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Irrigating Slopes

ELIZABETH LEXAU | Miscellaneous

 

IN THIS ERA OF CONSERVAtion and environmental consciousness, there is little tolerance for irrigation that wastes water or causes erosion and runoff. Irrigation professionals are expected to create systems that keep plants green while meeting strict criteria for environmental responsibility. This task can be challenging enough on a flat landscape. Add to it the extra variable of a slope and the challenge reaches a new level.

The simple fact that gravity pulls water downhill can impact many aspects of the performance of an irrigation system, all of which contribute to its effectiveness, efficiency, and environmental impact. Even a gentle slope can result in too little water for some plantings and too much for others. A slope can cause damaging erosion and runoff and make a system cost much more than it has to. It can also put water where you don?t want it, presenting a liability issue for you and your customers.

Our developed landscapes aren?t getting any flatter. As housing and commercial expansion continues to advance into the hills, the call grows for irrigation systems that do a thorough and efficient job despite the ups and downs.

Irrigating hilly terrain requires well-trained professionals. ?It?s much more difficult to irrigate slopes correctly than level turf grass,? says Dave Davis, president of the American Society of Irrigation Consultants, and of David D. Davis and Associates in Crestline, California. ?When more sophisticated housing moves into an area, it requires more sophisticated people to deal with it. The better training people have in maintenance and operation, the more successful slopes are going to be and the better reputation they?ll have.?

Even in flatlands, hills emerge because we humans tend to like them. They make the landscape more interesting and provide privacy. ?People want to be vertically separated,? says Davis. ?We?ve seen areas where the ground was flat and the developers brought in soil to create different elevations.? In this market, it makes sense to develop expertise in slope irrigation.

Changing goals

In years past, the goal for irrigating slopes was simply to provide ample coverage. People didn?t pay as much attention to conserving water or distributing it evenly. ?People just didn?t think you had to be that efficient,? says Davis. ?Now we are learning that it?s even more important on slopes than on flat ground.?

Efficiency is just part of the story. Keeping all the water on the hill is another. ?Controlling runoff while providing good coverage is one of the biggest issues in slope irrigation,? says Steve Hohl, president of Water Concern, an irrigation consulting firm based in Orange, California. ?You need to provide enough water to keep plants healthy but not so much that it will run off.? Without appropriate runoff control, irrigation water can pick up sediment from the property and carry it to nearby watersheds. This is especially of concern when landscapes are fertilized or when irrigation systems use reclaimed water. In many locations run-off is simply not tolerated; all irrigation water must be kept on the property. ?The more populated the state and the more watersheds they have, the more of a concern it is,? says Davis.

To meet these standards, irrigation specialists have had to perfect their techniques for watering hills. They have done so by developing systems that are flexible, fine-tuned, and highly customized to meet the needs of each landscaped area.

?How you irrigate a slope will depend in great part on how it?s being used,? says Davis. ?Is it a slope separating homes in a development? Is it a slope that will be used as a lawn? Is it a steep slope that?s not intended to be walked on? What are the rules and regulations in the area?? All of these questions impact system design.

Even within one slope, there are different areas with different needs. One part of the slope may contain lawn, another part may be scattered with trees and shrubs. One area may be shaded, another may be subject to direct sunlight all day. Some parts of the slope may not need any irrigation at all. ?We don?t rely on one system to try to do all parts of a slope,? says Davis. ?For example, it?s not uncommon to have parts of the slope irrigated with sprinklers and other areas with drip.?

Fortunately, today?s products and systems make it much easier to address a variety of specific needs even within one slope. More sophisticated controllers make it easier to deliver different amounts of water to different areas. Pressure regulating devices help maintain constant pressure despite elevation differences. More efficient nozzles provide uniform, slower coverage to prevent runoff and conserve water. Low-volume drip technology has been successfully integrated into systems to use much less water while maintaining optimal plant health.

There are many strategies for ensuring waterwise irrigation on a slope. Here are a few of them.

2_9.jpgAccurate measurements

Remember that when you look at an irrigation plan you are looking at a flat surface, and looks can be deceiving. The angle of the slope means the actual area is bigger than it appears on flat paper. Decisions need to be made based on the actual distance of the slope, not the distance shown on plans. Divide and conquer

Use multiple small zones within a slope to provide more flexibility in meeting varying water needs. Different elevations call for different amounts of water. Hohl puts it simply: ?Water flows downhill with gravity, so you need to irrigate the top more than the toe. The top, middle, and toes should be on different valves.? Group sprinklers from side to side across the slope and schedule the groups differently. This way, you can decrease the run times to the toes and other areas that don?t require as much water.

Remember other factors that impact moisture requirements within a slope, such as type of plant material, soils, and microclimate. ?We separate slopes based on their sun exposure,? says Hohl. ?It?s amazing how much less we have to water the north-facing slopes.?

