and why it plays an important role in the health of your grass

One of the most important aspects for natural grass sports field construction is designing the field for maximum drainage to remove excess moisture from rainfall prior to and/or during an event. If fields are constructed without any type of underground drainage system and they do not have an adequate crown to remove excess moisture from the field rapidly, the topsoil on the field becomes saturated and the water will do one of two things, pond or run-off. In most cases, the water starts ponding because there is little or no crown on the field. Playing games when the soil is saturated will cause severe damage to the field and will increase the amount of soil compaction, which will only make the drainage problem within the field worse. Over the years of working with sports field construction, I have always been amazed at how little attention is given to this critical phase for sports field construction or how many times at the last minute, the underground drainage system was removed from the original design specifications to try and save money. Key factors to consider when designing drainage systems for a sports field include; movement of water onto the field from surrounding areas, type of soil and soil depth, ability to remove excess water away from the field ( storm sewer, creek, etc.) and weather factors such as rainfall.

One of the first steps in assuring that a proper drainage system will be designed for a new or existing sports field, is to hire an architectural firm and/or contractor that has extensive experience in the drainage of sports fields. Just because someone knows how to run a backhoe, doesn't mean they can drain a sports field. During the design phase, it is important to hire someone such as an experienced sports field architect to develop a very detailed set of specifications for the sports field to be constructed or renovated. I am also amazed at how many sports fields are built without any design specifications for soil type and depth, drainage system, irrigation, etc. Then, when a problem arises with the field, the school, college or city that contracted to have the field built has no recourse to have the problem corrected.

Excess moisture is generally removed from the playing surface of a sports field in one of two ways, surface run-off and through an installed underground drainage system. For sports fields built out of a native soil, surface drainage is going to be the main means for removal of excess moisture from the field to the sidelines where the underground drainage system picks up the excess water.

For football fields, the general guidelines is to provide a 2.5 % slope from the crown in the center of the field to and past the sideline areas of the field. It is important to install a permitter drainage system on the sideline areas of the field to remove the excess moisture from these areas of the field. You may lower the crown of the field to 1.0 % slope from sideline to crown, but then an underground drainage system under the whole field will be required. Note, many high school football and city park fields are also being used for soccer following the football season. If this is the case, then the slope for the field needs to extend well beyond the normal sideline area of the football field. If this is not done, then the sideline areas of the football field were the players stand during games becomes torn up and this will make a very poor playing surface for the soccer games. For soccer fields built out of native soils, the recommendation is to provide a 1.0 to 2.5 % slope to the crown. While a 1.0 % slope to the crown is preferred by most soccer players, they also don't like playing in a swamp. If an underground drainage system cannot be included in the design of the soccer field then it would be best to use a 2% crown for the soccer field to insure good surface drainage, but a perimeter drain system is required along the edges of the field. While a crown in the center of football and/or soccer field is the most effective method to provide for surface drainage, it should be recognized that for some fields, surface drainage can be provided for by sloping the entire field from sideline to sideline or from end zone to end zone. However, the further the surface water has to flow to get off the field, the less effective the surface drainage becomes. Any low areas in the grade will disrupt the flow of surface water from the field and a drainage system is all but required. 

For baseball fields, the high point of the field should be the pitchers mound, which is normally 10 inches higher than home plate. Then the field should be sloped in all four directions away from the mound at a 1.0 to to 1.5 % slope. As with football and soccer fields, this slope needs to continue to the sideline areas of the field and then some type of drainage system installed to remove excess moisture from this area of the field. Also, for baseball fields it is very important that this 1 to 1.5% slope continue across the skinned portions of the baseball field and into the outfield. If constructed correctly, there will be  very little movement of water down through the soil profile in the skinned area of the field. Surface drainage is the main method to remove excess water from this portion of the baseball field. Another problem that often occurs on baseball fields is the build up of  lips on the grass areas next to the skinned area. These lips will prevent water from flowing off the skinned area and into the grass portions of the field. Regular maintenance practices should be used to prevent the occurrence of lips around the skinned portion of  the field. From the skinned area, the field is then sloped at a 1.0 to 1.5% slope using a cone laser to the outfield fence. Even with the slope in the outfield an underground drainage system will remove the water up to 6 times faster then without it. Which in turn brings the field back to playable conditions after a large rain just hours before.

Another step in sports field drainage that is often overlooked by the inexperienced contractors, is removal of the excess moisture from the subgrade. Movement of water into the subgrade of the field can only occur once the topsoil profile becomes saturated. Then, drainage of the field occurs at the rate of water percolation down through the subgrade, which as a rule is much slower than the percolation rate for the topsoil. If means are not provided to assist in the removal of excess water once it hits the subgrade, then the field will tend to hold water at the interface between the subgrade and the topsoil. During periods of heavy rainfall, the field then acts like a bathtub and fills up with water, thus dramatically increasing the amount of time required for the field to drain. On native soil fields, it is very important that the subgrade not be heavily compacted prior to placing the topsoil mix over the subgrade. Use of heavy equipment is often one of the big mistake inexperienced contractors make in building a sports field. Once the field is roughed in, only flotation tire and rubber track equipment should be permitted on the field surface. Compaction of the subgrade will further reduce the flow of water from the topsoil profile down into the subgrade, thus increasing the time required for removal of excess moisture from the topsoil. That is why it is important to install an underground drainage system in the field to remove excess water from the subgrade area of the field.

Correct installation of the drain lines in a new or existing sports field is critical to the successful removal of excess moisture from the field during heavy rainfall periods. Spacing for the trenches should be at a 20 to 30 foot spacing for sand based fields. However, on native soil fields I would recommend a maximum of 15 to 20 foot spacing. The grade on the trench should have a minimum positive slope of 0.5% with 1.0% grade if all possible. The use of trenchers such as the one shown below with a laser attachment can insure that a uniform, continuous grade occurs in the base of the trench with minimal mess do to the “no spoils” design of this unit. If any low areas occur in the drainage trenches, it will impede the flow of water through the drain lines and thus allow the fine soil particles, silt and clay, to settle out in the bottom of the drain lines and eventually plug up the drain lines. If the soil is not stable, then it may be necessary to line the drainage trench with a filter fabric in order to keep the silt and clay soil particles found in the subgrade from moving into the trenches and thus slowing down the movement of water into the drainage pipe. Next, place a 2 inch layer of USGA spec pea gravel in the bottom of the trench. Place the drain pipe used for drainage on top of the gravel layer and then top off the pipe with a additional 2 inch layer of the same pea gravel material. For most sports fields, a USGA spec sand layer is then placed over the gravel to the top of the drainage trench in the subgrade. Again, remember that it is just as important to verify the bridging between the pea gravel and sand layer to make sure that the sand does not filter into the pea gravel and then into you drainage system. A certified lab will be about the verify the bridging effect. Most sports fields today use the corrugated HDPE pipe with fine slits ( perforated ) in the plastic for the drainage system. Note, the common size for drainage trenches in the subgrade of sports fields is a eight inch width with a minimum eighteen inch depth to clear the irrigation system and to make sure that the pipe is placed deep enough in the field so that aerification with a deep tine aerifier will not pierce the pipe. You might be surprised how often this actually happens. For most new or existing football fields with the proper slope from crown to sidelines, the drain lines are installed lengthwise in the field. This requires much less trenching and also allows for maximum interception of water flowing from the center of the field toward the sideline areas of the field.