Irrigation on a Slope: What is your experience?
This was the question posed on LinkedIn’s group for the American Society of Landscape Architects. I was intrigued by the question about how to irrigate a slope. As I was gathering my thoughts to share my uniquely professional and sage advice, I started reading the great responses already posted. Disappointingly, I soon realized I had nothing new to add and even read a few points I might not have suggested. Since the LinkedIn group is private and the information posted was so good, I decided to edit the responses and share here.
- 4 ac site in Oregon (Willamette Valley)
- Slope varies 25-35%
- New construction with many retaining walls
- Soil profile is approx. 18″ deep loam
- Plant bed widths vary between 4’ and 35′ with mixed shrub, groundcover and tree plantings
- Pump on site will assure 55 PSI min. at the top of the slope
Tim May, PLA, LEED AP is a Landscape Architect at Teague Nall and Perkins in Fort Worth, TX offers several good points for consideration on how to irrigate a slope. In order to minimize erosion, design the zones to be parallel with the contours and schedule the zones in a staggered or skipped fashion to allow percolation. (i.e. 5 parallel zones from top to bottom and order the operation in a 1, 3, 5, 2, 4 sequence. The irrigation zones at the bottom of the slope should require less water than the zone at the top of slope due to runoff and gravity moving water down the slope (surface and/or subsurface).
Drift from spray irrigation will be compounded with slopes so use a low angle or flat trajectory spray nozzle and specify a controller with multiple start times and “cycle-n-soak” options. Several quick syringe-type irrigation cycles will apply the proper amount of water for the plant material while minimizing runoff.
William Valdez, President and Founder of FreeFlow Products.com in Oceanside, California and a landscape contractor for over 30 years, agrees with Tim’s recommendation of specialty spray heads and suggests using Hunter’s MP Rotators for best percolation. William also points out the cycle and soak will help fertilizers migrate deeper into the soil. When this is not done plants at the bottom of the slope will be verdant while the top of the slope will be chlorotic as the fertilizer leaches from high ground to low ground.
Michael Cook, Vice President, Planning & Landscape Architecture at Firma Design Group in the San Francisco Bay Area writes, “Remember to check with the local water supplier for their regulations. In some areas overhead spray on steep slopes is not allowed and drip irrigation is required.” If overhead irrigation is allowed use a very low precipitation rate sprinkler such as the Hunter MP Rotator or Toro Precision Nozzle. Keep the precipitation rate under ¾” or 1″/hour.
John Woods, Principal with Revival by Design in Salt Lake City, UT points out the importance of the angle of installation for spray heads. If the heads (particularly rotors and impact heads) are installed vertically instead of perpendicular to the soil surface/slope there will be a wash-out condition on the uphill side of the head where a concentrated spray is aimed directly at the soil uphill. This also creates a reduced precipitation rate of coverage of in downhill areas where the spray is projected upward away from the soil.
Geometry has to be considered in the design when designing the irrigation for a slope. Head spacing on a slope might appear to be adequate in plan, but when the additional surface area created by the angle of slope may result in greater than optimum spacing and result in dry spots. Gravity also comes into play as spray distribution pattern down slope. Since the spray pattern is not vertically oriented like it would be on flat terrain, the distribution point shifts toward the downhill side. The pattern will be circular relative to a plan view of the site and elliptical in relation to the actual soil surface, but the center point of the spray pattern is usually a little downhill from the physical location of the sprinkler head.
Separate from LinkedIn, I asked Eric Santos, Division Leader, Irrigation and Water Management at ValleyCrest Companies for his suggestions on how to irrigate a slope. He has started seeing a lot of regulation being mandated where shrubs can no longer be watered by overhead irrigation, regardless of slope. This limits the design options to bubblers, point source or line source drip. Some cities are also limiting precipitation rates on slopes to below 1 inch per hour, which means drip must be designed with low gph emitters spaced far enough apart so 1 inch per hour is not exceeded.
Eric personally prefers bubblers over point or line source. GPH Irrigation makes an excellent bubbler with a built in check valve that works well for shrub plantings on slopes. Zone trees separately from shrubs so the trees can be scheduled for deep root watering. The irrigation design for trees should account for future root expansion as the canopy and root zone expands.
What has been your experience designing, installing or maintaining irrigation systems on slopes?