it was washed away. Had the sand been pushed up into berms the outcome may have been more favorable.
San Diego beach replenishment projects have been successful
In 2001, San Diego County initiated its Regional Beach Sand Project (RBSP) with over two million CY of sand placed on 12 San Diego area beaches, encompassing six miles of coastline. The sand was mined from five offshore borrow sites. The project was completed again in 2012, at fewer beaches.
In the 2001 project, 160,000 m3 of beach fill was placed at Torrey Pines; seven months later a storm with 3-m significant wave heights eroded away most of the fill (Seymour et al, 2004). At this beach, the natural and nourished sand grain size were both fine-grained. The beach fills fared better at Imperial, Cardiff, and Solana, where the natural fine-grained sand was replenished with medium/coarse-grained sand.
At Imperial Beach, in 2012 project, the coarse-grained sand was placed subaerially in front of riprap. Movement created an "accretionary crown" and a depression between the crest and the rip-rap, where water accumulated, creating flooding in the homes behind the riprap. Sand spits formed and drifted southward in the winter and northward in the summer. One bar blocked off the mouth of the Tijuana estuary.. After one year, non-replenished beaches also grew in width by about 25', up to 1 km to the north and 2 km to the south of the initial placement. After four years, about 50% of the nourished sand remained (Ludka, Guza and O'Reilley 2018).