A flood claim goes to your NFIP or private flood carrier, not to your homeowners insurer — they are separate policies and a homeowners claim will be denied for flood water. Report it the day you find it. NFIP requires a signed Proof of Loss, normally within 60 days of the loss, and the documentation you gather in the first hours is what it is built from: video of every affected room before anything is moved, photographs of the high-water line, a list of damaged contents with what you paid, and receipts for emergency mitigation.
Texas homeowners should purchase a separate NFIP flood policy through the National Flood Insurance Program, as Texas Department of Insurance rules require separate flood coverage — standard HO-3 policies do not include rising water from Joe Pool Lake or Mountain Creek overflows. Mitigation is not optional — every policy requires you to stop further damage, so drying starts before the adjuster arrives, and you keep the invoices.