Clean by the Mound, Not by the Calendar: A Sediment Accumulation Rule for Rainwater Cisterns

Guidance on cleaning rainwater cisterns is usually a calendar interval — every year, every few years — offered without reference to how fast sediment actually accumulates in a particular tank. This paper computes accumulation from stated solids loads and shows that two corrections transform the answer. First, sediment does not spread evenly across the tank floor: the coarse fraction settles near the inlet, so the governing depth is the local depth over a deposition footprint taken as 20 percent of the floor, which grows five times faster than the whole-floor average. Second, loads differ by an order of magnitude between roof types. For a 100 square metre roof feeding a 10,000 L tank, the typical case — 60 g/m²/yr of solids, no first-flush diverter — builds local sediment at 3.57 mm per year and reaches a 50 mm cleaning trigger in 14.0 years, where a uniform-depth calculation would have said 70 years. An ageing shingle roof or heavy leaf fall at 200 g/m²/yr reaches the trigger in 4.2 years, while a clean metal roof with a diverter fitted does not reach it for 117.6 years — a 28-fold spread that no single calendar interval can serve. A maintained first-flush diverter multiplies the interval by 2.8 in every case. Cleaning should be scheduled from the roof's load class and the tank's inlet geometry, verified by a single depth measurement at the inlet, not from a universal interval. All results are simulated, not measured.