I don't know the particulars of lake P, but from the photos of the bridges damaged that I have seen, it looks likr nearly ALL of the fallen bridge sections fell at odd angles in to the lake, and are still there...
could these have a significant effect on outflow?
No because parts of the bridges are intact, and if there were a damn effect, you would see an overflow over the sunken sections. You did not see that in the pics. One fallen brige over two miles does not a damn make, plus the swamp area between the lake and the gulf is barely above sea level itself, and riddled with channels to the gulf. Still it would be great to see an aerial photo of the exit routes.
I dunno, but some of those slabs of I-10 would make dandy temporary additions to the levees...if we only had Paul Bunyan, Babe the Blue Ox, and the Jolly Green Giant to wade through the flotsam and jetsam and carry them over.
Full Disclosure: Anyone notice how much the Gulf Coast resembles Isengard after the Ents got done with it?