Splash, Trickle, And Flow

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.

Water Migration

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.

Pool Depth

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.

Color Selection

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.

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