Vegetated Filter Strip System Design Manual
Design of Distribution System

3.6 Design of Distribution System

• 3.6.1 Raised Distribution Pipe
  • Figure 3-6 Raised Distribution Pipe
• 3.6.2 Design of Raised Distribution Pipe
• 3.6.3 Distribution Channel

The purpose of the distribution system is to evenly distribute the runoff water across the full width of the top edge of the infiltration area. The recommended method is to use a raised distribution pipe. However, a distribution channel may be used under the following conditions:

• There is insufficient slope at the conveyance outlet to accommodate the raised pipe.
• The runoff can be directly discharged from the integrated storage/settling basin and flow is transferred to the infiltration area via gravity.
  

While the focus within much of this manual will be on the development of a raised distribution pipe, the design parameters associated with both a raised distribution pipe and distribution channel are presented below.

3.6.1 Raised Distribution Pipe

The conveyance pipe will discharge into the distribution pipe, as illustrated in Figure 3.6. In addition, Figure 3.6 describes a raised distribution pipe. The size of the distribution pipe should be calculated using Equation 3.9. A minimum pipe diameter of 100 mm (4 in.) is recommended. The conveyance pipe will penetrate the distribution pipe about midway along its length. A grade for the distribution pipe of 0.1 per cent to 0.3 per cent in both directions from the conveyance pipe connection is recommended. The distribution pipe will be raised 0.5 m to 1 m (1.6 ft to 3.3 ft) above the finished surface of the ground. The distribution pipe will be fastened to support columns (0.1 m × 0.1 m (4 in. × 4 in.) pressure-treated wood) using galvanized pipe clamps. The interval between support columns will be short enough to ensure that the distribution pipe does not sag between columns (typically between 2.4 m (8 ft) and 3 m (10 ft)). The column spacing should take into account pipe joints to allow for adjustment of pipe sections if required. The pipe joints should be situated between support columns. Additional support, e.g., distribution pipe attached to a continuous plank system running between the support columns, may be provided under the pipe in an effort to minimize the number of support columns required (depending on the diameter of distribution pipe). The distribution pipe installed on the top of each slope of the infiltration area will be loose-fitted using bell ends or couplings. The conveyance pipe and tee union connection to the distribution pipe will be hard-fitted. The distribution pipe joints should be screwed together to retain the distribution pipe position.

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Figure 3-6 Raised Distribution Pipe

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Orifices located at design spacing along the length of the pipe provide uniform application. The flow from each orifice is controlled by the size of the orifice and supply pressure (9 to 14 kPa or 1 to 2 psi). The application rate across the top of the infiltration area is determined by the orifice spacing.

Orifices may be placed on one side of the pipe or on both sides for use with back-to-back configurations. Orifices may be variable or fixed. The most common type of variable orifice-pipe is gated aluminum or PVC pipe, such as those used for furrow irrigation. Gates are plastic slide closures that can be adjusted by hand to achieve the desired flow and uniformity of distribution. A minimum gate spacing of 0.6 m (2 ft) is recommended. Fixed orifice pipes consisting of rigid PVC pipe with drilled holes (at a design spacing) can also be used, but provide less operating flexibility than gated pipes. The orifice capacity will be 25 per cent greater than the flow rate delivered by the conveyance pipe to allow for potential clogging of orifices.

A drain hole will be situated at the ends of the distribution pipe to allow all water to discharge from the pipe after delivery of runoff. The drain hole diameter will be 0.005 m. More than one drain hole at either end of the distribution pipe may be required depending on overall length of pipe and slope variations in pipe along length (e.g., no standing water should remain in pipe).

Splash Pad

The splash pad will protect the infiltration area from erosion and assist in even flow distribution to maintain sheet flow over the VFS. A splash pad will be installed beneath each orifice opening using a standard 0.6 m × 0.6 m (24 in. × 24 in.) concrete patio stone or other suitable energy dissipation measure.

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3.6.2 Design of Raised Distribution Pipe

The design requirements of the raised distribution pipe include the sizing of the distribution pipe, establishing the number of orifice openings, and the spacing of the orifice openings along the length of pipe.

