Chattanooga has sorely missed the Urban Design Studio. As an arm of River City Co., the studio's architects for years helped shape the environment for the city's downtown renaissance. They envisioned how the core downtown area could be restored and enhanced, and how plans for its new and rehabbed buildings, parks, blocks, streetscapes and, yes, new downtown residents, could come together with a sense of vibrant, cohesive redevelopment and architectural unity.

That valuable urban planning function withered after the studio was abruptly dismantled by Mayor Ron Littlefield shortly after he took office. Fortunately, the design studio's seed work -- and the obvious gaps that yet remain in downtown's urban redevelopment -- are set to get fresh attention.

The vehicle for the initiative is the River City Co.'s new Urban Design Challenge, a year-long contest in which six volunteer design teams have been selected to create a vision and redevelopment plan for six major downtown sites that clearly need such attention.

As the Challenge was announced Wednesday, each team was assigned to create a redevelopment plan for one of the six specific challenge sites. Their work will be unveiled in stages over the coming year, with one plan presented and discussed every two months in a public meeting.

The sites to be addressed are the 700 block of Market Street, the Civic Forum site at 11th and Market Streets, Patten Parkway, the 4th Street corridor from Chestnut Street to Georgia Avenue, the Vine Street corridor from Georgia Avenue to UTC, and Main Street between Broad and Chestnut Streets. Each site is critical to the core downtown and rich in opportunity, but clearly in need of visionary work.

The first plan to be discussed, in September, will cover the vacant east side of the 700 block of Market Street, downtown's most prominent eyesore. Redevelopment plans for the vacant land have faltered for more than a decade. Since Trey Stanley's proposed Mayfair-on-Market plan tanked three years ago, RiverCity has been trying to recover title to the vacant property. Until its lawsuit is settled and the city recovers and develops the land, there will be an unbridgeable hole between the waterfront and Main Street.

Barriers to redevelopment of the other sites are less contentious, but still formidable in the current investment climate. Redevelopment plans built on exciting vision and innovative design, however, should foster fresh interest in investment as the economy recovers and as downtown's urban revival and housing opportunities go forward.

There is good reason to believe in the latter. A Challenge kickoff address by Alex Krieger, one of the nation's most prominent urban planners and designers, offered convincing insight into why Americans are increasingly abandoning suburbs and returning to well-designed urban environments and city life, and the personal and societal benefits of that shift.

America's changing demographics -- families with fewer children and couples with longer empty-nest years -- has a lot to do with the shift. Among other factors are the revival of interesting cities, proximity and convenient access to expanding urban amenities and social interaction, and weariness with costly and time-consuming commuting.

To capitalize on the urban trend, however, cities must provide more than jobs. Krieger's list of principle ingredients for successful urban centers include easy and safe amenities and activities, affordable housing, convenient mixed-use environments and mobility, investments in public spaces and culture, pedestrian-friendly transit, green environments and ample recreation.

Chattanooga already offers much of that, and interest in downtown living and nearby housing reflects the city's expanding urban appeal. But continuous improvement is necessary. Redevelopment plans to be produced by the six teams competing for top honors in River City's Urban Design Challenge should greatly enhance the city's revival.