Design Criteria for Irrigation Canals - Sediment Transport in Irrigation Canals: A New Approach - page 76

• regulating structures: control structures and regulators maintain the

water level for different discharges: checks, regulating gates, control
notches and weirs, division works, lateral intake structures, proportional
distributors;

• measuring structures for an accurate measurement of the irrigation

discharges: a broad-crested weir, Parshall flume, Cipoletti weir,

Crump-de Gruyter orifice, baffle distributor, gated pipe offtake,

constant head orifice;

• protective structures: culverts, inverted siphons, overchutes, spillways,

waste ways, side drainage, sediment traps and settling basins;

• division structures at the head of (sub-)secondary canals;

• offtake structures or turnouts at the outlets to tertiary or end units:

tertiary offtakes, turn-outs, pipe offtakes;

• auxiliary structures: appurtenant canal structures, dikes, roads, opera-

tion facilities;

• miscellaneous structures: culverts, farm and other bridges, drainage

inlets, fish-control structures.
From an operational point of view four major types of structures will
be classified:
•

fixed (weirs and orifices);

• on-off (shutter gates);

• adjustable: stepwise (stop logs, modular distributors) or gradually

adaptable (undershot gates, movable weirs);

• automatic (upstream and downstream water level control structures).

4.4.2 Main hydraulic principles for irrigation structures
The type of flow over an irrigation structure can be either overflow
(weirs, drops, flumes) or undershot (orifices, gates).
Flow through irrigation structures can be a sub-critical flow (aqueduct,
flume, and culvert) or a critical flow (measuring structure, flow control
and division structure, drop). The design of a structure for sub-critical
flow aims at a minimal head loss with smooth transitions to minimize the
entry and exit losses. The flow through a structure with critical flow is
either critical or supercritical.
Many situations in hydraulics can be solved by applying two out of the
three main principles:
1. conservation of matter (continuity)
2. conservation of energy (all energy losses are known)
3. momentum principle (all external forces are known)

The continuity principle for a structure without any extra outflow or
inflow inside the structure results in the observation that the upstream
incoming flow is equal to the downstream outgoing flow:

Q = v 1 A 1 = v 2 A 2 (4.21)