Design Blueprint: Low-Water Crossing Flood Witness — Upstream Sensor, Crossing Beacon and the Barrier Decision

DESIGN BLUEPRINT — SIMULATED, NOT MEASURED — NOT BUILT. A buildable blueprint derived from the source paper 'A Flood Witness for Low-Water Road Crossings' (DOI 10.5281/zenodo.24109005). It specifies an upstream pressure-transducer sensing node, a radio link and a solar-powered warning beacon at the crossing, with a base cost of US$1,340 per crossing, sensor placement maximising warning per dollar (15.28 minutes on an 8.0 km2 small steep catchment, 40.58 minutes on a 35.0 km2 medium catchment, 68.93 minutes on a 120.0 km2 larger gentle catchment), the full trigger and false-closure trade-off (56.31 percent false alarms at the 50 percent reference trigger, 22.06 percent at 80 percent), assembly, siting, commissioning and maintenance procedures, and the failure regime printed at full size: on the smallest steep catchments (about 0.5 km2, slope about 0.08) the best warning is only 1.67 minutes, and a physical barrier, not a beacon, is the answer. All numbers are taken from the source paper or computed in the accompanying compute.py.