Dedicated To The Tribal Aquaculture Program
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September 2005-Volume 53 |
http://www.fws.gov/midwest/ashland/mtanhome.html
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Coordinator: |
Edited
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Topics Of Interest:
The following submissions are the second of a two part series regarding pond construction and repair. This information was received from the Southern Regional Aquaculture Center and has been edited.
Repairing Fish Pond Levees
By: Jim Steeby, Nathan Stone, H. Killian and Dennis Careman
Many fish farms have older ponds that need repair. Pond renovation is expensive, with costs ranging from $300 to 1,000 per acre, equal to one-third or more of the initial earth moving cost of pond construction. Ponds are rebuilt for two major reasons:
Soil type, wind speed, pond size and orientation influence the rate of erosion. Farmers have little control over these factors once ponds are built. To extend the useful life of a pond, farmers must rebuild the levees properly and take measures to reduce erosion by establishing vegetation as quickly as possible after construction.
Building Strong Levees
The goal in rebuilding (or building) levees is to make them strong and as erosion resistant as possible, given the engineering properties of the existing soils. The useful life of a pond depends on levee design and construction practices. Three design features are most important in increasing pond life:
A gentle slope is critical to
dissipating
wave energy before it reaches the bank
Waves damage levees by undercutting the bank and leaving a vertical face on the levee. When all the wave energy is directed onto the levee, the soil erodes rapidly as bank sections slough off into the pond. Increasing the slope of pond levees from 4:1 to 5:1 or 6:1 slows pond erosion on most clay soils. However, for a 6:1 slope, you need to weigh the increased costs against the reduction in levee erosion. For a 10-acre pond with a 6-foot-high levee, building the pond with 6:1 inside slope costs about $3,000 more than one with a 4:1 inside slope. There is more shallow water in ponds with flatter levee slopes, which increases the habitat for marginal aquatic weeds. The wider top lengthens the useful life of a pond by providing more levee area to erode before it becomes too narrow to permit traffic.
Strengthening The Soil
Two primary factors determine a Soils strength in a given levee:
Soil strength greatly affects how much a completed levee can resist erosion. Draglines or trackhoes that simply dump pond sediments in piles and tamp the top of the piles with the back of the bucket do not adequately compact the soil. The compactive force is too small, and the soil is usually too moist for good compression. In highway, dam and building construction, engineers test the soil before and during construction. In practice, experienced contractors can determine if the soil is acceptably moist by its behavior during construction. Soil should be moist enough that equipment does not stir up clouds of dust. On the other hand, it should not be so wet that tires leave ruts deeper than an inch or two in the compacted fill.
A sheepsfoot roller is often used to compact soils. It consists of a cylinder with protruding blunt spikes, designed so that when it has made enough passes to compact the soil adequately, the cylinder no longer touches the ground, but is supported on the protruding feet. Repeatedly passing equipment over the soil increases density, although each pass has less and less of an effect. Dirt pans compact more than dozers, as the wide tracks on dozers are designed to float on top of the soil. If you build a levee with a dozer, also use a sheepsfoot roller to compact the soil. For proper compaction, route equipment uniformly over the fill area during construction.
Rain can interfere with levee repair. Compared to a sheepsfoot roller, rubber-tired equipment leaves a smoother surface that allows rainwater to run off. Compact levees on a slight sideways slope to help them shed water during construction. To make the levee as strong and uniformly compacted as possible, it is critical to place the soil in 6-inch-thick layers and compact each layer before adding the next layer. Running equipment over thicker layers compacts only the surface layer, leaving un-compacted soil underneath that is weak and easily eroded.
