Does a survey require an ethical approval?
The question of what sort of ethical approval is required is a complicated one, depending on the local laws, funding sources, university policies, and journal policies.  I don't think anyone can give a definitive answer without knowing these details.  For example, see these charts for an indication of the rules imposed by the U.S. Department of Health and Human Services.


  Since we have the forms of informed consent, aren't they sufficient?


No.  If ethical review consisted of nothing but saying "be sure to obtain informed consent", then it would be replaced by a simple rule requiring informed consent.  Even that is tricky (what exactly are the risks you need to inform participants of?), and there are other issues for a survey.  For example, are you following best practices to ensure anonymity of survey respondents?


  If none work out, is there a repository for unpublished work like ArXiv in epidemiology?


I don't know, but I can't really imagine the community would look kindly on using such a repository to avoid ethical review.


  The institute ethical committee denied a post hoc approval.


It makes sense for ethics committees to be reluctant to approve research after the fact.  One reason is that it puts a lot of pressure on them to approve research that's not perfect but can no longer be fixed; another reason is that if post hoc approval is an option, it creates an incentive for researchers to skip the initial approval in the hope of forcing it through the process later.