Water Conveyance

Question 1

Describe the important characteristics of an irrigation conveyance canal

Answer 1

• The main objective of a canal is to convey a required discharge in the most economical, efficient and sustainable way from a source to the fields.
• Irrigation canals allow water to flow through regular man made cross sections by gravity, losses due to friction
  ​​- A rectangular cross-section is only possible with concrete and gabions. With any other material the risks of collapse are too large.
• When constructing canals in earth we always need to respect a minimum side slope, which can never be exceeded.
• Discharge calculated using Mannings (see question below)
• Deviations from the design discharge are possible. Due to rain events or because of errors in operation the design discharge could be exceeded -> install freeboard and side-spillways or safety syphons to avoid overtopping of the canal.
• To avoid erosion there is a maximum velocity that depends on canal texture (bed material and sediment load). Max velocity ranges from 0.5 m/s for sand filled with clear water to 1.5 m/s for clay with sediment in the water. Lined canals easily flow at 2 m/s.
• Sedimentation also needs to be avoided by maintaining velocity above a minimum. It is worth noting that seepage losses through the canal bed are a function of the wetted perimeter of the canal. Low velocities mean relatively more seepage losses. Therefore, not only a maximum (preventing erosion) but also a minimum velocity of 0.5 m/s is recommended.
  > Reduce seepage losses -> reduce wet P and water depth
  > Reduce wet P by increasing velocity (increasing slope)
• Concrete and other materials allow much higher velocities and therefore smaller cross-sections. Lining also reduces the seepage/infiltration.
• Additional safety precautions are needed to avoid accidental drowning of humans and animals. Animals drowning in water systems can also cause infestation of the irrigation water with pathogens.
• Lined canals
  > Less erosion
  > Less seepage
  > Higher velocity possible
  > Lower sedimentation
  > Reduced dimensions possible
  > No weed growth = less maintenance
  > Less permeable especially with reinforced concrete lining which is more expensive however soil can be compacted and a layer of stabilised sand on top as a cheaper alternative
• Earth canals
  > Short construction times
  > Small distribution canals
  > Weeds grow
• Primary and secondary canals are always flowing full so better to be lined whereas earth canals more suitable for tertiary and field canals which operate less frequently
• Always measure the conveyance efficiency in a canal, that is the proportion of the water starting from the resource that reaches the border of the field. These measurements should be repeated every few years or measured constantly at important points to ensure water isn’t lost through seepage or in large amounts by infiltration in earth canals
  > Conveyance = water at field border/water diverted (this is a classical efficiency, does not consider reuse)
• Best way to conduct measurements is using hydraulic structures where relationships between water at an upstream level and discharge are known (they translate water level to discharges)
  > Weirs which are cheap and easy to build but can trap sediment but can trap sediments and require a large head difference
  > Flumes which are good for long term measurement, have good sediment passage and require a lower head loss
  > Orifices which usually serve as an intake structure, are not sensitive and disrupt channel flow
  > Most sensitive to flow changes: V-notch weir > Flume X > Cipoletti Weir > Orifice
  > Important to account for backwater effects (leads to submerged structure)
  » Avoid by having lower discharge and not having other structure so soon downstream of first structure
  > Calibrate structures regularly with velocity measurement in the cross section
• Important that in a distribution, equitable distribution of water is allowed

Question 2

Describe the important characteristics of pressurized pipe conveyance systems

Answer 2

• Rain guns (high P), sprinkler (medium P) and drip irrigation (lowest P) requires pressurised water
  > Drip irrigation requires 100-200 kPa or 1-2 bar or 11 to 21m of water column
  > Small sprinkles requires 200-350 kPa or 21 to 35m of water column
  > Rain guns requires 350-800 kPa or 35 to 80m of water column
• Advised that losses in pressurised systems from source to drip/sprinkler is no more than 30%.
• Pumping needed unless there is sufficient topographic differences (such as Andes). If we don’t pump, reservoir needs to be at least 1.3x higher than water column in field
• Advantage of pressurised system:
  > less leaks, especially with good fittings and pipes. Also leaks are detected quickly
  > Flow can also be monitored much more easily (pipe flow is easier to monitor and systems are normally smaller)
• Con: they require an energy supply. Energy consumption can be minimised by lowering water pressure
• Surge tank should also be employed to minimise water hammer due to high pressures

Question 3

Explain the technical aspects of water distribution in an irrigation scheme.

Answer 3

• Equity is the biggest challenge - often not possible for all farmers to receive the full water supply simultaneously (supply is not enough)
• Ideal system is on-demand where farmer get how much water needed at the time of need - closer water delivery is to irrigation schedule, the better. However this is complex to employ as a sophisticated system of monitoring, communication and coordination is required as well as administration and management. At large scales there will be issues with over-abstraction and conveyance system capacity
• Rotational supply systems allow for water to be supplied at certain times with a fixed volume. This means timed access for water under a strict irrigation schedule that needs to be followed. As the system works with the opening and closing of a supply system, offtake structures are required. There is some operational flexibility - adjustments can be made especially in events e.g. droughts
• Proportional supply systems utilise primary to secondary to tertiary canals at fixed proportions under constant water supply. This means farmers receive water at a fixed proportions and changes in flow rate upstream have proportional effects downstream meaning no operational flexibility, especially difficult in periods of drought as flow rates vary with no remediation even when water is needed. However, this system is transparent and gives the farmer more flexibility for irrigation scheduling

Question 4

Explain what the importance is of the power in the discharge level relation of a hydraulic structure

Answer 4

• Using standardized structures, we have a unique, well-known relation between the water upstream point and the discharge. Weirs, flumes orifices have general equations that relates the upstream energy head, H1, to the discharge passing through the structure. Upstream energy head is the water level and kinetic energy.
• Depending on the type of structure, it has a different discharge coefficient Cd. K1 depends on dimensions - for an orifice, K = area of throughflow and for rectangular weir, K = width.
• Q=CdK1H1^u
• Very important for the sensitivity is the u. For an orifice, we have a “u” of 0.5, so we can call it the square root of the water level. And this has a very low sensitivity to changes in water level and is therefore not very sensitive or suitable as a measurement structure. The highest sensitivity is obtained by a V-notch, which gives us a ‘u’ of 2.5. The Cipoletti weir is an almost rectangular weir and has been especially designed to have exactly a 1.5 under all circumstances. Most flumes operate in trapezoidal canals, so quite often we have a power of 1.75; or something around that.