Mechanical Or Structural Treatments
Several mechanical or structural treatments also protect pond levees effectively, including rock riprap, geotextile fabric and soilcement. Unfortunately, such methods are prohibitively expensive, except perhaps for spot treatments. A 3-foot-high band (1 feet above and below the waterline) of 4 to 5 inch diameter rock riprap will greatly enhance the life span of the levee. Adding a geotextile base where the rock riprap is placed slows erosion significantly, but raises the total cost. Geotextile material used alone has been successful on the face of levees. The fabric should extend 1 feet above and below the pond waterline. Properly installed geotextile fabric should extend the life of a levee by about 5 years.
Soil-cement is a mixture of site soil and cement to resist erosion. The cement is usually mixed at about 10 to 13 percent by dry weight with slightly plastic clays or silts. The soil-cement mixture is placed in a 6-inch layer and extends 1 feet above and below the normal water line. Correctly installed soil-cement barriers control erosion for a long time.
Vegetation
Vegetation can stabilize stream, river and lake shores. Mulching reworked pond levees (about 100 pounds of straw per 1,000 square feet) works well and planting with rye grass, centipede grass or Bermuda grass as soon as possible to reestablish vegetative cover will help reduce erosion. Contact your county Cooperative Extension Service office for information on cover grasses best suited to your location. Levee-stabilizing vegetation must not interfere with harvesting. Any vegetation that reduces or impedes seining efficiency is unacceptable.
Rebuilding Ponds
Draglines can reach into the pond and result in less severe trenching at the levee base. This method may give relief from the worn levee for an additional 2 to 4 years. The best time to rebuild is during drier periods, which allow the sediment to dry and grass to become established before rains wash soil back into the pond. Late spring or early summer is best. For ponds older than 10 years, draining and drying may be more suitable.
A traditional way to rebuild levees is to drain the pond and wait for the pond bottom to dry enough to support the weight of a tractor and dirt pan. Typically, pond bottom soils crust over, forming a firm surface layer with moist soils below. Disking the surface exposes deeper soils and speeds drying. A bulldozer can break up and mound the bottom soils so that they dry faster. When weather permits, use dirt pans to rebuild levees, such as in the initial construction period. The principal advantage is that by this time soils have dried enough to compact well during rebuilding. Breaking the sediment crust the first time through the pond is essential for good drying.
Alternatively, after the pond bottom dries enough to support a wide-track dozer, use the dozer to push moist pond bottom soil to the levees. The dozer rolls up on the levee to pack the material back in place. Then use tractor drawn dirt pans to finish the pond to grade, leaving a smooth surface that slopes to the drain. Bulldozers and dirt pans can be used to rebuild ponds as small as acre.
For best results, compact the soils in shallow layers using a sheepsfoot roller or wheeled equipment. This method is usually reserved for ponds with deep sediments (more than 2 feet deep) in ponds not drained in 10 to 15 years. After all the water is gone, the dragline digs a 3 to 4 foot-deep drainout ditch along the levee base around the entire pond, which allows water to weep from the pond sediments and leave at the drain.
Slope the drainout ditch to the drain to carry out unwanted water. When the sediments have set up to a soft consistency (usually a week if no rain falls), fill back in a section of the ditch at the shallow end of the pond to form a ramp to allow a size 65D bulldozer to cross into the soft sediments. The bulldozer makes passes down the pond length, rowing up the soft sediment on either side of the blade on each pass.
The sediment should be soft enough to still flow slightly. If allowed to dry too much, the mud pushes down the pond rather than flowing to the sides. The goal is to row up the mud (pile the bottom sediments in rows) down the pond length for good drainage and quick drying. You may need to make more than one pass, several days apart, to complete this stage of the process. The rows will be 4 to 5 feet high and of equal width, with the parent soil showing slightly between the rows. Rains will drain off the rows and flow down the channels left by the bulldozer.
Allow the rows to dry for 2 to 3 weeks, depending on rainfall, or until the rows are a heavy plastic consistency. Then, bulldoze across the rows starting at the pond center, moving toward the nearer levee. The push should pack the entire distance to the levee. When all the sediment has been packed back into the levee slopes, top a small amount of the original clay from the pond bottom back over the packed sediment material. This minimizes erosion during the early period when the pond is refilled.
