PLAN THE YEAR
A five-year flood, more or less
Design peak 700 m³/s. Forecast without works is in the panel below.
Raise a berm between the channel and the town.
ALDERWAKE
A river is not a pipe. Every year it wants to drop its load somewhere, and you decide where. Put a levee up and the town stays dry while the gravel bar starves; cut the berm and the flood takes the field but rebuilds the bar. Every year the channel walks somewhere new, so this year's work is next year's problem.
- KEEP THE TOWN DRYMill Row sits 2.9 m above datum. Watch the gauge, not the rain.
- KEEP THE SALMON INThey need a shallow fast riffle over clean gravel, in August, every year.
- LEAVE A FIELDThe farmer wants silt, not scour, and not another metre of water over the beans.
The gauge came to 2.84 m.
The river did what the model said it would.
Something goes.
The peak is bigger than the works. The county has money for one of these and the mill has already been over twice. Choose which one you are giving up, then build for the other two.
The water stands still.
Nothing moves while you are here. The reach, the works and the year counter are where you left them.
How the reach is worked.
- CLICK THE GROUND
- Puts the selected work where you clicked. The section cursor follows, and the cross-section instrument shows you exactly what you are standing on.
- 1 – 5
- Levee, weir, gravel pit, reconnection, remove. The slider under the tools sets the crest, sill, rate or cut; the second slider sets the length.
- SPACE
- Releases the year. Hold it while the flood is running to push the river through faster.
- ARROWS / DRAG
- Turn the view. [ and ] walk the section up and down the reach.
- ESC · M · H · R
- Pause, mute, these notes, start over.
The model is a real one, not a dice roll. Water surface comes from a backwater calculation on the whole section; the bed changes by the Exner equation from the bedload the flow can carry, with outer banks cut on the fast side of a bend and the material dropped on the point bar. That is why the channel walks, and why a levee built in year 2 is standing in a different river in year 5.