Engineers outline phased drawdown, dredging and permitting schedule for Lake Mitchell project

Barr Engineering engineers presented a multi‑phase plan to the Mitchell City Council for a controlled drawdown, dredging and reconstruction of the Lake Mitchell spillway and a permanent drawdown outlet during a council meeting update.

The firm described an initial, controlled drawdown that will use siphons, pumps and temporary cofferdams to lower the lake slowly for structural and environmental reasons, then a targeted removal (a roughly 20‑foot‑wide saw cut) in the spillway to complete the first phase. After dredging, crews would construct a permanent intake and an outlet pipe through the rebuilt spillway with a valve to permit future drawdowns on an as‑needed basis.

Barr Engineering said the team completed coring of the existing spillway and observed variable concrete thickness: cores showed about 26 feet of concrete depth on the south side and about 19 feet on the north. The firm said it selected the south side for the initial breach because of the thicker concrete there, and that contingency measures will be included in the design in case underlying conditions differ from core samples.

The engineers emphasized a slow drawdown to protect shore stability and avoid an abrupt release of sediment to Firesteel Creek, which contains the Topeka shiner, a species discussed with U.S. Fish and Wildlife Service. The firm said downstream sediment controls will be installed and that the U.S. Army Corps of Engineers has been engaged; staff reported the project appears to qualify for a nationwide permit rather than an individual permit, which the city and consultants said should streamline federal review.

Key technical points provided by Barr Engineering during the presentation: the permanent outlet intake will be placed away from the spillway in deeper water so it can function when the lake is partially lowered; the anticipated controllable drawdown from the permanent outlet was described as on the order of 8–10 feet; a temporary notch during initial work will be wider and deeper (20‑foot wide saw cut) to provide a controlled flow path during construction; deepest parts of the lake were described in the presentation as roughly 25–26 feet; and Barr said a 24‑inch outlet pipe was shown in design concepts, with the firm noting that a pipe of that size would require long durations to lower the full lake volume and therefore the installed system is sized for periodic maintenance drawdowns rather than full emptying.

Barr Engineering said it is completing endangered‑species, cultural resources and special aquatic site evaluations required for federal permitting. The firm recommended avoiding dredging immediately adjacent to a mapped prehistoric site (referred to in agency materials as the Mitchell site) and said it will perform an archaeological field check at Firesteel Park where native ground disturbance is expected. Consultants told the council they have concluded, after coordination with agencies, that Topeka shiner impacts are unlikely if the drawdown is managed and sediment controls are used.

City staff and the consultant outlined a two‑contract construction approach in concept: an initial contract for drawdown and stabilization and a second contract for dredging, disposal and reconstruction. Barr said the city could award the initial phase this fall if permits and documents fall into place and the drawdown could begin before lakes freeze; the larger dredge/reconstruction phase would follow in a subsequent construction season and could extend over multiple years. The firm noted some dredging operations may benefit from frozen conditions but said the total material volumes make a multi‑season approach likely.

Mayor and staff invited residents to a repeat presentation the following day in the same council chamber, and staff and the consultant said they will continue monthly coordination meetings with the mayor and city staff.

Ending

City staff and Barr Engineering framed the schedule and next steps as permit‑driven: submit the Army Corps preconstruction notification (nationwide permit) in the near term, complete agency consultations and finalize bid documents after initial drawdown data become available. No formal council action on contracting or funding was taken at the meeting; staff said formal bids and permit approvals will come back to council for action when ready.