Anyone who lives anywhere near a reservoir are familiar with the absolute necessity of having a dam. They know how essential that structure is to hold back and control storm water that can cause devastating damage to life and property. They also know dams supply water, power and other benefits to communities and residents.
According to Illinois State Conservationist Ivan Dozier, "Johnson County can celebrate their two small dams that just turned 50 years old and are still going strong protecting land and crops from floodwaters -- as well as providing vital drinking water."
Currently, more than 1600 dams are listed on the National Inventory of Dams in Illinois -- there are actually many more dams in our state, but these are those large enough to be included on the inventory, have high or significant hazard potential or both.
The dams were built for different purposes. Most watershed protection dams in Illinois were built a long time ago when communities and government leaders took steps to protect property and capture water for other uses.
Between 1950 and 1969, 555 inventory dams were built in Illinois. Twenty-nine -- almost half -- of the 66 inventory-size watershed protection dams planned and built with USDA assistance were completed during this period.
Like all intricately designed and well-engineered flood control structures, dams require specific standards and specifications be met to ensure they are built correctly. By meeting these "standards," builders and community members can have greater assurance the dam will successfully do its job.
"Water is heavy, and flowing water is a very powerful force. It requires great strength to hold water back. Most dams in Illinois use an earthen or soil-based structure to contain and hold back waters," explained Dozier. "The dams include principal, or primary, spillways usually made of concrete or metal. Spillways convey the water when the time comes to release or manage water levels, and to keep the water level at its planned pool level. And in case of a huge storm event, the dams also include auxiliary, or emergency, spillways to safely pass the extra storm water without causing failure of the dam.
"Once established, earthen dams need to be protected from erosion. In climates like Illinois where we can easily grow grass and other perennial vegetation, dams are planted with grassy vegetative cover to secure the soil in place. That vegetation protects the dam from damage that could impact its strength or compromise its integrity," added Dozier.
Vegetation protecting the integrity of a dam can face serious risks and must be carefully monitored, managed and maintained by the people in charge of taking care of the dam and all it protects.
When planning and designing a flood control dam, engineers establish a certain length of time called a "lifespan," or evaluated life, that the dam can normally be expected to remain strong and maintain full performance.
For the U.S. Department of Agriculture's (USDA) small watershed program, that evaluated life typically ranges from 50 to 100 years and is dependent on how well protected the watershed is from sedimentation.
A well-maintained structure can last much longer than the evaluated life for which it was designed.
There are four dams near Vienna that protect land in Johnson County through the USDA small watershed program.
Two of these, called "Structure #8" and "Structure #10," were designed and strategically built in 1969 as part of the Little Cache River watershed project. Both dams collect water from the area around Vienna and provide flood control, recreational opportunities, water supply and wildlife benefits.
These benefits, created 50 years ago, are still being realized today.
"We've had a very wet month of May this year, with more than 7 inches of rain in Johnson County. Conservative estimates show that this system of dams provided about $20,000 in benefits for the month, preventing flooding and damages that would have otherwise happened," stated Dozier.
Structure #10 is a multi-purpose, municipal water supply and flood control structure, sponsored by the City of Vienna. Structure #8 is a floodwater retarding structure, originally sponsored by the Vienna Drainage District.
Both were completed in 1969 when sponsors took responsibility to care for them for their intended lifespan and beyond. Part of the original agreement signed with the federal government to pay for and build the dams required sponsors to maintain and fix any problems, so dams would work and function properly. According to Dozier, the sponsors of these dams have kept their commitment to conduct detailed inspections at least once a year, and staff from USDA's Natural Resources Conservation Service (NRCS) help as well.
"I'm happy to report that both dams are in excellent shape. The sponsors have been doing a great job of maintenance over the years," says NRCS Area Engineer David Webber.
"Each dam currently has a few minor maintenance items that NRCS discussed with sponsors, but both dams are strong and have more life and protection to offer the people of Vienna and Johnson County," Webber adds.
NRCS District Conservationist Rick Street explained that even though the formal agreement between NRCS and sponsors expires this year, locals can still ask NRCS for technical assistance on the dams if they are interested.
"We still highly encourage these sponsors to continue yearly inspections and keep up with all the high-quality maintenance as they've done for fifty years," adds Street. "Their commitment really shows." Structure #10 still serves as the water supply reservoir for the City of Vienna.
To learn more about NRCS programs and get ideas for solving natural resource issues in your area, visit https://www.nrcs.usda.gov or anylocal field office.