I built a model river in my garden. Part VI

By Stephen Rodgers

River Bed Section, An Engineered Junction, A Natural Junction ~5.2K GIFs

The main channel

The wildlife pool was by now seething with, well, wildlife. The sump was full and holding water, the pump was humming along 24 hours per day, the lake was brimming and the lake overflow drain was gurgling nicely. It was time to construct the river bed, and link the wildlife pool to the rest of the system.

The visible water in a natural river acounts for only about one third of the actual volume of water that is really flowing. The rest of the water moves through the rocks and soil of the river bed. This suits the model river engineer fine as it allows him/her to lay a fairly wide and deep polythene liner and then refill most of it with soil. The soil can then protect the liner from damage (sunlight and children), soak up the water and conceal the polythene.

I had already made a false start at laying a section of river bed, the mistake I had made was to make it simply too wide and deep. I became convinced that the water would all flow through the mud, never being visible. One afternoon I expended an enormous amount of energy removing this bit of polythene, the rain had collected in the bottom and produced the 'puddin' effect in the soil.

I fact I have since learned, when filling the reed bed, just how impermeable my soil is, I could have probably left it alone.

In particular the construction requirements of the river course were :-

I intended the wider, quieter pool to be fairly formal and quite shallow, stocked with small goldfish, orfe and fingerling trout. Two full length (8 feet long) gravel boards would supply the sides, with stones placed at the entrance and exit (for the water to gurgle around and over) and a gravel bottom.

It was at the downstream end of this pool that I resolved to make my first connection of two underwater sheets of polythene. The lower piece was already connected to the waterfall head by being laid right into the brickwork.

I excavated all of the river bed first, keeping in mind at all times that quite a lot of the soil was going back in after the liner was laid. Across the bottom of the pool I laid a piece of 50mm * 15mm timber, set slightly into the heavily compacted soil. Two pegs of the same material were hammered into the soil at either end, forming a long flat bottomed 'U'. I was then able to lay the downstream sheet into this 'U' and apply a lot of bathroom sealant to the upper surface, the upstream sheet could then be laid on top and a smaller piece of timber used as a baton, nailing through the baton, both sheets of polythene and into the bottom timber with 20mm roofing felt nails. I don't think it leaks.

This sort of junction was all right for the bottom edge of a fairly formal, straight sided pool. The other junctions (out of the wildlife pool, half way down the river and at the head of the middle pool) could not/did not need to be so heavily engineered. I effected these junctions by relying on a huge overlap, bathroom sealant and gravity.

I have spent a lot of time studying (well, drifting down) the sort of artificial rivers found in large swimming pool complexes in this country and 'water parks' in Spain. They all look very dramatic, with water swirling in deep pools, but they are managing to achieve this effect with quite low flow rates. The secret is deep sections connected to each other by shallower runs of much faster water. By arranging to have the 'dams' at the right places in my river, I was able to ensure that water would have to flow uphill, if it managed to pass the bathroom sealant, to be able to escape.

Having shaped the river bed as an even channel, with an even fall rate, I scooped up the soil into a bump, laid the downstream section of polythene on the bump, cleaned it thoroughly and applied a 30cm thick band of sealant. The upstream polythene sheet was then laid on top, with at least one metre of overlap. I don't think it leaks.

All of these pieces of polythene were laid in the following sequence:-

With the river bed laid, the sump and the lake full, and the pump running, the excitement was enormous as I announced that the river would shortly be flowing properly. I had partially drained the wildlife pool to build the junction from it to the river, so I ran a garden hose from the middle watering point into it, opened the tap, and waited.

There were a lot of other small boys around for the occasion and we all scampered around noting the levels of the various sections of the system as they filled, until a trickle arrived at the falls. This water was, of course, none too clean and rather scummy with all the buoyant particles picked up on its journey, but after ten minutes the flow had picked up considerably, and clean water was hurtling over the falls. After a day and night the water in all the system was crystal clear.

Note that at this stage the bathroom grey water was not connected to the system - all the water was from the skies or from the tap and had been circulating between the sump and the lake for many days.

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