What are culvert/ bridge upgrades: As the title implies it is the modification of a culvert or bridge by increasing the size of, repositioning, and/or adding additional culverts or bridges to increase water flow. This can be useful to increase the discharge capacity of the channel particularly through a structure, and may reduce the severity of culvert/bridge blockage during a flood event. The negatives may include increasing the velocity, timing and severity of flooding downstream and potentially causing stream bed and bank stability issues if not designed correctly.
Culvert/ bridge upgrades are generally neutral in improving community access and recreational use. Why? Culvert and/or bridges upgrade typically occur in the footprint of an existing structure and as a result neither limit community access and recreational use nor improve it. Some Council’s however, incorporate riparian vegetation management into these upgrades increasing recreational use of the stream or river and provide access points.
Culvert/ bridge upgrades may slightly disadvantage individual members of the community. Why? Private land either upstream or downstream of the structure may be acquired to allow water flow transitions through the structure. However, generally culvert and bridge upgrades typically occur in the footprint of an existing structure on public land and as a result usually do not disadvantage individual members of the community.
Culvert/ bridge upgrades generally increase safety to the community during flooding. Why? The upgrade can reduce debris build-up that may cause the culverts to block and re-divert flow. However they can also increase the velocity, timing and severity of flooding downstream so flood management offsets are typically required such as detention basins or increased vegetation.
Culvert/ bridge upgrades do not typically raise community awareness and understanding of the local flood risk. Why? Culvert/ bridge upgrades are usually perceived as a local drainage measures and not as a flood reminder.
Culvert/ bridge upgrades typically improve habitat for local plants and animals. Why? Culvert/ bridge upgrades are generally undertaken to increase undersize culverts or bridges that typically restrict natural flow paths and cause an increase in water levels upstream. Upgrades typically improve this previous restriction allowing opportunities for increased animal migration and potential habitat creation. However, poorly designed structures have the potential to increase conveyance causing erosion and scour leading to bed and bank instability both upstream and downstream of the structure, and possibly destroying habitat during and after construction.
Culvert/ bridge upgrades can potentially improve water quality. Why? As noted above culvert/ bridge upgrades are designed to enlarge the water passage partially aiming to restore natural flow conditions which can improve riparian corridors that in turn buffer pollutants increasing water quality. If the upgrade is poorly designed however, the structure can increase velocities and flows in the river channel which can lead to increased scour and erosion diminishing water quality.
Culvert/ bridge upgrades can have major initial costs to council Why? Culvert and bridge upgrades generally require the removal and replacement of existing structures or require an additional structure to be built. This generally involves geotechnical, flood behaviour studies, survey, feasibility studies, detailed designs and construction. A small culvert upgrade would typically cost thousands of dollars where as a bridge upgrade could exceed a million dollars. One significant aspect that can increase costs particularly with bride upgrades is the moving of existing utilities such as water/ electricity/ sewer that are typically housed on, in or under the structure.
Culvert/ bridge upgrades have minor ongoing costs to council post implementation. Why? Maintenance generally involves routine inspection for deficiencies such as cracking, slumping movement, and concrete spoiling. Further works may also be necessary to remediate riparian corridors, reduce bed and bank stability issues and remove debris that may be located in and around the structure.
Culvert/ bridge upgrades are generally undertaken to reduce the annual averages damages of flooding to the community. Why? Culvert/ bridge upgrades are only undertaken if they reduce potential flood damages to the community. As mentioned although culvert/ bridge upgrades can reduce flood levels in critical locations they significantly increase the velocity, timing and severity of flooding downstream. If flood modelling indicates there will be no adverse impact downstream as a result of culvert/ bridge upgrades, then this can reduce annual average flood damages to the community.
Culvert/ bridge upgrades can potentially cause adverse flood impacts to other areas if poorly designed. Why? As mentioned previously culvert/ bridge upgrades are designed to increase water flow through the structure. This amplified conveyance therefore moves flood waters downstream quicker and with more energy, causing possible damage to downstream assets unless offsets have been undertaken such as detention basins or riparian vegetation management. Furthermore the upgrade could reduce the potential for debris blockage reducing flood impacts for upstream areas. As a result feasibility and detailed design studies for a full range of flood events (from regular to extremely rare floods) are required to assess the upstream and downstream impacts of culvert/ bridge upgrades.