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... Tiberi subvecta[v]issent, onustae rudere decurrerent, aedificiaque ipsa certa sui...

utique naves...decurrerent: the ships which had conveyed ("subvectavissent") grain upstream on the Tiber to Rome should sail ("decurrerent") downstream to Ostia and the sea laden with rubbish/debis," a substantive/noun clause." Note the imperfect subjunctive following the pluperfect, "subvectavissent."