How We Do It: Nov. 2000, Designing Irrigation Systems at Morrell Group

Designing an irrigation system is done through a detailed and well-thought-out process at The Morrell Group, a division of OMNI Facility Resources, Atlanta, Ga.

The main focus in designing an irrigation system is separating the zones based on the water requirements of each particular landscape area. Plant material, turf and seasonal color beds should all have their own individual zones to provide flexibility for property maintainence.

Zone separation by water requirements is also the most efficient form of water conservation. Installing systems that water everything at the same time is a lot less expensive, but it could prove more costly in the long run (increased water costs, disease and fungus problems, nutrient problems, potential plant material loss, etc.).

Contractors should use a controller with multiple programming features for zone separation. Electromechanical clocks are great for durability, but pointless if you do not have the ability to run different zones at different times.

No more than one type of head should be installed on a zone. Too many times we see contractors install one lone gear head on a spray head zone because the radius on a spray head doesn’t quite cover all of the landscape in that area. Zones with spray heads should be all spray heads, zones with gear heads should be all gear heads and zones with drip should be all drip – no exceptions. The amount of water dispersed from a spray head is significantly higher than from a gear head or drip.

MAINTENANCE MATTERS. Many irrigation contractors install systems and don’t concern themselves with what happens after the job is complete. Some of the practices that facilitate future systems maintenance are more costly but give customers huge long-term savings.

One such practice is to install at least one extra control wire for each control valve that is installed. This is not a costly or time-consuming practice, and it allows for easier system upgrades in the future.

Isolation valves are also an under-utilized component. We install an isolation valve in front of every control wire. This greatly expedites future valve service and reduces overall system downtime in the event of valve failure. Also, spacing isolation valves periodically throughout the main line network reduces system downtime in the event of a main line failure. Control valves, as well as any other valve, should be installed in a properly-sized valve box, easily accessible for future maintenance. Plenty of excess control wire should be coiled inside the control valve box.

When designing a system, it is important to keep the velocity of water moving through the pipes less than 5 feet per second (FPS). In most manufacturers’ catalogs, there is reference data that guides the designer on how many gallons per minute (GPM) is allowed through various pipe diameters. Generally, for 1-inch Class 200 PVC pipe, no more than 16 GPM should be travelling through the pipe in order to stay below 5 FPS. The higher the velocity, the more wear and tear you may experience in your pipe and fittings.

No more than three pipes should be placed in any trench. Many contractors like to have the control valves in one central location. This creates a mess of pipe and wires – a maintenance nightmare. If there is ever a leak within the manifold area, plan on having to redo the entire manifold. Another problem with manifolds is that control valves should be located near the zone center to allow for more even flow and pressure distribution.

Finally, size the main line to allow for a shorter watering window. Some counties in our area can only water for seven hours every other day. Many of the systems we manage cannot water in less than seven hours during a cycle. The best-looking properties have smaller systems or the main line is sized to allow for the shorter window of time.