Planning the pond
Although excavated ponds can be built to almost any shape desired, a rectangle is commonly used in relatively flat terrain. The rectangular shape is popular because it is simple to build and can be adapted to all kinds of excavating equipment.
Rectangular ponds should not be constructed, however, where the resulting shape would be in sharp contrast to surrounding topography and landscape patterns. A pond can be excavated in a rectangular form and the edge shaped later with a blade scraper to create an irregular configuration (fig. 34).
The capacity of an excavated pond fed by surface runoff is determined largely by the purpose or purposes for which water is needed and by the amount of inflow that can be expected in a given period. The required capacity of an excavated pond fed by an underground waterbearing layer is difficult to determine because the rate of inflow into the pond can seldom be estimated accurately. For this reason, the pond should be built so that it can be enlarged if the original capacity proves inadequate.
Fig. 34. Geometric excavation graded to create more natural configuration.
Selecting the dimensions—The dimensions selected for an excavated pond depend on the required capacity. Of the three dimensions of a pond, the most important is depth. All excavated ponds should have a depth equal to or greater than the minimum required for the specific location. If an excavated pond is fed from ground water, it should be deep enough to reach well into the waterbearing material. The maximum depth is generally determined by the kind of material excavated and the type of equipment used.
The type and size of the excavating equipment can limit the width of an excavated pond. For example, if a dragline excavator is used, the length of the boom usually determines the maximum width of excavation that can be made with proper placement of the waste material.
The minimum length of the pond is determined by the required pond capacity.
To prevent sloughing, the side slopes of the pond are generally no steeper than the natural angle of repose of the material being excavated. This angle varies with different soils, but for most ponds the side slopes are 1:1 or flatter (fig. 35).
Fig. 35. Typical sections of an excavated pond.
If the pond is to be used for watering livestock, provide a ramp with a flat slope (4:1 or flatter) for access.
Regardless of the intended use of the water, these flat slopes are necessary if certain types of excavating equipment are used. Tractor-pulled wheeled scrapers and bulldozers require a flat slope to move material from the bottom of the excavation.
Estimating the volume—After you have selected the dimensions and side slopes of the pond, estimate the volume of excavation required. This estimate determines the cost of the pond and is a basis for inviting bids and for making payment if the work is to be done by a contractor.
The volume of excavation required can be estimated with enough accuracy by using the prismoidal formula (Eq. 6):
V = volume of excavation
A = area of the excavation at the ground surface
B = area of the excavation at the mid-depth (1/2 D) point
C = area of the excavation at the bottom of the pond
D = average depth of the pond
27 = factor converting cubic feet to cubic yards
As an example, assume a pond with a depth, D, of 12 feet, a bottom width, W, of 40 feet, and a bottom length, L, of 100 feet as shown in figure 35. The side slope at the ramp end is 4:1, and the remaining slopes are 2:1. The volume of excavation, V, is computed as follows:
If the normal water level in the pond is at the ground surface, the volume of water that can be stored in the pond is 3,996 cubic yards times 0.00061963, or 2.48 acre-feet. To convert to gallons, 3,996 cubic yards multiplied by 201.97 equals 807,072 gallons. The sample procedure is used to compute the volume of water that can be stored in the pond if the normal water level is below the ground surface. The value assigned to the depth D is the actual depth of the water in the pond rather than depth of excavation.
A summary of methods for estimating the volume of an excavated pond is provided in appendix A. This summary information is reprinted from NRCS (for- merly SCS) Landscape Architecture Note No. 2, Land- scape Design: Ponds, September 2, 1988.
Waste material—Plan the placement or disposal of the material excavated from the pond in advance of construction operations. Adequate placement prolongs the useful life of the pond, improves its appearance, and facilitates maintenance and establishment of vegetation. The waste material can be stacked, spread, or removed from the site as conditions, nature of the material, and other circumstances warrant.
If you do not remove the waste material from the site, place it so that its weight does not endanger the stability of the side slopes and rainfall does not wash the material back into the pond. If you stack the material, place it with side slopes no steeper than the natural angle of repose of the soil. Do not stack waste material in a geometric mound, but shape and spread it to blend with natural landforms in the area. Because many excavated ponds are in flat terrain, the waste material may be the most conspicuous feature in the landscape. Avoid interrupting the existing horizon line with the top of the waste mound (fig. 36).
Waste material can also be located and designed to be functional. It can screen undesirable views, buffer noise and wind, or improve the site’s suitability for recreation (fig. 37). In shaping the material, the toe of the fill must be at least 12 feet from the edge of the pond. In the Great Plains you can place the waste material on the windward side of the pond to serve as a snow fence for collecting drifts in the pond. These banks can also reduce evaporation losses by breaking the force of prevailing winds across the pond.
Fig. 37. Waste material and plantings separate the pond from a major highway
Perhaps the most satisfactory method of handling waste material is to remove it from the site. Complete removal, however, is expensive and can seldom be justified unless the material is needed nearby. Waste material can sometimes be used advantageously for filling nearby low areas in a field or in building farm roads. If state or county highway maintenance crews need such material, you may be able to have them remove it.