Flooding From Excessive Rain Downstream From Earth Dams

Rose Hill Dam FailureAn article from the Daily Republic in South Dakota talks about an earthen dam that recently failed there due to a 9-inch rainfall event on July 29th of this year.

This rainfall event “overwhelmed its capacity” causing the failure of the earth dam.  There was no report of injury downstream of the dam. This dam was built in 1935, as were a number of them during the Work Programs after the Great Depression.

A Department of Game, Fish and Parks Engineer said that they “were satisfied with the condition of the dam” during inspections in 2007 and again in 2008 and that the dam breach “was caused by an extraordinary natural event and not by any structural weakness in the dam.” (Photograph by Laura Wehde/The Daily Republic)

Earth dams are almost too numerous to count around the country. In fact, you probably live a lot closer to one than you might think. A large number of dams were built over 70 years ago and, in many cases, the owners of the dams are different than when they were built. This makes maintenance and inspection of the dams less regular.

FEMA estimates “there are over 80,000 dams in the United States”, and that approximately “one third of these pose a ‘high’ or ‘significant’ hazard to life and property if failure occurs.”

In the countries worst dam failure disaster, the South Fork dam failure in May of 1889 killed over 2200 people (almost half of which were under 20 years old) in the town of  Johnstown, PA.

This is known as the “Johnstown Flood.”  A 37-foot high wall of water hit Johnstown, located 9 miles downstream from the dam. The entire city was almost destroyed, including 1600 homes and 280 businesses.

After the failure of the St. Francis Dam in California in March 1928, legislation was enacted in and around California. This, and other later legislation led to life-saving advance warning when the Baldwin Hills dam near Los Angeles, California failed on December 14, 1963.

Only 5 individuals were killed because of the advance warning which enabled the evacuation of approximately 16,500.

Even though there have been far less loss of lives in the United States from dam failures since the 1970’s, reports that…

there were 132 dam failures and 434 “incidents” between January 2005 and January 2009.

Of course, I should note that the failure of the earthen levees near New Orleans, LA during and after Hurricane Katrina are purported to be responsible for killing more than 1000 people.

The KaLoko Dam on the Island of Kauai, Hawaii failed in March of 2006 killing 7 people. In November of 2008, the developer, James Pflueger, was indicted for manslaughter and reckless endangerment in relation to the dam failure. The county of Kauai and the State of Hawaii paid out over $9 Million in settlement of lawsuits after the failure.

Cause of Dam Failures

Heavy rains cause overtopping, which is by far the most common cause of dam failures. Dam spillways and structures are typically not designed for more than a 1-percent chance (aka 100-year) storm event.

When a rain event exceeds this, the water begins to travel outside of the control spillway. This leads to erosion of the soil on the dam from the excessive amount of water traveling over it.

It is also possible for overtopping to occur from smaller rain events because of debris blockage of the outlet structure or spillways or because of settlement of the dam crest.

Foundation defects, including settlement and slope instability, cause about 30% of all dam failures.

Seepage or Piping is the cause of another 20% of U.S. dam failures. Piping is the internal erosion caused by seepage under and through the dam. It often occurs around structures such as pipes through the dam and spillways.

Seepage can also be caused by animals burrowing in the dam, by roots of trees growing on the dam, and through cracks in the dam.  All earth dams have seepage resulting from water permeating slowly through the dam and its foundation.

But this seepage must be controlled or it will progressively erode soil from the embankment or its foundation, resulting in rapid failure of the dam.

What Should You Do To Protect Home?

Since the failure of a dam causes excessive flooding, one of the best courses of action is to avoid building in a flood zone, unless you elevate and reinforce your home. You should have your area investigated for flood determination or dam failure.

Do you live downstream from a dam? Is the dam a high-hazard or significant-hazard potential dam?

To find out, contact your state or county emergency management agency and/or visit the National Inventory of Dams. There are around 2,228 dams on the National Inventory in Alabama. Of those, 636 are listed as high or significant hazard potential dams.

If you live downstream from one of these dams, find out who owns the dam and who regulates the dam. This information should also be available from the National Inventory of Dams.

Next, find out if there is an Emergency Action Plan in place. Again, consult your state or county emergency management agency. (Alabama Emergency Management Agency)

Strangely enough, Alabama is the only state in the United States that has not passed dam safety legislation.

If you want help with investigating a piece of property you are considering purchasing or of one you already purchased, please call Mobile Land Surveying today at (251) 281-2081 or better yet send us a contact form request.

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