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Athletic Field Drainage Design Assistance

A. Designing a drainage system begins with estimating how much water must be drained from a field and in what amount of time.

1. To determine the amount of water the system must be designed to handle, consider:

a. the area to be drained

b. the greatest anticipated rainfall event

c. acceptable down time

2. Design a system that addresses established needs and considers the budget.
To calculate how many gallons of water a given area will receive in a 1 inch rainfall event, multiply: length • width • .623 (7.48 gallons per cubic foot)

3. System design must include a collector system and a transport system.

a. The Multi-Flow collector system is laid out in a pattern across the field and collects water from saturated areas.

b. The transport system takes water that is collected and carries it off the field to some acceptable off-field location. Varicore strongly recommends a rigid, smooth-walled pipe. (PVC, ABS, or dual-walled corrugated)

B. Designing a transport system
First, consider the total amount of water that will be collected and in need of relocation via the transport system. We will then come back to the Multi-Flow collection system.

1. If a fully saturated field 180 feet X 360 feet receives an inch of rain; 40,390 gallons of water will need to be removed. (180 • 360 • .6233 gal. = 40,392 gal.)

2. In order to evacuate that water in one hour, PVC carrying 673 gpm will be needed.

a. One 8-inch PVC (718 gpm) or

b. two 6-inch PVC (349 gpm each) or

c. six 4-inch PVC (119 gpm each) could accomplish this task.

3. Head pressure on the whole collection system will add to the flow rate. The greater the depth of water on the system the faster the water will flow.

4. Smaller transport pipes can be used at the “upper” end of the field, full capacity will be needed at the “lower” end.

5. In order to carry away more than an inch of rain per hour, more capacity will be required.

6. If there is any danger that the water level at the discharge point will rise higher than the level of the collection system, a check valve should be installed on the discharge pipe. This will avert water from backing onto the field and also prevent contaminated water from backing up into the collector lines and causing siltation.

C. Collector system

1. Transport systems can only carry away water that the collector system has gathered. At least seven factors will affect the rate at which the collector system will desaturate the soil and deliver water to the transport system:

a. Different soil types will release water at different rates:

  • Silt/clay loam (20-45% particles less than .1 mm) will have a hydraulic conductivity of .2 - .4 inches/hour
  • Sandy loam (<20% particles less than .1 mm) will have a hydraulic conductivity of .4 - 2 inches/hour

b. Distance between collector lines
Because water moves through the soil laterally very slowly, 10-15 foot spacings are preferred.

Opt for closer spacings if:

  • Maximum rainfall events are more than average
  • Faster response time is required
  • Long term performance is more important than short term cost.

Opt for wider spacings if:

  • Maximum rainfall events are less than average
  • Fast response time is not required
  • Budget is inadequate

In porous soils
Where soil is light and sandy, water seeps down to the collector pipes soon after a rain. Lines must be closely spaced to avoid excess demand on the system capacity.
           

In dense soils
Where soils consist of clay or heavy loam, water moves down very slowly. Closely spaced lines allow more frequent opportunities for surface and soil water to find its way to the collectors.

c. Depth of collector lines

1. Optimum depth will be determined by at least four factors:

a. anticipated maintenance practices
e.g. deep tined aeration

b. subsoil properties
e.g. irrigation lines, hard pan, clay, other obstructions?

c. performance requirements
e.g. How soon is the field needed after a rainfall event?

d. soil type
e.g. native or amended?,
sandy loam or clay?

2. Typically these factors lead designers to locate the top of the Multi-Flow somewhere between 4 and 15 inches below the surface.

3. Where deep installation is required, bring the sand backfill up to or near the surface to allow for a rapid entry way for surface water.

d. Length of collector lines
The length of the line must not exceed the line’s ability to handle the water. For example:

  • A line of 6-inch Multi-Flow can carry 1020 gallons of water per hour.
  • A saturated 20’X 100’ strip of land would accumulate 1247 gallons of excess water if it received an inch of water in one hour.
  • In this situation, a 100’ run would be inadequate if the goal is to evacuate all the excess water in one hour.

e. The configuration of the collector lines
Lines should be laid out in such a way that water will cross the lines in its natural flow.

