The last three times we have been at the Jettys - we have seen two people loose their anchor because they couldn't get the anchor off the bottom. Dewey found this anchor rigging in a magazine.
"To easily turn a standard Danforth anchor into a breakaway Danforth, drill a hole in the anchor's base plate. (The size of the hole will vary with anchor and chain size.) Remove the anchor chain from the top of the anchor shaft and reattach it to the hole you just drilled. This may require a new shackle.
Once the chain is attached to the new location, lay it along side the anchor shaft. Using two heavy duty electrical ties wraps, attach the chain link that lies closest to the top of the shaft to the shaft. If the anchor hands up in a wreck or rock pile, the tri wraps will break under pressure when retrieved and the forks of the anchor will easily pull away from the snag. Be sure your anchor has ample chain. Chain length and anchor rode are critical to successful anchoring. Also, be sure to have extra heavy duty tie wraps on board to re-tie if they break." Sports Fishing Magazine - June 2004 - page 91
Here is another link to another article about the Breakaway Danforth Anchor.