Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 78

From an operational point of view four major types of structures exist:

•

fixed (weirs and orifices);

•

on-off (shutter gates);

•

adjustable: stepwise (stop logs, modular distributors) or gradually
(undershot gates, movable weirs);

• automatic (automatic upstream and downstream water level control

structures).

From the sediment transport aspect, irrigation structures can be

divided into two categories:
• structures with sub-critical flow : examples are aqueducts, flumed

canal sections, super-passages, culverts, etc.; the design of structures
with sub-critical flow aims at a minimal head loss with smooth
transitions to minimize the entry and exit losses.

• structures with critical flow : the flow through these structures is either

critical or supercritical; examples are measuring structures, flow control
and division structures, drops, etc.; structures with critical flow have a
unique depth discharge relation when the flow at the control section is
critical or modular and the downstream water level will not influence the
upstream water level. If the downstream water influences the upstream
water level due to the sediment deposition, poor canal maintenance or
ponding up of the downstream canal reach, it turns into a drowned or
submerged flow.

Structures may have a free overflow or an undershot flow. Some free
overflow structures may be used to convey or regulate the water level
or measure the discharge. Structures with critical flow present a unique
head discharge relationship when the flow at the control section is critical
or modular; this is the case when the downstream water level does not
influence the upstream water level. The flow turns into a drowned or
submerged flow once the downstream water influences the upstream water
level due to the sediment deposition, poor canal maintenance or ponding
up of the downstream canal. Some important overflow structures are a
broad crested weir, sharp crested and short crested weirs and flumes.
Hydraulic references give more detailed information on these structures.
The head-discharge equation for a broad-crested weir (Figure 4.5) with
a rectangular cross-section and free flow conditions ( h 2 < 0 . 7 h 1 ) is (Bos,
1989):

C d B 2

3 g 2 3 H 3 / 2

Q = (4.25)

1

Sometimes it is difficult to estimate the total head ( H ) and the water
depth h 1 is used:

C d C v B 2

3 g 2 3 h 3 / 2

Q = (4.26)

1