Chapter Hazard Analysis

Dam failure: Dams are made to hold back large amounts of water. If they fail or are overtopped, they can produce a dangerous flood situation because of the high velocities and large volumes of water released. A break in a dam can occur with little or no warning on clear days when people are not expecting rain, much less a flood. Breaching often occurs within hours after the first visible signs of dam failure, leaving little time for evacuation.

Dam failures are usually caused by either structural problems with the dam or by hydrologic problems. Structural problems include seepage, erosion, cracking, sliding and overturning that are a result of the age of the dam or lack of maintenance. Hydrologic problems typically occur when there is excessive runoff due to heavy precipitation. A dam failure can occur if the dam has to impound (hold back) more water than it was designed to, or if the spillway capacity is inadequate for the amount of water needing to pass downstream.

A dam can suffer a partial failure or a complete failure, but the potential energy of the water stored behind even a small dam can cause loss of life and great property damage downstream. The following factors influence the impact of a dam failure:

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  Level of failure (partial or complete)
  
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  Rapidity of failure (sudden or gradual)
  
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  Amount of water released
  
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  Nature of the development or infrastructure located downstream.
  

In Illinois, dams are categorized in one of three classes, according to the degree of threat to life and property in the event of dam failure:

Class I – Dams located where failure has high probability for causing loss of life or substantial economic loss in excess of that which would naturally occur downstream of the dam if the dam had not failed.

Class II – Dams located where failure has moderate probability for causing loss of life or may cause substantial economic loss in excess of that which would naturally occur downstream of the dam if the dam had not failed.

Class III – Dams located where failure has low probability for causing loss of life or minimal economic loss in excess of that which would naturally occur downstream of the dam if the dam had not failed or where there are no permanent structures for human habitation.

The Illinois Department of Natural Resources (IDNR) Dam Safety Section has 20 of Kane County’s dams in its inventory. IDNR has identified the Tara Lake dam on Jelkes Creek as a Class I Dam due to the high probability of life or property loss should a failure occur. Six dams are rated as Class II dams and 13 dams are Class III.

All of the dams in the Office of Emergency Management inventory are listed in the table below. Other Kane County dams that are not included in the IDNR inventory, are low hazard dams. These dams were not included in the inventory primarily because their height was less than 25 feet and less that a 50 acre-foot impounding area, If these dams were added to the inventory they would be Class III dams.