I built a model river in my garden. Part III

By Stephen Rodgers


Post Cross Section, Plan View Patio/Sump, Grey Water Supply System, and Overflow Drain ~17k GIFs


The Sump

While working on the wildlife pool, I had been researching the sump. At one point during her TV programme Kirsty McColl had pointed to an enormous polypropylene tank and shouted to camera "that used to contain concentrated orange juice, not bad for fifty quid eh?". I simply could not track down these tanks, letters written to Kirsty had gone unanswered (perhaps they didn't reach her), and I didn't even know the capacity (* see HINDSIGHT)

I considered all sorts of alternatives:

All of these options were to enable a hole to be dug and the vessel/s then buried, leaving me to lay a patio, in the conventional way, on top. Somebody else contemplating a similar project not be constrained by patio complications and could just bury it - lucky you!. As I started to toy with the idea of producing a open topped tank, with the patio on a supported roof, I realized that whatever I decided, 'the mother of all holes' was going to be required - so I set to and started digging

Big, complicated projects tip #1.

'The best laid plans, of mice and men...' also refers to river engineers. No matter how much thought you give a task, when you get out there, on the ground, it can be very difficult to know where to start. PUT DOWN THAT POLYTHENE CUTTER!, in all projects there is always something to be done that needs no brains, just brawn. Do that first and when you do decide to 'just trim the polythene a little', you are less likely to ruin the previous days/weeks work.

My soil was a delight to dig, pure clean topsoil to a depth of 60 cms, on top of increasingly clayey subsoil. The topsoil I deposited with the spoil from the spring pool, onto the old lawn near the end of the projected patio. I put all my spare topsoil, from all the digging, here, as I wanted to build up the height thus giving a greater drop to the falls. The subsoil was wheelbarrowed up to the top end of the garden and dumped - out of site is out of mind. For some reason it was put onto sheets of polythene, (I think to separate it from the good soil below), it was then removed altogether, into a skip.

Soil storage tip#1.

Don't store soil, or sand, or gravel, on sheets of polythene. Soil fills up with rainwater, becoming like 'puddin' and incredibly heavy to shovel. Also the sheet tends to ruck up when digging, the shovel catches on the folds and makes it very difficult to establish a rythym. If you need to separate a granular material from a substrate, store it on flat boards, perhaps plywood shuttering, or some old doors recycled from skips.

Skip tip (groan!).

I have found that the difference in price between the smallest and largest of these aesthetic horrors is trivial compared with the dispair at finding yourself with no more space. Get a big one.

Skip tip#2.

Normally they cost no more to hire one for one day or one week. Get it for as long as you can stand the sight of it - everybody else does.

Having completely failed to discover a suitable, cheap and buryable vessel I had settled on the open tank with a patio ceiling option. At first I was going to use wooden joists, perhaps from a demolition site, arranged in a lattice with paving slabs on top. Friends, neighbours and family all said that they thought wood would tend to flex too much and would rot. It was also pointed out that 12, 60cm paving slabs are pretty heavy in their own right - and I was going to call it a patio? I soon realized that nobody was going to come to any of my barbeques, so I took me off to the library.

A diagram of a domestic cess pool caught my eye. 'Underground, concealable water container - seems like just the job', I thought... The recommended design called for a 15cm deep concrete base, double width brick wall supporting a roof of builders' lintels. With a sinking feeling I made the builders' merchants my next stop - the estimated costs exceeded my budget (which was about £100) by 500% !

Considering the structure as suggested, it seemed to me that the floor was there for the benefit of the walls, and the walls were there for the benefit of the roof (perceptive, aren't I). I concluded that if I continued with my idea of a scooped hole with a lid on, insured that the roof did not flex, and was decently supported, I might be able to barbeque in company.

In the next aisle along from the lintels (cheapest £30) were eight feet long concrete fence posts (£8 each). People had obviously been quite selective about the ones they took, because most of the ones I could see were chipped, cracked or distorted. The following day a new pallette load revealed that most of them were crisp and clean and even - quite high quality control.

My neighbour had had experience of these posts - he had cut them up and all sorts of things. I produced a sketch of the proposed roof support and he passed his approval 'yea, that ought to do it'. To my shame I have still never tested one of these posts to destruction, to see exactly what load they can support. All I can say is that at the last barbeque, ten people were seen clustered around somebody executing a card trick - all atop 2,000 litres of water.

