Scott County authorizes Prior Lake–Spring Lake Watershed District diversion and iron filter on County Ditch 13

Chair (Scott County Ditch Board) opened a public hearing June 3 to consider a petition from the Prior Lake–Spring Lake Watershed District to divert, impound and modify a portion of Scott County Ditch 13 for water‑quality treatment.

The board heard engineering testimony, staff confirmation of legal notice and public comment before adopting findings and an order granting the petition as Resolution No. 2025‑156.

The project, presented by Kyle Crawford, project engineer for EOR, would install a reinforced earthen berm and a removable diversion weir at the Swamp Lake outlet on the upstream end of County Ditch 13. Runoff diverted by a 12‑inch diversion pipe would pass through a biofiltration pretreatment basin and then an iron‑enhanced sand filter; treated water would return to the ditch through underdrains. Crawford said the watershed has an easement with the landowner at the project site and that construction would be on agricultural land adjacent to Redwing Trail.

Why it matters: Attorney Kolb (drainage authority counsel) told the board its decision must follow Minnesota Statutes §103E.227, which allows a drainage authority to authorize impoundment or diversion if the engineer’s report and other evidence show the project will provide public or private benefit and will not impair the drainage system’s utility to affected owners. Kolb said the primary benefit asserted by the watershed district is downstream water‑quality improvement.

Design and impacts: Crawford described two main components: a biofiltration pretreatment area planted with native vegetation and an iron‑enhanced sand filter for phosphorus removal. He said the project is expected to remove about 89 pounds of phosphorus per year over the project life and that sediment removal would also reduce downstream impacts. Crawford said the outlet and diversion are sized so the project will not raise water levels in Swamp Lake, even in large storms, and that flows not captured by the diversion will pass over the weir or the stabilized berm overflow and continue through the ditch.

Crawford described stabilization work at the outlet where a roughly 4‑foot head cut has formed; the plan calls for extending riprap, restoring the channel profile with on‑site material and using an erosion control blanket. The county requested, and the project includes, a maintenance bench along the southern bank at least 20 feet wide (25 feet on bends) to allow heavy equipment access for future ditch maintenance.

Operations and maintenance: The diversion weir will be removed by watershed staff each late fall and reinstalled in spring to avoid ice or debris problems near Redwing Trail, Crawford said. He also said the iron medium limits the filter life; the watershed expects the filter to meet the 25‑year minimum service life required by the clean‑water grant, and the watershed will replace the iron medium or rehabilitate the facility if needed.

Public comment and clarification: Ted Kowalski of Spring Lake Township asked whether all water would be treated; Crawford replied that normal, low‑flow conditions would be routed through the treatment train, and the structure is sized to treat roughly the one‑year event so higher flows would bypass via overflow. Crawford estimated the iron filter life in the two‑decade range and reiterated the grant requirement that the facility be maintained or replaced for a 25‑year project life.

Easement and next steps: County staff confirmed the watershed’s easement is recorded with the county recorder. County counsel and staff noted the board was required to consider the engineer’s report and other evidence; the board adopted the draft findings and order authorizing the modification with conditions. Crawford said the watershed would finalize contractor agreements and hoped to begin construction late summer or early fall, subject to contractor schedule.

Deliberation and action: After limited additional discussion the board voted to adopt the findings and order as presented. The order authorizes the modification of County Ditch 13 to permit diversion, pretreatment and an iron‑enhanced sand filter, and imposes conditions recorded in the adopted findings and order.

The board’s order and the engineer’s report will be part of the record for the project; work on contracts and construction scheduling will follow the board’s authorization.