According to the National Climatic Data Center, a division of the National Oceanic and Atmospheric Administration (NOAA), nearly 30 percent of the contiguous U.S. is experiencing moderate to extreme drought conditions. The part of the nation affected by “moderate to sever” drought extends from the Great Plains to the West Coast. There are two epicenters of “extreme to exceptional” drought: one encompassing central to southern California and extending into Nevada, and the other centered in the Texas panhandle but reaching into central Texas, New Mexico, Oklahoma, southeast Colorado, and western Kansas. Even Hawaii is experiencing a moderate drought.
Parched vegetation, low reservoir levels, and water conservation measures are familiar daily reminders of the drought. But prolonged drought conditions can also be responsible for significant impacts on the lands that support our buildings, roads and highways, water systems, and other critical structures. Among the most notable and damaging effects of drought are desiccated expansive soils and ground subsidence.
Desiccation of Expansive Soils
Expansive soils are soils that swell when wet and then contract or shrink as they dry. Expansive soils typically contain a large percentage of silt or clay; sandy soils are less likely to have expansive characteristics. As soils swell and shrink they can stress and damage buildings and other structures that they support. Measures commonly employed to protect buildings from the damaging effects of expansive soil movement include deep foundations, deeper than standard conventional foundations, use of non-expansive materials below the structure, and post-tension foundations. Damage related to swelling of expansive soils is fairly well-known phenomenon, but often the most severe effects of expansive soils are seen when the soils become dry and undergo pronounced contraction, or “desiccation.”
In California, the swell-shrink cycle tends to occur seasonally due to the climate pattern of we winters and dry summers. Usually, these seasonal moisture fluctuations affect only the upper couple of feet of soil, as soil moisture tends to be more stable below this depth. During periods of prolonged drought, however, the lowering of the groundwater table typically results in decreased soil moisture to greater depths. In response to the drier conditions, soils contract to a greater extent, and to a greater depth, than during a normal rainfall season. When these soils are supporting a foundation, they can shrink away to the point that the foundation is left unsupported. Even deep foundations such as caissons or driven piles can be affected as they rely on “skin friction,” i.e., the friction created at the contact between the soil and concrete, wood, etc. When soils become desiccated due to low moisture, they contract way from the caisson or driven pile, compromising the frictional bond. Damage to buildings due to soil desiccation is expressed most commonly as cracks in the structure, sloping floors, and difficulty opening doors or windows.
Besides buildings, retaining walls, bridges, sidewalks, and pavement are also vulnerable to damage from expansive soils. Although asphalt concrete pavement is more flexible than rigid concrete and therefore more able to tolerate soil movement, it is still subject to stresses imposed by expansive soils. During periods of drought, cracks upwards of three feet deep can develop due to desiccation of expansive soils. Warping and cracking of pavement has resulted in billions of dollars worth of damage to highways and streets, leading the Federal Highway Administration to consider re-routing some highways to avoid areas of highly expansive soil. Expansive soils are such a concern at airport taxiways and runways that methods of soil stabilization are being developed to counter their effects.
Subsidence of land occurs when the extraction of large amounts of groundwater from and aquifer exceeds the aquifer’s capacity to recharge. Aquifers are permeable, water-bearing geologic units that store groundwater. During periods of drought, water availability from surface sources typically decreases, and groundwater supplies are tapped by more frequent intensive pumping, drilling of deeper wells, etc. When water is pumped out of an aquifer, the water pressure is reduced. In some cases, the reduction in water pressure results in a loss of support for the overlying soil, and compression occurs. This compression is what causes ground subsidence. Depending upon the soil conditions, sometimes the compression can be reversed when the groundwater is recharged, but often the compression and associated subsidence become permanent.
Problems associated with subsidence include damage to buildings, roads, railroads, storm drains, canals, levees, bridges, and wells. Due to concerns about subsidence damaging the California Aqueduct, the U.S. Geologic Survey has used satellite tracking data to monitor changes in ground elevations in the San Joaquin Valley since 2009. In past drought years, excessive pumping of groundwater caused the ground in the San Joaquin Valley to sink several feet, resulting in bowing of the aqueduct structure and necessitating costly emergency repairs. According to State officials, the cost of repairing such damage is approximately $1 million per mile.
A seemingly incongruent yet frequent effect of subsidence is flooding due to changes in drainage patterns. This is particularly common in low-lying areas adjacent to bodies of water, where even minor changes in ground surface elevations can have devastating effects. For example, at the San Jacinto Battleground State Historical Park near Houston, Texas, ground subsidence of over 10 feet has caused approximately 100 acres of the park to become submerged due to flooding from adjacent Galveston Bay.
Light at the End of the Tunnel?
With NOAA predicting a good chance of an El Niño. The current El Niño has strengthened over the summer with a strong event this fall and early winter. Drought-weary Californians are hoping that the forecast will prove correct, and spell and end to the drought cycle. The term “El Niño” refers to warmer than usual sea surface temperatures that periodically develop in the central and eastern tropical Pacific. In California, a strong El Niño event is generally accompanied by heavy rainfall. Moderate or weak El Niños, however, produce wetter than normal winters only about half of the time.
Even Dry Clouds Have a Silver Lining
As with most bad situations, even the prolonged drought is having some positive effects. According to the environmental advocacy group Heal the Bay, the overall quality of ocean waters off the California coast has greatly improved over the last couple of years. The improvement is attributed to the drier weather condition which produces less surface runoff, the main source of pollutants affecting coastal waters.
Note: This article was originally published in an Earth Systems newsletter in the Summer of 2014. It has been updated to show current drought graphics and El Niño predictions.