The Reservoir Is the Sand: Extractable-Storage Accounting, Evaporation Suppression and Worked Sizing for a Sand Dam on a Seasonal River

Sand dams are sized by rule of thumb, but the reservoir is the sand body itself. With porosity 0.35-0.40 and specific yield 0.20-0.30, a village of 400 people at 20 L/day through a 180-day dry season needs 1,440 m3 of extractable water, requiring a corrected sand body 147.4 m long, 20 m wide and 2.0 m in mean depth (5,895.8 m3). Vapour diffusion through a 0.30 m dry sand crust loses 34.0 m3 over the season against 2,907.5 m3 for open water over the same plan area, a saving of 2,873.6 m3 (359.2 days of supply). Stokes settling shows staged 0.30 m lifts keep only grains coarser than 57.8 micrometres, while a full-height wall traps grains down to 33.3 micrometres. One filling stores 0.88-8.84% of the seasonal flow scenarios examined. All results are simulated, not measured, and reproducible from the deposited compute.py.