In 2011, almost all of the anchors used were 14 pound anchors (TODO - verify that!) that came with the houseboats. The came attached to 50' of line (TODO - verify that!).
As we can see from the info above a 14 pound anchor is only strong enough to hold a 31' boat in a stiff breeze while they were being used to hold 40' houseboats.
In addition with 50' of line, assuming 20' depth, the anchors only had about 40% of their holding power (2.5:1 scope). As such, they didn't hold at all. After we tied two ropes together, we had closer to 5:1 scope and the anchors mostly held.
The picture below shows the approx. strategy used for anchoring:
Even with all of the anchors, the city broke free once and floated downstream.
It took a very long time to assemble the city and to take it apart. Once boats were in place very few of them moved until the end of the event.