Pipe diameters and orifice openings must be large enough to prevent clogging with solids. The minimum standard size for gated irrigation pipe is 152 mm (6 in.), but 100 mm (4 in.) diameter pipe can be specially fabricated.

The design of the raised distribution system will be based on the following design parameters:

• Calculate the pipe diameter using Equation 3.9, assuming slope of pipe between 0.1 and 0.3 per cent and that the pipe will be full. Verify that the velocity of flow is at least 0.6 m/s (2 ft/s). Use a minimum 100 mm (4 in.) diameter pipe.
• Increase orifice capacity in distribution pipe by 25 per cent of flow being delivered from the conveyance pipe to the distribution pipe to account for blockage of orifice holes in distribution pipe.
• Assume the head at orifice holes to be 0.9 m (3 ft).
  

If the supply rate exceeds the capacity of the perforations at minimum head, then the pipeline will flow full, the head will increase, and the flow through the perforations will increase due to the higher available head.

With a pumped system or through a dosing siphon, it is possible to ensure the pipe is fully charged to provide better flow distribution down the pipe. Either the pumped or dosing siphon alternatives require that the distribution piping not be oversized for the planned supply flow rate.

Step 1 - Establish Discharge Capacity of Orifice in Distribution Pipe

Use orifice discharge capacity equation (Equation 3.7) and solve for Q assuming an orifice size of 10 mm (0.4 in.) and a head of 0.9 m (3 ft).

Q=CA(2gh)^0.5

For example, the following values would be used for the equation variables: C of 0.61, A of 7.85 × 10^-5 m² (84.5 × 10^-5 ft²), g of 9.8 m/s² (32.2 ft/s²), h of 0.9 m (3 ft). The resulting discharge would be (0.61) × (7.85 × 10^-5 m²) [(2) (9.8 m/s²) (0.9 m)]^0.5 = 2.0 × 10^-4 m³/s (0.007 ft³/s).

Step 2 - Establish the Number of Orifice Holes to Meet Flow Capacity Being Delivered Plus an Additional 25 Percent

For example, assuming that a flow rate of 5.3 × 10^-3 m³/s (0.19 ft³/s) is being delivered to the raised distribution pipe from the conveyance pipe, then the target discharge volume for the distribution pipe increased by 25% would be 6.63 × 10^-3 m³/s (0.23 ft³/s). Divide the target discharge rate for the distribution pipe of 6.63 × 10^-3 m³/s (0.23 ft³/s) by the orifice discharge rate of 2.0 × 10^-4 m³/s (0.007 ft³/s) to determine the number of orifice holes required along the length of discharge pipe. The result would be (6.63 × 10^-3 m³/s)/(2.0 × 10^-4 m³/s per orifice) = 33. Ensure that there is an even number of orifices, so make 34 orifices. Do not place an orifice in the tee (centre of the distribution pipe). If you use a larger diameter orifice, then you will need fewer orifices.

Step 3 - Determine the Spacing of the Orifices Along the Distribution Pipe

For example, if the distribution pipe is 100 m (328 ft) long then the spacing between the 34 orifices would be equal to 100/34 = 2.94 m (9.7 ft). Start drilling the orifices from the centre of the pipe run (intersection of tee of conveyance pipe and distribution pipe) and place the first orifice half the spacing distance from the centre of the run (tee). This will evenly distribute the orifices on either side of the distribution pipe, placing the last orifice on each side of the pipe, half the spacing distance from the end of the pipe

3.6.3 Distribution Channel

A typical distribution channel will consist of a 0.3 m (1 ft) deep channel with banks that have a slope of 1:1 and a flat bottom that is 0.3 m (1 ft) wide, spanning the width of the infiltration area. The downgradient edge of the channel will be graded so that the edge will be level along the entire length of the channel. The channel will be filled with 30 to 50 mm (1.2 in. to 2 in.) diameter gravel that will extend downgradient of the channel edge about 0.5 m (1.6 ft). This design will provide erosion protection and reduce the potential for blockage of the distribution system as a result of freezing temperature.

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