News

Flow modelling and design details

17 December 2024

Renmark Paringa Council in partnership with BMD Constructions and Tonkin are pleased to share the below information on the design for the Twentyfirst Street Bridge Replacement.

The design portion, including flow modelling, for this major project has evolved since February 2024 following the options phase completed in 2023.

To highlight the key improvements of the new bridge construction, please see below comparison against the existing infrastructure

For comparison of the existing infrastructure against the designed bridge:

Existing BridgeNew Bridge Design
Bridge Dimensions10m (wide) x 15.5m (long)11.9m (wide) x 24.9m (long)
Culvert/DrainageNorthwestern (Renmark West side) - 8 drainage pipes
approximately 15m long x 450mm diameter

Northeastern (Township side) - Nil		Northwestern (Renmark West side)
41.5m long x 8.1m wide x 1.2m high

Northeastern (Township side)
25.6m long x 8.1m wide x 1.2m high
  • Flow modelling for the bridge structure utilised current research and modelling inputs created early 2024 by the Department for Environment and Water and incorporates the future Bookmark Creek upstream regulator structure.
  • This assessed the existing bridge conditions against the proposed conditions. For reference the 1956 Flood was 20.61AHD at this location
  • The design developed detailed modelling outputs for both water depth and water velocity in the Bookmark Creek channel.
  • The depth and velocity outputs are used to confirm road and bridge levels and to design and specify scour protection.
  • Hydraulic modelling of Bookmark Creek and the Twenty first Street bridge has been undertaken to assess the performance of the overall structure (including the bridge, high flow culverts and the road embankment).
  • Flow modelling allowed a culvert blockage factor of 20% in accordance with design Australian Rainfall and Runoff (ARR) 2019.
  • The modelling was completed in HEC-RAS 2D version 6.5 (using the SWE-ELM solution set) with a time-varying flow hydrograph as the upstream boundary condition and a time-varying water level as the downstream boundary condition.
  • The flow capacity of the culverts diverts water that would otherwise be forced through the bridge, and thereby reduces flow velocities and hydraulic forces on the bridge for all flood events.
  • The alignments of the culverts have been chosen to be aligned with the direction of flow coming from upstream on the floodplain unlike the previous drainage pipes that were misaligned.
  • The northwestern culverts were shifted south towards the bridge consideration was given to the potential of high-velocity flows coming through the northwestern culverts to cause scour on the flood levee immediately downstream.
  • The base width of the channel under the bridge has been designed to be the same as the existing channel, in order to not reduce the hydraulic performance of the bridge during low flow events.
  • The bank alignments under the bridge and immediately upstream and downstream of the bridge have been altered slightly to be better aligned with the direction of flow from upstream and proceeding downstream, and to provide a smoother transition between the broader floodplain, upstream channel and the contracted section of the bridge.
  • The batter slope of the spill-through abutments has been flattened compared to the existing abutments.
  • This provides a wider cross-section for the channel under the bridge which results in a significantly greater flow capacity under the bridge. It also provides greater slope stability for the erosion protection to be placed on the abutments. This change resulted in a larger bridge span being required.
  • The proposed works include the raising and widening of the existing Twentyfirst Street formation at to provide greater flood resistance to the existing carriageway. The works are required because of recent flooding events which saw the overtopping of the Twentyfirst Street and significant damage to bridge, road formation, and road surface.
  • The new vertical alignment seeks to tie into the existing road on either end. The northern extent will tie in prior to the existing roundabout circulatory carriageway on Bookmark Avenue. The new road then maintains a flat grade for approximately 60m where the road height increases in order to interface with the new bridge over Bookmark Creek.
  • A 0.5% downgrade has been designed across the bridge to mitigate any possible surface runoff and drainage issues on the bridge. Following the bridge, an increased road height continues until it returns back down to existing road levels as quickly as possible.
  • The new bridge has been raised to provide flood immunity for the 120 GL flood event which was the optimal level against the other options presented in 2023.
  • Any additional increased height would have resulted in significant road upgrades at the intersection of Bookmark Avenue and Cucumunga Street at a much higher cost due to third party utilities such as NBN & Telstra being present.
  • The proposed road has made provision for a 1.2m wide on-road cycle lane in each direction of travel. An unsealed 2.5m wide path has also been provided for adjacent to the eastbound lane to provide connectivity to the school east of Bookmark Creek. The 2.5m wide path has been designed to accommodate pedestrian and cyclist access.