OC: 26 sections flow-direction flipped by v17c pipeline (SWOT-validated)

[jameshgrn]

Summary

The v17c pipeline re-run for OC (2026-03-13) flipped flow direction on 26 sections based on SWOT slope validation. This changed 180 topology links in reach_topology and required dist_out recomputation for 116 affected reaches.

Details

What changed

• 26 sections flipped (HIGH and MEDIUM confidence from SWOT junction slope validation)
• 180 topology links had direction swapped (up↔down) in reach_topology
• 116 reaches had dist_out recomputed via iterative propagation from boundary reaches (11 iterations to convergence)
• 2,881 nodes had dist_out updated to match parent reach
• 636 sections flagged for manual review (LOW confidence — not auto-flipped)

Backup

Pre-flip topology saved to reach_topology_backup_OC_3811541ae0d6

Validation after fix

• T001 dist_out monotonicity: 0 violations (was 66 after flip, before dist_out fix)
• V001 hydro_dist_out monotonicity: 0 violations (was 78 before pipeline re-run)
• V010 main_connection_integrity: 0 violations (was 86+66 before pipeline re-run)
• F006 facc_junction_conservation: 5 violations (down from 480 in v17b)

Sections flipped

22, 230, 303, 413, 448, 515, 1042, 1116, 1209, 1436, 1671, 1911, 1978, 2086, 2207, 2215, 2421, 2425, 2788, 2798, 2801, 2830, 2831, 2860, 2905, 2941

Risk

• dist_out is now recomputed for these sections and may diverge from v17b values
• The 636 manual-review sections were NOT flipped — they may contain additional direction errors
• Node dist_out was linearly interpolated, not recomputed from first principles

Follow-up

• Spot-check flipped sections in QGIS to confirm SWOT direction matches satellite imagery
• Consider running flow verification module (Flow verification module: automated reversed-section detection #198) across all regions for consistency

[jameshgrn]

OC topology flips retained and verified across 0.0.2 and 0.0.3 releases. 119 OC flow-corrected reaches in pipeline.