Mandeville Resurgence Channel Upgrades

The Mandeville area has a history of flooding, including more recent storm events in 2014, 2017, 2022 and 2023.

While this flooding is generally caused by heavy rain, the extent and depth of the flooding can be exacerbated by high groundwater levels and resurgence flow.

Following flooding in June 2014, budget was allocated in outer years for the Mandeville Resurgence Channel Upgrade and Diversion Project.

Investigation work identified that there are options to improve the drainage in this area.

Three potential options for Stage 1 and one potential option for Stage 2 have been identified.

We are planning on implementing upgrades to improve the management of stormwater and resurgence flow through Mandeville.

Potential options for upgrades:

Stage 1 - Upgrade of Existing Channel Options

Option A

Existing Drainage Path Upgrade - Upgrade existing main drainage channel through Redfern Lane, over Tram Road, through Millfield subdivision into Bradleys Road drain. This option increases the total capacity of the existing main drainage channel to 1.0 m3/s from the existing capacity of 0.5 m3/s (average).

Option B

Drain to Water Race – Install culvert to convey stormwater flow to an open drain on the south side of Tram Road, then utilise the stock race to convey flow. This option provides an additional 0.5 m3/s capacity for the existing main drainage channel. When combined with current capacity of the drainage channel, which is an average of 0.5 m3/s, the total drainage capacity would be increased to 1.0 m3/s.

Option C

Pipe Options to Bradleys Road - Install new 900mm diameter pipe from Tram Road culvert outlet to Bradleys Road drain. This option provides an additional 1.0 m3/s capacity for the existing main drainage channel. When combined with current capacity of the drainage channel, which is an average of 0.5 m3/s, the total drainage capacity would be increased to 1.5 m3/s.

Click map or table to enlarge.

Stage 2 – Diversion Option

No. 10 Road Drain Diversion - Construct a new open drain to divert stormwater (1.0 m3/s) along No. 10 Road south to the Old Bed Eyre River. The Old Bed Eyre River discharges into the Silverstream which flows into the Kaiapoi River.

Click map or table to enlarge.

**Two Chain Road Drain Diversion Option

The Two Chain Road Drain Diversion included constructing a new open drain along Two Chain Road and south to the Eyre River Channel. This option is no longer being considered as it has been determined unfeasible based on the following factors:

• Gradient - Two Chain Road generally runs uphill (from Long View Lane to the Eyre River) and does not provide sufficient gradient for natural gravity flow without pumping.

• Health and Safety - To provide sufficient gradient would require a drain with significant depth that would be problematic from a road safety and maintenance standpoint, and would still need to be pumped over the stopbank into the Eyre River.

• Cost - This option would require significant amount of cost ($8M+) for deep channel earthworks and a permanent pumpstation.

Considering these factors, the decision to no longer pursue the Two Chain Road Drain Diversion option was made.

Other Options

Alternative alignment options to divert flow to the Eyre River have been considered, but discounted due to the significantly higher costs.

Benefits:

These upgrades will increase the capacity of the drainage systems from approx. 0.5 m3/s to 2.0 m3/s (about four times the existing capacity depending on the recommended option). While this is a significant improvement, surface flooding will still occur in larger rain events.

Project Programme: