Integrated Catchment Management

Flood gates - implications for management

Many of the tidegates around the Motueka area have been installed by private interests for an unknown purpose and are a headache for TDC engineers since they are called on to maintain them. In some cases this may lead to the removal of floodgates.

Tidegates seem to bring out the extremes in people. Some people love them, while others are actively involved in reducing their effectiveness by jamming them open.

Given the likelihood for sea-level rise associated with climate change, it may seem crazy to be tinkering with floodgates. However, there may be a need in the future to build more floodgates so it would be useful to have a better understanding of their effects and how to mitigate them.

The challenge here is to understand the exact role of floodgates and if floodgate management can be altered to allow improved fish passage/habitat quality without threatening productive land upstream. Decisions will probably be needed on a site-by-site basis, since every floodgate is a bit different, but there will undoubtedly be some generic lessons too.

What is TDC doing?

Over the past 3 summers we have surveyed 321 structures in waterways (culverts, weirs, fords, tidegates, floodgates etc.) within 10km of the sea for fish passage. Through this process we have found 157 barriers (not all the barriers occur at all flows, e.g. many are only at low flows). Adding in barriers identified by Fish and Game and DOC we have a total of 183 barriers in our inventory. This database currently excludes any dams and very little survey has been carried out on private farmland.

These are all characterised and have been measured up. We hope to have this in a proper database in the in 2007. It is anticipated that the database will be incorporated into the "Waterbodies Database" that DOC and Fish and Game have access to (and possibly in future allow wider access). The most common problem is installing culverts in soft or mobile material and erosion scouring the downstream pool until the water level drops. Many of the structures are owned by TDC but currently there is no remediation programme.