Expect the levee to settle some as the packed material compacts in the next few months. With this method, a pond drained in late spring or early summer can be back in use by midsummer in most years. Again, compaction is less than optimum using this method.
Rainfall determines the time needed to finish any process, but the initial trenching and rowing process of this method allows rainfall to exit the pond rather than soak into the sediments. Ponds rebuilt this way should last 10 years.
Renovating Leaky Ponds
By: Nathan Stone
Prevention
Proper site selection and pond construction methods will help prevent pond leakage. Landowners should consult with the National Resources and Conservation Service (NRCS) before building a pond. The Service has soil surveys with valuable information on soil properties and their suitability for water management. Taking core samples from a proposed pond site will help identify areas of high permeability and prevent future problems.
Cutting too
deeply during construction
is a common cause of leaky ponds
As adequate compaction and suitable moisture content are critical, a pond basin should be in the middle layer of a soil profile. This middle layer is called the B horizon and typically has the maximum accumulation of clay. The parent material underneath the B horizon may have little clay.
Finally, pond maintenance is important in preventing leaks from developing. Levees should be mowed to prevent tree growth and to discourage burrowing animals. Trees growing on levees are a frequent cause of leaks in older ponds. However, once trees with substantial root systems become established it is better not to cut them down, as leaks typically develop where dead roots decay.
Identify the Problem
In some cases the cause of a seepage problem may be apparent; in other cases, the cause may be unknown. When there is excessive seepage, the first step is to carefully examine the pond dam or levees. Seeps, wet spots or wetland vegetation on or below the dam are indicators of leakage through or under the dam or levees. Water also can leak through muskrat or crawfish burrows or through channels left by rotten tree roots. Older ponds can develop leaks around drain or overflow pipes. If the water drops rapidly, carefully check the perimeter of the pond at the waterline for holes.
In addition to inspecting the pond, gather as much information as possible about the pond history, site characteristics, and construction practices used. Important questions include:
If possible, ask the person who built the pond about potential problem areas. This may help identify and reduce treatment costs. Before undertaking expensive pond renovation, consider that the problem may not be seepage. If the ponds watershed has been altered or reduced in size, less water may be flowing into it. This would give the misleading impression that water is being lost from the pond.
Sometimes ponds only appear to be leaking, or they may leak only temporarily. Ponds built in shrink/swell clays will develop deep cracks when dry. As they are refilled, water runs through the cracks until the soil swells again. A considerable volume of water may also be lost during the initial filling of a pond as it soaks into the surrounding soil, giving the impression of a leak. It is not uncommon for new ponds to have high initial seepage rates that decrease as microbial activity and the accumulation of fine sediment and organic matter help seal the pond bottom. If ponds that have sealed in this manner are dried or re-worked, they may again leak excessively until the seal re-establishes.
Bentonite
Sodium bentonite is a highly plastic clay that expands 8 to 20 times in volume when wet. It is used to fill in voids in porous soils. Bentonite has been used successfully on relatively sandy soil (at least 10 to 15 percent sand) where there is adequate support for the bentonite-treated layers.
as sodium bentonite and should not be used
The amount of bentonite needed varies with soil type and laboratory testing is recommended to determine optimum application rates. Given its high cost, bentonite is more suited to spot treatments or small ponds.
For best results, spread bentonite over the pond bottom at the recommended rate and mix it in with a disk or tiller to a depth of 6 inches. Moisten the soil if necessary, and compact the pond bottom. Seepage will not decrease immediately because the clay swells slowly. Once treated, the pond bottom should be kept moist to prevent the clay from cracking and shrinking. If a pond cannot be drained, granular bentonite can be used as a spot treatment over a suspected seepage area. The granular form allows the bentonite to sink to the pond bottom before dispersing.
