​5   Leeston Drainage District

5.1       Scheme Summary

5.2       Key Issues

The following key issues are associated with the Leeston Drainage District. A list of district wide issues are located in 5Waters Activity Management Plan: Volume 1.

Table 5‑1 Leeston Key Issues

5.3       Overview & History

The Leeston drainage district is segregated into two management areas – 'Leeston Rural' and 'Leeston Township'. The 'Leeston Township' budget was amalgamated with the 'Leeston Stormwater' budget.  The Leeston scheme has a rating area of 10,400ha, servicing a total area of 21,150ha between Brookside and Irwell Road and with drainage outlets for this scheme discharging to Te Waihora (Lake Ellesmere).

The Leeston Township management area sits within the greater Leeston Rural area and as the name suggests is limited to the bounds of the Leeston Township.

The leeston urban drainage district is managed by the leeston urban drainage committee. The boundary of the district includes leeston creek, pound road drain down to the diversion structure at the outlet of the show grounds culvert. Flooding occurred by leeston creek in June 2013 and a diversion is planned for the town which is discussed in the Leeston stormwater Activity Management Plan. Regular maintenance on the creek is required to cut back vegetation and remove obstructions from the bed. This was undertaken in December 2013 subsequent to the floods once water levels had dropped.

The Leeston Rural management area sits outside the Leeston township boundary.

In 2011, a variation to the Te Waihora Water Conservation Order was made, bringing a strong cultural focus into place.  The Selwyn - Waihora Zone Committee (established under the Canterbury Water Management Strategy) also released its Zone Implementation Plan (ZIP) and ZIP addendum.

Hanmer's Drain

Hanmer's drain was deepened for flood relief measures in 1945 in conjunction with the general upgrading of drains within the Ellesmere area.  The Hanmer's drain is different in that it was designed for both drainage and flood relief. 

Reinforced concrete drop structures were installed at 14 locations to reduce flood flow velocities.  On account of the shortage of steel, concrete access bridges were designed with mass concrete abutments and only the decks were reinforced.  Some structures are beginning to experience spalling and concrete erosion.

Engineering inspections of the drop structures are undertaken periodically.​

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​​ Figure 5-1
LEESTON Land drainage - Scheme map
Open larger map: 'Land drainage Scheme'

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5.4       Resource Consents

There are existing resource consents for Leeston urban land drainage scheme. Historically flooding in the township has inundated low lying houses.  Consent has been granted consent for a flood diversion which incorporates stormwater in a development area of Leeston North (Manse Road; CRC143914).

To also assist with flood mitigation a flood diversion structure (gate) was constructed at the downstream end of the Leeston township which when opened discharges stormwater down drain 58 which ultimately discharges to the Tramway reserve drain above Lochheads Road.  The consent for this diversion is covered under CRC930163.

Table 5‑2 Resource Consents

Land drainage activities are controlled through Rules 5.57 and 5.58 under the Land and Water Plan. An application was lodged in November 2011, with a request that it be put on hold pending discussion with stakeholders.

5.5       Scheme Assets

There are three main scheme assets within this scheme:

1. Land drainage channels - excavated open channels that intercept, convey and discharge groundwater to allow productive use of land.
2. Drop structures - engineered weirs will stilling basins designed to dissipate energy and minimise erosive velocities.
3. Flood Level Gauge - Telemetry linked water level pressure transducer used to prelude alarms during rising flood levels.

A summary of material and diameter for pipes, where known, is shown below in Figure 5‑2 and Figure 5‑3.

Figure 5‑2 Pipe Material - Leeston

Figure 5‑3 Pipe Diameter – Leeston

5.6       Operational Management

Council delegates some aspects of management of the Land Drainage network to 9 Land Drainage Committee's comprising of local residents with an interest in the Land Drainage network.  Council Service Delivery Staff work alongside the Committee's to prioritise and facilitate maintenance activities which are undertaken by a number of local contractors.

5.7       Photos of Main Assets

​ Photo 1: Hanmer Drop Structure

​ Photo 2: Tramway Reserve Drain

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5.8       Risk Assessment

A risk assessment has been undertaken for the Leeston scheme. The key output from the risk assessment is the identification of any extreme and high risks which need to be mitigated. In order to mitigate these risks they have been included and budgeted for in the projects within this LTP. Table 5‑3 details the risk priority rating, Table 5‑4 outlines the risks and the list of key projects is found in Table 5‑8.

Table 5‑3 Risk Priority Rating

Table 5‑4 Risks – Leeston

The list of district wide risks can be found in 5Waters Activity Management Plan: Volume 1.

5.9       Asset Valuation Details

The total replacement value of assets within the Leeston Land Drainage District is $22,312,345 with further details in Table 5‑5 below.  The majority of value is made up of channels.

Table 5‑5 Replacement Value, Leeston (Rural and Urban)

Channels are broken down into drains, pipes and stock banks. The Leeston land drainage district is made up of drains and pipes.

5.10       Renewals

The renewal profile has been taken from the 2019 5 Waters Valuation. Renewals are scheduled for Leeston are shown in Figure 5‑4 below.​

​Figure 5‑4 Leeston Renewal Profile

5.11       Critical Assets

The criticality model for Leeston has been updated for the 2021 AcMP. The methodology of the criticality model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the criticality has been calculated for the reticulation assets. Table 5‑6 and Figure 5‑5 below show​ the calculated criticality for all of the assets within this scheme that have a recorded known length.

​Table 5‑6 Length of Assets per Criticality Level

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​​ Figure 5-5
LEESTON Land drainage - Criticality map
Open larger map: 'Land drainage Criticality'

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5.12       Asset Condition

The asset condition model was run for Leeston in 2021. The methodology of the model can be found in 5Waters Activity Management Plan: Volume 1 and it provides details of how the model has been calculated for the reticulation assets (particularly pipes). There is no known recorded condition for assets within this scheme.

5.13       Funding Program

The 10 year budgets for Leeston are shown by Table 5‑7​. Budgets are split into expenditure, renewals, projects and capital projects. Expenditure and renewals have been reported on a district-wide basis in Volume 1. 

All figures are ($) not adjusted for CPI “inflation".  They are calculated on historical data, and population growth where relevant.

Table 5‑7 Leeston Budget Summary

An explanation of the categories within the budgets are as follows below:

• Expenditure consists of operation and maintenance costs;
• Renewals are replacement of assets which are nearing or exceeded their useful life;
• Projects are investigations, decisions and planning activities which exclude capital works; and
• Capital projects are activities involving phys​ical works.
  

There is one ​project for this scheme.

Table 5‑8​ Key Projects

* LoS refers to Level of Service

​The list of district wide projects can be found in 5Waters Activity Management Plan: Volume 1.

Discussion on Projects

Projects have been determined based on their:

• Relevance to the scheme
• Requirement to be completed under legislation
• Ability to bring the scheme up to or maintain the Level of Service required under council's Asset Management Policy. 

Many projects are jointly funded by more than one scheme and activity.  Each scheme pays a pro-rata share only, equivalent to the number of connections.

​Discussion on Capital and Projects

Where relevant, Capital (Levels of Service) and Capital (Growth) projects have been included in the scheme financial details. 

Levels of Service Projects and growth splits have been provided to ensure the costs of population driven works are clear