07-01-04 | News

Re-Creating
Nature’s Kettle Ponds
in Jackson Hole

By Stephen Kelly, managing editor

The largest of the three ponds perfectly simulates the area’s natural ponds and lakes. Water was pumped from the Snake River Canal in eight hour intervals at 1,200 gpm onto the top of the liner and geotextile, which slowly submerged it. A foot of sandy soil was then placed over the top of the protective geotextile and a long-reach excavator deposited soil around the perimeter.

The key to successful PVC geomembrane-lined pond building is site preparation, according to Rohn McKee, owner of GreenSpace, Inc., in Jackson, Wyoming. Ponds need to be excavated and groomed before deployment of the PVC liner, which then requires proven building techniques acquired over the years to assure structural integrity of the liner, i.e., making sure the liner doesn’t leak.

Several years ago, a private landowner in Jackson Hole, Wyoming, which boarders the Teton Range and is about the prettiest country you’ll ever see, contacted landscape designer Rohn McKee to create some water amenities on his 35 acre property for use by his family and the extensive wildlife. The property owner selected Mr. McKee based on his expertise in the valley at designing and building ponds.

Interviews with the client and visits to the site ensued. Designing a landscape of this size was not a typical undertaking. The scale of the land, Mr. McKee felt, required large water features. The decision was made for three ponds: a main pond of 120,000 square feet, and two smaller ponds, 90,000 square feet and 30,000 square feet.

 

Large factory-fabricated 30 mil PVC geomembrane panel liners are brought together for welding. The substrate was compacted with heavy compaction equipment to smooth the soil surface and minimize the number of sharp rocks in which the liner would come into contact. A geotextile fabric is under the liner and will also be placed over the liner to protect it.

The geology of the Jackson Hole Valley consists of glacial till, a sandy, rocky substrate. The largest obstacle, however, was the area’s high water table, a mere six feet down in the spring. Mr. McKee had previously designed a pond in ground with a high water table. Even children digging dug a hole at the beach understand the principle. You’re happily scooping out sand with your hands or a little shovel and shoring up the sides as you go. Then you strike water, and slowly the hole begins to fill. You can bail the water, but it’s a losing battle.

One solution for dealing with a high water table is heavy-duty pumps running 24 hours a day and an underwater basin collecting system until the water table basin sufficiently lowers to allow dry conditions for the PVC geomembranes to be welded. One such pumping at a previous site, recalls Mr. McKee, cost $125,000.

To excavate three ponds on the Jackson Hole land with its high water table, Mr. McKee believed he needed a less costly and less time consuming method of dealing with the groundwater.

For starters, the first phase of the earth work began during the winter months so the water table would be lower and there would be less ground water to contend with.

Dykes and causeways were created within the groundwater basin of each hole as a dry staging area for installing and welding the liners. The subwater level was dropped by pumping directly from the basins.

The ground was excavated 10-12 feet down. The subwater level was dropped by pumping directly from the basins. The substrate was then carefully compacted with heavy compaction equipment so the surface of the soil was smooth. It was important to reduce and minimize the number of sharp rocks that the liner would come into contact.

“We created internal dykes for a staging place for installing the liners,” explained Mr. McKee. Causeways were constructed to a grade slightly above ground water level to weld the geomembrane panels together. Mr. McKee enlisted the help of Stuart Lange to assist in laying out, fabricating and installing the PVC pond liners. Mr. Lange is an active member of the PVC Geomembrane Institute and president of Lange Containment Systems in Denver, Colorado.

Polyvinyl chloride (PVC) geomembranes, according to Mr. Lange, are nonreinforced materials with a high puncture strength, excellent abrasion resistance and resistant to a large number of industrial chemicals. In buried applications, as in the case of ponds, the material has a life of 20 plus years. With the availability of large factory-fabricated PVC panels and diverse field seaming capabilities, which include chemical, hot wedge, and hot air processes, PVC is easy to install. It can also be custom sized to fit the variety of internal basins within the ponds. In addition to standard grade, PVC geomembranes are available in oil resistant, UV resistant, fishgrade, reinforced, and potable water formulations.

Jackson Hole’s glacial outwash plain geology presents a nearly flat sage-brush covered terrain interspersed by streams and “kettle” holes, created when buried ice blocks were covered by sand and gravel; when melted, the overlying sediment collapsed to create depressions that the water table filled in as ponds or lakes. The soil is till, an unconsolidated mixture of clay, sand, gravel and boulders. To begin re-creating such ponds, the ground was excavated 10-12 feet down beginning in the winter, a period when the water table was lower.

To line the Jackson Hole ponds, Mr. McKee selected 30 mil PVC geomembranes. The largest pond consisted of five panels with the largest panel being over 6,000 lbs. The lining material is manufactured with high quality standards to prevent leakage, and special care is taken to protect the liner via an eight ounce nonwoven geotextile layer, specified to go above and below the PVC liner. The geotextile layer protects the liner from any rocks in the substrate and the “cookie-cutter hooves” of the area deer and moose. Mr. McKee has learned important lessons from colleagues who have struggled to fix failed systems. Extra efforts early on obviates the expensive call-backs for the landscape contractor to deal with repairs and disgruntled clients.

The PVC geomembrane and the geotextile protective layers were deployed over the ground water and the edges were placed in a perimeter anchor trench. The PVC geomembrane and the geotextile protective layers were held in the anchor trench with sandbags placed at intervals around the pond. No equipment is allowed on the geomembrane after it is installed. Water was then pumped from the Snake River Canal in eight hour intervals at 1,200 gpm onto the top of the liner and geotextile, slowly forcing it to the bottom of the pond. After the lining system settled, a foot of sandy soil was placed over the top of the protective geotextile and a long-reach excavator, which extended out 52 feet, deposited soil around the perimeter. In some places the ponds were more than 200 feet across, which meant the middle of the pond would not have soil on it. In those places a six-inch layer of one-inch minus washed rock was placed evenly over the bottom of the pond with a crane and cement bucket. The washed gravel helps create a sustainable habitat for aquatic insects. This, in turn, makes for healthy trout and a happy owner.

The pond building in Jackson Hole took over two years. Two ponds were built in the first year; the third pond and a stream were built in the second year. In all, about 30,000 square feet of PVC panel were welded together.

“It was one on the most interesting and complicated projects I’ve seen,” Stuart Lange noted.

While this installation method is more time consuming and certainly more expensive, the ponds are still leak-free after several seasons in service.

For more information about PVC geomembranes, visit the PVC Geomembrane Institute (PGI) website at www.pvcgeomembrane.com.