Jan. 10—OLIVIA — Renville County is moving forward with a drainage improvement project after hearing concerns expressed about it by the city of Olivia as well as the Izaak Walton League.
The County Board of Commissioners, acting as the drainage authority, unanimously approved a report and appointed viewers for a petition by landowners to improve a 2.4-mile underground tile line outletting into Renville County Ditch 63.
Known as Lateral N, the tile line drains 667 acres of cropland in Troy Township in the vicinity of the Olivia Municipal Airport.
The line is in very bad shape, and provides inadequate drainage for crop production. Fields have experienced ponding and overland flow of water during rain events, according to a report by engineer Shaun Luker with Bolton & Menk of Sleepy Eye.
At the public hearing on Tuesday, Luker said the drainage authority could simply have ordered repair of the line due to its condition. With the landowner petition for improvement, the line can be improved and the entire Ditch 63 system will be assessed for $670,516 of the overall estimated costs of $1,314,490.
The line now consists of underground tile ranging from 18-inch diameter to 20-inch diameter at its outlet. It will be replaced with a dual wall, polyethylene pipe ranging from 24-inch to 36-inch diameter. It will increase the flow coefficient from one-tenth of an inch per day to one-half inch per day.
The project will also include the development of four water and sediment control basins that will be able to temporarily hold up to 50 acre-feet of water.
The city of Olivia is concerned that the additional carrying capacity of the improved system could increase the potential for water to back up in the ditch the city uses for its wastewater plant discharges.
Dan Coughlin, Olivia city administrator, said the city has experienced "pretty critical situations with high rainfall events."
Three times in the last decade it has been forced to temporarily shut down the wastewater plant when waters in the system have backed up.
"Our concern," said Couglin, "is will it increase the volume going downstream? Is it going to back up all the faster when big rainfall events happen?"
David Minge, a former congressman and judge representing the Izaak Walton League, cited an increased potential for backed up sewers and flooded basements in Olivia as among the organization's concerns.
In a statement read to the drainage authority, Minge said engineers have estimated the improvement will increase the discharge capacity of the lateral by more than 400%.
The Izaak Walton League is concerned about the environmental harm of added drainage. The Minnesota River is at its capacity, and this project promises to add more water to the Minnesota River system during times of high flow, according to Minge.
He urged the board to require an environmental assessment worksheet to consider the potential adverse environmental consequences to areas downstream of the project. He also said the county should not consider the project unless best management practices are implemented to increase the ability of lands in the watershed to retain water.
He also cited concerns that the increased flows could affect the stability of downstream ditch banks; their erosion will increase sedimentation. He also urged that the potential impact on groundwater infiltration be analyzed.
Project engineer Luker said the overall water volumes into County Ditch 63 will not be affected by the project: Due to overland flows, "the water is getting there one way or the other."
He said the water and sediment control basins should reduce the peak flows during rain events. He also said the improved drainage should increase the ability of the land to hold water during rain events, since the soil structure is less likely to be saturated.
The project should also reduce the amount of sediment loss from the area drained by the proposed project, he said.
The initial project design calls for non-perforated tile for portions of the line at a depth of 6 feet or more, which would be for the majority of its length. Landowners have expressed a desire for a perforated line. The commissioners agreed to have that change considered for the final plans.