Koi

Incorporating a waterfall into your setup can be beneficial to the eye, as well as the health and happiness of your Koi. Peter May guides us gently through the process of creating a cascade in your pond…

Building a waterfall from scratch may require some practise, even for those of you who are adept at flourishing trowels full of mortar. If the basic principles that I’m about to lay out seem quite understandable and logical, then go ahead. But I’d also be prepared to go back on your workmanship and make some minor or even fundamental adjustments to the structure.

Before you start, look at the pool and the environment and think about what would be suitable. The water has to flow over something back into the pond. It could be as simple as a set of brick steps or even a pile of logs.

Consider your physical and financial resources. To build it from scratch out of natural rock needs a certain familiarity with stone and mortar. There are some excellent preformed and moulded units on the market, but you’ll need a certain amount of stone similar to the preformed unit in order to blend it into the slope.

Stone can be expensive to buy. The quantity that you’ll require is astonishingly large and there is generally a fair bit of wastage. Also remember to factor in the costs of liner material.

Which pump?
The pump is the key to the size of your waterfall. To choose the size or power of pump required, bear in mind the ultimate height of the waterfall and, by looking on the pump performance table, see how much water will be delivered to that height.

In terms of gallons per inch, I’d estimate that for every inch of sill width, you need a pump that can deliver 100gph to that height. In centimetres and litres, this works out at roughly 200l per cm.

If we’re linking the size of pump to the filtration system, or we want a rule of thumb that will give us a practical formula that will suit any size of pond and not disturb the life therein, we should aim for a pump that can deliver half the volume of the pond water per hour to the top of the waterfall (or through the filtration system), using the maximum bore of pipework that it can take.

That will then dictate the width of the waterfall. For example, a pond that’s roughly 15ft x 9ft x 3ft could have a waterfall that’s 1m high, with a pump delivering 1,500gph or 7,500lph. This would mean the waterfall could be 15in wide, but I know from practical experience that a 12in waterfall is more spectacular.

Go for a modern pump design that has been ‘continuously rated’, ie designed for non-stop use. Modern filter pumps are exceptionally economical and you’ll save several times over in electricity costs and the life of the pump.

Blocks and liners
Creating natural-looking waterfalls with liners and a concrete-block skeleton is a technique to be used in made up or unstable ground, or on loose sandy soil. Start from the bottom, working up. As you’re excavating into the bank down which you want this cascade to run, imagine the water coming down in a series of drops.

Even if there’s only one drop and it’s vertical, or the scheme is near horizontal and it looks simply like a stream, regard it as a series of pools flowing from one into another for the initial skeletal construction.

Building a framework
This project is intended to be permanent and as such there will be a considerable amount of stone involved. In which case, it’s best to create a construction which will hold everything in place with the liner at the right level and the water being contained where necessary, even if there isn’t any stone there.

For the series of waterfalls, leave enough room to lay a skeleton of 4in concrete blocks. These effectively face the soil with a hard framework that supports the liner and the stone facing that covers the liner. This stream will have a series of waterfalls cascading down at some speed. Already there’s a block-work pool at the bottom.

A line of bricks will raise the face of the other two drops to contain the water. When they’re filling with water, the lower front edge will allow water to cascade from one pool into another.

The blocks must be laid level, apart from the outlet at the front, which will be at least 10cm (4in) lower. This is where the water flows into the next section and has to be low enough to allow for the thickness of a sill stone cemented into place on top of it.

I often describe these pools as ‘armchairs’; the arms contain the water within the chair as it flows over the seat, until it reaches the water in the next pool. Make sure that the face of the waterfall is well back behind the end of the side blocks at that level, ie the face of the seat of the armchair is well behind the arms.

In this way, the blockwork projecting out beyond the sill and the fall, while supporting the liner up the sides of the waterfall, will contain any sideways seepage.

The blocks are laid on cement on a well-consolidated soil base. A footing would be ideal, but not always practical. After the cement is dry, the blocks are backfilled with soil, then the bases of the pools are smoothed out and consolidated gently.

A 1in layer of sand can be laid throughout the system with an underlay over the blocks. Lay the hose from the pump along the most sensible route. Excavate a trench for it, but save the backfilling for the point when you are finishing off the rockery.

