In parts 1 (link) and 2 (link) of our drainage series, we discussed getting water off the field and through the profile. In part 3 we’ll talk about getting water away from the field once it’s removed from the surface. A common strategy is the installation of a drainage system.
First, a reminder that installed drainage systems are not a solution for a wet infield skin – to address this issue, install a quality soil material and establish a positive surface grade. See Part 1 of the Maintenance Minute drainage series for more information about wet infields. This article will focus on installed drain systems for soil-based, natural grass portions of sports fields.
The first question to ask is whether the wet conditions are due to surface water or to subsurface water. As discussed in the two previous articles, excess surface water (i.e. rainfall) must exit the field via a positive surface grade. Only then can installed drains be considered as a possible upgrade. However if the drainage issue stems from the presence of subsurface water (i.e. high water table), the installation of closely spaced drain lines may help lower the saturated zone. This practice has been used by farmers for over 100 years to make otherwise wet land suitable for cultivation. An easy way to differentiate between surface and subsurface drainage issues is to dig a test pit 1-2 feet deep. If the surface is dry but the hole fills with water within 24 hours, you probably have a subsurface water problem.
Figure 1. The top profile shows poor surface drainage due to a low spot, while the bottom profile shows a drainage problem caused by a high water table and capillary rise.
Even when a field is crowned correctly and is devoid of a high water table, it may remain wet after heavy rain. Often the soggy conditions persist because “heavy clay” soil is present and retains too much moisture (this is why we call it heavy clay; when dry, clay is actually lighter than sand). In this case a system of interceptor drains can be installed to expedite the drainage process. These drains still rely on a proper surface grade to feed them, but minimize the distance water must flow across the surface before being removed. A drop of water falling on the center of the field now must only travel 25-30 feet before it can infiltrate downward, rather than the 160 feet which would be required if the crown were the only means of removing water from the surface.
Figure 2. The top diagram shows the distance water must flow on a crowned field without installed drains. The bottom diagram shows shorter arrows, indicating the lesser distance water must flow across the surface before it percolates into a drainage trench. Both systems rely on a surface crown to move water- either totally off the field or into the drains.
Trenches are cut at an oblique angle to the surface grade and a perforated pipe is laid at the bottom. The trench is then filled with coarse sand. It is critical that the coarse material is backfilled all the way to the surface! By far the most common mistake with these installations is covering the trench with a layer of native soil. The soil will limit downward movement of water and will create a “perched” water table above the trench. This area will remain soggy even after the field has drained to field capacity. Contrary to popular belief, the grass does not need this soil layer to grow. In fact, the grass atop the sand is normally the healthiest turf on the whole field, due to the improved oxygen content of the rootzone.
Figure 3. The top diagram shows a properly designed interceptor drain, with the coarse sand filled all the way to the surface. Water flowing across the surface infiltrates rapidly into the trench and is carried away by the perforated pipe. The bottom diagram shows an incorrect installation where soil has been placed above the drain – water cannot percolate through the fine-textured soil and the drainage system provides little benefit.
Installed drain systems are not the answer for every field. Maintenance costs for fields with installed drain lines are higher – aerification plugs must be harvested and the field must be topdressed with sand to prevent clogging of the trenches. If these practices are ignored, the drain system will fail within a few years. In addition, installing drainage lines as a “band-aid” fix without first re-establishing a proper surface crown will not prevent water from collecting in low areas. Before the installation all materials used as trench fill should be evaluated by a soil testing laboratory.
Check out other editions of the H&K Maintenance Minute for tips on turf management and ways to keep your fields in great shape! Contact Evan Mascitti at evan@HKSportsFields.com for more information on correcting drainage issues with your field.