The Hazards Of Water Migration
By Scott Cohen

Water passing from one level to another will always be a popular feature in watershapes - from the familiar spillways from spas into pools to the intricate flows found in many fountains, reflecting pools, runnels, rills and even koi ponds (to name a few possibilities). Depending on the application, these features might be called scuppers, weirs, cascades, edge overflows or just plain waterfalls. For our purposes here, let's call them spillways. One of the common characteristics of spillways of all sorts is that they create turbulence, especially at their edges. If you don't take this special behavior of water coming out of a spillway into consideration, you can wind up with surprisingly serious issues on your hands.

Scenario: With most common spillways, the water flowing over the edge has a strong tendency to migrate laterally at the edges - that is, away from where it's intended to flow - sometimes causing problems as it goes. Indeed, it doesn't take much water migrating out of the system to empty a pool, saturate the surrounding soil, damage concrete or, most common of all, attack any stone it happens to encounter.

Take flagstone, for instance: Although it's frequently used in spillway applications, the fact that it is sedimentary and was laid down layer by layer through hundreds of thousands of years can make it extremely susceptible to water damage. While it may be something of an oversimplification to declare that flagstone is made by water, you can accurately surmise that water can destroy it - and quite easily, depending on the specific material at hand.

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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.