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Hermine - Hurricanes and Groundwater

Posted on September 1, 2016 at 1:55 PM Comments comments (0)
As Tropical Storm Hermine, possibly soon-to-be Hurricane Hermine, bowls into Florida, we are expecting several inches of rain. Its been a pretty average summer for rainfall already, and we've been dealing with some pretty wet sites for drilling just lately. Our Florida "wet and dry" seasons can lead to some dramatic changes underground too. Daily rains and tropical systems can be a driving force for sinkhole formation, particularly in areas where stormwater runoff is concentrated, such as dry retention ponds and ditches. The localized increase in drainage down through the soil below a dry retention pond leads to a process called "suffusion", which is the removal of fine particles from the soil. If there are holes in the limestone below and the water can flow into the limestone, then this can be a mechanism for sinkhole formation. Sinkholes can also form in the aftermath of heavy rainfall. As groundwater levels rise through the rainy season, the load that the soil resting on the limestone at depth "feels" reduces, through a process akin to buoyancy. As the groundwater level falls back again, then the load of the soil is reapplied, and sometimes, if there is a cavity in the limestone close to the top of rock, then that lifting and relaxing of the soil can cause the soil and/or the top of rock to weaken and fail, leading to sudden sinkhole formation. Tropical Storm Debby in 2012 was responsible for dozens of collapse sinkholes, many of which formed in this way. We at IDGS can help. We can act as "first responders" with our drilling and GPR equipment if a sinkhole either forms or is suspected after heavy rain. Knowing how high the water table can rise is also a skill that helps in the design of stormwater ponds. If a pond is designed to be dry, then no-one wants to see cat tails growing in it at the end of the wet season. Also, if the pond is designed to be wet, no one wants to see a swampy mess instead of a nice body of water at the end of the rainy season. Predicting the "seasonal high groundwater" is key, and its as much art as science! We at IDGS can help here too. We can estimate seasonal low and high groundwater table by direct measurement or by installing monitoring pipes in boreholes, to see where the "stabilized" groundwater rests. We can also find signs of high groundwater, such as staining and mottling of soils. GPR can also be used on a broader scale, since the GPR signal is damped down when it enters saturated soil. Hope that everyone stays safe during the storm! David Wilshaw, MS, PG IDGS