There is a relatively simple way to do this, although it involves a mod (and admittedly, I wrote it). If you go to the Saints Row Mods forum there is a simple mod that I created for precisely this reason; it checks on start up whether or not you've completed the full storyline and if you have, it drops the drawbridges, restores the statue and cleans up the zombies.
There is no achievement at the end of the game to lower the drawbridges despite earlier posts. It just doesn't happen. This is for several reasons as it turns out; the first is that many of the drawbridges make up the stunt jumps that the player can find. These are not attached to any challenge or the like, so unless you're a 'collector' there's no benefit to finding these. Also, one of the final survival (dial in) missions is a zombie cleanup mission, and just in case you haven't done it by the end of the story they leave the zombies in Arapice Island, quarantined by the drawbridges.
If (like me) you just want the drawbridges down and don't like zombies (or some variation of this combination) take a look at the mod, read carefully what happens (you have to finish 3 Way both times for instance) and then install and use at your own risk of course.
Best suggestion is to hang around and wait for the zombie survival mission to come through before starting 3 way so that you can close out your area at 100% complete, but all other challenges that are in the Saints Book section can be completed through alternate sources.
I should also note that there is a much more comprehensive mod (Things to do in Steelport) that allows you to toggle drawbridges and zombies as you desire, as well as does a large number of other things. If however you just want a lightweight mod that covers this specific function, then follow the link above.