How to Calculate the Drainage Slope to a Percentage

The slope of a drainage pipeline is a measure of its steepness. This is an important factor in designing the pipeline because it determines the rate at which water drains from the land. You can calculate the slope of a drainage pipeline from its length and the change in its elevation. Surveyors typically express the slope in terms of a percentage. This procedure requires the use of a line level, a device that allows you to measure the difference in height between two points over a distance.

2 surveyor stakes
Hammer
Line level
Tape measure
Calculator

Instructions

- 1
  Hammer a surveyor stake into the ground at one end of the drainage pipeline. Hammer an identical stake at the other end of the pipeline. Ensure the stakes are in the ground at the same depth.
- 2
  Hold a line level at a specific height on the highest surveyor stake and record this as Height A.
- 3
  Take a sighting with the line level on the lower surveyor stake. Record this as Height B.
- 4
  Hold one end of the tape measure on the first stake at the height recorded in Step 2. Hold the other end of the tape measure on the second stake at the height recorded in Step 3. Assume for this example the distance between the two stakes is 115 feet.
- 5
  Subtract Height B from Height A to determine the difference in height between the two surveyor stakes. Assume for this example the difference between the two heights is 6 feet.
- 6
  Divide the height difference between the stakes by the distance between the stakes to get the slope. This value is 6 / 115 = 0.052 for this example.
- 7
  Multiply the slope by 100 to obtain the slope percentage. The slope percentage is therefore 0.052 x 100 = 5.2 percent for this example.

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