Weirs
There are many types of weirs or spillway edges. Structurally, there is a wall that dams up water until the water rises above the wall and overflows to the other side. Rectangular weirs have level edges and some form of sidewalls that are generally used to control the width of the spillway. However, natural streams have water flowing over uneven boulders, and perimeter overflow spas or pools don’t have sidewalls. V-notch weirs have no bottom edge and instead rely on only two angled sidewalls to limit and control flow.
For dam walls in spas or vanishing edges, we are also faced with a multitude of wall cap options, such as level surfaces, submerged surfaces that cut back into the pool side and surfaces that cut away from the pool. The wall cap configuration does not affect the flow much, but it has downstream performance implications and esthetic considerations, generally decided by the site and water shape design.
When referring to downstream performance in the context of weirs, the effect of the nappe, or face of the spillway, is being described. With a lot of flow, water can break away from the outside of the dam wall, creating a clear-sheet effect. At low flow rates, the water is likely to adhere to the wall. If the wall cap is cut away from the pool, the slope of the cap naturally accelerates the water in a direction that jumps off the wall.
There are lots of details and considerations for weirs and vanishing edge wall design. More information can be learned from the Genesis 3 Edge Program presented by Randy Beard and Skip Phillips. To determine how much water is flowing over the weir, measure the depth of water over the edge and refer to Table 3.
Table 3: Rectangular weir performance (Francis formula) |
|
|---|---|
| Weir Head | Flow rate per unit of weir width |
| 1.6 mm (1/16”) | 7 lpm/m (0.56 gpm/ft) |
| 3.2 mm (1/8”) | 20 lpm/m (1.61 gpm/ft) |
| 4.8 mm (3/16”) | 36 lpm/m (2.91 gpm/ft) |
| 6.4 mm (1/4”) | 56 lpm/m (4.48 gpm/ft) |
| 7.9 mm (5/16”) | 78 lpm/m (6.26 gpm/ft) |
| 9.5 mm (3/8”) | 102 lpm/m (8.19 gpm/ft) |
| 11 mm (7/16”) | 128 lpm/m (10.3 gpm/ft) |
| 13 mm (1/2”) | 157 lpm/m (12.6 gpm/ft) |
| 14 mm (9/16”) | 187 lpm/m (15.0 gpm/ft) |
| 16 mm (5/8”) | 219 lpm/m (17.6 gpm/ft) |
| 18 mm (11/16”) | 252 lpm/m (20.3 gpm/ft) |
| 19 mm (3/4”) | 287 lpm/m (23.1 gpm/ft) |
| 21 mm (13/16”) | 323 lpm/m (26.0 gpm/ft) |
| 22 mm (7/8”) | 361 lpm/m (29.0 gpm/ft) |
| 24 mm (15/16”) | 400 lpm/m (32.2 gpm/ft) |
| 25 mm (1”) | 440 lpm/m (35.4 gpm/ft) |
| 32 mm (1-1/4”) | 612 lpm/m (49.3 gpm/ft) |
| 38 mm (1-1/2”) | 801 lpm/m (64.5 gpm/ft) |
| 45 mm (1-3/4”) | 1,005 lpm/m (80.9 gpm/ft) |
| 51 mm (2”) | 1,222 lpm/m (98.4 gpm/ft) |
Information has been published stating a certain flow rate should be used for vanishing-edge design. Unfortunately, without qualifying all the key variables, a single recommendation is misleading and wrong. It is preferable to break things down into the influent and effluent systems.
The influent system would include the returning water that is spilling over the edge back to the pool. Obviously, there are many variables to size that system, including weir length, desired downstream effect (if any), pool usage, prevailing wind direction and strength, and even the edge detail. Esthetically, the design is typically configured for a low flow rate requirement of 7 lpm per linear meter (0.5 gpm/ft) of edge. Hydraulically, the system is typically designed to run at 25 to 37 lpm per linear meter (2 to 3 gpm/ft) of edge. This means more than 7 lpm per linear meter (0.5 gpm/ft) may be used if necessary, though this can be slowed down as low as possible to achieve the effect at low operational expense, and with less evaporation and potential splash out.
Finally, the effluent system (gravity lines to a surge basin, if present) is designed to handle twice the flow rate of the edge system—in this case, 25 to 75 lpm per linear meter (4 to 6 gpm/ft) of edge. This way, there is a 2:1 safety factor to ensure the gravity system will not be exceeded by normal operation of the edge system. This also provides extra capacity for surges caused by swimmer displacement and wave action.
All of this, of course, may vary significantly depending on the project. A competition pool probably needs a more robust system to keep the pool’s gutter flowing with all the wave action. Every project is different and has its own set of variables that need to be considered.
