Moab presents 30% design for Third South bridge replacement; estimate near $10 million

Moab City staff and project engineers presented a 30% design update Oct. 28 for the 300 South (Third South) bridge replacement, describing a bridge and channel redesign intended to pass a 100-year flood with two feet of freeboard while working inside a constrained right-of-way.

The presentation said the current construction estimate is just under $10 million and could rise as the design is finalized. Designers are proposing a continuous bridge span (no intermediate culvert columns), a deeper channel section with a concrete-lined channel, and a shared-use path on one side with a sidewalk on the other. Because of the required structural depth, the road deck will rise roughly 4.5 feet at its center, requiring longer ramps and grade transitions on either side.

Why it matters: the Third South bridge is a known bottleneck for flood flows through downtown Moab. Staff said a plugged culvert at this location can cause urban overtopping and downstream impacts; the replacement aims to increase capacity and reliability while adding pedestrian connectivity.

Key details and tradeoffs - Capacity and elevation: the design target is to pass the 100-year storm with 2 feet of freeboard. The two‑foot freeboard is consistent with FEMA best practices and improves the city’s chance of meeting federal funding and mitigation criteria. - Footprint limits: private development directly adjacent to the bridge constrains horizontal expansion. Staff said that because the existing building and property layout are fixed, the design must fit within a limited envelope and tie smoothly into upstream and downstream channel geometry. - Channel treatment: engineers proposed a concrete-lined channel beneath and immediately upstream of the bridge to establish a predictable conveyance elevation and simplify maintenance and debris removal. Staff noted a tradeoff: concrete provides capacity and ease of future scour control but reduces riparian character and will require measures to lessen visual impact. - Pedestrian access and safety: the north-side staircase that currently provides access to the path cannot be accommodated in the tighter footprint. The shared-use path will be longer, with fewer options for near-term egress; staff proposed a 42-inch concrete safety wall at a low section of the trail to buy time in a rapid flood and signage to close the trail when flows rise. Pedestrian safety measures under consideration include rapid rectangular flashing beacons (RRFBs) and additional bridge lighting; photometric reviews are underway. - Utilities: the project will upsize a key waterline at the bridge to meet future demand so that the city will not have to rework the bridge for larger mains later. - Construction impacts and schedule: staff said construction will likely require closure of Third South for the bulk of the work. Preliminary duration estimates range from 12 to 18 months; phasing to keep traffic partially open would increase cost and complexity. - Funding and next steps: staff has submitted a congressional funding request and said the project is being positioned to qualify for federal appropriations; an open house is planned when plans are further developed.

Stakeholder outreach and mitigations Staff reported an adjacent-property stakeholder meeting and a capital project review with city departments and utilities. Where property geometry forced changes, staff said they worked directly with property owners to reduce impacts; for example, a modified driveway tie-in and a relocated sidewalk ramp were described as negotiated outcomes. For sections where tree removal is unavoidable, staff noted that trees have historically been lost in floods and pledged to pursue replacement plantings in less flood-prone locations.

What remains to be resolved Designers and staff highlighted several decisions before final design: whether to use a concrete channel through the downtown segment or combine concrete bottoms with vegetated side treatments (gabion baskets, pocket plantings), the final lighting and RRFB locations and specifications, and the tradeoffs of retaining versus removing trees and other riparian elements. Staff said the bridge was the top priority because it represents the tightest bottleneck; downstream sections will be sequenced as funds and plans allow.

Next steps and public outreach Staff said they will continue design toward 60% and 90% milestones, hold an open house this winter, and coordinate with emergency services and utilities on phasing, detours and emergency access. The council requested follow-up information about timelines, visual mitigations for the concrete channel, and a more detailed construction-phase traffic plan.

A funding decision from Congress could affect schedule and scope; staff indicated that outcome will determine how quickly the project advances into construction.