Types of outlets and flexibility

Types of canal outlets (4 marks)

An outlet is a structure that feeds the water course from a distributary. By dependency of its discharge on water levels, it is:

Proportionality (2 marks)

An outlet is proportional if its discharge changes in the same proportion as the discharge in the distributary, so every outlet takes its fair share of any supply. This needs the flexibility to be one.

Flexibility (2 marks)

Flexibility $F$ is the ratio of the rate of change of outlet discharge $q$ to the rate of change of distributary discharge $Q$:

$$F = \frac{dq/q}{dQ/Q}$$

With $q = CH^{m}$ for the outlet (head $H$) and $Q = KD^{n}$ for the distributary (depth $D$), and $dH = dD$, this gives $F = \dfrac{m}{n}\cdot\dfrac{D}{H}$.

• $F = 1$: proportional outlet (shares any change in supply in the same ratio).
• $F > 1$: hyper-proportional (takes more than its share when supply rises).
• $F < 1$: sub-proportional (takes less than its share when supply rises).

Example: for a wide distributary $n \approx 5/3$. A free pipe outlet ($m = 1/2$) at bed level ($H \approx D$) has $F \approx 0.3$ (sub-proportional); an open flume ($m = 3/2$) is proportional when its crest is set $0.9D$ below the water surface; a rigid module has $F = 0$.