A spring-fed stream tumbling down a bluff and into the St. Croix River has been significantly damaged by an accident at a recently-closed gravel quarry in Scandia. Stretches of Middle Creek are buried in a foot or more of sand.
An earthen berm holding back a pond at the site of the former Tiller-Zavoral mine was breached after last week’s torrential rain, when more than four inches fell on the area in about 12 hours. The overflow sent many tons of sand down Middle Creek, also damaging the bluff, a wetland at the base of it, and the railroad embankment.
The site’s current stormwater flow was shaped by a controversial gravel pit which operated there between 2013 and 2016. Concerns about such a catastrophic failure of stormwater retention ponds were voiced by local opponents and the Take Action-Conserve Our Scandia (TA-COS) organization during environmental review and permitting hearings.
The land is owned by Dr. James Zavoral and his wife, who leased it to Tiller Corp. for the quarry. Part of it is now being leased by BHE Renewables for a large solar panel facility. BHE Renewables is a subsidiary of Berkshire Hathaway, and is responsible for stormwater management at the site. The company largely adopted Tiller’s plans as its own.
‘Much worse than expected’
The Carnelian-St. Croix-Marine Watershed District is responsible for issuing a stormwater management permit for the site.
District administrator Jim Shaver visited the site during the rainstorm late on Thursday afternoon and said the pond was not overflowing at that point. He came back Friday morning and it was “overtopped and flowing down the bluff.” There was not significant damage to the bluff visible from the overflow point.
The rain gauge at his office, about two miles to the east, showed 4.33 inches of rain had fallen.
Shaver said models of rainfall amounts used in the stormwater plan stated it would be safe even in the extraordinary event of almost six inches of rain, two days in a row.
The district quickly notified BHE Renewables, sending an official Notice of Deficiency, requesting urgent action.
“The District requests correction of the deficiencies as soon as possible because runoff discharging via the emergency overflow is flowing down the Saint Croix Scenic Riverway bluff, which consists of very erodible soils,” the notice reads. “Continued discharge could lead to formation of an eroded ravine with downstream deposition of sediment in natural areas, wetlands and the Saint Croix Scenic Riverway.”
Over the weekend, the company repaired the berm and placed sandbags to prevent further overflow. But, in the span of 24 to 48 hours, a lot of water carried a lot of sand and soil down the bluff. The extent of it wasn’t fully known until Wednesday.
After a neighbor who lives at the foot of the bluff called the city of Scandia about damage to the railroad embankment between the bluff and the river, Shaver and a watershed district board member visited on Wednesday morning. It was then they saw how not only had the railroad been damaged, but the bluff and creek was also much worse than expected.
“I was very surprised to see the amount of sediment coming out over the outflow, with noticeable deposits all the way down the bank, and notable erosion issues,” Shaver said.
He added that the railroad embankment was severely damaged, and was clearly a source of some of the sediment that clogged the lower creek.
‘Most likely to overflow’
The point where the overflow occurred was near the southeast corner of the site. It’s the lowest point below a large open hillside and the solar panels. Draining what’s called “Depression Area 7,” it was identified in the gravel pit’s environmental review as vulnerable, but computer models predicted it would only overflow after highly improbable amounts of rainfall — much more than what fell on September 20 .
“Depression Area 7 would be the most likely to overflow under existing conditions,” the environmental impact statement said. “Further calculations, conservatively completed by ignoring infiltration, evapotranspiration, and interception, indicate that Depression Area 7 would overflow if a 100-year 24-hour storm (5.9 inches) would be followed the next day by another 100-year 24-hour storm. This result indicates the likelihood of an overflow event from existing depression areas to an off-site area is small under existing conditions.”
The design for stormwater flow through the site included the low spot where the breach occurred as an “emergency overflow,” but the site was supposed to drain entirely by water soaking through the gravel and sandstone below.
“The approved stormwater management plan includes three retention basins in series with an emergency overflow from the southern retention basin. This emergency overflow was never intended to be realized,” the watershed district wrote to BHE Renewables.
The permit issued to BHE Renewables requires the company to ensure stormwater storage capacity is “at or above the design volume and so that each facility continues to operate as designed and erosion or structural problems are corrected.”
This included a requirement to grade the basins and remove accumulated silt, which would let water quickly drain down through the sandy soil underneath. The pond was supposed to store up to 12 inches of rainfall.
The district asked BHE Renewables to repair the breach as quickly as possible, and also investigate what caused the overflow and prepare a plan to prevent it from being repeated.
Six days after the storm, crystal clear water was trickling down Middle Creek, over the thick bed of sand, and into the St. Croix. The water formed patterns like tire tracks on the sand. A large delta extended into the river for at least 10 feet.
Anything that had been living in the creek, from dragonfly larvae to algae to little darting fish, must have been totally suffocated under the sediment.
The overflow incident may be connected to climate change. More intense and isolated rainstorms are a key part of the climate forecast for the upper Midwest. Rather than spreading rain out over large areas and many different days, it’s more often falling in short, heavy downpours.
“We appreciate the quick action by neighbors, the landowner and solar company, and the watershed district,” said Deb Ryun, executive director of the St. Croix River Association. “Unfortunately, it didn’t take long for a lot of damage to be done. This shows how fragile the St. Croix and its tributaries are, and the need to engineer these types of projects to take into account our changing climate and the now-common intense rain events.”
The incident is reminiscent of another sediment discharge into the St. Croix River. In 2012, a containment berm at a frac sand mine on the edge of the river in Grantsburg, Wisconsin failed. It dumped massive amounts of fine sediment into a creek, which flowed into the St. Croix, for three days until a passerby reported it.
That mine is also owned by Tiller Corp., which designed and operated the Scandia gravel pit.
The Scandia gravel mine was first operated in the 1960s by Tiller’s predecessor, Barton Sand & Gravel Co. Similar blowouts of the pit and sediment discharges into the creek and the St. Croix occurred at that time.
Short, steep, groundwater-fed streams like Middle Creek are an important part of the St. Croix River. They provide habitat, spawning beds, and cold, clean water all year long. This one won’t function naturally for a long time.
Now that the overflow has been stopped, and the damage inspected, Shaver says an investigation will come.
“We’ll be looking at making sure it doesn’t happen again,” Shaver said.
The watershed district has already notified other agencies, including asking the Washington Conservation District to assess the impacts to the wetland at the bottom of the bluff.