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Mayoral Taskforce On Flooding – Report To Council Meeting

Mayoral Taskforce On Flooding – Report To Council Meeting Of 12 May 2014

[Full document: MayoralTaskforceOnFloodingReportToCouncilMeetingOf12May2014.pdf]


2.1 The large number of recent floods in Christchurch is due to a run of major storms. It is clear that the severity of the floods has been aggravated by earthquake damage to the land and stormwater network, particularly due to land lowering in some places such as Flockton Basin.

2.2 The Taskforce focused on the most vulnerable of those households affected by regular flooding since the earthquakes. Vulnerability has been assessed using both physical and social factors. It was determined that the most vulnerable were those with two or more instances of flooding above the floor level. The next most vulnerable were those houses with two or more instances of flooding beneath the house but not up to the floor.

Restricted access to dwellings defined the third category of vulnerability.

2.3 The Taskforce examined a range of short-term flood defence measures to reduce impacts of regular flooding. These included house defence and local area schemes which benefit more than one dwelling. Where neither of these options was practicable then short-term relocation of the household was considered an option.

2.4 House defence typically involves raising the house (permanently), tanking the house (waterproofing just above the level of frequent flooding), bunding (either raised mounds or sandbags) or property re-grading to improve drainage of floodwaters away from the house. Local area schemes are more complex, but typically involve diversions, bunding and pumping, and would typically be located on the street or within drainage easements.

2.5 Local area schemes provide wider benefits in terms of addressing the issues experienced by the most vulnerable houses, as well as those considered less vulnerable in the surrounding areas. This helps preserve occupancy within affected areas, which is an important component of strengthening these communities and improving the quality of life.

2.6 The field teams had limited time to carry out all of the above work and so a large amount of judgement and interpolation was necessary. It is considered that the process was sufficiently robust to provide a high level of confidence in the outcomes. However there may be further vulnerable properties within the city which have not yet been identified.

2.7 The information gained by the field teams allowed an assessment of which houses were most vulnerable, and a comparison of the costs of individual house defence and a local area scheme. In most instances it was found that the local area schemes were not only more cost effective, but also provided benefit to a much wider area.

2.8 In summary, based on recent storm event data and community feedback, there are approximately 994 households assessed as vulnerable to regular flooding in Christchurch. Of those 56 are considered the most vulnerable due to impacts of flood water entering the house. The second most vulnerable are those households that have experienced frequent flooding under their houses. There are 451 households in that category.

2.9 The Flockton area has 30 of the most vulnerable households.

2.10 Any proposed long term flood defence schemes will not necessarily prevent such flooding in more extreme events, beyond the one in 50 year flood, as experienced on 5 and 6 March 2014.

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