Smooth glass or tumbling rapids?
With a solid understanding of flow, resistance, velocity and the plumbing choices that affect these factors, one can begin planning the specific looks and sounds that make every water feature unique. The following example will focus on vanishing edges.
Like any water feature, when planning a vanishing edge design always start with the flow requirement. If the design calls for sending 6 mm (0.25 in.) of water over a square edge, plan for 26.5 to 38 lpm (7 to 10 gpm) per lineal edge foot. By aiming for the higher end of this range, more leeway is given if the edge is not perfectly level. If the client wants 25 mm (1 in.) of water to flow over the edge, 136 lpm (36 gpm) per foot may be needed.
The shape and surface material of the weir (i.e. what the water flows over) also has an impact on flow requirement. The smoother and more level the edge, the less flow is needed. An edge built to a 1.6-mm (0.0625-in.) tolerance will need 3.175 mm (0.125 in.) of water to consistently cover it, while an edge built to a 6-mm (0.25-in.) tolerance will require 12.7 mm (0.5 in.) of water. A weir with a rounded edge will need at least 25 mm (1 in.) of water.
If an insufficient flow rate is used, the water will not effectively break the plane and could wrap under the edge, rather than flowing smoothly over. This may be desirable if designing a wet wall, but not if the client wants a waterfall on the backside.
When designing a vanishing-edge water feature it is also important to consider where the falling water will land, as this is critical in designing the catch basin. A very slow trickle from a thin film of water will drop nearly straight down, while greater flow will propel the water further. If 6 mm (0.25 in.) of water is flowing over a wall that is 1-m (3-ft) high on the back end, the water will meet the catch basin roughly 0.3 m (1 ft) from the wall. By the same example, 25 mm (1 in.) of water will meet the catch basin approximately 0.7 m (2.5 ft) from the wall. Remember, however, if six swimmers cannonball into the pool, more water will flow over the edge. As such, the catch basin should be designed to accommodate a sudden surge of water, too.
When sizing the catch basin, the area of the main pool is the critical measurement. Some people assume the spillover length is the key factor but they are wrong. A tiny pool with perimeter overflow will require a smaller catch basin than a large pool with mostly standard walls and a simple 0.7-m (5-ft) spillover. The catch basin needs to be large enough to hold all the water that would spill into it when the circulation is off, in addition to whatever would spill into it during the pool’s heaviest use. A good rule of thumb is to design a large enough catch basin to hold the number of litres (gallons) in 51 mm (2 in.) of the pool’s water.
For example, a 4.9- x 9.8-m (16- x 32-ft) pool has a surface area of 48 m2 (512 sf). Multiplying the surface area by 28.4 L (7.5 gal), the volume of water in a cubic metre (foot), will determine there are 14,498 L (3,830 gal) in the top 305 mm (12 in.) of water. By dividing this result by six, it can be determined there are 2,415 L (638 gal) of water in the top 51 mm (2 in.) of the pool. Therefore, the catch basin for this pool must be able to hold at least 2,415 L of water.
Obviously, the larger the pool’s surface, the more litres (gallons) must be accommodated, no matter how many linear feet of spillover.
Other bodies of water, such as an elevated spa, should also be considered. If the spa’s check valve fails, spa water will flow into the catch basin; as such, it is important to factor in the spa’s water volume when designing the catch basin. Also, to avoid vortexing (i.e. pumping air instead of water), the catch basin must always contain at least 0.3 m (1 ft) of water.
In total, the catch basin must be large enough to hold the water coming over the edge, including bather surge; the volume of any raised spas or other bodies of water flowing into the pool; and the vortex-prevention reserve.
