N = y + R

y = N + R

R = N + y

R > (N + y)

air relief valve

reflux valve

pressure relief valve

sluice valve

5 litres/sec

50 litres/sec

500 litres/sec

1000 litres/see

more depth

less depth

more discharge

less discharge

suspended impurities

dissolved impurities

both suspended and dissolved impuri¬ties

none of the above