The proposed boardwalk along the river side of the Liffey quays was a great idea, dreamed up by Dublin's ebullient City Architect, Jim Barrett. With the quays choked by heavy traffic, including an almost endless procession of juggernauts, it seemed an inspired notion to create a promenade for leisurely strolls.

As originally envisaged, the boardwalk was to be cantilevered from the quay walls along Bachelors Walk and Ormond Quay Lower at a level below the existing footpath, so that anyone sitting down on a bench would not be seen above the parapet and would thus be insulated, to some degree, from the traffic noise.

However, a full-scale mock-up of a section of the boardwalk, installed near O'Connell Bridge, suggests that it will be at exactly the same level as the footpath. Not only is this a significant departure from Mr Barrett's original idea, but the absence of any difference in level also renders the quay wall superfluous, even meaningless.

The high level of the boardwalk means that its 1.1-metre rail protrudes above the quay wall, which admittedly is very low on Bachelors Walk, creating a discordant feature when seen from, say, the traffic island in front of the O'Connell monument. If the boardwalk was dropped by two or three feet, this would not be a problem. Given that it is proposed to have kiosks and stalls along the boardwalk, these would be even more obtrusive, if its present level is maintained. An Taisce has also expressed concern that the kiosks and stalls, apart from reinforcing the boardwalk's "utilitarian ethic", could very soon start to look "shabby and tawdry".

In a submission to Dublin Corporation, it described the design by award-winning architects McGarry Ni Eanaigh as "anti-urban and gimmicky" and said it was unfortunate that this modern intervention was being made on Ormond Quay Lower, "arguably the only surviving stretch of historic quayscape". There had also been a "scandalous lack of consultation" on the scheme, with the corporation deploying the "Part 10" mechanism which precludes public appeal - a process that was also used in the case of the Millennium Spire for O'Connell Street and the remaking of Smithfield, another scheme by McGarry Ni Eanaigh.

According to the Government's Millennium Committee, which has allocated £2 million in funding for the boardwalk and another project to light up 11 Liffey bridges, this new promenade would provide "an attractive amenity for walking, strolling, relaxing with refreshments and a new venue for weekend markets".

Obviously, it is important that its height over the water is sufficient to ensure that the surface is not washed over by a high tide. But the level proposed is significantly higher than that and seems to have been determined by two factors - the need to avoid ramps and a wish to expose as much of the structure as possible.

Though there is already some green slime at the base of the diagonal struts, the City Architect said he would "revisit" the height issue to see if the boardwalk could be dropped lower. Another issue to be resolved is that there is too much "spring" in the timber surface and this may deter a lot of people from using this new amenity.

Meanwhile, An Taisce has voiced its concern over another major intervention in the Liffey-scape - the proposed parabolic-arched road bridge between Blackhall Place and Ushers Island, designed by Santiago Calatrava, the Spanish engineer-architect whose dynamic sculptural forms have made him world-renowned.

As An Taisce noted, Calatrava's offering is "radically different" to three bridge design options proposed for the same location in 1998. These were of a similar "slim-line concrete construction" as the other modern bridges across the Liffey - the Talbot bridge at Memorial Road and the Sherwin bridge near Heuston Station.

It said the "highly assertive design" now being proposed would create a "serious visual barrier in the line of the Liffey, cutting off one of its most important vistas - the first view of the quays, with the tower of St Paul's Church and the Four Courts dome providing a memorable visual composition for visitors arriving from the west.

An Taisce acknowledged that Calatrava had designed some fine bridges, notably his dramatic structure in Bilbao, which had enlivened a bleak stretch of that city's very wide industrialised river. But his proposal for the Blackhall Place crossing was at odds with the important civic design character of the Liffey quays.

"The huge curved arch with its suspended cables is an aggressive conceit which has no structural logic, as demonstrated by the manner in which it meets the piers at each end. This is a case where Calatrava is supplying one of his hallmark highly-profiled bridge designs in a location and context for which it is inappropriate."

An Taisce recalled that An Bord Pleanala had refused permission for a controversial footbridge linking Ormond Quay with Wellington Quay in 1995 on the grounds that it would be "a discordant and obtrusive feature in the townscape quality of the river by reason of its height, curved profile, mass and lack of transparency".

This bridge, designed by McGarry Ni Eanaigh for Temple Bar Properties, was nicknamed the Wibbly Wobbly bridge because of the way it seemed almost to stagger across the river. It has since been replaced by a simpler, more subtle proposal by Howley Harrington Architects, which won a design competition for the project.

The City Architect, Jim Barrett, believes that An Taisce is not comparing like with like because the context of Blackhall Place is "very weak" compared to the scale and strength of enclosing buildings on Ormond Quay and Wellington Quay. As a result, he is convinced that Blackhall Place "can take a much stronger statement".

Indeed, that's why Calatrava was given the job, because he was very unlikely to design something bland and self-effacing. (He has also been commissioned to design a much larger swinging bridge on the axis of Macken Street, in the Docklands area; this will apparently be suspended from a central pylon, 120 metres high).

"We could have settled for a simple structure, but we wanted to make a statement here. It might be controversial, but this bridge is going to be a very positive feature for that end of town - the western end of the quays - because we want to pull city that way. It will also benefit the National Museum at Collins Barracks," Mr Barrett said.

He denied An Taisce's claim that the bridge, with its splayed arches rising to a height of 8.5 metres (28 ft), had no structural integrity or that its form goes beyond what is absolutely essential. "It's not contrived, in my opinion. If you straightened up the arches, it would be a lot higher. So in a way it could be seen as minimalist."

The City Architect said there was more to the Blackhall Place bridge than "just conveying X number of cars from one side of the river to the other"; it was also essential as a traffic relief route to enable O'Connell Street to be remade. "I wouldn't in any way be apologising for it because I believe it's the right thing to do," he declared.

It will be a matter for the Minister for the Environment, Mr Dempsey, to adjudicate on whether Jim Barrett and Santiago Calatrava are on the right track.