Story originally printed in the Vernon Broadcaster or online at www.vernonbroadcaster.com
Published - Wednesday, August 29, 2007
Flood control dams handle twice their designed capacity
Vernon County officials said Friday that the county's 22 flood-control dams were battered and bruised, but still standing after handling nearly twice the rainfall for which they were designed.
A number of federal, state and local officials met at Jersey Valley dam to review the performance of the county dams following rainfall on Aug. 18 and Aug. 19 that dropped between 6.5 and 11 inches of rain on Vernon County. The rainfall was referred to at the meeting as a "1,000-year flood event."
"No dams in Vernon County failed," Resource Conservationist Phil Hahn. who oversees the dams, said. "We have two dams that suffered damage to one of their components, their auxiliary spillways."
Hahn said the spillways are normally a "cut" or lower portion in the top of the dam that allows water to come over it in the event of filling to capacity.
"The one at Runge Hollow was severely damaged, the one at Hidden Valley, that auxiliary spillway failed," Hahn said. "In other words the gullying unraveled all the way over the top of the dam."
Hahn said much of the problem at the Hidden Valley Dam had to do with 160 round hay bales that had washed into the dam and eventually to the spillway that caused the water to be diverted. Hahn said the DNR was instrumental in flying in pumps via helicopter to relieve the pressure on the dam.
Hahn emphasized that the dam structures performed well.
"All the dams performed above their design standards," Hahn said.
Hahn said there is about $6-$7 million in damage to the county's dams, but that they protected about $35 million in private and public property.
"Vernon County was fortunate the county board of supervisors and soil and water conservation district back in the 1960s decided to take on the most aggressive flood control program in the state by building 22 of these structures," Hahn said. "These are flood-control structures not flood prevention. You cannot prevent floods."
Hahn said there are 35,000 acres of land that had to have conservation practices in place above the dams before they were even built. Hahn said the objective is to keep "as many raindrops in place" as possible and those that aren't are captured in a dam and "metered out" over time.
Hahn gave credit to former County Conservationist Jeff Hastings, who convinced the county board to add his position, which is dedicated to dam maintenance, as a full-time position.
"I have been very aggressive in my maintenance and the engineers have told me they believe that has made a difference," Hahn said.
Natural Resource Conservation Service watershed specialist Tom Krapf said the dams are generally designed for around 6.5 inches of rain and some took as much as 15 inches over a three-day period. Krapf said there is a difference in the amount of rainfall and the amount of runoff. Krapf said the initial 15 inches was not all runoff, because most of the watersheds were dry in the initial stages of rainfall and at least some of that moisture was absorbed.
What that means for residents downstream of the structures is that they are now in a vulnerable period while the dams slowly empty, but are not able to absorb much more rainfall if another storm comes along.
"The watersheds are totally saturated and it is almost like falling on concrete because it just runs off," said County Conservationist Kelly Jacobs.
"I would say a 4- to 6-inch rainfall would be cause for concern," Krapf said.
Third District Congressman Ron Kind praised the work of the NRCS, DNR and local conservation officers for their action and the forward thinking of past officials that invested in the structures.
"But we do realize that there will have to be some repair and maintenance at the end of the day," Kind said.
NRCS State Conservationist Pat Leavenworth said it is a credit to those who engineered the structures that they were able to survive the initial "jolt" that was twice their designed capacity and said NRCS will be releasing funds from its Emergency Watershed Program to "stabilize" the structures that have had damage. Leavenworth said NRCS will also be looking at putting together numbers for long-term repairs.
"If we at least shore them up that means that people can at least get back to their places," Kind said.
Leavenworth said she has about $200,000 available now for emergency measures. Leavenworth said money secured for dam maintenance in the past by Senator Herb Kohl was put to good use to make sure the structures were in good condition when they were called on the control flooding.
Krapf said NRCS has already worked out an agreement with Vernon County that will allow it to start letting bids for emergency repair work this week. Krapf said the priority structures are Hidden Valley and Runge Hollow.
"We have had engineers working steady for two days, so… we are ready to go," Krapf said.
Krapf said Runge Hollow will get a temporary fix that will repair the spillway. The Hidden Valley Dam will get a permanent fix that will put it back the way it was before the flooding, Krapf said.
Hahn said other structures like the Seas Branch, Sidie Hollow, Primmer/Yttri and Tally dams will all need to be assessed for suspected problems. Hahn said those dams were suspected of leaking through the hillside and past catastrophic failures like the Dahlen Dam in 1978 were because of leaking through the hillside.
Hahn said eventually there will be about 75 acres of flood debris that will have to be cleaned up.
Jacobs said nearly all of the structures will need some maintenance even if they were not "compromised" by the flooding.
All stories copyright 2006 Vernon Broadcaster and other attributed sources.