Turfgrass Drainage

Gary Forrester,
Senior Extension Agent,
Horry County, Clemson University.

HTG 1117

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Water is essential for all plant material to thrive, including turfgrass. However, too much water can have a negative effect on plant growth that causes thin areas in lawns or possibly loss of an entire stand of grass. While irrigation is a key component to proper turfgrass management, drainage can be just as important. Turf loss due to excessive moisture usually occurs when the addition of water exceeds the ability of the soil to drain properly. As with all plants, turfgrass roots need oxygen to grow. Saturated soils will have little air available for root growth, and results in a poor stand of turf.

This lawn has a poor slope to allow for runoff and poor soil porosity. The result is inadequate soil drainage and unhealthy turfgrass.
This lawn has a poor slope to allow for runoff and poor soil porosity. The result is inadequate soil drainage and unhealthy turfgrass.

Water Movement within Soil

The movement of water into and throughout a soil profile is characterized by the soil infiltration and percolation rates. As water is applied to a soil, it will infiltrate into the upper levels and then percolate through the soil down to the water table. The force driving this is gravity, and these components can be measured through various tests. Factors that can influence infiltration and percolation rates include the soil surface condition, soil structure, soil texture, and organic matter content.

Soil texture affects soil porosity, and this has a large influence on how well a soil drains, as well as how much air is available for root growth. Sandy soils have a greater percentage of big pore spaces or open spaces, while clay soils have a greater percentage of small pore spaces. Therefore, sandy soils will drain rather freely, and clay soils will drain slowly. In contrast, sandy soils have poor water and nutrient holding capacity, whereas clay soils will retain water and nutrients better. The addition of organic matter into a sandy soil will help with moisture and nutrient retention, but adding organic matter into a clay soil may help with drainage issues.

Another inhibition to soil drainage is a stratified soil condition. When soils of differing textures are placed one on top of the other, water will not move into the underlying soil until the top layer has become saturated. An example of this would be adding a 2 to 3 inch layer of sand on top of a clay base. Water will not move into the clay until the sand on top has become saturated.

Since water is essential for turf growth, there must be a certain amount of available water in the soil for the roots to absorb. Adequate turfgrass growth will occur with a soil moisture content somewhere between the field capacity of the soil and the wilting point of the plant. Field capacity is defined as the water available in a soil once gravitational water has left the lawn area, while the wilting point is when the turfgrass can no longer extract moisture and begins to wilt. This level of moisture is mainly supplied through irrigation and rainfall.

Issues arise in turfgrass management when the addition of water through rainfall, flooding, or irrigation exceeds surface runoff, evaporation, gravitational flow, and plant use. This results in wet soil conditions that can lead to the following problems:

  • Air is excluded from the root zone, which can suffocate and kill turfgrass roots.
  • Saturated conditions provide a good environment for root rot pathogens to flourish and spread, and subsequently to infect and kill turfgrass.
  • Wet soils can prevent the use of the turf for recreational and sporting activities.
  • Overly wet soils can increase the chance of turfgrass winterkill.
  • The soil becomes softer, which increases the chance of soil compaction.
  • Additional water cannot infiltrate into a saturated soil, and this may increase the chance for runoff and possible soil erosion.

Surface Drainage

Turf grown on well-drained soils generally do not need any supplemental drainage to thrive. However, turf grown on poorly drained sites probably will, especially in areas that receive frequent rainfall. Turfgrass drainage occurs in two basic ways: surface drainage and subsurface drainage. Good surface drainage is essential in removing excess water after heavy or prolonged rainfall. Proper surface drainage of a lawn is created prior to turfgrass installation by grading a 1% slope away from any structure on the property. Sports fields should have a 1 to 2% crown running down the center of the field. Low areas or depressions on the site should be filled. Dry wells, slit drains, or surface drains can be installed in areas where depressions cannot be avoided. Ditches or small channels running between structures to carry surface water off site should also have a 1 to 2% slope to convey water away.

Subsurface Drainage

Natural slopes incorporated into landscapes will assist with the remove of water through surface drainage, but there are situations where an adequate slope may need to be constructed. In this case, if the internal soil percolation rate is poor, the installation of subsurface drainage will also be necessary to remove excessive water.

Drain tiles are made from different materials including concrete, clay, and corrugated plastic pipe. Each material has its advantages and disadvantages; however, corrugated plastic pipe (incorporating both perforated and solid pipe) is the most common material used for small drainage systems.

When the surface grade cannot be raised to allow for surface drainage, isolated depressions in the lawn can be drained in several ways. A dry well dug in this area acts as a water storage system. This removes the water from the root zone and stores it underground until the water has enough time to drain deeper into the soil. A dry well size is dependent on the amount of water that needs to be stored, and its depth should reach a better-drained subsoil. Drain tile can laid in close proximity to the dry well in order to further aid in water directed there. If there are numerous small, poorly drained areas in the lawn, random drains can be built using subsurface drain tiles that lead away from the depressions, or by installing surface catch basins. Connect these small areas through subsurface drain tiles into a main line running off site. It is important when designing a subsurface drain system to bury and slope the drain lines so there is a positive flow away from the poorly drained area to the outflow.

Large areas with uniform slopes can be drained using the gridiron or herringbone design. This type of subsurface drainage utilizes a main line running down the center of the poorly drained area with laterals running off at angles and spaced at certain intervals. The main line then runs off site as an outflow. The number of laterals and the spacing is determined by the soil type and soil wetness. Poorly drained clay soils will require more laterals that are spaced closer together than for better drained soils, which require fewer laterals that are spaced further apart. Calculations can be made using various formulas to determine how many lateral lines will be needed and their spacing.

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