I'm often asked why we don't sod over our French drains, leaving the gravel open to the surface? And why we will actually go back in the winter and remove the turf that had grown over them?
Without going into an in-depth discussion of how water moves in the soil, the answer comes from the primary type of water we're trying to capture - surface water.
We have really gummy, slow-draining, clay soil; especially on the upper holes. With the open drains, we are trying to maintain the best possible path for surface water to enter the drainage system. We are trying to catch water sheeting across the sloping turf before it has a chance to soak into the clay soil below. Most golf courses do cover the French drains, but their rate of capture is far slower than if the drains are left open. Turf grass quickly forms a mat of thatch that impedes rapid water movement into the drains. We do try to keep the open channel to less than 4” in width, because the drains do come into play. Thankfully, the Rules of Golf do allow free relief from these areas, whether open or covered by turf.
With our heavy soil, we’re trying to do everything in our power to provide the best possible route for water to enter the drainage pipes below. If impeded even slightly, the water would spill over the drains and slowly soak the clay soil. Wet clay soil forms an unplayable, unmaintainable muck that is very slow to dry, especially in winter. This year has been very bad, with over 10" of rain in just the first 15 days of December.
Sand topdressing is one practice that could be beneficial. Regular fairway topdressing has proven to be very successful in the Pacific Northwest. Unfortunately it is very expensive, a bit disruptive, and must be done regularly throughout the playing season for a number of consecutive years.