those white pipes were used by MWD to fill temporary water tanks filled by water trucks to replace ground water lost during construction of the 10 ft diam tunnel bored pipeline. MWD helicoptered them out when the const was complete. Kevin Williams gathered them up in hoping they could be used for future projects.
The old old system used black 1.5 in diam pipe that got burned in the 03 fire. I think there was like a couple thousand feet of it too.