Spot Treatments
If seepage appears localized, dig out the suspect area and cover that section with a 1-foot-thick layer of compacted clay soil. Form the clay blanket with two layers of soil, compacting each layer separately. When repairing leaks in dams or levees, avoid making vertical cuts, as they are difficult to seal. Dam or levee cuts should be made in a broad V shape so the new fill will bond with existing soil as it is compacted. If a spot treatment will not work or if leakage cannot be localized, the entire pond basin will require treatment. Methods for complete pond basin work-overs include simple compaction, compaction aided by additives, or the addition of a liner.
Liners
Properly installed, a liner is 100 percent effective in stopping seepage
For soils with high gypsum or sand content, a liner is the only option. Because of the relatively high cost, liners have been used primarily for ornamental ponds, water storage, fish hatcheries, and research facilities. However, advances in liner technology achieved through their widespread use for landfills, hazardous waste, and water storage facilities have caused prices to fall. Proper liner installation is important. A liners edges usually are anchored into a trench built around the pond perimeter.
Many factors influence the selection of a liner material, and consumers have a wide choice of products. Modern plastics are environmentally safe, withstand freezing, and are UV-stabilized for long-term exposure to the sun. With care, a pond liner can last 20 to 25 years.
Liner thickness is measured in thousandths of an inch (mils). Twelve to 20-mil liners can be used in sandy soils, while thicker liners are required to prevent punctures in rocky soils. In some cases, an inexpensive geotextile mat is applied under the liner to protect against punctures. Liners 20 mils or less in thickness are more suitable for temporary uses. For permanent aquaculture facilities, 30 to 40-mil liners are recommended. Thin liners will be punctured by plant sprouts along pond edges unless a sterilant is applied before the liner is installed. Obviously, even thick liners must be protected to a reasonable degree. For example, cattle or deer should not be allowed to trample lined ponds.
Liners can be made from high density polyethylene (HDPE) or low-density polyethylene (LDPE). LDPE is more flexible and is used where there is more shifting or movement of the soil, while HDPE is more resistant to chemicals. Polyethylene is both economical and durable and can be reinforced for greater strength.
Seams on polyethylene liners are best sealed by heat welding or extrusion, which calls for custom fabrication for smaller ponds or professional installation on larger ponds. Vinyl can also be used for liners and, though less resistant to aging than polyethylene, it can be sealed and patched with solvent adhesives.
Liners will become slippery with time, which can create a safety hazard where pond slopes are steep. Using a textured plastic liner or embedding small stones in the liners sides with a liquid urethane rubber product will provide a non-slip surface.
New Innovative Aerator
for Pond Aeration
By:
Aquaculture
Research/Environmental Assoc. (AREA), Homestead, FL.
After 5 years of
development and being tested by universities, Modern Air Aerators are a
non-corrosive airlift driven aerator that will move approximately 9 million
gallons of water per day. This amazing aerator has no moving parts allowing for
maintenance free operation that will run continuous duty for at least 5 years.
Another benefit is that there are no electrical components in the water which
makes Modern Air Aerators harmless for employees.
The list below shows several advantages of the Modern Air Aerators over traditional pond aerators. They include:
Moves 9 Million+ Gallons Of Water Per Day
Transfers 3.162 Lbs Of Oxygen Per Hour Low Operating Costs (Approx. $ 0.12 / Hour) No Moving Parts Continuous Operation And Maintenance Free For At Least 5 Years 100% Heavy Duty Pvc Plate And Fitting Construction (Non-Corrosive) No Petroleum Lubricants |
Low Noise Level ( 60 Dba) Easily Portable Due To Being Lightweight Relieves Animal Stress In Production Ponds Total Destratification At Any Water Depth Operates In A Minimum Of 4' Of Water 5 Year Limited Warranty |
Reference Material from the American Fisheries Society (AFS)
The books listed below represent a sample of the online fish culture references published by AFS.
Also included are selected titles from other publishers.
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