  • Crowned rectangular fields are most efficiently laid out in a herringbone and drained to the sideline.
  • Flat rectangular fields are most efficiently laid out in parallel lines.

f. Shape of the drainage product

  • Optimum drainage occurs when the collector pipes allow water to enter as fast as it can carry it away.
  • Multi-Flow drains provide substantially more surface area and consequently, significantly better “in-flow” rates.

g. Carrying capacity of the drainage product
Multi-Flow has far and away the best flow rates of any panel shaped product

  • 6-inch 17 gpm
  • 12-inch 29 gpm
  • 18-inch 45 gpm
That is two to three times the rate of other panel shaped drains.

2. Slope of drainage media

a. For most drainage:
.5% is minimum acceptable slope
1% slope is recommended

  • .01 slope
  • 1 foot of fall in 100 feet of pipe
  • 1 inch of fall in 8 feet

b. Additional slope will increase the flow rate.

c. Wherever possible, fall should correspond to the natural fall of the land. This prevents the collector lines from becoming excessively deep.

d. If collector pipes are becoming too deeply buried, they should be emptied into a transport pipe and a new collector line should begin nearer to the surface.

e. A laser level is essential for establishing and maintaining proper grade.

3. Design for natural turf field

a. Layout

i). Crowned or sloped fields
If the surface of the field has a consistent slope (greater than .5%), it is advised that the collector lines be placed such that they intersect the water flow direction. Placing the lines at a 90 degree angle to the flow direction and running toward the end zone is usually not the best policy. The resulting lines would be too long and, in order to maintain fall, excessively deep. A 45 degree angle running toward the sidelines works well because it allows the lines to maintain grade while also intercepting the direction of surface water flow. The resulting herringbone pattern complements the existing field contour, providing effective drainage as well as an uncomplicated installation.

ii). Flat or irregular fields
If the field is flat, or has less than a .5% slope from the center, then a parallel drainage pattern may be appropriate. Since the water will basically stand on the surface of a flat field, there is no advantage in attempting to place drainage lines in the flow path. Collectors could run toward the sidelines or the center of the field. Proper grade for the collectors will need to be maintained by increasing the depth of the trench.

b. Sideline drain

i) A line of 18-inch Multi-Flow set back 5 to 10 feet from the sideline on each side of the field, makes an excellent sideline drain. This is true for at least two reasons:

a) Sidelines are heavily trafficked. Because a lot of time is spent running, standing, and sitting on the sidelines, drainage is critical in these areas. This is especially true if the field is surrounded by an inward sloping track that might channel additional water to the sideline area.

b) Making a connection from 6-inch Multi-Flow to 18-inch Multi-Flow is less expensive and time consuming than connecting 6-inch Multi-Flow directly to the transport system. This provides intensive sideline drainage at little or no extra cost.

ii) An 18" to 18” to 6” (at 45 degree angle) connector must be placed in the 18-inch sideline drain at each point where the 6-inch collector lines intersect. These will be 1800P or 1800Q connectors depending on whether it is the left or right side of the field.

iii) To avoid overtaxing the 18-inch collector lines, they need to be emptied into a transport system occasionally. To calculate that interval, assume the following:

  • 6-inch Multi-Flow = 17 gallons per minute
  • 18-inch Multi-Flow = 47 gallons per minute
  • 1800P or 1800Q = 112 gallons per minute
  • 4-inch PVC = 112 gallons per minute

Based on the above assumptions the 18-inch line should be emptied into the transport system once for every 6 or 7 lines of 6-inch Multi-Flow. This is accomplished by attaching an elbow or a tee to the bottom of every sixth or seventh 1800P or 1800Q. The 18-inch perimeter drain can be installed without a slope so the outlet to the transport line will be accepting water from both directions. When the system is at full capacity there will be substantial head pressure on the system which will increase the flow rates of all components.

