All recent rainfalls have been welcomed, but a lot of rain must fall to put an end to a drought that is now in its fifth year.

According to U.S. Drought Monitor data as of April 2, part of Crawford County is listed as abnormally dry, and part is in a moderate drought.

To help counteract the effects of the drought, the Denison Municipal Utilities (DMU) Board voted at its most recent meeting to pursue the permitting process to place a rock riffle dam on the Boyer River north of the Crawford County Fairgrounds.

The purpose of the dam is to raise the static water level in the alluvial aquifer, from which DMU’s north well field draws water.

As described by Martha Silks with LRE Water, a firm hired by DMU, the water level in the Boyer River has an indirect effect on the water level in the alluvial aquifer. The effect is indirect because the river’s ability to recharge the aquifer is slowed by a clay layer in the soil.

The DMU Board authorized Peter Baudhuin with DGR Engineering, Rock Rapids, to pursue the permitting process with the Iowa Department of Natural Resources and the Army Corps of Engineers for two four-foot-high rock riffle dams or one six-foot-high rock riffle dam.

Baudhuin explained in an email to The Denison Free Press that a riffle dam structure is taller on the upstream end. The backslope of the dam is gentler than the foreslope to allow for fish and paddle craft passage.

A riffle dam would span the channel of the river, and the length, from the foreslope to the backslope, may be up to a few hundred feet long.

A factor that was somewhat disconcerting for DMU is that the permitting process may take up to 18 months.

But even if the drought ends before the rock riffle dam is constructed, the dam will help the wellfield when droughts occur in the future.

Baudhuin said if the permitting process started right away, completion of a dam or dams could be sometime in 2026. The construction timeline is short compared to the permitting and funding process. Construction could take a month or two and maybe three to four months if two structures are installed in the river.

The DMU Board’s decision to pursue the permitting process followed a presentation of a draft document on the effectiveness of rock riffle dams.

To demonstrate the effectiveness of a rock riffle dam, LRE Water built a computer model and reviewed the area’s geology that focused primarily on DMU’s north wellfield. Four simulations were run – one four-foot dam, two four-foot dams, one six-foot dam and one 10-foot dam.

The 10-foot dam was used only for comparison and was not seen as feasible; both Silks and Baudhuin said the DNR was unlikely to permit a dam of that height.

In the model for the two four-foot-high dams, the distance between the dams would be about 3,500 feet, Baudhuin told The Denison Free Press.

One four-foot dam placed in the river would result in an increase in the aquifer’s static water level of only 215 to 363 gallons per minute, depending on climatic conditions, said Silks. She added that the range of the lowest amount of increase at 215 gallons per minute isn’t that good.

Two four-foot dams would yield an increase of 876 to 1,450 gallons per minute, which is the highest of the probable dam constructions.

The next highest is the six-foot dam which would yield 753 to about 1,200 gallons per minute.

Increased water pumping isn’t the only consideration. Cost of construction per gallon increase was also examined using DGR’s opinion of probable cost and the lower end of water yield increase for each size of dam.

Estimated costs for the various size of dams (excluding the 10-foot dam) would be $1,716,000 to $3,219,000, with nearly a $1 million increase in the projected cost going from one six-foot dam to two four-feet dams.

The highest cost dam based on the increase in water yield was the single four-foot dam.

The two four-foot dams had a projected cost of $2,218 per increase in pumping rate.

The best economical dam was the six-foot dam at $1,878 per increase in pumping rate.

Silks said LRE’s conclusion was to proceed with permitting for a six-foot dam or two four-foot dams.

Board member Chad Langenfeld asked if any dams had been considered for placement on the river south of the discharge from the wastewater treatment plant, which is about one million gallons per day.

Silks said the factor in placing the dam or dams on the Boyer River north of the fairgrounds had to do with constructability. Farther south, the constructability gets a little more complicated, she said.

Baudhuin added another factor is that the north well field was targeted for the construction of a rock riffle dam because that’s where most of DMU’s water production comes from. Putting a dam farther south would not benefit the north well field enough for it to be worthwhile.

He also said that the DNR would have to consider the likelihood that a dam would increase the possibility of the river overtopping the banks.

Baudhuin said DGR would use river modeling to show what those flood events could look like with a dam in the river.

A rock riffle dam would not significantly change the amount of water flow downstream, Baudhuin also said.

“If you build a dam and water starts to back up, you get a real temporary downstream reduction in the flow, but then as soon as the water overtops the structure, you’re back to the baseline flow of the river to what it was before,” he said.

In addition to pursuing the permitting process with the DNR, Baudhuin will have discussions with USDA Rural Development about funding possibilities.

He said that recently, for Regional Water Rural Water Association based in Avoca, DGR was able to get a $1 million USDA Rural Development grant through the agency’s emergency community water grant assistance program.

He explained what is needed is an emergency event and that right now Iowa is in a drought. DMU must also have to be able to document that the emergency has caused a reduced capacity and show that the reduced capacity has caused the water system to not meet the community’s demand or be in danger of not meeting the demand. Baudhuin concluded that DMU has a qualifying event for an emergency community water grant.

Baudhuin continued that with a cost ranging from $1.7 million to $3.2 million, depending on the size and number of the dams, and after considering a $1 million grant, the balance needed to fund the project would be from $700,000 to $2.2 million.

He said that USDA Rural Development would provide a low interest loan for the balance of the funding.

DMU could also seek other sources of funding for the balance of the cost. The Hungry Canyons Alliance was mentioned as one possible funding source.

Weis said if a dam is built, DMU would have to see if any water rate increases would be needed to fund the construction.

Langenfeld pointed out the community needs water.

“There’s no question about it,” he said.

Baudhuin had prepared some rough figures relating to rate increases to fund a rock riffle dam, with the $1 million grant taken into consideration.

He said the USDA Rural Development loan rate for a community with DMU’s income level is currently 2-1/4% on a 40-year loan. Based on a customer base of 3,000, and not considering any other grants to lower the cost, it could mean an increase in the per-customer monthly bill of 76 cents for a single four-foot dam, $2.35 for two four-foot dams and $1.31 for a six-foot dam.

Langenfeld asked if the construction of a rock riffle dam would have any downsides attached to it.

Silks didn’t see any downsides.

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Baudhuin said the only short-term downside he saw was the cost of the project compared to the amount of capacity DMU could gain by drilling more wells. He said DMU could probably get more capacity by drilling more wells in the wellfield; however, if the drought continues, it’s going to continue to affect all of DMU’s wells.

“The long-term benefit of a rock riffle project like this is that it gives you a minimum water level in your aquifer, so it’s a good security measure,” he added.

Other advantages of the dam compared to drilling more wells is that the dam would not involve the installation of more power lines and the use of more power and would not require the installation of more water pipeline.

Maintenance to the dam would be deferred for a number of years, Silks said.

The price estimated for the dams was based on using Sioux Quartzite, a durable rock, which would give the dam an estimated 50-year lifespan. Local limestone could be used but has a lifespan of only 10 years. The cost savings by using the less expensive limestone was only 10%.

The DMU Board’s motion to proceed included payment of LRE Water’s invoice.

Baudhuin was directed to provide DMU with DGR’s cost proposal for the permitting and funding phase of the project.

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