The hole was now about 2 metres by 1.7 metres and the alignment and solidity that was crucial to success lay in the setting of the two end cross beams (sides B and D). The other six were all going to rest upon them and a patio is meant to slope away from the house (for the rain to run off). I didn't like the idea of these beams sitting only 30cms from the slightly ragged edge of the hole, so I decided to set one 60cms back from the edge (side D), and build a brick foundation for the other end (side B). My first ever bricks were laid, 15 paving slabs were delivered - and two weeks of fairly tiring concrete jigsaws ensued.

Materials tip #2.

Never attempt to shape, shorten or otherwise alter moulded concrete (posts, slabs etc) with a hammer and chisel. It simply will not chip nicely and dirty great cracks will appear after a few clouts. Use the next door neighbour's angle grinder (and his goggles!).

Materials tip #3.

Concrete paving slabs are far weaker than their rediculous weight would suggest. They break if trodden on when improperly supported and they break if dropped from higher than 4 cms. They can be drilled, but subsequent use of rawlplug and screw will crack them.

Materials tip #4.

Never throw any bit of wood, broken slab or half a brick away - you can always use it. Throw most plastic refuse away immediately, particularly polythene trimmings.

With the hole dug, the posts beginning to look like they were designed to support a patio and a river engineer gaining confidence as he skipped across them (my four year old and I even had a picnic underneath it!), it was time to line and test-fill the sump - this is when you find out how good you were at using the spirit level.

Lining tips #1 and #2.

It looks pretty, but there is no need for the bottom of this sort of hole to be flat - nor is there any need for the botton corners to be square.

At about this time, 50mm pipes had been laid from the rainwater downpipe and to the main drain (though not attached), and in moments of confusion (see 'big, complicated projects tip #1') a lake hole had been started.

The project was beginning to look feasible.

The existing patio is actually rather too high already, this means that the pipe to take the overflow away from the sump is nicely inclined, but the supply pipe from the house points uphill, this means that it will always have standing water in it, which may yet reveal a freezing problem. A bypass to divert all waste from the house and into the drains is essential anyhow.

Materials tip #5.

The plumbing of my entire project was executed in ABS plastic plumbing, an excellent material. It cuts easily, it is very cheap, it flexes enough, it can even be shaped (pour boiling water into it). When you want to make a permanent joint you use the sort of solvent that cannot be sold to adolescents, it sets in minutes and producing water tight joints is easy.

Suppliers tip#1.

Builders merchants, and D.I.Y. superstores may look like they carry a good range of rainwater and overflow pipes and connectors - but not enough! A dedicated plumbers' merchant will carry every imaginable fitting, adapter and diameter, at one quarter the price of D.I.Y. palaces. Moreover you will not be the first non-plumber to walk in with an uncertain air, koi carp keepers discovered them long ago. Make friends with them, their advice will be invaluable.

The drain pipe was laid into a concrete collection box, the supply pipe was allowed to pass over the top of the maximum level and down into the main body of water. This silences any splashing noises, and by angling the delivery tube, produces a swirl effect in the tank when a bathload arrives which aids dilution.

The polythene was left with at least 30cms spare all round, and at the overflow point this was pulled hard into a recess cut in the tank edge, then concrete (sharp sand and cement, with a waterproofing additive) was sculpted and modelled to allow water to leave freely. This is the 'out of sight is out mind' school of river engineering - of which more later.

Materials tip#6.

Resist the urge to buy sands and gravels in pretty bags at D.I.Y. palaces. You may have to prepare a storage area (see soil storage tip #1), but delivered loose by a builders merchant these materials can be one quarter of the price. You buy them by the cubic metre, 0.4 cubic metres is at least 6 wheelbarrows worth.

Concrete collection box tip#1.

Never attempt to mould or sculpt a concrete edifice without first preparing proper, solid and obvious edges. This is where materials tip#4 proves its worth. Even if you remove these edges after the concrete is set, it needs to know where its limits are, and so do you.

Pipes tip.

Within a couple of weeks of starting a project of this size I had two, sometimes three bodies of water at different heights, up and down the garden. These are both an asset, (you can store water in them), and a responsibility (things may have set up home in them). A model river engineer MUST be equipped with a good range of pipes. You need thick ones and thin ones. Flexible and stiff ones. You need some transparent ones and you need a high level of interconnectability.

As an example, my sump was full, so was the wildlife pond, and rain threatened. I was able to rapidly construct a flat dish pond out of a couple of old doors and some slabs. The pump from my old pond was then used to park 1000 litres 'up the garden'. Over the following days I was able to use siphons to 'flow test' a new drain and make up the losses revealed by these tests.

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