Saturday, July 11, 2009

Sand Budget: Common Knowledge Wrong?

Encinitas Sand 2009

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%)...

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.
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.

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.

13 comments:

  1. good read, sir. We appose ourselves so easily to the gentrification of our home that it's refreshing to find evidence counter to said pessimistic assumptions.

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  2. 90% of our sand comes from rivers, unless we're talking about Ponto (where it comes from the car lot of Pacific Station) or Moonlight Beach (where it's imported near and far from ash trays in train stations ).

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  3. Just spent 6 hours at Cardiff. The lagoon is tidal, the water clear, tons of fish at the rivermouth and lots and lots of kids playing on the massive sandbars. The thick kelp beds are keeping the surf glassy all day. (The parking lot is jammed btw.)

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  4. Just got back from Cardiff beach myself. Took a 3 mile walk, did a little swimming, and made sure I stayed out of the way of the surfers! Sand is good and the best part for me is it is in walking distance of my house. It doesn't get much better than that, IMHO. Enjoy everyone! If you don't have too much stuff to carry, the best parking is up the hill a bit. I am on Oxford and it only takes me about 5 minutes to walk to the Kook.

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  5. Wishing for cobbleJuly 12, 2009 8:46 PM

    Less sand means less people to use and abuse the beach and ocean.
    What about all the brake pad lining and tire tread wear that comes from the millions of cars that travel the freeways and roads and end up on the beaches and in the ocean here.
    What about the dog poop that most people don't pick up and the fertilizers and pesticides and who knows what else that ends up on the beach? There seems to be more plastic each year in the water also.
    These forms of pollution are just as important or more so than sand.

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  6. What about all the wax from the surfboards, Sunscreen worn by the surfers and paddle boarders and the plastic from the paddles. Surf wax, paddles, sunscreen, fiberglass (not natural) surfboards should be banned. Heck, the only surfing instrument should be made from natural wood.

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  7. "Knowledge" is spelled wrong in your title...

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  8. what a killer weekend! We sure live in a nice area!!!

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  9. miserable piece of shit who leaves lame comments on blogsJuly 13, 2009 10:27 AM

    What is the most environmental way to kill yourself?

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  10. Not many people know this, but the Core of Engineers are working on a plan to open up the San Elijo lagoon north of seaside reef. Apparently Surfrider is heavily involved.

    The plan involves a massive bridge out of Solana Beach that would come down somewhere around Las Olas.

    Surfrider is hoping it'll create a new Ponto and leave Cardiff Reef and Seaside alone, but with the lessened sand flow out of the current inlet, sand bar that makes south peak great will likely disappear.

    I think it's too risky, and that they should move the 101 back a quarter mile if the HAVE to do something.

    Anyone have any hard info on this?

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  11. 911,

    Yes we live in great area and we should appreciate it enough to get off our asses and ensure that our city doesn't get raped.

    Thanks for the blog JP.

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  12. Good point 1:00,

    Los Angeles pretty much has the same weather as San Diego.

    Tijuana has the same weather as San Diego.

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  13. Whaddaya mean? Even San Diego seldom has the same weather as San Diego.

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