The soil dug out in forming the pool excavation may well be used as the basis of a rock garden beside the pool in which a watercourse can be formed. Note. However, that a mound of soil on the very edge of the hole will be a confounded nuisance while pool construction or installation is in progress. You will need some elbow room round the hole whether you are using concrete or plastics. Stretch-fitting a liner, in particular, needs several feet of unimpeded space all round the excavation if you are to spread the liner adequately and get it in with the minimum of wrinkling. So the excavated soil needs a temporary resting place well clear of the pool-making operations, after which it is moved back to the edge to form the rock garden-cum-watercourse.
The extent of the watercourse will depend on your ambition and the overall scale of the layout. It will consist of a few steps, a sort of shallow staircase in which the treads are little pools or stream sections and the risers falls of water pouring clear from one step to the next. Unless you have a naturally steep site, 12-in. Drops are likely to be the deepest you will need; even 6 in. is enough to make a splash. For a small pool where space is limited a single-step waterfall can be completely effective. Where space permits three steps, with 6-in. Drops between them, the highest level will be only 18 in. above the water level in the main pool. Even for a pool of 100 sq. ft. or so, a vertical lift between pool surface and the top of the watercourse of between 2 and 3 ft. should be ample. This means that the rock garden can be kept low and wide and look like a reasonably natural rocky outcrop, which is very much more to be desired than a conical pimple in flat surroundings.
Keeping the vertical lift as low as possible also means greater effectiveness from the pump, because the higher it has to lift the water, the smaller the volume of water that emerges from the end of the pipe. Or, looking at it in economic terms, if you wanted a vertical lift of 5 ft. you would need a more powerful – and thus more expensive – pump to get a respectable flow of water than would be necessary to produce the same flow at 3 ft.
In forming the watercourse a more natural effect will be achieved if the stages are angled or mildly zig-zagged than if the ‘staircase’ runs in a straight line. And how do we go about making such a ‘staircase’, with the essential quality of retaining all its water and returning it undiminished to the pool?
A series of stream channels and cascade pools made with concrete often proves a bitter disappointment, losing so much water through leaks that the pool level drops all the time the system is working. One common cause is failure to consolidate the soil thoroughly before concreting; the concrete is seldom used thickly enough, or with sufficient reinforcement, so that fractures result very quickly from later soil settlement. Another mistake is to imagine that you can make a watercourse by slapping concrete down in scrapes and hollows between rockery stones. Concrete will not make a watertight bond with stone, and steady seepage will inevitably occur wherever a stone sticks up through the concrete.
To make a really reliable concrete watercourse it is necessary first to remove sufficient soil and stone . and thoroughly consolidate the base, and then form a continuous concrete shell to underlie all the watercourse stages from the top down to the pool edge. This watertight shell, solidly bedded on really firm soil, provides the foundation on which you thenrockery stones, with more concrete worked between them, to form in detail the pools and channels of the watercourse system. In operation, any water that seeps down the superficial rockwork will get no further than the underlying shell and it must ultimately find its way back to the pool.
As with pool construction, a good deal of labour can be saved by making use of plastics, PVC, reinforced or otherwise, and butyl rubber make admirable waterproof linings for watercourse channels. It may require some fiddling to form satisfactory pouring lips between one stage and the next in the watercourse, and in concealing raw edges, but with a little care and ingenuity it can be made to produce a very satisfactory result. The secret lies in the use of stone, smooth-faced rather than ‘knobbly’, to build up the vertical steps and flank the edges; in flat thin pieces on the sills; and, in the form of pebbles, to cover the plastic floor of each section. Beach shingle can be used if well washed and free of oil. The ideal material is flat, smooth, water-worn stone of the sort you can find in upland rivers.
An even simpler, though more expensive, method is offered by shaped plastic units in the form of basins or troughs with pouring lips. Some of them are unfortunately too shinily plastic ever to be adequately disguised but there are others, made in glass-fibre, which have craggy angles and a rough rock finish and colouring that combine admirably with Westmorland rockery stone. They are in several shapes, imitating rocky pools and stony streams, and they can be combined to form a series of waterfalls of any required length, and with any convenient vertical distance between them provided, of course, that they overlap sufficiently to allow a clear fall of water from one to the next.