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Basic drainage design is relatively simple by following this easy 10-step process.

 

Before we start there are 4 important points to remember:

1. Excess water always collects at the lowest elevation points.
2. Design for worst case conditions and not for average conditions.
3. Although obvious, water can only flow downhill.
4. Be generous in the design-if in doubt add an extra collection point (grate) and rather oversize than undersize a component.

In order to help understand the basic steps of designing a drainage system, consider the following example:

A contractor needs to install a drainage system in the backyard of a house in Atlanta, Georgia. The backyard is comprised of 3 areas, a loamy soil flower bed, a grass area and a concrete deck. The flower bed area is 6&#; wide x 20&#; long (area of 120 sq ft), the grass area is 20&#; wide x 24&#; long (area of 480 sq ft) and the concrete deck area is 40&#; wide x 20&#; long (area of 800 sq ft). The distance from the backyard to the street is 100&#;.

STEP 1:
Determine the Rainfall Intensity of the property.

Different parts of the country have varying rainfall intensity profiles. Use Table 1.1 below to determine the Rainfall Intensity (I) of the property. Keep in mind that the table below has been simplified (more sophisticated data is available on the web that contain 100 year one-hour rainfall maps, etc.)

Table 1.1

Rainfall Intensity Zone States in the USA Rainfall Intensity Zone A AL, FL, GA, HI, LA, MS, NC, OK, SC, TN, TX 5.0 inches per hour Zone B AR, CT, DE, DC, IA, IL, IN, KS, KY, MA, MD, MI, MN,MO, ND, NE, NH, NJ, NY, OH, PA, RI, SD, VA, WV, WI 3.5 inches per hour Zone C AK, AZ, CA, CO, ID, ME, MT, NV, NM, OR, UT, VT, WA, WY 2.0 inches per hour

As the property in our example is in Atlanta, GA, it falls into Rainfall Intensity Zone A with a Rainfall Intensity of 5.0 inches per hour.

STEP 2:
Calculate the maximum run-off potential flow for each area.

In order to correctly size the drainage pipe network, the maximum run-off flow for each area must be calculated. A frequently used formula for calculating the maximum runoff potential from a small area (under 200 acres) is the Rational Method. The Rational Method Formula is:
where:
Q = peak runoff flow (gallons per minute)
C = coefficient of runoff
I = rainfall intensity (inches per hour)
A = drainage area (square feet)

For our example, we already know the area (A) of each zone to be drained. The flowerbed is 120 sq feet, the grass area is 480 sq feet and the concrete deck area is 800 sq feet.

Next, we will determine the coefficient of runoff (C) for the surface type of each area to be drained using Table 2.1 below. The coefficient of runoff for the loamy soil flowerbed is 0.45, for the grass area is 0.35 and for the concrete deck area is 0.90.

Table 2.1

Surface Type Coefficient of Runoff Concrete or Asphalt 0.90 Clay 0.60 Gravel 0.50 Loam 0.45 Sand 0.40 Grass 0.35

Lastly, we will use the Rational Formula to calculate the peak runoff flows for each area.

For the flowerbed: Q = (0.45 x 5.0 x 120) / 96 = 2.8 gallons per minute
For the grass: Q = (0.35 x 5.0 x 480) / 96 = 8.8 gallons per minute
For the concrete deck: Q = (0.90 x 5.0 x 800) / 96 = 37.5 gallons per minute

Adding these together, the total combined area maximum runoff flow (Q) is 49.1 gallons per minute.

 

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STEP 3:
Choose the number of drainage zones.

When designing a drainage system, attention must be paid not to exceed the maximum flow capacities of the drainage pipe (as show in Table 3.1 below). Using either 3&#;, 4&#; or 6&#; drainage pipe is recommended on most residential and light commercial projects as these pipe sizes and applicable fittings are readily available and easy to install.

Table 3.1

Pipe Size Maximum Flow Capacity 3&#; 44.0 GPM 4&#; 75.0 GPM 6&#; 175.0 GPM

In our example and using Table 3.1 above, there are two choices to accommodate the maximum property run-off flow calculated in step 2 of 49.1 GPM.

1. A single 4&#; or larger pipe may be used (a single Drainage Zone system).
2. Two separate 3&#; or larger pipes with two discharge points could be used (a two Zone Drainage system).

For the example we are using, as 3&#; pipe is easier to install and cheaper than 4&#; or 6&#; pipe, we will select a two Drainage Zone system using 3&#; pipe with two independent discharge points. However, if 4&#; or 6&#; pipe is more readily available than 3&#; pipe, you may choose to use that instead.

Drainage Zone 1 will comprise the flowerbed area and the grass area (maximum run-off flow for that combined area is 11.6 GPM) and Drainage Zone 2 (maximum run-off flow is 37.5 GPM) will comprise the concrete deck area.

 

STEP 4:
Locate the basin and/or grate positions.

Locate the low spots on the property where excess water could accumulate. Once these are finalized, they become the basin and/or grate locations. In the example above, let us assume that each of the 3 areas described has a single low point within its confines. The first basin and/or grate is located in the flowerbed, the second basin and/or grate is in the grass area and the third basin and/or grate is in the concrete deck. If there is more than one low point within an area, each of these would be a collection point and a location for a basin and/or grate.

 

STEP 5:
Decide whether a flat or atrium (domed) grate is appropriate.

Flat grates are typically used in areas where there is pedestrian, wheelchair, or light duty auto traffic. Atrium grates are domed and typically used where there is no pedestrian traffic and where mulching, leaves, or other debris could be present that could plug the flat grate.

In the previous example problem, assume that the flowerbed has been mulched and leaves may also be present. Therefore an atrium grate (Grate 1) is recommended. For the grass area, a flat grate (Grate 2) must be used as there will be lawnmower and foot traffic present. For the concrete deck area, a flat grate (Grate 3) must also be used.

STEP 6:
Select the appropriate grate size for each area.

Using the maximum runoff flows from each area that we calculated in Step 2, divide each area&#;s flow by the number of grates locations in that area to calculate how much flow each grate needs to handle. Once you have calculated each grate&#;s flow requirements, use Table 6.1 below to select the grates.

Table 6.1

Grate Maximum Flow Capacity 3&#; Round Flat 3 GPM 4&#; Round Flat 6 GPM 6&#; Round Flat 16 GPM 6&#; Square Universal 11 GPM 9&#; Square Flat 50 GPM 12&#; Square Flat 70 GPM 18&#; Square Flat 120 GPM Grate Maximum Flow Capacity 3&#; Atrium 12 GPM 4&#; Atrium 20 GPM 6&#; Atrium 36 GPM 9&#; Atrium 40 GPM 12&#; Atrium 65 GPM Grate Maximum Flow Capacity 3&#; Brass 4 GPM 4&#; Brass 7 GPM

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