My recently installed wildlife pond with a flexible liner is filling rapidly
with rain and it can only be few weeks before it overtops. I want to
control where the water overflows rather than let it settle anywhere. So I
think I need a sluice. I say 'think' because none of the books I've seen
mention the need for a sluice in a pond which is not stream-fed. But what is
the best way to make a sluice in a flexible liner?

Some details of my pond: the edge of the pond slopes gently for about a
metre and is covered with pebbles. The liner rises 1" vertically all round
the perimeter and nestles among large pebbles - this allows me to fill the
pond right to the edge. In winter the water table reaches within about 4"
of the ground surface. Beyond one end of the pond the ground runs level for
about a metre then falls away.

I don't mind a temporary seasonal sluice.

many thanks, Davy