BROOKINGS —City councilors received an update at Tuesday night’s meeting on an effort to mitigate the impact of Six-Mile Creek, which has a propensity to overrun its banks every so often, putting lives and property at risk.

Known formally as the Six-Mile Creek Feasibility Study, the three-year effort by the city and its consultant, RESPEC Engineering of Rapid City, has put forth five options to mitigate the impact of the creek’s floodwater — options that would cost millions of dollars apiece to implement and are likely years away from actually being built.

The options focus on an area from the U.S. Highway 14 Bypass on the north to Sixth Street on the south and from the U.S. Highway 14 diagonal on the west to Medary Avenue on the east. Roughly 230 structures are contained within that zone.

The options are:

Lengthen north U.S. Highway 14 bridges

• Goal: Allow for better water flow by lengthening the bridges from 100 feet to 500 feet and lowering the existing terrain beneath the bridge.
• Cost estimate: $20.2 million.
• Proposed benefits: $7.4 million to $11.5 million.
• Challenges: Downstream impacts.

Channel connectivity plus U.S. Highway 14 bridges

• Goal: Allow more water to flow through the north channel in order to alleviate flooding along the south channel that passes through the city.
• Cost estimate: $24.1 million.
• Proposed benefits: $10.1 million to $15.7 million.
• Challenges: Wetlands, easements and downstream impacts.

Channel bypass

• Goal: This bypass would redirect Six-Mile Creek’s flows around Brookings.
• Cost estimate: $48.8 million.
• Proposed benefits: $19.5 million to $30.4 million
• Challenges: This option would require two to three new structures. Other concerns include wetlands, property acquisition/easements, downstream impacts and, frankly, complexity.

Upstream detention

• Goal: Detain water upstream and slowly release it to minimize adverse impacts downstream. The dry dam would be north of the U.S. 14 Bypass, between Medary Avenue and Interstate 29.
• Cost estimate: $11.8 million.
• Proposed benefits: $18.7 million to $29.2 million.
• Challenges: High hazard dam classification, property acquisitions/easements and wetlands.

Levees

• Goal: Protect homes and businesses from floodwater within the study’s focus area. Levees would be built near both of Six-Mile Creek’s channels.
• Cost estimate: $27.5 million.
• Proposed benefits: $19.2 million to $29.9 million.
• Challenges: Complexity, no question. Also, levee classification along with land acquisition/easement.

Don’t feel alone if the mind-boggling cost estimates leave you a bit rattled. That’s why grant options are being pursued, including, Richter said, two in particular: A hazard mitigation grant and a BRIC grant.

The BRIC grant is a more competitive one, but South Dakota also provides a contribution as well, he said. Normally, the hazard mitigation grant is typically a 75-25 split, while the BRIC is either an 80-20 or 85-15 split, depending on what percent the state contributes, which, Richter said, could be 5% or 10%.

“We would have to navigate that and see which one would be more obtainable,” he said.

Furthermore, additional refinement of the five mitigation options is likely — a solution, for example, could be one of the five options, or a hybrid of two or more options, according to Richter.

“As you can see, these solutions are pretty extensive both in cost and magnitude,” he said. “We are working with FEMA right now — with their technical assistance portion — to navigate that. It’s not every day that communities tackle projects such as this.”

He continued, “We don’t take these options very lightly. There’s obstacles with each one of them that we need to navigate, and we’ll have to carefully navigate that so that everyone’s interest is realized.”

The upstream water detention option drew the attention of Councilor Bonny Specker, who asked about the potential impact on the city’s wellhead field.

Eric Witt, the water/wastewater and engineering manager for Brookings Municipal Utilities, said it would put water over the city’s wellhead protection area — but that the area already experiences periods of high water levels as well.

“My understanding is that, as a dry dam, it’s not retaining the water there permanently; it would just increase the depth of the flood event and then meter itself back out,” he noted. “That’s an area that definitely floods already, so I have that circumstance with a flood event now.

“There’s been times that there’s been 4 feet of water over that whole area, let’s say. If we do a dry dam and it retains more to protect the downstream creek channel through town, maybe it goes up to 6 feet,” he continued. “I don’t know what the incremental difference in risk to the wellhead is for that additional volume of water that’s retained there, but as that event goes through just like any other flood event, it will drain back out and function like a normal creek.”

Witt said he’s not necessarily opposed to the detention option due to the occasional water issues at that location he’s already dealing with, but he would want an evaluation and more details before committing to the upstream detention.

Beyond the Six-Mile Creek floodwater mitigation options, discussion also centered briefly on plans to develop a maintenance program along the creek. It’ll start small, but could grow from there.

“The area west of the Western Avenue bridge is being targeted for debris removal possibly this fall, with a possible larger project in the future,” Richter said.

— Contact Mondell Keck at mkeck@brookingsregister.com.