Canals require geotechnical engineering to last for decades with minimal maintenance, but irrigation canals in particular tend to be built informally or with at least under-engineering. When the wrong liner materials are used or past flood records, soil stability, and various forces that impact wall strength are not considered, there is a great risk that installations may fail soon after completion. A single mistake, like filling the canal before a concrete liner has completely set, can result in massive failure with property damage and high repair costs. Taking extra care during the design process for canals results in long-lasting structures that require less maintenance.
Determining Required Water Supply
Water delivered by a canal is generally measured in the form of cumecs, which is the flow of a cubic meter of water in one second. Minimal flow amounts, for irrigation, is determined by the open trench, sprinkler, sprayer, or drip tape systems you choose for delivering the water. Liner materials, or a lack thereof, can reduce the total cumecs a canal can deliver. Unlined canals are at risk of collapse due to erosion if total flow recommendations are exceeded, while properly lined canals can handle overloading for at least short periods during flooding periods. Determine the flow output needed from each canal and pick a liner based on its ability to support that flow without damage.
Testing the Soil
Soil conditions also greatly determine the size and total capacity of any new canal. Foregoing proper soil testing and engineering calculations can result in a canal that collapses, leaks, or supplies far less water than expected due to silt washing in from surrounding areas. Even the slope, average soil temperature and moisture levels can affect the stability of a canal. Hiring a separate geotechnical engineer is a good idea even for relatively minor storm water and irrigation designs. Many farmers and landowners designing their own canals assume that the light weight of the sloped soil and geomembrane or thin concrete liner materials won’t have much effect on the soil, but this isn’t true. Clay soil is considered expansive and tends to absorb a lot of water, causing swelling and canal deformity. Then, rapidly settles again leaving voids behind. Sandy, dispersive soils cause similar issues by suddenly shifting away and settling causing cracks or even a serious wall collapse. Both types of soils, and many other challenging conditions, can be dealt with by installing a geomembrane as the primary liner or as an underlayment.
One of the most important considerations of soil condition for canal design is the natural angle of repose. Each soil has a specific angle at which the earth behind the slope exerts no pressure against the surface, giving the highest resistance against collapse and landslides. For loose and expansive soils, this may be a very gradual slope. With this angle, resulting canal sides can be dozens of feet wide. Geomembranes help bind soils together that are prone to movement. Thus, preventing collapse and allowing you to potentially increase the slope of the canal walls. If the slope angle can be increased, the extent of grading and related costs is greatly reduced.
Categories of Canal
Few irrigation, transportation, or power generation canals are designed as a single continuous run of the same size and depth. Most canals are part of larger water distribution systems and connect to many other individual trenches and canals, especially for irrigation uses. The five major categories of canal are:
- Main: Main canals are those capable of providing 10 cumecs or more. These canals connect directly to water sources or discharge points, making them the most high-risk for damage during flooding events. These canals are most commonly lined with concrete due to size, but with very large liners now available in continuous pieces, it’s possible to even line main canals with geomembranes with minimal seaming. BTL Liners has the capacity to manufacture liners to your specific needs and assist with even the largest canal installations.
- Branch: Smaller channels running in various directions off of the main canals are classified as branches if they carry a flow output of at least 5 to 10 cumecs. Like the main canal, branches aren’t used directly for irrigation or water delivery. This means that loss, through seepage, is particularly likely to cause damage.
- Major Distributary: A discharge of at least 0.028 to 15 cumecs qualifies a canal as a major distributary. This is the largest canal category used for direct irrigation to fields. They’re also used for feeding smaller channels or supplying large-scale pump and sprayer or sprinkler systems.
- Minor Distributary: If discharge is under 3 cumecs but still over .25 cumecs, the canal is classified as a minor distributary. These minor canals do not have to have major distributaries between them and branches to qualify, just the right amount of flow. Many sprinkler and drip tape systems are run with these canals.
- Field Channel: The smallest canal size, the field channel, includes all the other trenches supplying less than .25 cumecs of discharge at the end. Direct trench and flood watering systems rely on hundreds of these field channels, while irrigation systems with other equipment tend to require far fewer. Field channels still need lining to reduce crop damage from oversaturation and water loss.
