Flood Mitigation Plan For New Library Shows Manageable Flood Risk At Riverside Precinct Site
Further flood modelling on initial concept drawings of the proposed new Nelson Central Library has shown there would be minimal flood risk of developing the proposed site on the surrounding area if specific mitigation measures are incorporated in the design brief.
The Tonkin & Taylor report, presented to Council on Thursday, 23 September, compared five different scenarios against two flood events (present-day 1% AEP and 1% AEP in 2130 incorporating RCP8.5M climate change impact) against a baseline model that represents the current building configuration.
The modelling results showed that any on-site and off-site flooding effects could be mitigated by design features such as a minimum floor level of RL 4.0m, with the ability to raise the floor level of the building in the future, and by creating a secondary flow path. In two of the scenarios modelled, the expected flood depths in some areas were lessened.
The five tested scenarios were:
- A ‘worst case’ model, where the entire site, bordered by Trafalgar, Halifax and Tahaki Streets, is built up. Note - this scenario is not anticipated.
- The Burger King building and the building between Findex and the Riverside Youth Pop-up Park are removed, and two new library buildings added as per the initial concept drawing. No secondary flow path incorporated
- Scenario 2, but with a secondary flow path
- Scenario 3, with the additional removal of the existing Elma Turner Library building
- Scenario 4, with the inclusion of a Climatorium.
While scenario 1 and, to a lesser extent, scenario 2 showed limited upstream flooding, this was reduced by the introduction of a secondary flow path in scenario 3. Scenarios 4 and 5 showed a positive benefit, decreasing flood levels to Civic House and Halifax St. The addition of a Climatorium for the purpose of the current modelling had a negligible impact on the positive outcomes seen from scenario 4.
Council accepted the report’s recommendation that these design elements are factored into the design brief for the new central library building, but also noted that no commitment to the building footprints used in the modelling, nor any specific building shape or layout, has been made. Further flood modelling will be undertaken when the final building footprint is developed.
The current flood modelling does not take into consideration information that will be available from future planned flood mitigation engagement and the benefit from potential future works in the central city.
Mayor Rachel Reese said: “This is not the first risk assessment we have done of the site, and I am confident in the professional advice we have received so far. We have seen here that our due diligence is paying off.”
Group Manager Infrastructure Alec Louverdis said Council was also aware of the research regarding insurance retreat discussed in a recent report for the Deep South National Science Challenge.
“The report states that insurance companies work with annual exceedance probabilities (AEPs) to decide what to charge for coverage and when to stop insuring a property. It refers to anecdotal evidence from the insurance industry that suggests partial insurance retreat begins to occur when the likelihood of an event reaches the 2% AEP threshold, and full insurance retreat will have occurred by the time this reaches 5% AEP (one-in-20-year occurrence). New buildings designed to be above 1% AEP flood levels at 2130, like the library, are not expected to be subject to insurance retreat over this time frame.”
A timeline for the library project is expected to be discussed at a Council meeting on 5 October.