Cardinal Rule #1: Proper seepage drainage must have an adequate airspace
If we could only explain one principle to improve people’s understanding of proper seepage drainage, it would be that proper seepage drainage must have an adequate airspace to drain to in order to be effective. If a person absolutely understood this one critical concept, 90% of failed drainage installations would be eliminated. Hardly a day goes by that we don’t have some type of discussion with a golf course superintendent, engineer, or golf course architect whereby it is obvious that cardinal rule #1 is either not truly understood, or is underestimated in its importance.
The basis of all seepage engineering is a formula for the proper design of drainage envelopes called the “Terzaghi formula.” This formula was adopted by the USGA in the rewrite of its recommendations for greens construction in 1993 (current “USGA Recommendations for a Method of Putting Green Construction” has made a minor adjustment to the formula) . Most superintendents are familiar with the left side of the equation; D15 (filter) < 4 to 5< D 85 (soil). This deals with the selection of a properly sized gravel in order that the greens mix “bridges” with the gravel. However, there is also a right side of this equation; D15(filter) >4 to 5> D15 (soil). This part of the equation deals with the necessity that proper seepage drainage always has adequate permeability to accept seepage flows during the rain event. Inadequate permeability or capacity within a system that would be used in the construction of a dam or highway can cause catastrophic failure with loss of life. In a golf course, it will cause an otherwise well constructed drainage system to fail. The definition of proper seepage drainage is “proper seepage drainage only occurs when water moves from a lower to a higher permeability, with the end of the system being the ultimate permeability created by a free flowing pipe.” A proper relief is always the most critical part of a seepage system. This means that it must have an airspace at all times.
“proper seepage drainage only occurs when water moves from a lower to a higher permeability, with the end of the system being the ultimate permeability created by a free flowing pipe”
Part of the misunderstanding comes from the fact that different rules apply to surface water than they do to seepage water. Surface water can be moved as long as there is an exit at any elevation that is lower than a basin that collects water upstream. Therefore, things such as bubblers can be employed at the end of a pipe transporting surface water. However, a bubbler implies that the end of the pipe is full of water, and the water will move to the surface whenever water in the pipe reaches an elevation higher than the ground elevation where it is surfacing. The hydraulic head within the pipe will send the water to the surface. However, look how this would work if the bubbler was placed at the end of a seepage pipe. The water that was required to backup into the pipe to an elevation high enough to force the water out would fill the voids in the pipe, as well as the aggregate or drainage envelope that surrounded the pipe. Not only would this cause the water to seep out of the pipe into the profile, but the airspace that is required for the water that is trapped on the profile would not be able to move down from the force of gravity.
Another example is a fairway that drains to a fluctuating water level. This could be tidally affected, or it could just be a stream or lake that backs up with heavy rain. Often a contractor or superintendent will install a seepage system when the water is at resting level, and feel that he has more than enough space for the system to be relieved. He never considers that when it rains, which is when the system is needed, the airspace that is normally available will go away, and no longer be available. Therefore, we always stress that when a relief is chosen, the decision must be made on where the water level is at its highest, not its resting level.
Our favorite, however, is the use of “dry wells.” The theory is that the “dry well” will fill up with water and bleed out to the surface. If the definition of proper seepage drainage is as stated above; “proper seepage drainage only occurs when water moves from a lower to a higher permeability, with the end of the system being the ultimate permeability created by a free flowing pipe,” a “dry well” fails to meet this definition. The “dry well” filled with gravel is a higher permeability surrounded by a lower permeability material that is present in the native soil. As the result, during a rain, the “dry well” will collect water from the surface, the walls surrounding the well, and the seepage drainage that is draining material of the same permeability as that which surrounds the “dry well.” As the result, there has been no actual drainage constructed anywhere in this configuration. Only a temporary storage compartment was created that will quickly fill with water. Once it fills with water, there is no airspace left, and all seepage drainage will cease.
These are the more obvious issues regarding how a free flowing airspace affects the performance of seepage drainage. However, the more subtle issues revolve around what we consider when deciding what is a proper relief. A proper relief will vary in terms of depth for different soil types and water types. We will discuss this further in another newsletter.