news from the waterfront by Tim Matson


The most common flaw with earth ponds new or old is failure to achieve the designed waterlevel. It's acceptable for a pond waterlevel to drop in very dry weather but then perform properly when water supplies recharge. A lot of ponds work that way. But if a pond never fills that's a big, or should I say small, disappointment.

Some chronic low water ponds fail right from the start. Some start out by filling but then for some reason the waterlevel drops and never recovers. Some new ponds slowly build up a decent seal as sediment accumulates over the newly excavated earth. But once you accept that you have a problem, what to do?

A drastically low water pond is usually the result of construction failure. Usually this is down to failure to properly test the soil and water supply. That means test pits before building, especially when monitored in dry weather so you know how poor the water supply can get in extreme drought. A contractor or property owner who doesn't dig test pits is risking failure. When signing on a contractor, owners should be asking for the builder to create an earth pond that fills and overflows most of the time. When it doesn't, the contractor should be obliged to make it right.

When an earth pond does fail, here are several ways it can be modified to work.

Adding Water

Perhaps the water supply can be improved
to make up for leakage. This is usually done
by finding other sources of water from the
upstream watershed: springs, ground water,
runoff, streams. Ideally such a source or combination of sources will flow by gravity to the pond without need of pumping.

Sometimes extra water needs to be pumped to the pond. This usually requires getting grid electric to the water source, or perhaps using solar or wind power for pumping. Hydraulic rams can also play a part
in moving water to a pond above the water source, depending on the lay of the land.

Supplementary water is sometimes added from
a dug or drilled well. If the well is overflowing and at higher elevation than the pond, the supply might flow by gravity with no need for power. Otherwise power will be needed.

Another source can be runoff from buildings. Roof runoff can be captured in gutters, downspouts, foundation drains, etc. Like watershed runoff, this works best if the water can be piped to the pond by gravity flow. Since depending on rainwater can be iffy, captured water might be stored
in a cistern for controlled feed to the pond. This is rare in my experience, but more common out west.







Liners: Clay

If adding water isn't an option, or doesn't solve the problem, improving the pond seal is your other choice.

Pond lining can be done with soil additives like clay, or installing a membrane liner. As long as ponds have been built, clay has been a traditional sealant. Sometimes it works, sometimes not so much. Often clay is sourced locally, so success depends
on finding good quality material close enough to make trucking practical. Good moist clay should clump and hold together when squeezed into a ball, and minimize sand, stone, and organic matter. Clay particles are smaller than silt soil, and range in color from white to gray to red. Clay type and quality varies, and it's important that your contractor knows how to source good quality material, and properly apply it. Some of the important factors in  use of clay are correct moisture content when installed, proper compaction, and making sure pond slopes are not so steep the clay will slump downhill. It is often not enough to simply layer clay on top of porous soil.

The pond slope may have to be dug out so the layer of clay is built into the basin. A layer of small stone is often applied over the installed clay. This protects the clay from drying out if the pond waterlevel drops and exposes the clay; stone can also prevent clay particles from clouding the pond due to
stirring by water currents, fish, swimming, dogs, etc. If there are springs in the pond, it may be necessary to install drainage under the clay so upwelling ground water doesn't break up the clay seal. Another dimension to clay use is loss of pond volume due to thickness of clay (the pond gets smaller). Bottom line: when considering clay, make sure your contractor is experienced in getting good material and properly using it.

Coming in May: Part Two: Sealing with Bentonite Clay, and Membrane Liners.

April 1, 2016

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