Working with soil
In order to build a waterfall in well-consolidated soil, carve the shape of the waterfall into the soil, like steps into the slope. Each step must have a trough out of which the water will flow, and these will provide reservoirs of water that will steady the flow and prevent most it from disappearing under the rock.

The lips over which the water will flow can be reinforced with a rendering of cement. Once they’re hard, you should continue to build as if you have a block-work framework by lining the excavation with both sand and an underlay.

Laying the liner
Lay the liner over the blockwork and underlay with a large overlap right down into the pool (with the stream liner on top of the pool liner). Carefully push and fold the liner into place, gathering as many creases together as possible, while fitting it right into the contours of the blockwork.

Carefully trim off some of the excess liner where it won’t be required, but leave plenty spare at the top until the last possible moment.

The side and base of the pools and stream can be lined with pool underlay to lend the liner a bit of protection from the stone that you’ll face the inside of the pools with.

Be careful not to have this underlay folding over the top of the blocks, either at the sides or down through the stream or series of waterfalls. It’ll function as a wick and siphon the water out of the stream pools into the surrounding soil or down into the bottom pool, which may cause it to overflow.

Remember, you want the waterfall/stream pools to stay as full as possible, even when the stream isn’t running, so that when you turn the stream pump on, there’s a minimal amount of water taken from the main pool at the bottom to get the stream/waterfall flowing again.

Pull out the liner between the side facing stones, not so much that it can’t be disguised, but enough to prevent water travelling out sideways behind the stone.

The stonework
Line the pools and stream with stonework. Start from the bottom and work up, laying base stone on a thick layer of mortar. While saving the best flat stones for the sills, concentrate on the face of each waterfall and work outwards and slightly forwards into the rockery or wall, retaining the soil at the side of the waterfall.

Always make sure that you think in terms of how the water will be retained in the stream within the liner as the water comes over the sill and falls down the face.

For a large part, particularly around the sides of the stream and waterfall, try to lay the stone with its strata as it would have laid naturally in the ground, ie following the general strata of the rockery (if there is one).

However, this is a rule to be broken because, as you will see in nature, very often a waterway cutting is strewn with a variety of rocks and boulders heaved up and cast aside in more geologically or meteorologically violent times.

Filling the gaps
In order to use the minimum amount of mortar, the best technique for the long term is to make the stonework as self-supporting as possible and only use mortar around and behind the outlet, the sill stones and usually the facing stones on a large fall.

Using cement mortar gives you a really solid finished feature, but it’ll add lime to the water, so a waterproofing agent in the mortar mix may help to counter this. Fill in the gaps and backfill behind the stone with smooth pea gravel.

You’ll use a lot, but it’s much more healthy for the pool environment than cement, and often doubles up as a biological filter bed if the stream or waterfall is in operation for long periods.

There’s no point in trying to seal the stone to the liner with cement. In the longer reach of time, water will find its way between the liner and cement and seep down sideways, and, with the pressure of flowing water from behind, seep upwards, thus creating a hidden seepage you’d hoped to deter by using cement.

The pea gravel behind the stone works as a gentle cushion, growth medium, and filter medium, and holds the water static. Moving water, the stuff that we see, flows over the top of it.

Sill stone and pump
When placing the sill stone on a bed of cement, it’s very often best to project it over the lower facing stone, unless it’s very rough and patterned. Otherwise the result is that the water sticks to the facing stone surface and the effect of the fall is lost.

The sill stone is fairly critical – it needs to be reasonably thin and slightly dished. Other small flat stones cemented on the outer edges can help guide the major part of the flow. A random collection of large pebbles will also carry out the same function.

Stones cemented into place on top of the sides of the sill stone contain the water as it flows over. They can be on the stone if it’s wide and reaches right across the width of the outlet, or, they can be cemented to the side of it with the gap between sill and side stone pointed up with cement and dressed with pebbles or gravel.

Meanwhile, the hose from the submersible pump has been dug underneath the rockery stone to the side of the stream, disguising with rockery stone in front and over it as it goes into the header pool.

It must lie so that it’s just above the water level in the header pool when the waterfall’s operating in full flow. Rockery stone is then employed in order to continue the strata levels sideways into the bank, and it’s then backfilled with soil.

Where the pump for the water sits in the pond can be a matter of debate, but I think that if it’s incorporated in the main biological filter, then it should sit at the opposite end of the pond in order to provide a complete turnover of the pond water. Otherwise, it should sit at some depth below the base of the waterfall.