Purpose
The purpose of this work is to identify and implement solutions to improve navigation conditions by mitigating the increasingly dangerous outdraft condition within the upper approach to Lock and Dam 7 (LD 7).
Location
LD 7 is located in La Crescent, Minnesota.
Description
In 2017, a hydraulic study was launched to identify potential causes of the increasing outdraft at LD 7 and to investigate and implement measures to alleviate this condition. Flow measurements collected over the last 30 years indicate a significant shift in flows just upstream of LD 7, with nearly 20% more flow coming through the existing navigation channel, which must then exit through the dam, exacerbating the outdraft condition.
A two-dimensional hydraulic model was developed to analyze existing flow conditions and assess potential mitigation measures. The upper approach to LD 7 is uniquely shallow and narrow; therefore, many traditional river training structures are not feasible within this area. Several combinations of rock structures and dredging scenarios were considered, but only one showed promise — a rock weir extending off of the river wall constructed to a depth of 7 feet below the low control pool in late 2018. The weir was expected to reduce velocities throughout the approach and therefore the impact of the outdraft on navigation. The weir was then extended further upstream by 260 feet in 2020 following a request by the navigation industry.
Status
The navigation industry expressed additional concerns with the increasing outdraft at LD 7 at the Mississippi Managers Meeting in February 2023, classifying it for the first time as a life-safety issue. In April 2023, the U.S. Army Corps of Engineers St. Paul District engaged industry partners to collaborate in biquarterly meetings to work toward a solution. In June 2023, a design charette was conducted with navigation partners and regional experts to outline the near-, short-, and long-term options available and the associated timelines. It was determined that a future budget package must be submitted for a long-term structural solution, and Phases 1 and 2 of the current study were launched.
Phase 1 focused on partner collaboration, collecting updated hydraulic data and modeling design alternatives that were identified in collaboration with the navigation industry to improve present conditions. Phase 1 is pending conclusion following a successful Engineering Research and Development Center (ERDC) Ship Simulation exercise, performed over three separate weeks in 2024, that allowed industry pilots to test the navigation conditions for each design alternative and allowed the St. Paul District to make the most informed decision on the approach to Phase 2. The ERDC-developed Navigation Improvement Study Report is currently in final review and will officially conclude Phase 1 of the project when approved. Phase 2, short-term implementation, which includes preconstruction engineering and design, was initiated in May 2024 and is currently in 95% Review. Construction is planned to begin in late August 2025 and finish in December 2025.
Authority
Congress authorized the Upper Mississippi River 9-Foot Channel Navigation Project as part of the River and Harbor Act, approved July 3, 1930.
Funding
To date, the project planning and design phase are using 100% federal funds. The dogleg single-weir design alternative, with the best short-term positive results during the ERDC Ship Simulation, has a life-cycle cost working estimate of $16 million. Project-specific construction funds were submitted for fiscal year 2025 earmarks in preparation for a fiscal year 2025 contract award to reduce outdraft conditions and improve navigation safety.