const COAST_SHAPE: { 5 fields }
The coast: how wide the shelf and the plain are, and how flat each is.
Widths are fractions of the hypsometry band's share of the budget, which is
the scale the sea datum itself is set on. shelfFlat and plainFlat are
the slope at the waterline as a fraction of the landform's own — a shelf a
fifth as steep as the ground that made it is a shallow sea a long way out,
which is what a continental shelf is, and the plain behind the beach is
flatter than the hills behind that.