Many fish farms have older ponds that need repair. Because soils in the major catfish-producing areas of Mississippi, Arkansas and Louisiana are weak and erode easily, fish farmers should expect to repair pond levees every 7 to 10 years.
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. Potential income from fish sales is also lost when ponds are out of production.
Ponds are rebuilt for two major reasons:
- To repair levees that have become so thin that maintenance and feeding equipment cannot pass
- To dry pond bottoms to make harvesting more efficient.
Soft, flocculant (fluffy) sediments accumulate at the bottom of older ponds and impair seining. The sediments are mostly silt and clay eroded from pond levees. Soil type, wind speed, and 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. Establishing vegetation as quickly as possible after construction is especially important.
Building strong levees:
The goal in rebuilding (or building) levees is to make them as strong and as erosion-resistant as possible, given the engineering properties of the existing soils. For a given soil, the useful life of a pond depends on levee design and construction practices.
Three design features are most important in increasing pond life:
- flattening the inside slope of the pond levee (Fig. 1);
- widening the top of the pond levee (Fig. 1);
- strengthening the levee by compaction.
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 (a slope equal to 4 horizontal feet for every vertical foot) to 5:1 or 6:1 slows pond erosion on most clay soils. However, for a 6:1 slope, weigh the increased costs against the reduction in levee erosion.
For a 10-acre pond with a 6-foot-high (on average) 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.
You may need additional grass carp to control submerged aquatic vegetation along extended shallows in ponds with slopes greater than 4:1. The wider top lengthens the useful life of a pond by providing more levee to erode before it becomes too narrow to permit traffic.
For a 10-acre pond with a 6-foot-high (on average) levee, building the pond with a levee top 6 feet wider costs about $3,000 more than a pond without the extra 6 feet.
Strengthening the soil:
Two primary factors determine a soil’s strength in a given levee:
- Force and degree of compaction;
- Soil moisture at the time of compaction.
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 compact the soil much. The compactive force is too small, and the soil is usually too moist for good compaction.
In highway, dam and building construction, engineers test the soil before and during construction. The American Society for Testing and Materials (ASTM) sets standards and procedures for testing.
The most common laboratory test of compaction, the Proctor test (ASTM D698), involves compacting a soil sample under standard conditions (a set number of blows from a given weight hammer dropped from a specific height). Water is added repeatedly and the soil retested to determine the optimum moisture content for compaction (that which produces the densest soil).
For example, a typical clayey soil in the Delta (Alligator series) that is highly plastic (high shrinkswell soil) has an optimum moisture content ranging from 23 to 28 percent. Tests of compacted soils from a job site are compared to these results.
Specifications often call for on-site compaction to meet or exceed a certain percent, often 95 percent, of the optimum compaction (soil density) obtained in the laboratory. Thus, pond levee soil compaction is typically discussed in terms of 95 percent Proctor or 95 percent ASTM standard (ASTM D698).
In practice, experienced contractors can determine if the soil is acceptably moist by its behaviour 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.
This is referred to as the feet “walking” the roller out of the fill. A sheepsfoot roller should “walk out” of the fill in four to five passes. Soil that is too dry can be watered with a tank truck to increase soil moisture.
For proper compaction, distribute the water uniformly throughout the soil by deep disking repeatedly before compacting the soil. Clayey soils need time for the water to soak into the soil before compaction. This can be done by watering the borrow area rather than wetting the fill.
Work overly moist soils with a harrow or disk to dry them faster. Where soils tend to seep, reduce permeability by compacting soils that are wetter than optimum. Tractors have been used to pull large rollers in ponds with 1 foot of water to help seal newly constructed, leaking ponds.
For some clay soils, a few percent change in soil moisture content can alter the permeability by one or two orders of magnitude. Strengthen the soil by increasing the force of compaction, by increasing either equipment weight or compaction frequency.
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 (lifts) and compact each layer before adding the next layer.
Running equipment over thicker layers compacts only the surface layer, leaving uncompacted soil underneath that is weak and easily eroded.
Authors:
Jim A. Steeby , Nathan Stone , H. Steven Killian and Dennis K. Carman
