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The Green Scene in Watershapes, April 2011

Water In Transit
By Scott Cohen

Understanding how to move water from one level to another is among the most surprisingly complex challenges in all of watershaping. Indeed, there are lots of ways to get into trouble — even if you’re confident that your design is correct.

Case in point: I was called in as an expert witness by California’s Contractor State License Board to investigate a project in which a contractor had convinced his clients that the dry river bed running across their property could easily be turned into a functioning waterfall, stream and pond without much effort or expense.

The client loved the idea and naively agreed simply to pay whatever it cost to have the work done. The contractor came back with a price of $18,000, which probably would’ve been fine had the contractor had the slightest idea what he was doing when it came to water in transit.

Scenario: The waterfall and stream traced a path about 80 feet long, descending a slope that rose to a neighboring property. There were three collection basins along its course, and it emptied into a 20-by-20-foot pond at its base. From the pond, the contractor used a pair of four-horsepower sewage pumps to push the water back to the waterfall.

When the pumps were activated, it took seven or eight minutes for the system’s top basin to fill completely and overflow. Unfortunately, the volume of water needed to initiate the falls lowered the pond’s level by a good 14 inches — a visual consequence the client didn’t like at all. To accommodate the differential and handle evaporative losses, the contractor installed a mechanical auto-fill system plumbed to the home’s main line. The unit operated with a large arm-and-ball valve — similar to what you’d find in a toilet, except the ball float was about the size of a basketball!

So as the pumps were moving water up the system, the auto-fill system would dutifully fill the bottom section of the pond back up at the same rate that water was being pushed up the slope. Amazingly, the contractor hadn’t considered that when the pumps were turned off, the water in the waterfall system would empty back down into the bottom pond, which would consequently overflow and send about a foot of water over the edge and into an adjacent downhill property.

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Scott Cohen is president and supervising designer of The Green Scene, an outdoor design and construction firm based in Northridge, CA. He provides consultation for clients nationwide and gives seminars on designing landscapes, swimming pools and outdoor kitchens.