The city of Black Diamond has agreed to assist in a potential flooding event at Horseshoe Lake, located just west of the city.

Horseshoe Lake is fed by ground water and has no outlet, making it prone to elevated water levels and has flooded on five occasions since 1990, according to Don Althauser, King County Stormwater Capitol Projects Manager. The largest event, in 1990, flooded the public access road, which covers approximately 35 homes. Pumping for the previous two events went to a nearby infiltration pond, but Althauser said this event had a larger magnitude and required an alternative site.

“This is a complex one,” Althauser said. “This ground water has a delayed reaction.”

Because of its flooding susceptibility, Althauser said the county has been plotting the lake since February and saw the lake rise from 503 to 512 feet in one month. Typically, it would take 2 1/2 to three months to raise nine feet. The elevation coincided with one of the highest precipitations for March on record.

“We are seeing a direct response to the March rain events,” Althauser said.

The elevation jump triggered an emergency pumping request from the county, which Althauser said comes when multiple homes could be flooded and access to the homes by emergency officials is compromised. The county’s stormwater division director made it an emergency determination, which essentially speeds up the permitting process.

Seth Boettcher, Black Diamond Public Works Director, said the city may need to process its own permitting papers, but that it wouldn’t stop them from assisting.

“That takes a while,” he said. “But that doesn’t mean we are going to wait and let people flood while we are processing permits. We can handle that kind of paperwork afterwords.”

Althauser said there are six homes on the perimeter of the lake that would be directly impacted by the flooding. He said 3,800 feet of 12-inch pipe will carry the water from the northeast corner of the lake to an unused gravel pit located between the Black Diamond Highway and Green Valley wall, which is part of the Yarrow Bay development property. The pipe will follow roadways and existing trails and is not expected to impact any vegetation or the public.

Althauser said the process could take between three to four weeks, but if all goes as planned, to have it under control by mid-to-late May.

“If we can pump at a higher rate we can provide relief more quickly,” Althauser said. “We can also cost the public less.”

In order to pump, the county entered into a temporary construction agreement with Yarrow Bay. The county worked off verbal permission to enter the site, with written permission to be given before any action would be taken on the property. The city will be monitoring the downstream areas daily to ensure no groundwater surfaces. The county will pay for Black Diamond’s services in the matter.

Mayor Dave Gordon said in an email that city staff is working to expedite the process with the county.

“We have a plan and thank Yarrow Bay for their assistance,” he said. “Without Yarrow Bay this wouldn’t be possible. This is technically outside of city limits. But everyone is coming to together in a common goal of helping these people that live around Horseshoe Lake.”

Althauser said this was a unique emergency because it required substantial “preparing and stage time.” The large pump takes up to two weeks to get in position. Black Diamond officials were concerned about routing water to Rock Creek, fearing flooding in Lake Sawyer, Althauser said.

Gordon thanked the county for declaring a state of emergency quickly.

“This is a perfect case for why the county wants to build the regional storm facilely,” he wrote. “But that is still several years away.”