Your svn merge syntax is wrong.
You want to checkout a working copy of trunk and then use the svn merge --reintegrate option:
$ pwd
/home/user/project-trunk
$ svn update # (make sure the working copy is up to date)
At revision <N>.
$ svn merge --reintegrate ^/project/branches/branch_1
--- Merging differences between repository URLs into '.':
U foo.c
U bar.c
U .
$ # build, test, verify, ...
$ svn commit -m "Merge branch_1 back into trunk!"
Sending .
Sending foo.c
Sending bar.c
Transmitting file data ..
Committed revision <N+1>.
See the SVN book chapter on merging for more details.
Note that at the time it was written, this was the right answer (and was accepted), but things have moved on. See the answer of topek, and http://subversion.apache.org/docs/release-notes/1.8.html#auto-reintegrate