Monday, February 1, 2010

The importance of coastal bluffs in supplying San Diego area beaches with sand


Figure 10. Estimated percentage of beach-sediment contributions to the Oceanside Littoral Cell for the study area between April 1998 and April 2004 (a statistically dry period). (from Adam P. Young, Scott A. Ashford (2006) Application of Airborne LIDAR for Seacliff Volumetric Change and Beach-Sediment Budget Contributions. Journal of Coastal Research: Vol. 22, No. 2, pp. 307-318.)

I've been thinking a bunch about this topic over the past few weeks as a long string of larger than normal surf have coincided with some very high tides. Easily every beach I've seen in the past two weeks has shown signs of erosion. Also see Andrea's posts below which do a great job of documenting the problems in Oceanside. The addition of the heavy rains have complicated things even more, and where they are not restrained by seawalls, beach cliff erosion is very active. If you've been down the paved trail to Black's Beach, you can see this pretty clearly.

In 2005, UCSD grad student Adam Young and his advisor Scott Ashford published "Application of Airborne LIDAR for Seacliff Volumetric Change and Beach-Sediment Budget Contributions" in Journal of Coastal Research. The important result from this work is summed up in the figure above. Classical thinking had been that rivers provided up to 90% of beach sand in the Oceanside littoral cell. Young and Ashford's work found that seacliff (or bluff) erosion contributed 67% to beach sediment in the Oceanside cell.

The reception of this work was varied. Many who fight against seawalls, saw this as a concrete (pun intended) example of the detrimental effects of seawalls, which are blocking sediment into our littoral cells. Obviously, the seawall advocates were not so pleased and many were quick to dismiss the work.
"We think it's pure hogwash," countered Paul Santina, president of the Beach and Bluff Conservancy. (from Study: Bluffs contribute most of the sand on local beaches, By: Paul Sisson North County Times, Oct 16, 2005)
Most of the scientific criticism about Young and Ashford's study was based on the short period of time that their analysis included. They looked at a six year period of time, which also was during a dry period. Six years in the geological sense is short, but their methods are accepted as valid by many in the coastal research community.

To help put this academic research into a real life example, head down to Black's Beach or any of the beaches in front of non-armored bluffs. You're likely to see patches of large amount of sediment eroding off the bluffs (bluff failures), which will soon make it's way into our littoral cell.

Here are some additional links regarding Adam Young's work:

3 comments:

  1. In the context of CEQA, the City of Solan Beach reviewed Young's work. Below is info pasted from the City's website.

    Analysis of Beach Sand Contributions from Coastal Bluffs at Solana Beach - Scientists at Coastal Environments have recently reviewed existing reports regarding sand contributions from Coastal bluffs. Please click the link below to view the July 2006 Drs. Flick and Elwany report.

    Flick and Elwany Report

    I also would urge a review of the following paper by Grandy et. al.

    Proceedings of Coastal Zone 07, Portland, Oregon, July 22 to 26, 2007, “VARIABILITY OF SEDIMENT SUPPLY TO THE OCEANSIDE LITTORAL CELL”, Carla Chenault Grandy, Gary B. Griggs, University of California, Santa Cruz, Earth and Planetary Science Department and Institute of Marine Sciences

    In the paper they find that sediment from nourishment actually exceeded natural sand input from either cliffs or rivers so there still is erosion even with more than adequate sand supply.

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  2. Thanks for the comment Jim. See, people read blogs!

    Also, thanks for the links to the other studies. They are important to consider, and have relevance in understanding this complex topic.

    In the conclusion of Flick and Elwany, they state:
    "Based on the results presented in Young and Ashford (2006), it is evident that the amount of sand found to have been eroded from the cliffs at Solana Beach during their study period is actually smaller than the average amount considered in the DFMEIR (AMEC 2002b)."

    Could this be due to the amount of seawalls installed between 2002 and 2006? How can I find that info?

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  3. Tom,

    One proabably an accurate approach would be to use the website:

    California Coastal Record Project Time Comparison

    and look from Seaside down to the white picket fence before Rivermouth. As for people reading the blog, it would be nice if someone from the chapter would link it to Facebook and the Chapter homepage.

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