Quick Facts...
- To control a pond for unwanted aquatic vegetation or fish
you must first determine the volume of the pond.
- Surface area (acres) and average depth (feet) of a pond are needed to calculate pond volume.
- Surface area is based on whether the pond is considered regular or irregular in shape. Regular-shaped ponds are either rectangular, circular or triangular. Irregular ponds can be a combination of semicircles, rectangles or triangles.
When chemical treatment of a pond is needed to control
aquatic vegetation or poison unwanted fish, the first step is to
determine the volume of the pond. Two characteristics of the pond
must be known to calculate volume: surface area (acres) and
average depth (feet).
Determine surface area by contacting the agency or
construction company that built the pond. If this is not
possible, the Soil Conservation Service or county planning and
zoning commissioners might be able to determine the area based on
aerial photographs.
If you have to determine surface area for yourself, the
following examples may be helpful. Many ponds are shaped closely
enough to rectangles, circles or triangles to use simple
formulas. Determining the area of an irregularly-shaped pond is
more difficult.
Regular-shaped Ponds
Rectangle: The area of a rectangle is found by multiplying
the length (L) by the width (W). (Figure 1). Many borrow pits are
this shape.
Circle: The area of a circle is calculated by multiplying
the radius (one-half the distance across) times itself, then
multiplying that answer by 3.14 (Figure 2.) Not many ponds are
round, but often a cove resembles a semicircle. The area of a
semicircle is simply the area of a circle divided by two.
Triangle: The area of a triangle is one-half the base (b)
multiplied by the height (h) (Figure 3). Ponds built by damming a
gully are often this shape.
Irregular-shaped Ponds
Irregularly-shaped areas often can be reduced into
combinations of rectangles, semi-circles and triangles. Calculate
the area of each and add them together to obtain the total area
(Figures 4,5,6).
Areas with irregular shapes require more side to side
measurements. The average of the side measurements can be used as
the width. Then the area is calculated as a rectangle.
An area such as shown in Figure 6 can be converted to a circle.
Measure the distance across the pond intersecting at the pond
center. Take at least six to 12 measurements; the more taken, the
more accurate the measurement. Remember, area of a circle uses
the radius which is half the distance across the circle. So the
average distance across the pond divided by two is equal to the
radius.
After square feet are determined for the pond, it is easier
to make further calculations if square feet are converted to
acres. The total area in square feet divided by 43,560 gives the
acres covered by the pond.
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Figure 1: Calculation of the area of a rectangle. |
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Figure 2: Calculation of the area of a circle. | |
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Figure 3: Calculation of the area of a triangle. |
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Figure 4: Area calculation for an irregular-shaped pond. | |
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Figure 5: Area calculation for an irregular-shaped pond. |
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Figure 6: Area calculation for an irregular-shaped pond. | |
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Figure 7: Maximum depth measurements for a pond having a uniform bottom slope. |
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Figure 8: Use of a series of depth measurements to determine average depth. |
A convenient measure of pond volume is acre-feet, which is
one surface acre that is 1 foot deep. To determine acre-feet,
multiply the acreage by the average depth (in feet). Bodies of
water having uniform bottom slope have an average depth that can
be approximated by dividing maximum depth by two (Figure 7).
Make more accurate determinations (Figure 8) by taking a
series of measurements along an s-shaped line across the length
of the impoundment. Total the figures and divide by the number of
measurements taken to determine average depth.
Take these measurements with a satisfactory sounding device
made by attaching a brick or similar weight to a rope marked at
one foot intervals.
Determine total volume by multiplying surface area in acres
times average depth in feet. The resulting product will be in
acre-feet. With the volume known, calculate the correct amount of
chemical needed from dosages given on the chemical label.
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