Pond problem

Hi there,

Our land slopes from the road to the back of the garden by 3m. As we are situated on good old London Clay, one section of the back of the garden gets very boggy when it rains.

As SWMBO wanted a pond, we dug out a 2.5m diameter x 30cms deep hole with an 8cm perimeter shelf, lined with pond membrane. Below the membrane, I put a pea-shingle / sand mix / felt fabric to assist drainage below the membrane.

All was well for the last year or so, until the recent heavy rains. Ground water hydrostatic pressure has buldged the liner around the shelf, forcing the contained water away. Now that the back pressure water has reduced, and the liner is back to its true shape, but the pond is about half full, rather than full.

Obviously drainage is the issue, but since there is nowhere significantly lower to drain water to, .....!

Is there a solution? Grateful for any advice and/or suggestions.

Have attached two photos; RearDrainge is the before, and 2257 is the work in progress, ready for the surrounding to be completed, hopefully, this year (if I can solve the back pressure issue first.)

Many thanks,

Rex

 

I see a couple of options, none of which are cheap / easy / popular

1) Move the pond.
I only wish I was joking, but I am not, since the current location is not suitable, you could move the pond to somewhere more suitable.

2) Sump
Construct a ground sump, install a pump and pump the water elsewhere. (You can't pump it down the sewer as that is illegal)

3) Raise the pond?
You may be able to build a raised pond instead, BUT if the groundwater level gets too high it will still cause problems.

Points of note.

Its not common, but it has been known for whole ponds (both flexible liner and rigid GRP to be lifted from the excavation when the water table rises.)

Houses built in flood areas are built on stilts, since the water will come sooner or later.

Fish / livestock can escape when the water table rises, this can be costly as they will die when the water level drops. (We know that, but fish don't)

I wish it was an easier thing to answer, but you can't beat mother nature.

 

Thanks for your reply.

2 is probably a no-no.

3 is a possibility but then it would be above ground which was not the object of the exercise.

1 most like thing to do, but a/ moving it will not guarantee that the same will not happen and b/ the whole point of putting it where it is, was that it was a naturally wet area.

Am thinking about just removing the liner but then the pond would dry out in summer. Although it is a clay soil are, it is not so clay that water does not drain away.

We are not planning fish, unless they arrive by themselves, just plants and whatever else comes along.

 

Have you had a problem before?

Have you had non stop monsoon rains? (We have)

On the basis of that you might consider doing nothing as it may be a rare event.

The soakaway from our cesspit is not working because the land is saturated - which is creating its own problem, as I'm sure you can imagine!, but I've never know that to happen before

 

Not non-stop monsoon rains, lots of drizzle until last week when there was a downpour.

We have a 6000l tank sunk n the garden for collection rainwater for toilet flushing. That ran out about one month ago, and the last few weeks gradually took it to about half full, then last week, the top half was filled in about one day!

As for the pond, I haven't seen it happen before, but then, I'm not wandering down the garden when everything is wet. I think it has been happening to a lesser extent as even after heavy rain fall, the pond has not been full to the brim. I figured that there may be evaporation of even a small hole, but now I know differently.

SWMBO suggested putting stones and pebbles around the shelf to give added weight. May give that a try. May think about digging a deeper drainage channel, but I don't know where to drain it to. There is woodland about five feet from the pond but it's not ours. On the other hand, the woodland is very slightly lower and that is the direction surface water goes without any assistance.

Guess there is little I can do until it all dries out a bit.

Rex

 

I fear some stones / rocks / bricks on the liner will be a complete waste of time since the water pressure will just lift the liner and they will move out of the way. If you use something heavier the liner will simply lift all around it.

You can have what is called a puddled clay pond, but its not ordinary garden clay, and even then it has to be "puddled" into place and it has to be at least 6 inches thick all round, and kept wet all year. (It does work but its a lot of effort for little gain, and you already have a liner)

How about a water feature or fountain instead?

 

Thanks for the information; sound like the sort of issue that I have but don't think I will go the whole, 'puddled clay' hog.

Am considering later in the year, digging a drainage channel, filled with peas shingle, from as low as I can get it, and then cover it with soil again. Hopefully, this may allow drainage from below the liner. The overflow from the pond would still follow the same course.

It's the only thing I can think of, other than filling it in. which would just create a very boggy area during rains.

Rex

 

If you do that put some perforated drainage pipe in the trench (before you put the gravel in) as it will hugely increase the efficiency. It comes on a roll, and isn't very expensive.

 

What I figured I would do it to dig a trench below where the current overflow is. Try to get as deep as I can that will allow a flow into the woodland that is all for 2.5m from the pond. Hopefully, there will be sufficient drainage into the soil in the woodland.

Put some terrain fabric into the trench, then some pea shingle, the perforated drainage pipe, as you suggest and more pea shingle, cover it all with the fabric and replace the soil. On the woodland end, dig a larger hole and fill with pea shingle, put the soil back an cover with woodland debris.

The woodland is owned by the local wildlife trust, but as it is slightly lower than my pond, there is nothing I can do to prevent water draining in that direction anyway. Not strictly 'legal' but that's where the water wants to go, so I will just help it.

Rex

 

(In the woodland bit) I would not cover the shingle with soil. When it rains it will wash the soil down into the stones, making the soak away less efficient and exposing it, cover the gravel with some weed fabric or similar, that way it will let the water through, but not the soil.

 

Yes, that is the plan. Probably did not make my self clear, but when I said "Put some terrain fabric into the trench, then some pea shingle, the perforated drainage pipe, as you suggest and more pea shingle, cover it all with the fabric and replace the soil" I mean to wrap the whole package with fabric to prevent the soil getting in.

Rex

 

You can use fabric to keep the soil out of the drain, but I've never bothered. I just use a decent sized (i.e. oversized) pipe and it will be decades before it has silted up.

If roots are a significant issue that might be a consideration, but I think that using fabric increases the cost and the time to lay the drain by quite a bit. Although for a short drain it probably only makes pennies difference.

 

Roots are not an issue whatsoever.

I do have a length of the brown, 4" underground drainage pipe laying around and thought of using that. The perforated would possibly be better, but since my aim is to drain the hole where the pond is rather than the surrounding area, a 4" pipe would probably do that rather well.

Rex