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Projects
Orakei Basin sluice gates projectIntroduction | History | Ecology of Orakei Basin | Options considered | Proposed management approach | Technical reports Options consideredInitial assessment | Options selected for investigation | Discarded options Initial assessmentAn investigation has been undertaken to identify the most suitable way to manage water levels within Orakei Basin. This investigation has built on previous work and has considered a number of potential management options including:
Each of the options were assessed using the following criteria:
After an initial feasibility assessment, options one, two and three were selected for further investigation, while options four and five were discarded. Options selected for investigationThe following options were selected for further investigation.
Option 1 - Fixing the sluice gates and fortnightly flushingThe sluice gates are currently opened approximately 27 times a year and are left open long enough to allow three changes of the tide (three tidal cycles) before they are closed. With each tide, the basin empties and then refills with seawater (this is one tidal cycle). This means the water in the basin is refilled with seawater approximately 81 times a year. This allows for recreation approximately 78 per cent of daylight hours. Under this option, the basin ecology will continue to have low biodiversity and be dominated by species that are tolerant to disturbance. This is typical of a modified environment. Option 2 - Fixing the sluice gates and maximising flushingThe number of times the basin gates are opened could be increased to 43 times a year. However, some of the time they would not be able to stay open for as many tidal cycles as in option one. This would mean the basin would only be refilled with seawater 94 times a year. This would allow the basin to be used for recreation approximately 76 per cent of daylight hours. No significant improvement in water quality would be expected, and the ecology would continue to be dominated by species tolerant to disturbance. There would be a minor increase in the area of mangroves on the northern side of the basin. Option 3 - Fully tidalReturning the basin to a fully tidal environment would allow the basin to empty and refill with seawater with each change of the tide (approximately 710 changes of water per year). However, this would only allow for recreation approximately two per cent of daylight hours. Minor improvements in water quality could be expected, and mangroves would quickly re-establish throughout the basin. This would lead to increased sedimentation and possibly mangroves completely colonising the basin. Pacific oysters may spread to central areas. However, the basin will remain a highly modified environment due to the ongoing restriction of the rail embankment. Discarded optionsOption four was discarded because very small reductions in the height of the gate would drop the water level in the basin to a point where recreation would become unviable. This option would not provide any significant improvements in water quality or environmental values. Option five would reduce the time the basin could be used for recreation by approximately 50 per cent, while not providing any significant improvements in water quality or environmental values above options one or two. Reviewed - March 2007.
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