August Hazards of the Month: Drought and Expansive Soils

Sneaky and Silent, Drought and Expansive Soils can impact our community in ways that are less visible than their flashier stormy cousins.

DROUGHT
Drought can sneak up on a region over time (compared to flooding) but can be aggravated by high temperatures, high winds, and low relative humidity. The multiple factors related to drought make it hard to give accurate, reliable, and timely estimates of drought severity and effects. Looking at past records, Tulsa experiences a drought about every 5 years. It has less drought than most parts of Oklahoma.

There are 4 stages of drought: meteorological (less precipitation than the average), agricultural (not enough precipitation for vegetation and crops), hydrological (drop in water levels, such as groundwater and streams), and socioeconomic (when water is scarce enough to cause social or economic hardship).

The main impact of drought in Tulsa is economic, as crops are lost due to lack of precipitation. In severe drought, there is a risk of fire, as vegetation becomes easily flammable. Tulsa has prepared for the event of drought by getting water for the city from different watersheds. Even still, rationing may also be put into effect, as needed.

The main danger to buildings/city infrastructure is drought’s effect on expansive (shrinking/swelling) soils. For example, a combination of extreme heat and soil movement caused above average numbers of water line breaks in Tulsa during the summers of 2011 and 2012.

EXPANSIVE SOILS
Expansive soils are often referred to as swelling clays because clay materials attract and absorb water, then shrink when the clay dries out. Buildings on expansive soils suffer cracking and buckling with the shift in soil volume. This map shows the expansive soil distribution in the Tulsa area. Soils with high to very high shrink/swell potential cover 30.84 % of Tulsa, especially in some isolated pockets (see map). The shrink/swell can damage buildings, roads, and other structures.

Expansive Soils, from 2014 City of Tulsa Multi-Hazard Mitigation Plan. The darkest areas have very high expansive soils. Developed by Flanagan and Associates.

Expansive soil is a silent hazard. Because the damage develops gradually and seldom presents a threat to life, it has received limited attention. Many problems are not recognized as being related to expansive soils or may be considered only nuisances and therefore are never repaired. However, expansive soil is one of the most-costly hazards in the United States, in terms of property damage from shifting soils.

Property damage can vary greatly, based on the long-term weather conditions, the type and quality of construction, materials used in construction, and, most importantly, the soils the structures are built upon. Damage can range from cracks in walls to water-main breaks and leaks from hazardous materials pipelines. One way to avoid damage from expansive soils is to use post-tension slabs in new residential construction.

For more information on drought visit the USDA Drought site at https://www.usda.gov/topics/disaster/drought. You might also want to check our August Key Message on low impact development (LID), which includes techniques that can be used as tools in water conservation: http://disasterresiliencenetwork.org/august-key-message-low-impact-development/.

For more information on expansive soils, visit the US Inspect Guide to Expansive Soils: http://www.usinspect.com/insights/guides/expansive-soils.

The Disaster Resilience Network is providing these hazards of the month in conjunction with the City of Tulsa’s Storm Water Drainage and Hazard Mitigation Advisory Board. These hazards are identified in the 2014 City of Tulsa Multi-Hazard Mitigation Plan.

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