Note that HEC-RAS uses the Federal Highway Administration (FHWA) culvert nomographs for computing flow through a culvert. Therefore, modeling flow through a 1300 ft long culvert would stretching the model limits by quite a bit.
I would recommend analyzing the culvert with something like SWMM.
Sincerely,
Chris E. Maeder, M.S.
Senior Technical Engineer
chris.maeder@bossintl.com