How to Calculate Drainage

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Drainage is the amount of water absorbed by the ground in a given area. If you are planning to buy or use a piece of land, then knowing the drainage is important. Using a few simple calculations, you will be able to calculate surface runoff and work out the drainage of an area. You can then use this information to plan for heavy rainfall, perhaps by irrigating the land or improving storm drainage.

Things You'll Need

  • Calculator
  • Measuring tape
  • Divide the area you are surveying by surface type. The three types of surface you should divide your land into are paved or other waterproof surfaces; lawns; and rain gardens or water.

  • Calculate the areas of each category by measuring the length and width of each section and multiplying them together. For example, if you have two paved areas, one 10 feet long and 10 feet wide and one 20 feet long and 6 feet wide, then the total area is 10 x 10 + 20 x 6, or 100 + 120. This means your total paved area is 220 square feet.

  • Research the average rainfall for the area. Use The Weather Channel website (see Resources) to determine how many rainfalls per year occur in your georgraphical area. Insert your ZIP code or town name into the website. Divide the rainfall in inches by 12 to find rainfall in feet. For example, 36 inches of rainfall is 3 feet. You can now work out how much rain falls on each of your surfaces by multiplying surface area by rainfall. The 220 square feet of paved area will receive 3 feet, meaning that in total there will be 660 cubic feet of water on the area per year.

  • Work out drainage by determining the surface runoff. On a paved surface, no water is absorbed; all is surface runoff. On a lawn, 40 percent of water is drainage, and 60 percent is surface run. On a rain garden or body of water, all water is drained; none is surface runoff. Use these statistics to work out how much drainage you have or will need. The 660 cubic feet of water that falls on your paved area will all be surface runoff, and you will need to accommodate this with drainage. If you have 660 cubic feet of water per annum on a lawn, you will have 264 cubic feet absorb into the ground and 396 cubic feet as surface runoff.

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