Taking Flooding Back in Time

I attended the South Dakota School of Mines seminar "A History of Especially Large Floods for the Black Hills Area" by Dan Driscoll from the USGS (United States Geological Survey). I found it very interesting and thought it would fit nicely into my blog topic. I was also very excited when Mr. Driscoll spoke about hydrographs, flow velocities, gage stations, statistical analysis, and a few other concepts we learned about in our hydrology class because I understood to what he was referring.

Mr. Driscoll started the presentation with some hydrographs, pictures, and other data on the recent large flash flooding events in the area for which we have recorded data, including the 2007 Flood in Hermosa, the 1972 Flood in Rapid City, and a few other floods throughout the Black Hills from the early 1900s. As with any dangerous and destructive event minimization of structure damage and elimination of loss of life is the goal. This task becomes difficult due to recorded data only dating back just over 100 years and the infrequent nature of these floods. This is where geology and mathematics come into play. Currently large floods leave behind evidence, such as deposits of plant debris, boulders, gravels, sands, and fine-grain silts. Typically the fine-grain silts and sands settle out at lower velocities (slack-waters). Flood events should behave in a similar manner in the past. So, these "slack-water" deposits can aid in detailing flooding events within the last few thousand years, but only if they have not been eroded away by wind and water. Rock overhangs or "alcoves" and small caves protect these deposits from erosion and are the focus of study for the USGS and other local, state and federal agencies.

Holes were dug in a number of alcoves and caves within the canyons draining toward the eastern edge of the Black Hills. The different layers of sediment, including organic matter, were documented and a schematic diagram was created to detail the deposits. Samples of the deposits and organic matter were used to date the ages of the deposits going back a few thousand years. The geology of these canyons are known to erode very slowly; thus the height of the deposits from the canyon floor allow for a close estimation of water height. This can then be used to estimate water velocities. Many of the canyons studied showed evidence of much larger floods then the gage records. Mr. Driscoll stated the largest gage record was the 1972 flood at 50,000 cfs but some of the paleofloods showed flows of at least 75,000 cfs with evidence of at least five floods over 50,000 cfs in the Black Hills area. This indicates we haven't experienced the largest possible flood.

This information can assist in more accurately determining the 100-year and 500-year flood magnitudes. This in turn can help governing bodies to create policies to reduce the damage and eliminate loss of life for future flood events. The entire study can be read at the link below which includes some great images to further understand the information provided.

http://www.sddot.com/business/research/projects/docs/SD2008-01_Fact_Sheet_06-11-12.pdf