By Dave Fletcher

An artist uses just the right amount of paint in just the right spot for his desired effect. Too much paint here, too little there can ruin his masterpiece.

Such is the case for irrigating a planted slope. Too much water at the bottom, too little water at the top can cause great harm. That’s why slopes require skill in irrigating. Changes in elevation can wreak havoc on water pressure, sprinkler distribution, water movement and water absorption. The result? Uneven water distribution that can lead to dry conditions at one extreme and flooding at the other, both resulting in poor or no growth.

But, with the right palette of equipment and techniques, you can master the art of elevation. Your skills will ensure safe, even, efficient water distribution for healthy plants and dazzling slopes—a truly awe-inspiring scene.

Crafting Uniform Coverage
Uniform coverage is all about distributing water evenly over a zone, and doing so at the application rate the plant material needs and the soil can absorb. This is, of course, critical in all areas of a landscape, but much more difficult to achieve on slopes. The trick lies in determining all of the proper irrigation zones.

Along with the usual zones that relate to differences in solar exposures, plant material and physical separations, on slopes you also need to determine differences in elevation. Different elevations of the slope have different water requirements. Because of sheer gravity, some of the water applied at the top of the slope will eventually cross the middle of the slope and end up at the bottom. So it’s important to group sprinklers in areas of similar water needs.

To create these zones, you will group sprinklers from side to side across the slope rather than up and down the slope. That means sprinklers along the top of the slope are grouped separately from the middle, which should in turn be separate from the bottom. It’s also important to be able to have individual control over these zones for irrigation scheduling.

The next step toward even coverage is choosing a sprinkler nozzle with a radius that will cover the actual linear distance of the slope or zone. Keep in mind that the actual linear distance of the slope is longer than the distance shown on the plans. For example, on a 2:1 slope, the distance from top to bottom might measure 30 feet on the plans but the actual linear distance is 34 feet.

For a two-row system (top and bottom rows only), choose a sprinkler nozzle with a radius of at least 34 feet to cover the entire distance. Remember to space sprinklers head-to-head based on the calculated 34-foot radius rather than on the plan-measured 30-foot distance. To determine the actual linear distance of a slope, multiply the plan distance by the following factors: for a 1:1 slope, multiply by 1.4142, for a 2:1 slope multiply by 1.1180 and for a 3:1 slope, multiply by 1.0541.
To fine-tune coverage even further, adjust the lateral line spacing to compensate for gravity’s effect on the sprinkler’s pattern. Because of gravity’s pull on the sprinkler’s discharge, more water will be distributed toward the bottom of the slope than the top. To compensate for this, where a slope has three rows (top, middle and bottom), adjust the lateral spacing so that the middle row is closer to the top row than it is to the bottom row. A rule of thumb is to add a minimum of 10 percent to the normal distance between the bottom and middle rows to ensure more even coverage.

For example, a 2:1 slope measures 60 feet from top to bottom on the plans, so the actual distance will be 67 feet. For a three-row system (top, middle and bottom) the normal spacing between rows would be 33.5 feet (half of 67 feet). To adjust the middle row so it is closer to the top than the bottom, add 10 percent or approximately 3.5 feet to the distance for a total of 37 feet between the bottom and middle rows.

Once proper zones, appropriate sprinkler nozzles and lateral spacings are established, you can take an additional step to ensure even coverage by using a sprinkler with built-in pressure compensation. Even with proper separation of top, middle and bottom lateral rows, there will probably still be some elevation differences within a zone, because some sprinklers may be slightly higher or lower than others. These small elevation differences will cause changes in pressure, flow and application rates and uneven coverage. Pressure compensation evens pressure out so that all sprinklers on the zone apply the same amount of water regardless of the pressure changes due to elevation.

Finally, make sure you don’t sabotage even coverage by mixing full- and part-circle sprinklers in the same zone unless they have matched precipitation rates. Sprinklers with differing precipitation rates in the same zone will have widely varying operating times, which can lead you to overwater one area to sufficiently water another.