4. Design for synthetic turf fields

a. No trenching required
In most synthetic turf situations, it is most efficient to install Multi-Flow flat. Collector lines can be positioned horizontally directly on the compacted base, or on top of the geotextile soil separator if one is used. No costly and time consuming trenching is necessary.

b. Layout pattern
Synthetic turf fields typically have a consistent center to sideline slope. Collector lines should be placed such that they intersect the water flow direction. Placing the lines at a 90 degree angle to the flow direction and running them toward the end zone is not the best policy because the resulting lines would be too long. A 45 degree angle running toward the sidelines works well because it allows the lines to maintain grade while also intercepting the direction of surface water flow. The resulting herringbone pattern complements the existing field contour providing effective drainage as well as an uncomplicated installation. This drainage pattern should extend all the way to the edge of the synthetic turf so it will include the intensively used sideline area. Transport lines should be located at the edges of the field.

c. Collector line spacing
The coarse sand and rock used in synthetic turf fields is capable of absorbing substantial amounts of water. Consequently, it is acceptable to allow more time for desaturating the base of a synthetic field than a natural turf field and as a result, collectors can be spaced farther apart. Placing collector lines 15 feet apart, outlet to outlet, provides excellent reaction time and uniform drainage performance. Twenty feet apart provides an adequate system. A field employing 25 foot spacings will require significantly longer to drain after a rainfall event. It would be unwise to space them farther apart because allowing water to remain on the compacted base for prolonged periods of time will soften the subgrade and destabilize the base.

d. Sideline drain

i) A vertically installed line of 18-inch Multi-Flow set back 5 to 10 feet from the sideline on each side of the field, makes an excellent sideline drain. This is true for at least two reasons:

a) Because sidelines are heavily trafficked, drainage is critical in these areas. This is especially true if the field is surrounded by an inward sloping track.

b) Making a connection from horizontally installed 6-inch Multi-Flow to vertically installed 18-inch Multi-Flow is less expensive and time consuming than connecting 6-inch Multi-Flow directly to the transport system. This method provides valuable sideline drainage at little or no extra cost.

ii) An 18” to 18” coupler (18009) must be placed in the 18-inch sideline drain at each point where the horizontally installed 6-inch collector lines intersect. Each 6-inch collector line will terminate in a single sided coupler (0600M) which attaches to the 18009 with a 3” elbow.

iii) To avoid overtaxing the 18-inch collector lines, they need to be emptied into a transport system occasionally. To calculate that interval, assume the following:

  • 6-inch Multi-Flow = 17 gallons per minute.
  • 18-inch Multi-Flow = 47 gallons per minute.
  • 1800P or 1800Q = 112 gallons per minute.
  • 4-inch PVC = 112 gallons per minute

Based on the above assumptions the 18-inch line should be emptied into the transport system once for every 6 or 7 lines of 6-inch Multi-Flow. This is accomplished by attaching an elbow or a tee to the bottom of every sixth or seventh 1800P or 1800Q. The 18-inch perimeter drain is installed without a slope so the outlet to the transport line will be accepting water from both directions. Keep in mind that when the system is at full capacity there will be substantial head pressure on the system which will increase the flow rates of all components.

e. Other ways to transfer water to the transport system

i) In some systems, the Multi-Flow collectors empty directly into a perforated perimeter drain pipe located in a stone filled perimeter trench. This can be a very effective system but will lack the longevity of an 18-inch Multi-Flow sideline drain.

ii) In still other systems the Multi-Flow collectors empty into a drained perimeter trench without a direct connection. This can be quite economical but lacks the effectiveness and longevity of an 18-inch Multi-Flow sideline drain.

5. Backfill

a. Clean, very coarse sand is the ideal medium for trapping clay and silt before it reaches the geotextile filter.

b. According to the USDA system of classification, very coarse sand has an approximate particle size of between 1.0 and 2.0 mm.

If sand is being manufactured, a sample top-of-the-line sand specification might read:

When passed over sieves very coarse sand will have:

  • less than 5% retained on a #10 U S standard sieve,
  • less than 5% passing a #30 U S standard sieve
  • no more than 1 % pass through a #50 U.S. standard sieve

c. While it will not be possible to find this exact sand backfill in every location, finding coarse or very coarse, clean sand backfill will be rewarded with increased drain life and performance.

d. It is an unwise and dangerous practice to backfill any wrapped drainage systems with native soil. Particles of clay and silt will build up on the surface of the geotextile eventually halting water in-flow. The rate at which this happens varies from a few days to a few years depending on the soil composition.

e. The same is true under synthetic turf. A blanket of filter sand covering each Multi-Flow run will dramatically extend the life of the drainage system.

f. See Selecting Backfill Material for more information on this topic.

How Artificial Drainage Works
Sample Athletic Field Drainage Drawings

Multi-Flow Installation Advice