From the Loveland Reporter-Herald (Saja Hindi):
The 60-foot-plus Nelson Big Dam, just west of Loveland’s water treatment plant, suffered major damage in the 2013 flood, and crews are in the process of not only reconstructing it, but making the dam more durable than it was before.
The dam was built in 1895 after a flood the year before destroyed another dam nearby. It supplies water from the Big Thompson River that irrigates about 20,000 acres of farmland in Larimer and Weld counties, most of the drinking water for the Johnstown treatment plant and diverts raw water for the city of Loveland to its treatment plant.
The dam, built on 15 feet of sand and gravel on the old river bed, cost $11,000 to complete at the time, and is considered a historic landmark, with its masonry stone arch shape.
“When people see it, they don’t really realize it’s not just a little retaining wall in the river. It’s a pretty big structure,” said Gary Gerrard, a board member for the Consolidated Home Supply Ditch and Reservoir Co…
Although the 1976 flood didn’t have a major impact on the dam, the 20,000 cubic feet per second of water flow going over the dam during the 2013 flood proved to be too much. To put it in perspective, Gerrard said average flows in September are typically around 100 to 200 cubic feet per second. Witnesses said the heavy water flow carried a lot of debris and large objects that struck and damaged the top of the dam.
Home Supply hired Gerrard Excavating to not only reconstruct the dam but to also complete deferred maintenance, unrelated to the flood — such as replacing the mortar between the rocks, which has eroded, and gate repairs — and constructing an additional spillway to mitigate future floods.
The Loveland City Council approved contributing funding to the dam repairs, and FEMA has also obligated funds. According to Gerrard, the cost of the project is between $2 million and $2.5 million.
Larry Howard, city senior civil engineer, said the city agreed to split the cost in half with the company of non-reimbursed flood repairs and maintenance (excluding projects on the dam that are only the company’s).
“The city has been just a great partner throughout the whole thing,” Gerrard said.
The dam is not only important to the city for its water diversion, but Howard said it’s also where the city has developed its treatment processes over the years. While Loveland has other sources of water, such as the Green Ridge Glade Reservoir, the dam is their main source of raw water.
With more than 5-feet of the dam knocked off as well as some of the stones from the arch, crews had their work cut out for them.
Immediately after the flood, Gerrard, who is also the project’s construction manager, said the goal was to fill all the reservoirs and move the river. Then, crews built concrete abutments, or concrete structures on both sides of the dam to hold the arch secure.
“The first thing was to get the repairs made to the dam,” Howard said. “That work began last winter after the flooding and was carried on throughout the winter.”
Construction took a hiatus when flows became too high to allow for work in the spring, but work was continued on the arch during the summer months. After Aug. 1, construction was underway to fill in the rest of the crest with concrete and the stone portion was repaired.
Howard said the project is unique in that stone construction is rarely seen anymore.
“The goal from the beginning was to maintain the historical appearance of the dam, and I think we’ve been able to do that,” he said.
To replace some of the stones, crews were able to get more stones from the Arkins quarry, on North County Road 27, which is where the original stones were from.
Crews added the new spillway, which is electronically controlled and is brand new technology, Gerrard said.
“We’re excited to see that work,” he said. “Once that was completed and the rest of the arch was complete, we moved into the Home Supply system,” which included repairing gates and deferred maintenance.
The reservoirs, he said, were filled early this year, making it easier to manage the river for construction and not diversion.
Flood-related repairs are planned to be completed this winter, and then re-mortaring between rocks will begin.
More Big Thompson watershed coverage here.