Splash, Trickle, And Flow
By Pawel Gradowski
Mistakes in design and installation give water features a mixed reputation, despite the fact they can provide valuable elements in landscaping. Several key issues are often overlooked during the conceptual phase by designers who may not understand the intricate patterns of flowing water, the limitations of mechanical components, or the complex environment of microorganisms living in water. This may result in future problems:
- Excessive waste of water
- Difficult installation
- Costly operation and maintenance.
This article is intended to describe a few issues most commonly overlooked during the water-feature design process. An understanding of splash, calculation of water in transit, implication of weir (waterfall) design, migration of water along the walls and edges, limitation for the pool depth, and selected color of the pond’s floor will reduce the number of flawed designs.
“Two” Many Designers
It is common for architects and landscape architects to develop conceptual drawings for a water feature. These professionals are usually focusing their vision on the feature’s shapes (pond, stream, water channel, infinity edge, etc.) and on the desired visual effects of water in motion (e.g., waterfalls, jets). When the conceptually developed project is approved for development, the detailed design of mechanical components is usually left to experts specializing in fluid engineering. At that stage, mechanical engineers, pool consultants, or skilled contractors are required only to provide a design for the mechanical components (i.e., pumps, filters, piping, and all fittings) and ensure that the selected equipment meets the initial vision of the designer.
Therefore, the engineers are rarely responsible for any unforeseen consequences related to the initial design. In some circumstances, a specialty consultant may decide the initial design is not possible or extremely difficult to build. However, in most situations a compromise is reached, and a feature similar to the one proposed by the initial designer is constructed. If the final effect is a success, both parties are proud of the accomplishment; but if anything goes wrong, nobody wants to take the blame for unforeseen side-effects. As a result, the faulty water feature eventually becomes an awkward planter or an eyesore.
One of the primary reasons for flawed installations is that neither party has a complete set of skills necessary to provide a creative design while fully understanding the implications of fluid engineering, resulting in a reluctance to take full responsibility for the design.
The following are critical issues most frequently overlooked during the initial design phase.
This overlooked characteristic of liquids frequently results in a significant waste of water and is probably the most common undesired side-effect of many water features.
Water splashing from waterfalls, jets, or overflowing ponds usually flows onto adjacent paving or into planters. Wet paving becomes either a nuisance or, when slippery, a safety hazard. If splashed water ends up in planters, soil becomes oversaturated, and plant material that cannot handle excessive moisture frequently dies or appears unattractive. There are too many variables affecting the extent of splash, making it difficult to accurately calculate. However, a general recommendation of many experienced designers is that the minimum distance between the source of splash and the edge of the pool should be a minimum double the height of the source. So, a 3-foot-high jet or waterfall should be located a minimum of 6 feet from the pool’s edge in order to eliminate splashing beyond the pool. Keep in mind that, in windy areas, that distance may need to be increased to compensate for wind action.
Water In Transit
This is the volume of water that needs to be pumped from one pool to another in order for a water feature (e.g., waterfall) to function. Initial designers often overlook the issue of the relationship between the surface areas of the pools connected by weirs in a multi-level water feature. When the pump is turned off (e.g., daily between 2 a.m. and 7 a.m.), the lowest pool has to be able to collect the “water in transit” in a short period of time. If the area of the lowest pool is significantly smaller than the combined area of all other pools, then provision for a surge tank may be necessary to collect the excess water. This common solution may significantly complicate the mechanical design of the system, and considerably raise the cost of installation and operation of the feature. When the pump is turned on, the collected water from the lowest pond or surge tank is necessary to rapidly raise the water level in the upper pools, allowing the waterfall to start flowing again. For example, raising the water level by a ¼ inch (a common measure in designed waterfalls) in a 100-square-foot upper pool requires lowering the water level by 1 inch from the lowest pool of a 25-square-foot area. Similarly, if the area relationship of the top pool to the lowest one is 50:1, the water level in the lowest pool will have to drop by 12-1/2 inches for the same ¼-inch waterfall increase. That necessary fluctuation of the water level in the lowest pond or the surge tank has to be anticipated during the conceptual-design development.
Weir Design And Construction
Difficulties of construction and required flow of “water in transit” (volume at a calculated velocity) are the two main implications directly related to length of weirs. A ¼-inch thick waterfall requires approximately 10 gallons per minute (GPM) for every foot of weir length. Therefore, a 50-foot-long weir requires pumping ± 500 GPM from the lowest to the top pond, which is a considerable amount of water (imagine water moving in a full 6-inch diameter line at ± 5-1/2 feet per second). If this feature is designed to operate every day, the monthly cost of energy required will be significant. Decreasing that flow will reduce the thickness of the waterfall, potentially turning it into a trickle.
Another significant issue is the contractor’s ability to provide the required precision while constructing the weir from specified materials (i.e., concrete or stone). Keep in mind that the entire edge of a weir has to maintain a constant level, with variations not exceeding half of the thickness of the waterfall. Running a 1/8-inch thick sheet of water (for a gentle waterfall flowing on a 3-degree battered wall) requires variations in the weir level to be less than 1/16-inch. Otherwise, the water may not be distributed evenly along the edge, leaving undesired, visible patterns of dry and concentrated water flow along the weir.
The cross-section shape of the weir edge also plays a significant role in the water-flow characteristics. A sharp edge (i.e., 90-degree angle) will (depending on velocity) allow for water to detach from the weir’s edge and freely fall. A gently rounded edge (e.g., 3-inch radius) allows water to “hug” the weir’s surface and keep flowing on a battered waterfall’s wall, thus significantly reducing the splash.
The electrostatic charge of water droplets allows them to migrate sideways along the smooth walls and horizontal edges, resulting in effects similar to splash. A vertical saw-cut or a corner forces water to run along its edge. Therefore, any patterns on walls (reveals in concrete or on stone facing) near the weirs will affect migration of water. Typically a sandblasted, vertical concrete wall with no reveals will allow water to migrate sideways from both sides of the weir at a 45- to 60-degree angle.
Shallow pools (e.g., 3 inches) create difficulties in sourcing mechanical fittings, which are shallow enough to be installed below or at water level. Also, the sun warms the water in a shallow pond faster than in a deep one, thus increasing complications related to algae and other water microorganisms. In most jurisdictions, pond depth exceeding 18 inches is considered hazardous and requires safety fencing. Designing pools between 12 and 18 inches deep appears to be the most practical approach.
The color of the pond’s walls and floor affects visual perception of the feature, and may also significantly impact the cost of its maintenance. Dark pools are perceived as deeper and also mask small amounts of dust and organic debris suspended in the water and on the floor, thus requiring less-frequent cleaning. Swimming pool-like colors invite people to wade and play with water, which may expose the owner to liabilities related to water sanitation. On the other hand, dark pools accumulate more heat from the sun, and are more susceptible to chlorine or calcium-deposit stains.
Other seemingly insignificant factors in pool design may considerably affect construction cost, visual perception, or maintenance issues. It is impossible to list in one article all of the issues and to propose appropriate solutions. If a talented but inexperienced consultant has a brilliant vision for a water feature, this vision should be developed in collaboration with an experienced pool designer at its earliest stage to allow for constructive coordination of work, and to navigate the outcome towards impressive, yet practical solutions.
Pawel Gradowski , BLA, CSLA, BCSLA, is a Partner and Landsacpe Architect at Durante Kreuk Ltd. in Vancouver, BC. Within the last few years, Pawel has been a key designer for more than 50 water features within complex urban developments, as well as for natural-looking ponds and rain gardens in a suburban environment, providing detailed working drawings and site review services during design implementation. For more information, visit www.dkl.bc.ca .