Once upon a time on a dam and levee not so far away, there lived a family of Basidomycetes fungi...
What at first sounds like a fictional tale is really the story of an interesting surface feature observed on two earthen embankments. Dams and levees should receive routine inspections that include visual checks for surface features that could indicate seepage and be warning signs of impending problems. Experienced engineers and inspectors know to look for patches of dark green grass on the downstream face of water-impounding structures that suggest ample moisture in the ground which can be an indicator of seepage.
During the spring of 2013, we observed localized areas of dark green grass on the downstream face of a dam and a levee in southeastern Indiana. Interestingly, instead of solid patches, these areas appeared to be individual and overlapping rings of lush dark green grass that were on the order of about 8 to 10 feet in diameter. Often, the rings were incomplete, but the breaks in the rings were not consistently located on the same place on the ring circumference.
Seepage through earthen embankments would be expected to cause localized saturation that would create solid patches of dark green grass – not the open-center rings that were observed on these two structures. To solve this mystery, we began some Internet detective work and consultation with a local biologist and lawn care experts. What we learned is that these rings are not due to seepage, but instead are caused by a fungus living in the grass vegetative cover on the embankments.
The southeastern Indiana dam and levee were constructed by compacting clayey soils, which are intended to resist seepage. The downstream faces of both structures have a thin veneer of topsoil for the purpose of establishing vegetative cover to resist surface erosion. The grass cover on both structures receives regular mowing and the grass clippings are mulched and left in-place to recycle nutrients. Over time, the un-decomposed grass clippings build a layer of thatch on the compact clayey soils. This provides ideal conditions for the saprophytic fungi of the Basidomycetes class, which feed on the grass clippings.
There are nearly 60 species of Basidomycetes of the order Agaricales. A common feature of these fungi is that they originate at a central point and spread out radially in all directions beneath the ground surface as a white, cotton-like mass of mycelium. The mycelium typically advances at a rate of about one foot per year. As they radiate outward, the mycelium inside the advancing ring release digestive enzymes with nutrients including nitrogen, which are used by the overlying grass and causes the dark green coloration of the turf.
Superficial fairy rings are caused by mycelium that live in the thatch and generally do not send up fruiting bodies (mushrooms). Other types of non-superficial fairy rings are classified as Types 1, 2, or 3. These non-superficial fungi cause a hardened layer of soil above the advancing ring, preventing soil percolation and causing the turf to die, which usually results in rings of brown, dead grass. Type 1 fairy rings are characterized by a ring of dead turf with adjacent stimulated grass. Type 2 fairy rings exhibit a ring of stimulated grass. Type 3 fairy rings typically do not release enough nitrogen to stimulate grass growth, but create rings of mushrooms on the turf. The mushroom rings have been rumored to delineate the paths of dancing fairies, giving these rings their namesake.
The good news is that the superficial fairy rings do not present a hazard for the embankments. In situations where aesthetics are important, fairy rings can be controlled by raking up and removing grass clippings or aerating and using a fungicide. Alternately, their appearance can be masked by regular fertilization and watering of the grass to maintain a more uniform, lush lawn. Such maintenance effort and expense for only aesthetics is typically not justified for dams and levees. Being able to differentiate between superficial fairy rings and signs of seepage is important for engineers and inspectors of water-impounding embankments so that appropriate actions can be taken.
Ron S. Lech, P.E., is a geotechnical engineer in the Cincinnati, Ohio office of Terracon Consultants, Inc. His duties include managing geotechnical studies involving slope stability, bearing capacity, settlement, foundation design, lateral earth pressure analyses, floodwall evaluation, landslide evaluation and remediation, reinforced slope design, mechanically stabilized earth and drilled pier wall design, seepage analyses, earth dam, levee and pavement design. He is a past chairman of the Cincinnati Section Geotechnical Group of the ASCE.
Peter W. Soltys, P.E., P.H., is a senior project manager in the Cincinnati office of Fishbeck, Thompson, Carr & Huber, Inc. (FTC&H). He has more than 35 years of experience in water resources engineering specializing in dams, surface water hydraulics, and watershed hydrology. He has been the president of the Cincinnati Section, ASCE; Cincinnati Post, Society of American Military Engineers; and the Water Management Association of Ohio. He was also a founding member and first chairman of the Ohio Dam Safety Organization.