At PV we do our best to be good stewards of the land. Toward this end
in 2008 we have embarked upon installation of a stormwater catchment
pond on our property. This will have the dual purpose of reducing
winter rainwater runoff erosion, while reducing our summer use of city
well water for irrigation.
On Bainbridge we are allowed to move 50 cubic yards of dirt in a given year without going through the slow and painful permit process, so that's the size we aimed for. (In the end we went ahead and got a permit anyway.) This comes out to just over 10,000 gallons.
We carefully chose the location to be where the Christmas tree property uphill from us has a dip, where much of it's surface sheet rainwater runs downhill onto our property. In previous years this water has been a constant problem running down and eroding our driveway.
Additionally, this spot is nearly the highest point of our property, thus we hope to be able to use the outflow for irrigiation without having to use a pump. Merely filling the outflow pipe to start a siphon should give us sufficient downhill flow to our vineyard and gardens. This is a twofold win as during the summer we will no longer be drawing down city drinking water for gardens, and we won't be paying for that water, or paying for the builtin sewage fees on it. Ideally, we won't use power for a pump if the siphon has sufficient downhill pressure for our drip irrigation lines.
We are also installing a pond liner as our soil does have significant
self-drainage. The liner is 45mm rubber, rather like a tire inner
tube in texture. The liner comes in a long roll, visible on it's side
in the foreground in the photo on the left.
Most importantly, we have also plumbed the roof downspouts out to the pond in order to capture what precious Spring and Summer rainfall that we do get here. This sounds difficult -- if you look carefully at Figure 1 (above), it appears that the house is downhill of the pond -- you're right, it is. However, the roof itself is actually higher than the pond. As long as the piping between the roof and the pond is watertight, the water will fill up the piping, then once full, will begin to flow out the lower end into the pond.
Normal downspout material is not sealed -- it's usually rolled aluminum and at each junction it simply overlaps (or more accurately, underlaps); so as long as the water within is falling, this is sufficient. If the pipe were to fill with water, e.g. if the bottom were blocked, water would shoot out the seams.
The ABS piping is used down into the ground, and under the driveway
all the way out to the pond. It ends in the air above the high water
level of the pond surface. As it is a sealed system, open only at
each end, it can fill up with water to no ill effects. As
the open end against the house is higher than the open end at the
pond, it does not matter that it is not downhill all the way -- once
the water has filled the piping, it will begin to flow out the lowest
opening, which is the pond end.
The pond end of the downspout ABS should be higher up than the highest level the pond surface can reach, so the pond doesn't try to empty itself down to the house via this pipe. It's also preferable to have it be above the pond surface as it is quite satifying to actually be able to see the water flowing! This is easily done by ensuring there is an overflow outgoing pipe at the top of the pond, ensuring that the level cannot exceed this pipe. The downspout feeder pipe will be higher up than this overflow pipe (see Figure 7 below.)
If you are considering doing this yourself, beware of one small caveat
of this method. As the downspout pipe does fill with water, it must
be emptied during serious freeze events (like we're having as I type
this!), so you must be sure to have a drain valve at the system low
point. If you're on Bainbridge, I highly recommend having a
consultation with Brian Bonafaci 206-842-8205 - his help on this
project was incalculable.
New additions 09-Feb-2009:
Although it's not visible in any of these photos, the 4 inch overflow pipe junctions to a drain pipe that is below the pond liner. This drain ensures that the liner will not puff up from below if there is significant flow from the soil below the liner.