Land Drainage

Made up of the Northern Area Land Drainage and the Raupo Land Drainage Schemes

Northern Area Land Drainage

The goals of the Northern Area Land Drainage (NALD) network are to achieve the following in a cost-effective manner:

·         Protect land from tidal waters;

·         Provide drainage of the soils to optimise agricultural production;

·         Control surface water during flooding; and

·         Divert run-off from inland hills.

 

In order to do this a number of drains, floodgates and stopbanks have been constructed in the past and integrated with naturally formed channels to achieve these goals. These assets are overseen by small; drainage committees comprised of representatives of the area. Sometimes this is a single individual. The local representatives are assisted by Council staff and a Land Drainage Co-ordinator.

Stopbanks

The original stopbanks were typically formed by mounding excavated tidal mud to form continuous banks from headland to headland.

Most stopbanks are topped up by regular drain cleaning. As a result, the heights of the banks are variable and are likely to be higher than currently required for flood protection. Over time many of the stopbanks have been built up to a height of 2.6 to 2.7m above mean sea level. 

The previous AMP suggests that the design standard for foreshore banks should be a crest level of 3.5m above mean seal level, which would include a freeboard of 0.5m above extreme tide levels. With predicted sea level rise and climate change this value may need to change and when a decision is made on this a programme of works will need to be developed and projects identified.

Stopbanks are not considered to require renewal at any particular time and are not depreciated. They do however require suitable maintenance to retain their form and functionality.

Floodgates

The NALD network includes 54 wooden floodgates and 17 concrete floodgates of varying description, size and configurations.

This AMP considers a floodgate to incorporate the inlet and outlet wingwalls, the pipes or barrels of the floodgate, the actual gate itself, and any chains, winch and gantry associated with the lifting of the gate. In the case of several of the larger floodgates, riprap material in the floodgate outlet is also considered to be a part of the floodgate.

Floodgates range from small diameter concrete pipes with small concrete wing walls and wooden gate, to large diameter, multi barrel steel channels, with extensive wooden wing walls and multiple steel gates. The most recent upgrades to the floodgate network within Raupo is seeing large pre-cast and poured in-situ concrete structures with large wingwalls and well protected channels. This also uses wooden piles and timber to form greater wingwalls areas if required.

The age of each individual floodgate is not known and to date a formal structural condition assessment has not been undertaken.  However, the condition of floodgates is regularly physically inspected. 

Due to their construction and the materials used, it is generally accepted that most, if not all, gates will leak during high tide. The resulting inflow of water is not significant, even at larger gates, and has not warranted remedial works. The use of rubber seals is being investigated together with alternative design and construction methods that may facilitate better seals between gates and barrels.

For renewal purposes, a floodgate is considered as having two components, the structure and the gate and these might be replaced individually.

Raupo Introduction


The goals of the Raupo Land Drainage (RLD) network are to achieve the following in a cost-effective manner:

·         Protect land from tidal waters;

·         Control surface water during flooding; and

·         Divert run-off from inland hills.

In order to do this a number of drains, floodgates, stopbanks and a storm pump have been constructed and integrated with naturally formed channels to achieve these goals. These assets are overseen by a self-managed; drainage board comprised of representatives of the district who are assisted by Council staff and a drainage co-ordinator.

Stopbanks

The original stopbanks were formed by dumping tidal mud to form continuous banks from Toka Toka to Te Kowhai. In 1927/1928, the Public Works Department carried out an extensive programme of stone pitching the river side of the stopbanks south of Ruawai.

Most stopbanks are topped up by regular drain cleaning. As a result, the heights of the banks are variable and are likely to be higher than currently required for flood protection. Over time the stopbanks have been built up to a height of 2.6 to 2.7 metres above Mean Sea Level. 

It is considered that the stopbanks are maintained to a specific design standard; however, there is currently little documentation available to confirm this standard.

The previous AMP suggests that the design standard for foreshore banks is a crest level of 3.5 metres above Mean Sea Level, leaving a freeboard of 0.5 metres above extreme tide levels.

In recent years, as part of stopbank maintenance, Council has undertaken erosion protection works by adding fresh spall material to riverside of the stopbanks from the Ruawai Township South.

Stopbanks are not considered to require renewal at any time, but instead require suitable maintenance to retain their form and functionality.

Floodgates

The RLD network includes 52 floodgates of varying description, size and material.

When discussing floodgates, this AM Plan considers a floodgate to incorporate the inlet and outlet wingwalls, the pipes or barrels of the floodgate, the actual gate itself, and any chains, winch and gantry associated with the lifting of the gate. In the case of several of the larger floodgates, riprap material in the floodgate outlet is also considered to be a part of the floodgate.

Floodgates range from small diameter concrete pipes with small concrete wingwalls and wooden gate, to large diameter, multi-barrel steel channels, with extensive wooden wingwalls and multiple steel gates (see Figure 5-7 for examples of difference in floodgate types).

The age of each individual floodgate is not known however a formal condition assessment has been undertaken and a renewals programme has been developed. 

Due to their construction and materials used, it is generally accepted that most if not all gates will leak during high tide. The resulting inflow of water is not significant, even at larger gates, and has not warranted remedial works. The use of rubber seals are being investigated together with alternative designs and construction methods that may facilitate better seals between gates and barrels.

For renewal purposes, a floodgate is considered as having two components, the structure and the gate.