Irrigating steep-sloped landscapes

Extremes provide irrigation designers most difficult challenges. The irrigation designer trying to cover a steep slope has arrived at what many consider to be the greatest challenge in irrigation design.

The steep slopes for which we design irrigation systems are generally the by-products of human activity. Many steep slopes exist in natural surroundings, but seldom call for irrigation systems.

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Most people think of steep slopes in regard to rolling terrain and mountains, but there are design challenges in areas of relatively flat topography-manmade features such as dams and bridge abutments, and artificial topography such as mounds in flat landscapes and cut areas for tunnels and underpasses.

Even in those areas that would not normally require irrigation, natural disasters and other phenomena can create a need for irrigation.

Handling the pressure Irrigation systems depend fundamentally on two processes: pressure and flow.

Pressure is directly related to gravity, which is directly related to elevation change. The more the elevation changes from a water source to a downhill outlet, the greater the pressure will be available for irrigation. The lesser the elevation change, the less pressure will be available.

Pressure can be gained only by moving a water source to higher elevation above its outlet point, or by installing a pumping device to increase the pressure.

Getting the pressure right isn't the only issue. Steep slopes are generally more prone to erosion, are more difficult to prepare for planting and can support only certain types of plants.

Designing irrigation systems on sloped ground consists of four basic steps: data collection, head selection and layout, circuiting and equipment selection.

Data collection A critical aspect in irrigating steep slopes is actually going to the site to "size up" the slope. Just looking at a grading plan isn't enough. Because of all the inherent irrigation challenges, steep slopes require a more intimate knowledge of the site.

You need to appraise the microclimatic, topographic and other site conditions. Examine sun and shade, wind, aspect, slope and gradient, elevation differences and the area's geological and soil makeup.

To determine slope and elevation change, the irrigation designer has to decide how to deal with the aspect of the design that will affect pressure. If the water source is at the bottom of the property, there will be maximum pressure at the bottom of the slope and minimum pressure at the top. If the water source is at the top, water pressure will be low at the top and high at the bottom. To arrive at a design standard of uniform irrigation across the slope, you must take into account the fluctuation between the low and high pressure and achieve balanced precipitation rates on the slope.

Even a novice irrigation designer knows pressure changes are related to elevation, but only those with more expertise know to adjust a design to fit a soil's textural qualities. The mechanical or textural qualities of the soil will influence how quickly water infiltrates and will therefore play a key role in determining frequency and duration of irrigation cycles. Thick or clayey soils will need smaller amounts of water applied more frequently, and sandy soils will need larger amounts of water less frequently to achieve deeper penetration.

Irrigation won't be effective if the soil can't produce healthy root systems and good plant material. In the case of steep slopes, you may have less opportunity to amend the soil. Thus, the soil report becomes a prime indicator of which plants can function in the soil without it being amended. You should know how much water future plantings will need and gather point-of-connection information (water-meter size, main size, etc.)

Head selection Look at the physical arrangement of the slopes, and try to match an existing irrigation head and related nozzles to effectively cover that area. Don't be concerned at this point about pressure and flow specifically, but the data you collected related to existing service for the irrigation system at the water meter will come into play. Pop-up irrigation heads require a base operating pressure to get the riser into the air, so the amount of water available to your system may affect which head you select.

In selecting heads, look at differences among heads and between spray irrigation and drip irrigation. The kinds of plants in the area will affect which heads you choose. Shrubs, trees and container materials can effectively be drip-irrigated, whereas groundcovers and grasses may need to be spray-irrigated.

The data you collect will be of great value in selecting heads. Trajectories must be coordinated with slope angles to make sure that the head does not "blast" into the side of a slope.

Precipitation rates for individual heads and nozzles also are critical, as will the size of the water droplet that each nozzle emits.

Determine specific head spacing based on the manufacturer's recommendations, either square-spaced or triangular-spaced.

Circuiting After selecting heads and determining their spacing, combine the heads into specific irrigation circuits. This depends primarily on the pressure and flow characteristics of your water source and irrigation system. But in the case of steep slopes, your decision also greatly depends on the site's contour. Elevation changes not only affect pressure, but they also affect the amount of water lost through gravity draining. By keeping irrigation circuits on the same elevation, gravity draining does not become a negative design factor.

Circuiting should provide the same water coverage for plants of the same water usage: the name most commonly given for this is hydro-zoning. This is where an irrigation designer should have some say in determining the actual landscape design. When confronted with a difficult-to-water planting design on the slope, you should consider rearranging the plant material to better suit the situation. If an irrigation system is wasting water due to inflexibility in the planting design, poor drainage and erosion can undermine the success of the project.

Equipment selection Once a design begins to take shape on paper, it is ready to be refined, finished and put out to bid. You need to select the equipment that will help the system function correctly and provide long-range efficiency and maintainability.

* Pressure-compensating equipment: Because of varying water pressures on a hillside, pressure-compensating equipment helps to balance an irrigation system and provide uniform coverage on the slope.

One of the most fundamental pressure-compensating devices is a pressure regulator at the point of connection. It can help reduce excessive pressure, especially at the top of the slope. That will allow the individual valves and heads to operate at a pressure that will not "mist" the water droplets as they exit the irrigation head.

The next point of pressure compensation is through the valve. As water or pressure is lost going up a slope or gained going down, pressure-compensating valves can balance the pressure.

* Pressure-compensating in-head devices: If a circuit cannot be laid out parallel to the slope and one of the heads needs to be lower than the circuit, pressure-compensating in-head devices help balance pressure.

Similar to pressure-compensating spray heads are pressure-compensating emitters. They have either variable orifices or pressure-compensating diaphragms within them that allow the heads to emit the correct gallonages.

* Check valves or anti-gravitational devices: Water escaping through the irrigation head after irrigation is finished may increase erosion on steep slopes. This is especially true where irrigation circuits are not accurately valved or circuited parallel to the contours of the slope.

Check valves or anti-gravitational devices prevent water from amassing on the slope and causing erosion on the upper portion of the slope, or from puddling and ponding at the bottom. The devices come in two basic forms: in-line and in-head.

In-line anti-gravitational devices prevent water from draining down a lateral line and are usually installed as the lateral line moves perpendicular to the slope, either up or down. An in-head check valve is installed at the base of an irrigation head and prevents water from leaking from the irrigation head after the valve is closed.

* Breakage and vandalism protection devices: The bottom irrigation circuit of a slope is one of the most problematic. Not only is it a collection point for gravitational water, but its proximity to walks and roads makes the heads on the lower part of the slope more susceptible to damage.

A high-flow shutoff valve is a device installed between the lateral lines and the irrigation head. It operates when an irrigation riser is broken and greatly reduces the amount of water coming out of the riser. This device does not entirely shut the water off but slows it down so that it can be detected by maintenance personnel on a regular inspection cycle

Another common device that reduces damage due to breakage is called a triple-swing joint. This device is a combination of PVC-threaded nipples and street elbows, generally Marlex, which is more flexible. The combination of these nipples and elbows allows an irrigation head to bend in any direction when struck. When a head on a triple swing joint is knocked over, it can be straightened again for regular irrigation operation.

A soil tensiometer senses available soil moisture. This device is generally connected between the irrigation controller and the irrigation valve. When soil moisture is above the limit set on the tensiometer, it will prevent an irrigation valve from opening and irrigating. In this way, you conserve water until it is needed.

In the end, steep slopes are no different than any other type of irrigation system. The challenges inherent in their design are the challenges that can only make a good irrigation designer better.

Steve McGuirk is a landscape architect for Madrone Landscape Group (Soquel, Calif.).

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