Soaking it in

To prevent runoff, it?s important not to put water down faster than the soil can take it in. Small amounts of water delivered in multiple applications are best. Choose controllers that allow for cycle and soak scheduling.

Integrating drip technology and other low-volume systems into your project can also help. Drip systems not only save the customer money by dramatically reducing the amount of water applied, they can also eliminate erosion and runoff, especially on steep sites. Because a low volume of water is applied at a very slow rate, it soaks in before it can flow downhill. ?In the last five years, drip irrigation has really caught on,? says Hohl.

?In the past, I designed a lot with drip for cities and counties. Now we are using it in almost all of our residential developments as well. We?re also seeing more landscape architects designing their plantings around it.?

?Drip irrigation works so great on slopes,? said Ken Berg, vice president, sales and marketing for Bowsmith. ?We provide precision irrigation. By installing a drip system with our forgiving emitter that provides water directly to the plant, we eliminate any run-off.?

Traditional sprinklers have become more water conscious as well. ?Nozzles are becoming far more efficient in their own design,? says Davis. ?We can get away with using much less water with more uniform, slower distribution.?

Even coverage

Several factors help ensure even irrigation on a slope. Piping is one of them. Lateral lines should run parallel to the slope, not against it. Piping that runs downhill can create a severe pressure differential that can cause several problems, including irregular water distribution and even damage to pipes and sprinklers.

Installing a check valve below each sprinkler or using sprinklers with built-in check valves can keep water from draining out of the lowest sprinklers on a line when the system is shut down. This is especially important on slopes. Not only does extra drainage waste water but, with the help of gravity, it can also quickly erode a path and cause runoff.

Pay careful attention to the sprinkler trajectory and angle of installation. Misaligned heads prevent even distribution and can also cause erosion as water digs into the ground because the spray cannot clear the slope correctly. Depending on the trajectory of the sprinkler, the optimal angle of installation can vary between the top, middle, and toe.

Generally, the bottom row of sprinklers should be tilted slightly back from vertical to allow them to spray up the slope. Middle sprinklers are typically placed at a perpendicular angle or with a very slight tilt toward the slope. At the top of the hill, low angle sprinklers are recommended to prevent wind drift. These should be installed vertically. However, if a standard angle sprinkler is used it should be installed nearly perpendicular to the slope and located slightly below the top edge.

While these are general guidelines, optimal angles can vary depending on the sprinkler trajectory and specific conditions at the site. Carefully following manufacturer?s guidelines, experimenting with different adjustments, and monitoring the results will help determine the correct tilt.

Remember that sprinklers installed on slopes will produce a distorted wetting pattern. As gravity pulls the water down slope, it wets the area in an ellipse instead of a circle. Uphill throw is decreased while downhill throw is increased. To compensate for this, sprinkler spacing may need to be adjusted. As a general rule, spacing across a slope is reduced one percent for every one percent increase in slope.

Managing pressure is also very important in slope irrigation. Make sure to factor in the pressure loss of .433psi per foot of elevation gained and determine whether there is sufficient pressure to accommodate the needs of the entire slope. If not, a booster pump may be needed.

Variations in elevation within the system will cause variations in pressure that could result in uneven coverage and poor operation of sprinklers. Using sprinklers with built-in pressure regulating devices will maintain constant pressure regardless of elevation differences.

3_6.jpgSafety checks

Steeper slopes generate steeper risks when it comes to problems Circle 154 on Reader Response Card Irrigating Slopes continued from page 32 with an irrigation system. On a flat surface, a broken pipe can result in a lot of wasted water. But on a slope, all that wasted water starts running downhill fast, which can result in major property damage or even personal injury. To prevent accidents, extra precautions are taken on hills.

Installing the mainline at the bottom of the hill versus the top is one of the first steps. A mainline break at the top of the hill can result in water gushing downhill, damaging the slope and everything at the bottom. A break at the bottom of the slope will be more accessible to fix, and any damage will be more contained.

A master valve should be installed on all slope zones. This will keep the mainline from being pressurized unless the system is on. When a cycle is completed, the master valve will shut the mainline down so that water will not flow even in the event of a problem with a zone valve.

Reverse flow valves are also a good idea. Any valve can fail. But when a reverse flow valve fails it sticks in the ?off? position, a much better option than failing in the ?on? position at the top of a hill. If a valve does not have a reverse flow feature, failure can result in a continuous flow until the problem is discovered.

A flow sensor on the mainline is another safety option. These can prevent trouble by detecting higher than normal flows such as those caused by a broken pipe or sprinkler.

Landscaped slopes are here to stay. Staying abreast of current techniques and taking advantage of the full range of technology available will help you irrigate them with precision and environmental sensitivity.

 
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