It was supposed to be the centerpiece of Palo Alto's biking renaissance -- a slender, elegant bridge that would span U.S. Highway 101 and offer cyclists and pedestrians a beautiful new entryway into the Baylands.

It was supposed to be at once subtle and iconic, sensitive to the expansive marshlands and yet fancy enough to attract attention and appreciation from passing motorists.

And in March, when the City Council directed city staff to negotiate a contract with Moffatt & Nichol for the understated, ribbon-like bridge the firm proposed, it seemed like money was no object, given that the city had already received about $8.3 million in grants for a project that had an estimated budget of $10 million.

Now, however, Palo Alto's hoped-for signature bike project is on shaky grounds. Its budget has swollen to more than $17 million and the city is preparing to sever ties with Moffatt & Nichol. With construction costs climbing, Public Works staff is also now recommending that the council consider a more traditional – an affordable – bridge and launch a search for a new engineering firm to build the structure.

If the council approves the new plan on Dec. 14, the bridge at Adobe Creek wouldn't be built until fall 2019.

According to Moffatt & Nichol, the main reason driving up costs is the volatility of the construction market. When its team submitted its design to the city-sponsored design competition, the estimated price tag for the structure was $9 million to $9.5 million -- well within the parameters of the competition, which the city launched in late 2013. But things changed dramatically this past spring, just after the selection of Moffatt & Nichol's low-key suspension bridge over a more glamorous, arched structure proposed by firm HNTB. Though the arched bridge won the design competition, the council ultimately went with the subtler option, arguing that it would fit in far better in the Baylands environment.

In May, the company submitted a new estimate showing that the total cost for the project would actually be $16.6 million, which includes about $12.2 million in actual construction costs and contingency spending and another $4.5 million for engineering, construction management and right-of-way purchases.

In a May 17 memo to the city's project manager, Elizabeth Ames, the firm cited the Bridge Cost Index, a quarterly report that is published by Caltrans and that tracks costs on transportation projects. The firm's initial $8 million target budget was based on a 2011 planning-level estimate, the Moffatt & Nichol memo states. According to Caltrans data, the "cost index" for bridge construction went up from 313 in 2011 to 456 in 2014, connoting a cost increase of about 50 percent.

"Therefore, it is reasonable to expect that a bridge cost as high as $12 million . . . may be realized by the time the structure is designed and prepared for bid," the May memo stated.

The revised numbers mean that if the city sticks with the design, the bike bridge would among the most the most expensive structures of this sort constructed in the Bay Area over the past decade, according to comparison data provided by Moffatt & Nichol. To date, the Mary Avenue Bridge that Cupertino completed in 2007 has been the most expensive of the bunch, with a total construction cost of $10.2 million (though the total project cost was $17.6 million). Construction of a bridge earlier this year in Larkspur cost $9.5 million; for a pedestrian bridge that Belmont completed in 2011, the figure was $7.6 million.

On Dec. 14, the City Council will consider whether to pursue the Moffett & Nichol design despite the larger cost or to look for a new contractor to work on the project. Public Works staff is recommending that the city cease negotiations with Moffatt & Nichol and pursue a more traditional process for bridge design: a "request for proposals" for a firm that would build the structure within what is now the anticipated budget of $11.4 million. Staff is also recommending that the city pursue additional grant opportunities, in hopes of ending up with a budget of $13 million to $17 million.

One potential funding partner that has already stepped up is Google, the Mountain View-based tech giant that leases offices in several Palo Alto locations. According to a letter from John Igoe, Google's director for real estate and workplace services, company officials held a meeting on Nov. 13 with city staff to discuss a possible contribution. At the end of the month, Google sent a letter to Assistant City Manager Ed Shikada confirming Google's offer of "up to $1 million" to directly support the Adobe Creek bridge.

The contribution, however, comes with a few strings attached. For one, it hinges on the council pursuing a "more affordable design" for the bridge, "with the low profile suggested in the past as well as being 'bird safe.'" While this shouldn't be problematic for a council that has already expressed a strong preference for a low-profile bridge, a second condition could be more controversial: It calls for Google to "receive credit as a traffic mitigation measure for any future development application." Thus, even though Google's letter refers to the $1 million as a "donation," it also makes clear that the contribution should be factored into future land-use decisions involving its applications.

So far, this condition has not deterred Public Works staff, which is recommending that the city enter into the partnership with Google. The city could also make a bid for the roughly $4.5 million in Santa Clara County funds that were given to Stanford University for construction of trails. Stanford has subsequently relinquished the grant and opted to build the trails around its campus with its own funding, leaving the county looking for a new recreation project to fund.

The council could also direct staff to negotiate with the team led by HNTB, the firm that actually won the design contest only to see its bid rejected by the council for being too showy for the Baylands. Even so, the arched design known as "Confluence" had its share of fans, including the majority of the design-contest jury and the city's Architectural Review Board. Even in selecting the understated Moffatt & Nichol design, several council members praised both finalists.

"I think the community was provided two exceptional designs, and I think the process in that sense did work very well and it will benefit the community greatly over what we might have had as an alternative over the process," Councilman Pat Burt said at the March meeting.

Given the community and council feedback, Public Works is not recommending this course of action. After conferring with other experts, officials concluded that unknown costs for the slender bridge are likely to further increase the project's budget. Complications, according to a new report, could include unexpected soil conditions, site complexities that require utility relocations, additional columns and new requirements imposed by Caltrans as part of the environmental-clearance process. The report also notes that staff has reviewed the Moffatt & Nichol design with "bridge designers and contractors who have expressed their opinions that the final cost would be significantly higher than $17 million."

The proposed suspension bridge included a host of bike- and pedestrian-friendly amenities, including a plaza on the Baylands side and a stairwell on the west side; a raised sidewalk to separate pedestrian traffic from bicyclists, benches and artwork.

Now, that the city is preparing to hit the reset button, some of these features are likely to disappear. The list of strategies submitted by Public Works for reducing costs and making the design more "standard" includes narrowing the bridge deck, making the plaza smaller, more columns, standard railings and "elimination of the stairwell, raised sidewalk, enhanced lighting, railing, benches, large plaza areas and similar amenities." A new request for proposals, the report states, offers the best choice to receive proposals that would meet the new criteria.

Brad Eggleston, assistant director of Public Works, told the Weekly that while the request for proposals would focus on "a more conventional, standard type of design," the city would try to integrate "aspects that came out of the design competition."

"We're still thinking of a low-profile sort of bridge, but probably not as slender as in the Moffatt & Nichol design," Eggleston said.

If the council agrees with the staff recommendation, the request of proposals would be released within days of the Dec. 14 meeting and design work would begin in April. Under the present schedule, the design work would conclude in November 2017 and the new bridge would be constructed in 2019.

The project is among the most ambitious and expensive in the bicycle and pedestrian master plan that the City Council unanimously adopted in 2012. The plan, which also includes myriad bike projects and trail improvements, identifies the overcrossing at Adobe Creek as "the preferred alternative for improving connections across Highway 101 from South Palo Alto to the Baylands and Bay Trail." Currently, the only access to the Baylands in the Adobe Creek area is an undercrossing that is generally only open from mid-April to mid-October. The master plan notes that the path surface in the undercrossing is "only one foot above dry-season water level and is regularly covered with mud and debris by even moderate storm flows." The new bridge, by contrast, would offer users a pleasant, year-round alternative for reaching the Baylands, according to the plan.

"An estimated 100,000 bicyclists and pedestrians would use the bridge each year, a figure that would rise as adjacent bicycle connections improve and area land uses adapt," the plan states.