In order to include the possibility of submerged flow conditions at the offtakes, three types of offtake downstream conditions (i.e., at the head of the secondary canal) can be modeled:

• a constant downstream water surface elevation

• a downstream water surface elevation Z2 that varies with the water surface elevation upstream of a free-flow weir:

Q(Z2) = µL (Z2-ZD)3/2 [25]

• a downstream water surface elevation that follows a rating curve of the type:

Q(Z2) = Qo( )n [26]

Figure 21