I don't need to know any specific law to stand by what I said. I agree that rouvis needs to contact his local government to determine if his chalk marks will violate any law or ordinance. In fact, I specifically suggested that. In addition, I never said, nor did I imply, that what rouvis intends to do might not well be illegal. What I said was that rouvis is not likely to find a law that specifically addresses chalk marks on public sidewalks. In point of fact, what I implied was that what he is thinking about doing is in fact probably illegal under some statute (such as prohibitions against "graffiti" or "vandalism" as you suggest), but that like many laws (eg. "jaywalking") any illegality would only be enforced (as you and I both suggested) based on what was actually done, where it was done, who did it, why they did it, and what effect it had on other people, etcetera.
BTW, rouvis made a point of stating that the chalk marks would not be "permanent". This might mitigate someone's umbrage over the chalk marks, but if someone wants to find a cause of action against rouvis for the chalk marks (eg. violation of some prohibition against "graffiti" or "vandalism"), then it probably won't matter that rouvis characterized the chalk marks as not permanent. For instance, if the city has to hire someone to deal with the chalk marks and their consequences (city cleaners to clean off the chalk marks, or police to direct pedestrians or traffic), then the city may try to find a way to recover those expenses from rouvis.
The bottom line is that public property is not yours, mine, or theirs to unilaterally "modify" as we see fit, even if it is not a "permanent" modification. What if rouvis thinks that it would be a cool art project to toilet paper the trees in the public square. Gee, it's not permanent and it didn't hurt anyone, whatever. My bet is that someone would find a way to make rouvis pay to have it cleaned up.
Again, at this point in the discussion this seems to me like a common sense question more than a legal question.