Does a survey require an ethical approval? We did a student project - which included a questionnaire and anthropometric measurements. Since it was a student project, and the topic was decided in a short time, we did not take an ethical approval from our institute committee. Our institute doesn't have a  provision for fast track approvals for student projects.

But we took a  written informed consent for each participant. Since we got interesting results (unexpexted) we thought of publishing our study. The institute ethical committe denied a post hoc approval. 

Do surveys actually require an ethical approval from a journal point of a view?

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

If none work out, is there a repository for unpublished work like ArXiv  in epidemiology?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  If none work out, is there a repository for unpublished work like ArXiv in epidemiology?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