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Retention Ponds
Introduction to Retention & Detention Ponds
A retention pond is a catch-all term for a catch-all pond. Retention ponds are basins that catch runoff from higher elevation areas. They are often created near development areas and have been gaining in popularity to the point they are required in many instances with new development of buildings, parking lots, roads, etc. Detention ponds are also created in these instances. The difference between the two is that a retention pond typically always has some water in it, hence the name retention pond. Conversely, a detention pond detains water during rainy periods. They are designed to help control runoff and limit flooding during high water times. A detention pond will hold water for a short period of time and slowly releases it. A retention pond will typically have an overflow pipe so it doesn't get too high, but there is typically water at all times.
Retention ponds are often fairly small in total acreage, typically less than an acre. They are also usually shallow with slow sloping bottoms. There is also typically a large area around the ponds that can accommodate high water during rainy times. This design is for function and safety. Since these ponds are in public areas, the shallow, slow sloping sides are needed for safety if people happen to fall in. The large surrounding areas will provide adequate area for water storage when heavy rains are present. This is essential since most are installed near developed areas where the rain cannot soak in due to pavement and buildings. Often times, retention and detention ponds will be installed near each other in a row. The water will get held up in the detention pond and slowly drain into the retention pond. The detention pond will help eliminate flooding and the retention will hold the remaining water.
Purpose of Retention Ponds
Retention ponds are developed to serve two functions, limit flooding and pollutant removal. As mentioned above, retention and detention ponds are often developed in areas with significant construction. Buildings, roadways, parking lots, drive ways, etc. are all impermeable surfaces, or material water cannot soak into during a rainfall. Since the rain cannot penetrate these surfaces, it has no choice but to runoff to areas of lower elevation. Therefore, the more buildings and pavement in a given area, the more chances of flooding because the rain cannot soak into the ground.
Retention and detention ponds are developed in these areas as both temporary and permanent runoff basins. The rain runoff from the impermeable surfaces will find its way to the retention and detention ponds directly or through storm sewers. The ponds are built to handle a quick influx of water and slowly release it into another pond, stream, wetland, or slowly back into the ground. Either way, the ponds catch the large amounts of runoff water that occurs quickly and slows its release. The lag time from input to output eliminates or minimizes the effects of large rains and flooding.
Retention and detention ponds also serve to remove pollutants and trash. Since retention and detention ponds are the drainage basin for an area, they are a magnet for items like garbage. Trash and debris are washed into these areas often after heavy rains or wind. Plastic bags and other waste also enters from storm sewers that typically drain into a retention or detention pond. The trash can create an unsightly pond and harm the fish or other inhabitants of the retention pond.
Retention and detention ponds also catch other pollutants from runoff such as petroleum products from roadways, fertilizers from lawns and fields, sediments, bacteria, suspended solids, and metals. These pollutants can have negative affects on the overall water quality, such as pH, turbidity, nutrients, and hardness. The retention and detention ponds collect the runoff pollutants and allow them to settle out of the water and also get used up through biological processes.
When the pollutants enter the pond during a rain event, the pond slows the water movement, allowing the heavier pollutants such as suspended solids, sediments, and metals to settle out of the water column and come to rest in the bottom sediments. T his greatly improves the overall clarity or turbidity of the water. Other pollutants such as the fertilizers are used in aquatic plant growth. Many of these nutrients are taken out of the water and used in plant growth. Bacteria can also be dealt with within the ponds biological processes. By retaining the water from runoff and allowing the pollutants to settle out or be used up, the water that is then returned to the ground or to neighboring streams, wetlands, etc. is of much better overall quality.
How to Have a Pond without Having Mosquitoes
Female mosquitoes like to lay their eggs (about 100 to 300 at a time) in a dark spot in still water. The eggs then float on the surface of the water for about 48 hours before hatching into larva. The larva will live in the water for about 10 days eating tiny plants and animals. It then changes into a pupa that floats on the surface of the water for about two days before the adult mosquito emerges.
Mosquitoes do not need much water to lay their eggs. However, the more water the more mosquitoes it can support and the more mosquitoes it will attract, which is why it is important to prevent your pond from becoming a mosquito nursery. This can be done either by making it undesirable to female mosquitoes or by preventing the larva from developing into pupas. There are several ways to do this.