A Portrait of Reduced Runoff
The rule of efficient use of water says there shall be little or no runoff or wasted water. That’s an easy rule to break on slopes. To help in this effort, choose a sprinkler with a low precipitation rate. The lower the precipitation rate, the easier it is for the soil to absorb the water. Always try to match the sprinkler’s precipitation rate with the soil’s infiltration rate as closely as possible.
To control the application of water even more, choose an automatic controller that allows watering to be scheduled in multiple, short cycles. Controllers with multiple start times allow the total run time to be divided into several short cycles, allowing the soil to absorb the water between cycles.
You’ll want to schedule watering times so that the zone at the bottom applies less water than the zone at the top. That way, when gravity pulls the water downhill from the top and middle zones, the bottom zone can absorb it.

To get that last bit of water savings, use a check valve under each sprinkler or, better yet, use sprinklers with built-in check valves. The check valve will keep water from draining out of the lowest sprinklers on the lateral line due to gravity after the system has shut down. If you only use one check valve on the lowest sprinkler on the line, the next highest sprinkler will drain. So do use a check valve on all sprinklers.

Safety by Design
If you overwater a non-sloped landscape, or there is a mainline break, there will be damage but it will probably be minor. With slopes however, these situations can spell disaster. That’s why it’s safest to use a master valve on all slope zones. The master valve acts as a backup in case the zone valve sticks open and causes overwatering. If the zone valve gets stuck open, the master valve will shut the mainline down at the end of the cycle to prevent unlimited watering.

If you’re not going to use a master valve, you should definitely use a reverse-flow valve. This way, in the event the zone valve malfunctions (such as a tear in the diaphragm), the reverse-flow valve will revert to the closed position and not allow irrigation to take place until the valve is repaired or replaced.

An even safer step is to add a flow sensor to the mainline to detect higher than normal flows, such as with a mainline break or a stuck valve.
The sensor can report the high-flow condition to the controller (if it has the ability to utilize this information), which can be programmed to shut the system down to prevent further damage.
If a mainline does happen to break, it’s better that it happen at the bottom of the slope rather than the top. When the mainline is installed at the top of the slope and it breaks, the ensuing flood from top to bottom can cause severe damage and erosion to the slope. By installing the main at the bottom, you’ll also have an easier time accessing the valves for servicing and manual operation.

Installation Inspiration
Here are general tips to keep in mind when installing your irrigation system on a slope:

one: Be conscious of differences in elevation and the corresponding differences in pressure. You’ll want to ensure that there will be sufficient pressure to operate sprinklers at the top of the highest slope. For every 2.31 feet in elevation upward, you lose one pound of pressure. To make sure there is enough pressure to operate the highest sprinkler at its optimal pressure, calculate all pressure losses and be sure to include pressure loss due to elevation. If the pressure loss exceeds available pressure, or if the residual pressure is less than the sprinkler’s pressure operating requirement, a booster pump may be required.

two: Install high pop-up sprinklers and rotors such as 6" or 12" at the bottom of slopes, to clear taller plants and to maintain the best coverage. Remember to use the appropriate equipment for the landscape’s maturity, not just for when it is newly installed. Three or four-inch pop-ups are not high enough to spray over tall-growing ground covers and shrubs.

three: Angle sprinklers to improve coverage and reduce wind drift. Install the bottom row of sprinklers so that they are tilted slightly back from vertical, allowing them to spray up and onto the slope rather than into it. The middle row of sprinklers should be installed at 90 degrees, or perpendicular to the slope. The top row of sprinklers can be installed vertically if located right at the top of the slope. A low-angle nozzle should be used to keep the spray low and away from the wind. A standard-angle nozzle can be used if the top row is installed slightly down from the top and at 90 degrees or perpendicular to the slope.

four: If piping is installed on grade, be sure to use an appropriate UV-grade pipe that stands up to ultraviolet (UV) rays of the sun. Conventional PVC will eventually become brittle and break
if exposed to the sun. Piping on grade should also be staked appropriately so that it is securely
fastened to the ground. The water surging through pipes carries enough force to jar and move
pipe, and can cause breaks if not securely battened down. Typically, the bottom row of a slope system can and should be buried to prevent tampering and vandalism.

five: Keep zones small to limit damage in the event of any kind of breakage. The more sprinklers on a zone, the higher the flow and the greater the chance of damage in the event of a mainline break. u

 

 

July 2002