Leith Flood Protection Scheme 

The Water of Leith rises in north Dunedin, and flows 14km through the City. It is a stream with the potential to inflict serious damage to property on the floodplainWe manage a flood scheme on the Water of Leith to reduce the risk and impact of flooding.


History

Before the 1929 Dunedin flood, concrete and stone walls had already been built along the banks of the river to prevent bank erosion and enable better use of adjoining land. The Otago Harbour Board had constructed a concrete channel from Forth Street to the harbour in 1913 -14.

Straight after the 1929 flood the concrete channel was duplicated from Forth Street to the harbour. Concrete and stone walls along the channel were strengthened and raised. In the early 1950s the Dunedin City Council extended the channel upstream of Forth Street.

In the late 1950s the Water of Leith channel from George Street to Great King Street was straightened by building a high velocity concrete channel. Boulder traps were built upstream of George Street and in the late 1960s a larger boulder trap was built upstream of the lower Malvern Street bridge.

History of Leith Flood protection scheme

Leith construction, 1950s

Clearing the bed of the Leith

Clearing the Leith riverbed after the flood, 1929

Hazard

The largest floods in the Leith typically occur when fronts bringing persistent heavy rainfall pass from east to west over the catchment. Flood events are caused by rainfall that can either be of a short duration and high intensity, low intensity and steady rate, medium intensity with a duration of several hours, or a combination of these conditions.

The relatively small and steep catchment can produce significant flooding in just a few hours after the onset of heavy rain, giving little warning. Likewise, the duration of peak flows is also brief.

There characteristics mean the Leith can easily become a raging torrent. The water surface under flood conditions is likely to be highly turbulent throughout most reaches, with large standing waves observed in some locations.

Damaging floods have been recorded in the Leith in 1868, 1877, 1911, 1923 and 1929. The 1929 flood was the most severe on record, with floodwaters sweeping away and damaging bridges and other channel structures, affecting numerous houses and flowing along the streets beyond lower Rattray Street.


Development of the current scheme

We manage a flood protection scheme on the Water of Leith which is designed to cope with a flow of 171 cumecs (cubic metres per second). This flow has an Average Recurrence Interval (ARI) of 100 years.

Extensive hydraulic modelling, using both computer and physical models was carried out to determine the level of a 100 year ARI design event for different reaches of the Leith. The hydraulic model's development and verification was based on observations and data collected during past flooding in the Leith. 

 The video below is a physical model built during the development phase of the current scheme.

The work on the development of the scheme has been spread over a number of years as outlined below:  

Stage

Year

Rockside Rd 2006
Anzac Av - Harbour (weir installation) 2011
Cumberland St - Dundas St 2012
Leith Walk - Forth St 2013
St David St - Union St 2014
Dundas St - St David St 2015/2016
Union St - Leith Walk (ITS building bend) 2017/2018
Dundas St Culvert 2018
Forth St - Harbour 2018-2020


Map of work programme

 


Union St to Leith Walk 2017/8 - Construction Programme

  1. Constructing site access from Clyde Street
  2. Construction of a new retaining wall and terraces along left bank
  3. Public access to riverbed from left bank
  4. Shaping, stabilizing and rebuilding weirs to stabilize river bed
  5. Raising the wall along the right bank to increase freeboard (the height of the channel wall above the flood level)

 

How much has the Leith Flood Protection Scheme cost? 

Below is an outline of the costings of the scheme from 2005 - June 2018.

Costings of the Leith Flood Protection Scheme from 2005 - June 2018, (advert in ODT, Friday 13 July 2018).

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