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Consultants present draft Main Street Bridge report; replacement favored on lifecycle and safety grounds
Summary
A 218-page draft engineering report presented to the Charles City council recommends replacing the historic Main Street Bridge (Alternate 5) after inspectors found extensive concrete deterioration and safety risks in a downstream flume; the consultant estimated $12.5 million for a new bridge versus $14.1 million for a major rehabilitation and warned of many unknowns until demolition exposes the arches.
Consultants for Calvin Burns and Associates presented a 218-page draft report on the Main Street Bridge at a city council workshop, recommending a replacement rather than an in-place rehabilitation and flagging immediate safety concerns at a downstream concrete flume.
The report summarized underwater and hands-on inspections, core sampling and chemical testing. Presenter L Miller said the dive inspection found the upstream low-head dam in “fair to good condition” but described the flume outlet as “in pretty poor condition,” including significant holes in the floor and missing wall sections. Miller told the council it may be necessary to close the flume to pedestrians because “we are starting to get concerned that thing could shift” during even moderately high flows.
Why it matters: the Main Street Bridge is on the historic register, and the report weighed two broad options: a major rehabilitation that would convert the earth-filled arches to an open-spandrel arrangement, or a new beam-style replacement. The consultant estimated a major rehabilitation at roughly $14.1 million (with a higher contingency to account for unknowns) and a new bridge at about $12.5 million plus allowances, and recommended proceeding with Alternate 5 (replacement) based on lifecycle cost-per-year projections and the expected extent of work required to make a rehab feasible.
What the report says: the rehabilitation approach would require extensive demolition (removing top fill and façade walls), cofferdams and scaffolding, confined-space and underwater access for repairs, and likely frequent inspection thereafter. Miller emphasized uncertainty: the arches have been covered since about 1910 and “you don’t know what the conditions of those arches are… until everything’s exposed,” meaning unforeseen repairs could raise costs once demolition begins.
Historic-review and permitting: because the bridge sits on the historic register, Miller said the State Historic Preservation Office (SHPO) and Iowa DOT will need to assess whether changes create an adverse effect on the historic property. He cautioned that an SHPO determination of adverse effect would not prohibit a project but would require additional coordination and could change whether the structure remains listed.
Council response and next steps: council members pressed the consultant for comparative bids and closer cost checks — citing a recent local replacement project with much lower costs — and asked whether a new structure could replicate the arch appearance versus a basic pre-stressed beam bridge. The consultant said beam bridges (pre-stressed concrete) are generally most economical for the spans discussed and that attempting an arch-style replacement could complicate DNR permitting because of backwater impacts.
Miller said the draft remains open for council input and that he would supply a final report after re-checking a cost opinion within about one to two weeks; the council agreed to schedule another workshop when the final report is available to consider grant applications and DOT coordination.
The council did not take formal action at the workshop; the item will return for further review and decisions on fencing, grant applications and design approach.
