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Climate Change and Infrastructure Design Case study: Wairau Catchment, North Shore City

Executive Summary

1. Introduction

2 Data and Methods

3 Results and Discussion

4 Implications

5 Conclusions and Limitations

6 Further Work

7 References

4 Implications

4.1 Upgrades

Figures 3-1 and 3-2 above can be used to display the areas where network upgrades would be recommended, using either the design storms or the dynamic modelling approach. In order for a comparison to be made, all upgrades have assumed a 10-year level of service requirement for the whole network.

It can be seen that more upgrades would be necessary under the future scenario than the present, and that the dynamic models identify fewer locations in need of upgrades than the design storms. However in both the dynamic and design storm models, the future scenario merely extends an area that is predicted to flood under the present scenario.

The total length of channel affected under each scenario is as presented in Table 4-1 below:

Table 4-1: Length of Channel Potentially Subject to Flooding in a 10% AEP Event

4.1.1 Costs

A range of options could be reviewed with respect to potential solutions for channel capacity and flooding issues in the Wairau catchment. For example, the issues in the Jonathon Lavery and Becroft sub-catchments may be addressed by an increase in efficiency of the current stormwater ponds. In these locations, the cost differences between the extent of works required under each modelled scenario may be minimal.

However, the lower catchment issues may require land purchase - many of the channels are closely surrounded by houses, and any works in the channel would require additional drainage easement purchase.

Figure 4-1 is a typical cross section through the Takapuna North area is J-13d, a trapezoidal channel, with a 'critical level' of 10.5m. It can be seen that this channel is in need of upgrade under any modelling scenario, in order to convey the 1 in 10 year flows without overland flow. It is also displayed in this diagram that the differences in water levels are quite small at this point between the different modelling scenarios.

To re-design this channel for the appropriate flows, additional land, excavation and re-instatement would be required. An alternative is to purchase flood - inundated land.

In order to look at the cost implications of the different modelling and climate approaches, and based on an assumption that 10m additional land each side of the channel would be sufficient to manage the flows, the difference in cost can be compared based on the length of channel required to be mitigated (refer Table 4-2 below).

Assumption: 10m additional easement each side of channel. Cost: $1000 / m²

Table 4-2: Potential Land Costs for Channel Upgrades in Takapuna North Section of Wairau

In this area, the extent of the issues is such that both the present design and dynamic models provide similar estimates of affected channel. The difference lies in the cost of allowing for the additional length at risk due to the future climate change. Elsewhere in the catchment, there are more significant differences between the "Present" design and dynamic model results.