Translation:By chance the dirty poop is on the bridge.
Yes, "sordidum" seems a rather unnecessary adjunct to "stercus," but it's useful for emphasizing that stercus is a (3rd decl.) neuter noun. (corpus meum, my body; magnum opus, a great work; would serve the same function)
I guess "sordidum" is a fitting 'standing epithet,' as it were, for stercus!