Adjustable orifice semi-module design

The Crump adjustable orifice semi-module (AOSM) has a rectangular orifice whose height $Y$ can be changed by raising or lowering a roof block, while a flume d/s makes the outlet semi-modular.

Data

$q = 0.22$ m³/s, working head $H = 0.7$ m, FSL of distributary $= 101.40$ m, bed $= 100.00$ m (depth $D = 1.40$ m). Discharge coefficient of the orifice $C_d = 0.7$ (assumed, as is usual for the AOSM).

1. Size of the orifice

$$q = C_d\,B\,Y\,\sqrt{2gH}$$

Adopt orifice width $B = 0.50$ m. Then

$$\begin{aligned} Y &= \frac{q}{C_d\,B\sqrt{2gH}} = \frac{0.22}{0.7\times0.5\times\sqrt{2\times9.81\times0.7}} \\ &= \frac{0.22}{0.7\times0.5\times3.706} = 0.170\ \text{m} \end{aligned}$$

Adopt $Y = 0.17$ m. Check: $q = 0.7\times0.5\times0.17\times3.706 = 0.221$ m³/s.

2. Levels

• Underside of the roof block $= FSL - H = 101.40 - 0.70 = 100.70$ m.
• Sill of the orifice $= 100.70 - 0.17 = 100.53$ m, which is 0.53 m above the distributary bed (this lets the outlet draw bed silt in proportion).
• The roof block is adjustable by $\pm$ some centimetres, which changes $Y$ and the discharge.

3. Other dimensions

• Bell-mouthed entry and a parallel throat about $1.5Y$ long (0.26 m).
• Downstream expanding flume (1 in 5 to 1 in 6 flare) discharging into the water course; a hydraulic jump forms in the flume, so the discharge is independent of the water course level.
• The jet should not be submerged: the minimum modular head is $H_{min} \approx 0.1$ to $0.15$ m below the roof block, so the outlet is modular for water course levels below the sill.

Answer: orifice $B \times Y = 0.50\times0.17$ m, sill at 100.53 m, roof block at 100.70 m.

 FSL 101.40 ~~~~~~~~~~~~~~~|
 roof block 100.70 ______|_______
                        <- Y ->  \____flume___ water course
 sill 100.53 ____________________\_________
 bed 100.00 _________________________________