Soil is used to construct levees and keep water in the pond. Ponds built with unsuitable soils will have levee erosion and excessive seepage, requiring more water to compensate for losses.
Soil texture is the most important physical characteristic related to pond construction. Texture depends on the sizes and shapes of soil particles and the distribution of those particles.
Soils contain coarse-grained particles (gravel and sand), fine-grained particles (silts and clays), organic matter, and water. A good soil for compaction has a wide range of particle sizes.
A clay content of 15 to 20 percent is preferred, although soils with 5 to 10 percent clay can be used if well graded. Coarse-textured sands and sand-gravel mixtures are unsuitable.
The amount of water in fine-grained soils determines how workable these soils are. Soils with too much water should be allowed to dry before levee construction. Conversely, soils that are too dry may need water added to aid compaction.
Seepage is the amount of water that exits the pond through the soil either at the bottom of the pond or through the levees. To prevent seepage, the pond must contain an impervious base layer that is 12 to 18 inches (30 to 45 cm) thick across the pond area.
Other soil characteristics that influence seepage include the amount of organic matter in the soil and the depth to the hardpan. The amount of water needed to maintain water levels in ponds with different seepage rates is illustrated in Table 3.
These water requirements are based on evaporation and rainfall for the various locations and assume that all rainfall is captured and no overflow occurs. Soil texture, water content, and other soil properties should be evaluated to the maximum depth of excavation.
Knowing soil characteristics over the entire site can be useful in determining whether particular areas need special attention or should be excluded. If soil conditions vary across the site, delineate the construction area and take core samples to the necessary depths along a 55-yard (50-m) grid.
Three or four borings per acre may be sufficient if the soils are uniform. Composite samples for analysis should be taken from the surface, mid-depth, and bottom of the cores. Ponds with sandy areas can be sealed with clay or with pond liners, although such measures can be expensive.
Topography:
One goal in selecting a site is to impound the largest amount of water with the least amount of earth moved. Levee ponds are most efficiently built by using soil from the pond bottom to simultaneously build levees.
In areas that are extremely flat (less that 1 percent slope), the pond bottom will be below the original ground surface, which may not allow the pond to drain entirely. On sites with average slopes of 2 to 5 percent, earth-moving cost can be minimized and ponds can be drained by gravity flow.
Levee ponds built on low-gradient slope sites usually require about 1,100 to 1,200 cubic yards of dirt to be moved per acre (2,078 to 2,266 m3/ha), although the actual amount of dirt that must be moved can vary greatly.
Wetlands:
Wetlands are areas that are normally inundated or saturated by surface or ground water and that usually support vegetation adapted for saturated soil conditions. Examples of wetlands are swamps, marshes, bogs and similar areas.
Although the availability of water and the relatively flat topography of wetlands make them attractive sites for ponds, wetland soils often have a high organic matter content (more than 10 percent).
Organic soils begin to decompose when exposed to air, making them unsuitable for constructing stable levees. Wetland soils are difficult to work with machinery, difficult to compact, difficult to drain and dry, and have a low loadbearing capacity.
Wetlands are, by nature, susceptible to seasonal flooding from heavy rainfall or severe storms. Some wetlands have acid-sulfate soils, which may be highly acidic when dry. Acid-sulfate soils can be used in aquaculture, but will need large amounts of lime to control low pH, which will be a continuing and expensive effort. Soils with more than 0.75 percent sulfur should be avoided.
Any clearing of vegetation or movement of soil in a wetland requires a permit from the United States Army Corps of Engineers. In some states, additional permits may be needed from one or more state agencies. Even if approval is granted, the process can be costly and time consuming.
Climate:
Climatological records for the proposed site should be evaluated to determine average precipitation and the likelihood of floods, droughts, and severe storms—all of which can cause problems. Sites for fish ponds should make good use of available rainfall whether groundwater or surface water is used.
Monthly and annual rainfall averages, minimum and maximum precipitation, and evaporation rates should be determined for each site to estimate water losses and gains. Absolute values as well as year-to-year variations should be assessed.
When rainfall storage capacity is maximized, the costs associated with pumping replacement water can be minimized. Evaporation rates, rainfall amounts, and seepage losses (Table 3) determine the amount of supplemental water needed to maintain pond levels.
Unfortunately, sites with ideal slopes for aquaculture ponds (2 to 5 percent slope) are often located in floodplains or valley floors that are subject to periodic flooding. Flooding of ponds can result in loss of cultured animals, contamination with wild aquatic animals, mixing of poor quality flood water with pond water, and damage to levees.
The NRCS can provide information on historic flood levels and how often they occur, how to avoid sites in flood-prone areas, and how to minimize the flooding of adjacent lands when ponds are constructed. Look for sites that flood no more than five times in 100 years.
Exterior (or perimeter) pond levees and drain pipes should be 20 inches (50 cm) above the historic high flood level when compared to interior levees. This allows the facility to be isolated from rising flood waters.
Droughts can have a significant effect on production, especially where surface waters are used. Sites where losses from evaporation and seepage cannot be replaced by available water resources should be avoided.
If pond water volumes cannot be replenished, water quality can deteriorate. This may cause cultured animals to feed less, which can delay harvest and increase mortality. Increasing a pond’s storage capacity may help prevent drought problems, but may be cost prohibitive and cause production problems associated with excessive water depth in non-drought years.
Severe storms can create prolonged power outages, limit access to facilities, and cause production problems. Severe storms and hurricanes can be commonplace along the Gulf of Mexico and Atlantic coasts. The Texas coastline is struck by more than one hurricane every 2 years. Rainfall records can be used to estimate the maximum amounts of rainfall such events may bring.
Ponds located in these areas should be designed and constructed with levees high enough to avoid over topping by unusually high tides, storm surges, and floods. Higher levees mean that surface water will have to be lifted several feet for transfer to canals and ponds, which increases energy use and pumping costs.
Thus, the best coastal sites for ponds are flat areas that are above the highest tide level.
Author:
Jimmy L. Avery
