All About Beaches
There are predictable patterns of wave movement throughout San Clemente dictated by bathymetry, swell direction, and swell period. These patterns determine how wave-driven currents behave and where the sediment they carry will go.
Where the sediment rests determines where we have big, healthy beaches...and where we do not.
San Clemente is blessed with near-year-round Southern Hemisphere swells that gradually push sediment northward. The most pronounced upcoast drift occurs from sharply approaching, shorter-period tropical swells or pre-frontal south-wind-driven seas, and the drift is felt most acutely on the south end of town due to the offshore bathymetry at Cotton’s. The lefts at T-Street also consistently drive upcoast currents.
Down-coast drift is strongest during short-period NW wind-swell events, but there is also a general down-coast drift during long-period WNW swells that arrive primarily in winter. The bathymetry that shapes T-Street’s rights encourages down-coast drift as well.
Sand movement follows a predictable energy-related pattern, shifting offshore and back onshore. High-energy wave events strip sand from the beach and move it offshore, forming bars. During low-energy periods, small waves push the sand from the bars back onto the beach, making it wider.
Finally, during extreme wave events, sand may be taken offshore beyond the point of return, resulting in a permanent loss of sand from the system, as was the case with Hurricane Marie during the summer of 2014.
When waves crash against anything hard, an explosion of energy occurs. The bigger the waves, the greater the impact.
Look at the videos above. Where waves batter rocks, a beach cannot form. It is not until waves get small enough and the battering stops that a beach will reform — and even that is contingent on having sufficient sand in the system, something San Clemente has lacked in recent years.
Note where the beach is reforming — it is always where there is no wave impact on rocks.
The solution to maintaining a healthy coastline is making sure we always have enough sand in the system (hello, sand replenishment!) so a nice, healthy beach is maintained in front of the rocks, preventing impact.
— Charlie Fox
San Clemente is located within an “embayment” (i.e. a bay) — bound by Cotton’s point to the south, and Dana Point to the north. The two ends of the embayment form a “mostly” closed sand transport system, leading to the retention of sand within the system.
A 2024 study (link in bio) on longshore sand transport mechanisms shows how the system works (see slide 5). Sand essentially sloshes back and forth within the embayment under the sway of wave events.
The integrity of this “mostly” closed system is also born out by history. The last large influx of sand to the system was at the completion of Dana Point Harbor — over 50 years ago! And it is only recently that San Clemente’s beaches have fallen into serious trouble.
If recharged with enough sand on a recurring basis (hello sand replenishment!), the system would work once again.
Next up…how would structures like groins and breakwaters impact our system? Stay tuned for Part 4.
Jetties, groins, and breakwaters are manmade structures, typically made from large rocks and/or concrete, utilized for varying reasons.
A jetty extends into the water perpendicular to shore, usually at inlets and harbors. Its primary purpose is protecting the navigability of channels.
A groin is like a jetty, but usually shorter and built on stretches of beach, not at inlets or harbors. They are often built in a series of parallel structures with the goal of trapping sand. But while groins may trap sand on their updrift side, increased erosion on their downdrift side is well documented.
Breakwaters can be built parallel or perpendicular to shore, with their primary purpose to protect coastlines from powerful waves. While sand may settle and accumulate behind the breakwater, increased erosion is likely to occur downstream.
San Clemente, in our opinion, is not a good fit for any of these structures as sand mostly stays within San Clemente’s “embayment” and moves both north and south within it. Structures like groins are typically utilized to keep sand from leaving a system up/down coast, not where sand movement is bi-directional like in San Clemente.
Finally, hard structures such as these create both intended - and unintended - consequences due to the complexity of the nearshore zone. One example is the potential negative effect any structures would have on San Clemente’s world-famous surf breaks. Jetties, groins, and breakwaters have a place, just likely not in San Clemente any time soon.
The San Clemente watershed is made up by creeks and streams that originate from the Santa Ana Mountains and surrounding hills.
Rainfall and runoff in these mountainous and hilly areas erode the soil and rock, breaking them down into smaller particles, including sand. Creeks then transport this sand to the coast with finer sand particles more often lifted and carried within the water column while coarser sand particles roll, slide, or bounce along the riverbed.
As the creek approaches the coast, its velocity decreases, causing sand to settle. But it often takes a heavy rain for San Clemente’s creeks to break through their bay-mouth bars, thereby allowing sand to finally reach the shoreline.
The San Juan Creek is by far the main source of natural sand replenishment to San Clemente, with the smaller creeks dotting San Clemente contributing lesser amounts. But the amount of sand naturally transported to the coast has been greatly diminished by development across the watershed and the armoring of creeks, all of which prevent the breakdown of soil and rocks into sand. One recent study found that natural sediment loads in SoCal have been reduced by as much as 91%.
With the natural sand supply choked off, it should come as no surprise that San Clemente is now in a sand deficit - and in need of replenishment.