Storing Water in the Valley Fill Itself: Storage and Seepage-Bypass Bounds for a Subsurface Dam Across a Buried Valley

A subsurface dam — an underground wall across a buried valley, keyed into bedrock, invisible at the surface — stores water in the alluvium itself. For a valley 80 m wide with 600 m of upstream fill 8.0 m deep, porosity 0.35 and specific yield 0.18, this paper computes a bulk fill volume of 384,000 m³, a pore volume of 134,400 m³, and a drainable storage of 69,120 m³, against a demand of 20.0 m³/day for 800 people at 25 litres per person per day. The seepage-bypass bound, from Darcy flow under and around the wall as a function of key depth, gives a 180-day loss of 1.1% of drainable storage with no key at all, falling to about 0.1% at full key. The honest conclusion is two-sided: storage and seepage both pass easily at this scale, so the design's real risks lie elsewhere — in abstraction, water quality, and construction — and money spent on an ever-deeper key beyond about half depth buys almost nothing. SIMULATED, NOT MEASURED.