ORC will this week focus its attentions on flooding around Lindsay Creek in Dunedin’s suburb of North East Valley, and possible investigations into mitigating the on-going problem.

ORC’s General Manager Science and Resilience, Tom Dyer, says the flood hazard risk is “significant” for large of the Lindsay Creek floodplain – within the suburb of North East Valley (NEV).

“Given the level of risk for North East Valley it’s prudent to take a fresh look at the flood hazard associated with the Lindsay Creek floodplain and to investigate options to reduce the flood risk for North East Valley,” Mr Dyer says.

Severe flooding and damage has been recorded in North East Valley at least 10 times since the late 1860s to the present. 

On Wednesday, 7 November, a report is going before ORC’s Safety and Resilience Committee meeting in Dunedin, outlining plans to investigate the issue further. The committee’s Agenda is available online.

Mr Dyer notes comprehensive flood protection schemes for NEV have been presented to Council previously, but it was decided not to pursue them, based on the economic climate at the time, feedback from community consultation, and the potential impact of costs on residents against the benefits provided by then proposed work.

“The flood hazard risk is higher than what is generally considered acceptable in New Zealand or elsewhere in Otago for an urban area, so we should regularly consider how best to manage mitigation,” Mr Dyer says.

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The Lindsay Creek channel is part of the Dunedin river management area and is actively managed by ORC to ensure flows and sediments are transported as efficiently as possible, such as routine monitoring, vegetation control, removing obstructions and bank erosion repairs where applicable.

ORC also maintains some rockwork and concrete walls designed to minimise erosion and help retain the creek within its existing channel, and in 2005 constructed a debris trap below Bethunes Gully to help reduce the volume of debris and the likelihood of channel blockage.

Proposed investigations phases

The proposed investigations phases are planned between financial year 2024/25 (year 1) until 2026/27 (year 3). During those phases, it is planned to complete a detailed assessment of the Lindsay Creek flood hazard which will inform the flood mitigation investigations but also the emergency flood responses. 

The investigation will consider all the sources of flooding; river, stormwater and surface runoff, account for the effects of climate change on river flows, and will assess the effects of “maximum credible” events and incorporate geo-hazards; such as landslides, due to their potential impact on the channel capacity and floods, Mr Dyer says.

One of the potential outcomes of the investigations phases would be the development of a flood mitigation plan for the Lindsay Creek catchment, he says.

It is proposed to have a decision point in 2027/28 (year 4) to consider the following options (as described in the Infrastructure Strategy 2024-2054):

• Status quo: ongoing maintenance of waterway and existing infrastructure, update of flood hazard information and collaboration with DCC as required, emergency response planning
• Implement flood mitigation options that focus on nature-based solutions where practicable
• Build a flood protection scheme that consists of hard engineering infrastructure

Mr Dyer says depending on the decision made, there is potential to start the implementation of the flood mitigation works from 2028/29 (year 5). The implementation would span several years.

BACKGROUND

Lindsay Creek rises on the flanks of Mount Cargill and runs southwest, draining North East Valley and joining the Water of Leith at the Botanic Garden. The catchment is compact with an area of about 12km2, and the main channel is relatively short being about 7 kilometres long.

Most of the stormwater network in North East Valley discharges into Lindsay Creek. During heavy rainfall events, stormwater overflows contribute to surface flooding and can compound the effects of river flooding.

In the creek’s upper catchment, the streambed is steep, and the stream has the characteristics of a mountain torrent. Under high flows, it can erode its banks and carry large volumes of debris and detritus. In the lower catchment, the slope of the streambed decreases through the urbanised area of North East Valley. 

During large flood events, the steep upper reaches of Lindsay Creek can produce large volumes of debris. Accumulations around bridges or other instream structures is likely, resulting in full or partial blockage of the creek channel. Floodwater would then back up behind the debris and could lead to floodwater spilling onto the floodplain.

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