Water depth at the shoreline should be at least 3 feet, with a minimum 3:1 slope. This will eliminate most aquatic weed problems and allow for emergency aeration anywhere in the pond. The bottom should be level and graded to the lowest point in front of the dam where the barrel pipe will be located.
The bottom should also be pulverized with a disc harrow and then leveled. If agricultural lime is required, spread it before filling the pond with water.
Installing the drainage pipe:
Typically, the barrel pipe with anti-seep collars attached is installed after the dam core has been partially constructed above ground (Fig. 5). The dam core is trenched down to the bottom of the pond and a barrel pipe with anti-seep collars is laid in the trench. Anti-seep collars are required because water will seek the path of least resistance.
This is most likely to be alongside a straight barrel pipe. Anti-seep collars are simply steel plates welded vertically and completely around the barrel pipe to provide the necessary resistance to laminar flow.
The connection must be watertight. If the dam is taller than fourteen feet, more than one collar will be needed. And be sure the barrel pipe has a slight fall to the outside of the dam so the pond completely drains.
Compaction around the barrel pipe is extremely important. This is the weakest area of the dam and the cause of most dam failures. The fill should be compacted with power tampers to ensure good compaction against the pipe and the collars. Manual compaction should continue until there is at least 2 feet of material over the pipe.
Riser pipes can be located inside or outside of the pond area. Locating pipes inside the pond area reduces the chance of vandals tampering with drains. One disadvantage of locating them inside the pond, however, is that it will interfere with seining.
A disadvantage of locating the riser outside the pond is that the riser and barrel pipe are hydraulically charged or pressurized at all times so all the welds must be good or leaks will develop and cause the dam to fail. Risers placed outside of the pond area will have to be larger because of the reduced head pressure on the pipe opening.
The dam’s freeboard may have to be increased also, adding to the expense. Large watersheds may preclude the outside placement of the riser for safety reasons. In colder, northern climates outside risers may freeze. Riser placement should be a carefully engineered consideration. A canal gate or alfalfa valve is installed to regulate discharge.
Concrete ballasts:
If an inside riser is installed, there must be a concrete ballast to counteract buoyancy forces of the water on the riser pipe. Steel pipes, like battleships, have a tendency to float. Buoyant forces are created when any object is submerged in water, whether or not the object is heavier than water.
When an object is submerged in water, a force equal to the weight of the water displaced acts upward on the object. A 24-inch riser 10 feet tall will have a buoyant force of 1,372 pounds. In order to counteract this force, 0.6 cubic yards of concrete must be poured around the base of the riser.
This amount of concrete weighs 2,430 pounds in the air but only 1,419 pounds under water (again because of the buoyant force on the concrete). Without a ballast, constant upward forces on the riser pipe will weaken any joints over time and may crack welded seams.
Also, water flowing through the pipe causes vibrations and movement. The ballast acts as a damper, adding longevity to the pipe system.
Sizing riser and barrel pipes:
The characteristics of the watershed and the typical rainfall in the area are used to size riser and barrel pipes. Riser pipes are larger in diameter than barrel pipes. They are welded together in a “tee.” Riser pipes are usually designed for 5-year storms, or rainfalls that occur normally once every 5 years for a 24-hour period.
For the west central area of Alabama, this is about 5 to 6 inches of rain. Of course, the larger the pond and watershed, the larger the riser must be. Barrel pipes are similarily sized. It is obviously better to oversize pipes than to undersize them.
Emergency spillways and dam freeboards:
Emergency spillways are required, particularly if ponds are located on large watersheds. Spillways can discharge large volumes of water around the dam. If properly designed, they will prevent water from overtopping the dam.
Spillways are designed large enough to handle a 25-year storm. This size storm also establishes the top-of-the-dam elevation that includes adequate freeboard—the distance between the designed flow of the emergency spillway and the top of the dam. This critical dimension is a minimum of 1 foot (Fig. 5).
There should be a vertical drop where the spillway discharges into a natural watercourse to discourage wild fish from entering the production pond. The spillway should be transversely flat, shallow, and grassed to minimize any channeling of water by erosion.
The proper sizing of emergency spillways should be determined by qualified persons, especially if dams are located in potentially hazardous areas.
Establishing vegetation:
Some new pond owners pay a lot for pond construction and then neglect to finish the project. Unless the soil is protected it may wash into the bottom of the pond. Gullies and rills can develop with the first rainfall, making maintenance difficult and even jeopardizing the integrity of the dam.
To prevent this, vegetation should be established as quickly as possible. If surface soil was stockpiled before construction, use it to top dress banks and dams. This will promote quick and stable vegetative cover. Soil on slopes should be roughed up with a disc or spike harrow. This encourages rainfall to soak in instead of running off and reduces erosion.
Distribute lime, fertilizer, seed and mulch at recommended rates. Depending on the season, plant warm or cool season grasses. Check with your county Extension office for recommendations. All-weather gravel roads to and on the dam are necessary for checking water quality, operating and maintaining aerators, and carrying out stocking and harvesting operations.
If banks have good sod cover, it shouldn’t be necessary to have gravel completely around the pond. A trash rack should be installed on top of the riser if it is located inside the pond. This keeps debris from entering the pipe and clogging it.
Author:
Gregory N. Whitis
