Where does our river-sourced sand come from?
From the Moffatt & Nichol (1990) Sediment Budget Report to the USACOE:
Major rivers discharge over 90 percent of the coarse fluvial sediments [ie sand] that reaches the coast. Small groups of coastal streams discharge the remainder mostly through ravines in the coastal terraces. UCACOE considers San Juan Creek the largest contributor of foothill and mountain-source sediment. In the Oceanside Littoral Cell, San Juan Creek yields an estimated 38% of the total average annual discharge of 82,000yd3/yr, followed, in order, by the San Luis Rey River (22%), Santa Maragrita River (12%), and San Mateo Creek (9%)...Most of our river sand comes from north of Carlsbad. Below is a graphic to show you the relative contributions of different river basins. Estimates of yields in several studies have San Elijo and Batiquitos providing a river discharge of zero to our beaches.
Over 95% of the coarse sediment discharged by rivers reaches the coast north of Oceanside. Before control structures reduced flows in the major rivers the average annual coarse sediment discharge to the coast was about 134,000yr3/yr.
A lot of people think that Batiquitos and San Elijo would be major continuous sources of sand if we would just remove the highways and rail road from the middle of the lagoons. They conclude that we would have expansive sandy beaches if it weren't for those infrastructure obstructions.
That conclusion is probably false for several reasons. First, there isn't much sediment moving down those drainages. There is over 20 times as much sediment flowing down the rivers north of Carlsbad than is coming into San Elijo. Second, the lagoons act as a big sediment trap. The capacity of the rivers to move sediment is very sensitive to the velocity of the moving water, which is a function of the slope. The lagoons are very flat and wide and velocities low. Coarse material does not get pushed through the lagoon very easily. Below is a great visual that shows what has been happening in our lagoons over geologic time scales (click to enlarge). Notice how the lagoons act to capture the sand.
One of the things that makes me smile about the above graphic is that it includes cobblestones as part of the wild system.
Maintenance of river-sourced sand on Encinitas beaches depends on resource utilization, infrastructure developments, and land use decisions in cities north and north-east of Encinitas.
As our population grows, the strains of balancing the use of our public resources will aggravate many of our current local issues. More reservoir dams on the northern rivers results in less sand. More transportation infrastructure (trains/roads) and more concrete and steel buildings means greater motivation to mine sand--there is already a stress on our industrial sand sources. These are issues that should be resolved before we commit to upzoning the region and a discussion of sand resources is either short-sighted or incomplete if it does not include a sophisticated integration of those larger decisions.