Physical Constraints

General

The structure should bridge the gap between the existing railway embankments, in one or more spans, leaving the river channel and banks as open as possible. The bridge should span from either the existing abutments (approximately 85m) or from new abutments up to 5m in front of the existing. The minimum gap between abutments is to be 75m. The overall length of the bridge is not to exceed 120m.

The Greenway

The bridge is to carry a cycleway/bridleway with a minimum width of 4m. The maximum gradient is to be 1 in 20 and a non-slip finish is to be provided.

1.5m high parapets with 300mm solid infill at the bottom and a handrail at 1m are to be provided.

Ramps linking the greenway to the footpaths running along both river banks will be provided outside the limits of the new bridge. Steps are to be incorporated into the bridge design on both sides of the river to give direct pedestrian access between the existing river bank footpaths and the new greenway.

River Douglas – Clearances, Intrusion into the River etc

i)    Deck surface level is to be 11.0m AOD at each end of the bridge to tie into the existing embankments. Deck soffit level is to be no lower than 10.0m AOD.

ii)    A minimum clear span of 25m is to be provided over the River Douglas, centred on the river channel.

iii)    No more than 2 support points will be allowed within the central 75m of the span. These must not reduce the waterway area (below 10.0m AOD, over a width of 75m) by more than 10%.

iv)    The existing footpaths on both banks are to be maintained with a minimum opening 5m wide and 2.7m high.

Plan Extents

The limits on the extent of the structure in plan are shown on the plan below. The structure is to be contained within a 120m long, 40m wide area centred on the original bridge location.

Elevation Extents

The overall length of the structure is not to exceed the plan limits, ie 120m long. The limits on soffit levels etc are as given in 3 above. The highest point on the bridge is not to exceed 71.0m AOD as shown on the elevation below.

Whole Life Cost/Quality

The judging will consider the Initial construction costs and likely maintenance costs in conjunction with the flair and scale of the design and the quality of the materials and detailing. Sustainability of the materials, construction methods and maintenance processes will also be considered.

Wildlife

Most of the bridge site and the approaches are Biological Heritage Site. The bridge design should seek to minimise the impact of the structure and maximise habitat connectivity across the greenway at both cycleway level and river bank level.