Drainage Physics for Mosquito Breeding Sites Near Schools: A Grading and Drain Specification That Keeps Standing Water Below the Aquatic Development Time, Priced per School

A design method that derives a grading and drainage specification for school compounds from the aquatic development time of malaria mosquitoes (7 days at 32 C, safety factor 1.5, design ceiling 4.67 days). Depression persistence is modelled by soil infiltration plus evaporation, with a seeded Monte Carlo rainfall-refill simulation of a 120-day season. SIMULATED, NOT MEASURED; nothing was built and no site, school or child participated. Ungraded, a 50 mm puddle on clay yields a 7-day standing spell in 98.1% of seasons; grading to a 10 mm residual (slopes 0.5-2.0%, drain spacing 50-20 m by soil class, ruts and borrow pits filled) reduces that probability to zero on sand, sandy loam, loam and clay loam at $1,124.40-$1,254.00 capital per 1 ha reference school, with 10-year totals $1,686.60-$1,881.00 against $3,536.00 for larviciding. Failure cases are printed: on clay infiltration alone fails for depressions deeper than 37.3 mm and the required piped drainage ($6,681.00 over ten years) loses to larviciding on price; a high water table collapses infiltration and leaves a 2.9% seasonal failure probability even when graded; a re-blocked unmaintained drain restores a 36.2% seasonal failure probability.