The Rapid City Council expects to hear a more detailed presentation of Canyon Lake Dam's leakage problems during tonight's special council meeting so it can decide what steps to take to fix the dam.

The special meeting is scheduled for 7 p.m. at the City/School Administration Center, 300 Sixth St. Robert Ellis, growth management director, said the council will review an HDR Engineering report and hear pros and cons of several options.

The HDR study recommended taking immediate steps to plug upstream sinkholes that have tunneled under the dam into the spillway. The report also recommends a major renovation or replacement of the dam within the next year or two.

An HDR representative attended the March 17 council meeting to answer questions, but the city council wanted more specifics about the analysis and why short-term fixes were recommended if the dam needs to be replaced.

Some council members wondered whether draining the lake now would be a better option to eliminate any potential danger from dam failure.

But draining the lake creates an entirely new set of problems, the biggest of which is silt washing into Rapid Creek and affecting fish habitat downstream.

When the city last drained the lake in 1995, sediment that flowed into the creek killed an entire age class of brown trout and violated state water-quality standards. The city spent $220,000 to improve trout habitat as part of a settlement with the state DENR.

As part of long-term prevention, the city is in the planning stages of a project to build a bypass that would divert creek flow above the lake through pipes that empty below the spillway. The bypass could be used whenever the lake is dredged or while the dam is repaired or replaced.

Ellis said draining the lake now does not address the silt problem. Allowing 3 or 4 feet of silt to essentially bake in the sun over the summer will likely create strong odors and a breeding ground for bacteria and mosquitoes, which happened the last time the lake was drained.

Mayor Alan Hanks thinks that after the council has an opportunity to listen to the experts, they will agree there is no need to drain the lake. Hanks believes the best option is to listen to the professional engineers and plug the holes first, draw down the lake level by 1 or 2 feet to ease pressure on the spillway, then design the bypass and repair or replacement project as quickly as possible.

Hanks believes language in the HDR report that refers to Canyon Lake being a "high hazard dam" is being misinterpreted by some people. He said every dam located above a populated area is defined as high hazard. It doesn't mean failure is imminent.

People should also keep in mind, Hanks said, there are big differences between the current situation and the 1972 flood. The city now has an established floodway, called the greenway, through the center of town, and the lake holds about half the capacity now as it did then.

"With the greenway, even if there's an event where we have major flow coming through there, we wouldn't see the loss of life and property like we did in 1972," Hanks said.

Ellis said HDR indicated the dam is a concern, but they do not believe it is in imminent danger of collapse. He said the firm believes, based on its experience with dams, that the city has time to make immediate repairs, which will buy the city several months while it pursues a long-term solution.

Plugging the sinkholes could involve pumping bentonite into them then sealing off both ends with rock and gravel. Ellis said bentonite expands when it absorbs water, which would seal the fissures.

A similar process can be used to seal the cracks in the concrete face of the spillway, Ellis said, using a polyurethane-type material. Ellis has been told by various professionals that those methods could shore up the dam for another one to five years.

Contact Scott Aust at 394-8415, or scott.aust@rapidcityjournal.com

If you go

What: Rapid City Council special meeting to discuss Canyon Lake Dam

When: 7 p.m. Tuesday, March 25

Where: City/School Administration Center, 300 Sixth St., Rapid City