Fish and Tadpoles
Koi may be beautiful, but are generally too large to prey on mosquito larvae. However, since Koi are known for their mellow nature, you might consider adding some smaller fish. Common goldfish, killifish, and guppies are three types of fish that are known to eat mosquito larvae and will get along well with other fish. If you have a natural pond that is not stocked with bass, bluegill, or catfish, you might want to consider adding some fathead minnows. Bass, bluegill, and catfish are all natural predators to mosquito larvae and will not need any additional help.
If you have a pond or any water source that does not have any fish, consider adding some gambusia affinis, better known as the mosquito fish. Mosquito fish are about an inch long and look similar to a guppy. A large female gambusia is capable of consuming over two hundred mosquito larvae in an hour. They are very aggressive fish and begin attacking the larvae when they are only a couple hours old. Unfortunately, they will also attack dragonfly larvae (which are another predator to mosquito larvae) and will be aggressive toward other fish and tadpoles.
Mosquito fish are very hardy and capable of adapting to different temperatures and salinity. They live naturally in the southern United States. However, some varieties of mosquito fish have been bred to survive colder temperatures, even down to -30oF. They are livebearers and can produce three to four broods each summer of about twenty-five to one hundred young each. You do not have to worry about them over-populating since they are also very aggressive towards each other.
If you have a large birdbath that is constantly filled with water, consider adding about ten fish. Larger ornamental ponds can support anywhere from 35 to 100 fish. For wastewater ponds, add about 1000 fish per acre. Minnows, guppies, and small goldfish can also be added to birdbaths and ornamental ponds, but they are not as hardy as the mosquito fish. You do not have to worry about feeding them either, since they sustain themselves on what they find in the water.
If you're not interested in having fish, consider tadpoles. Tadpoles not only eat mosquito larvae, but they grow up to become toads or frogs, which will eat mosquitoes. One toad can consume about a hundred mosquitoes a night.
Pond Design
Any area of your pond that has shallow or still water will be a breeding ground for mosquitoes. This includes the surface of aquatic plants, water in the top of submerged pots, and shallow water near the edges of the pond. Trim any vegetation that hangs over the edge of the pond that might provide shelter for mosquito larvae.
If your pond includes mosquito-eating fish, make sure your periodically raise and lower your pond level. This will dry out or deepen any shallow water where your fish can't go normally. Also, thin out any plants to ensure that your fish can swim around and through the vegetation. You may want to eliminate any aquatic vegetation around the edges of your garden pond.
If possible, agitate the water surface by using a waterfall or fountain. These elements can be combined with your filter set up, but this is not required. The movement of the water will interfere with the female mosquito's ability to lay eggs.
Bti, Methoprene, and Oil There are three chemicals that can be added to water to help reduce mosquitoes: Bti, methroprene, and oil. Only the first one is available commercially to pond owners. However, the other two may be used by mosquito control agencies so it is important to understand what they are and what they do.
Bti (Bacillus thuringiensis) is a naturally occurring type of bacteria that will kill mosquito larvae and some fly larvae but is not harmful to fish, pets, wildlife, or humans. It is commonly sold in doughnut shapes under the name "Mosquito Dunks" or "Mosquito Bits."
Methroprene is an insect-growth regulator used to control late-stage mosquito larvae. It is found in many flea and tick solutions. Although initial reports have indicated that this chemical is safe to humans, it is not currently available for use in ponds. There are also indications that this chemical may be damaging to frogs and other wildlife.
Oil is an older remedy for controlling mosquitoes. It works by putting a film over the water thus preventing the larvae from breathing. Unfortunately, oils can also be damaging to aquatic plants and other beneficial aquatic insects. Currently approved oils can only be purchased by mosquito control agencies. Contact one of them for this remedy as a last resort. Do not use regular cooking or vegetable oil. Not only is it not effective, it is extremely messy.
Whichever method you choose, it is important to check periodically that it is being effectiveness. This can be done by scooping water into a white cup or bowl. Any mosquito larvae should be visible against the white surface. If you see any wriggling little objects, you need to consider an additional method of mosquito control. If you find many larvae in your fishpond, it is possible that you are feeding your fish too much. Try reducing the amount of food for a couple of days and see if the larvae population does not decrease.
Follow these guidelines and you will be able to enjoy your pond all summer long without having to worry about mosquitoes.
Talk Like an Aquaculture Producer
http://www.ext.vt.edu/resources/4h/virtualfarm/glossary/aqua/aquaculturewords.html
Should You Attempt Fish Farming? Considerations for Prospective Fish Growers
http://www.ext.vt.edu/pubs/fisheries/420-897/420-897.html
Fish Supplies
http://www.dunnsfishfarm.com/stock_rate.htm
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