Lake control at Te Anau

Sign by Lake Te Anau

Dealing with multiple, conflicting, objectives is part of operational research.  I have written earlier about the Noah and Joseph problem and the conflicting objectives there.

When we were on holiday, I saw this sign on the bank of Lake Te Anau, in southern New Zealand.  The text is worth recording, as it describes control rules and the conflicting objectives.

Lake Levels
A Government appointed body, The Guardians of Lakes Manapouri, Monowai and Te Anau" ensures that the lakes are managed within their natural levels.
A complex set of guidelines determines how long a lake can be held at a certain level.
It it is too high for too long shoreline vegetation may 'drown'.
If it is too low for too long beaches are prone to sand and gravel loss and slumping.
The rate at which the lake level is lowered is also controlled. but weather determines how fast it rises.
The higher the lake level peaks above this mark (202.7 metres above sea level) the longer it is before the lake is allowed that high again.
for example, if levels reach 202.7 for up to 125 days, recurrence is prevented for another 20 days but if levels go higher to 204.3 even for just 1 day recurrence is prevented for 100 days (this in the high operating range).
As the lake lowers below this mark (201.5) fewer days can be spent at each level (this in the low operating range)

notes at the side read 
Aquatic plants are protected from drying by the lake level guidelines
Vegetation above mean lake level is protected from drowning and shoreline erosion

It is notable that the range of allowable lake levels is only 1.2 metres (4 feet)

Just stop and think about the modelling behind these rules.  Engineering (for the water release and control) meets biology (how long does it take to drown a plant, how long for it to dry out) meets geology (gravel loss) meets hydrology (modelling the lake levels) meets meteorology (rainfall) meets mathematics and operational research.  It is multidisciplinary, in the way that the best O.R. should be!  Well done!


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