Topic: The Case of the Sinking Pond!!!

[Garden Manager]

I need some advice on a problem i have with my pond.  I have a pond and bog garden built side by side in my garden. Strange as it may seem the bog garden was built first and the pond added later (about 3 years ago). All was well and both were/are maturing nicely. However in the last few months i have noticed the side of the pond next to the bog garden was starting to sink with the result being the water level in the pond dropped and lots of liner started showing even when the pond was full. This was barely noticable at first but now it is quite obvious, and leaves a purpose built wildlife 'beach' high and dry most of the time.

I realised that because the pond was built after the bog garden, with some of the bog garden being removed to fit the pond in, that side of the pond was only being supported by the soft soil of the bog garden. When the features were first built the weather was very dry so the bog wasnt as wet as it should have been, so was able to support the pond side. With all the wet weather these last couple of years the bog has become a true bog and now unable to support the pond.

This has made me wonder how other ponds and bog gardens work side by side without having the same problem. After all the two features are supposed to work side by side and compliment each other. i realised that in most cases either the pond is build first and the bog garden built later or both at the same time. in either case i reckon a hidden bank or ridge of dry solid soil must be left between the two to keep the features apart. This is what is lacking with mine.

What I need to do then is to try and either strengthen the side of the pond or install some sort of solid divider between the two features. However I am stuck as to how to acheive this. I dont want to go as far as building a wall or anything like that but i am not sure what will do the job. Can anyone make any suggestions please?

Has anyone else had a similar problem?

Thanks

[GrannieAnnie]

I'll take a stab at this.
I'd drain the pond and make sure you don't have a liner leak.

If no leak, has the ground actually sunk from maybe rainwater running under the pond? Or an underground spring?

If those aren't the problem you could get rocks or cinder block maybe and build some type of wall then run your liner over it.

My pond did not have a back wall on a downward slope so I built a dry stack wall with rocks (not hard to do just heavy).
Then I was afraid it might collapse so I built another wall near it and filled the space in between with dirt to plant things. It has held up fine and I imagine yours might also.
Or if there's room you could make it wide enough for a walking path between the pond and bog.

[froglets]

Not sure if this is of any help, but I garden on sand & love moisture loving plants.  Next to my very small pond, I have a series of bog pockets - ie "bags" made out of  pond liner with holes.  Based on the knowledge that the light soil would not support any pressure it workes on the honeycomb principle ( I trained in engineering) & probably will flex a bit over time, but stops the bog soil moving too far in any direction.

[Garden Manager]

Thanks.

Space is tight so not much room for large scale construction, or even much room to work in. Might try the breeze block option, dry laid and packed with (dry) soil. Would that be any good? Or could i just pack dry soil between the pond and bog liners to create a bank/wall between the two?

I ought to have said the pond is roughly 2 metres long by 1 metre wide and the bog garden similar dimensions. The join along their longest side (2 metres).

My soil is a clayey chalk soil in general although the area the the features are in contained a mixture of chalk rubble, subsoil, building sand and builders rubble which where undisturbed (was under lawn for decades) is quite solid and firm.  I removed most of this though when i dug out the features and the base of both the bog and the pond lie on top of the natural chalk bedrock. So i doubt either of the features has sunk in that direction, nor could there be a natural spring beneath it.

i still reckon the side of the pond next to the bog has sunk or colapsed slightly into the bog garden. Whether this is due to rain softening the bog soil or rainwater intrusion behind the liner somewhere I dont know.

[Garden Manager]

Time for an update I think;

Last Sunday decided to get stuck into sorting the problem. After removing water and lifting the edging stones, I dug a shallow trench/channel along the side of the pond where the problem was. On the bog side of this trench I dug a deeper slit trench and inserted 'bog liner' material (old compost bags cut up) overlapping it slightly with the existing 'bog liner'. this created a separately lined edge to the bog bed which will hopefully allow the strip of ground between the bog and the pond to dry out and firm up. 

To reconstruct the pond edge I dry laid a course of old bricks level with the untouched and correct edges of the pond. This created a firm level edge for the 'fancy' stones to rest on and be supported by. Behind the bricks i added a second row of old bricks and filled in the gaps with sand. Sand was also laid on top of the bricks to give the stones something to key into.

To finish the liner was relaid and the water replaced and topped up from the water butt before relaying the decorative edging stones. Oh and i had a good sort out of the pond plants while I was about it. Result: much improved pond!