Land drainage and flood control are important issues for many land owners in Hawke’s Bay. The Regional Council manages a number of drainage networks throughout the region on behalf of various landowner and ratepayer groups. These range from very small systems involving just a few landowners, through to the major scheme networks. The maintenance regime for all these networks is similar, but one scheme is noted for its comprehensive approach to the management of its drainage assets, and this is described more fully below.
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This is the largest drainage network in Hawke’s Bay, with almost 500km of streams and drains traversing much of the Heretaunga Plains. These range from major stream channels with large catchments and a high permanent flow, down to minor drains with little or no flow except in times of storm or extended wet weather. There are also 19 pumping stations and numerous floodgates, control gates and other hydraulic structures necessary for the system to work efficiently. Together these assets are valued at almost $30 million.
The system was founded on the principle that every rural property should have access to a community drainage outlet wherever this is possible, and many of the drains are sited on private property.
For ease of financial management the drainage system has been divided into 9 financially independent rating areas. Each area has its own works programme, which has been developed from an Asset Management Plan, and includes routine Maintenance Works, Capital Works to enhance the performance of the scheme, and Renewal Works to replace ageing or substandard assets. The total cost of these works is about $2 million annually.
Throughout the Heretaunga Plains, land values are high, and landowners rely on good drainage to protect their land and crops. Most fruit trees, and crops such as peas, beans, potatoes, tomatoes and squash cannot tolerate flooding or poor drainage. Because of this, a high standard of drain maintenance is vital, and the Council has a progressive maintenance regime to ensure the long-term success of the scheme.
| Because of the soil types on the plains, drain banks are often prone to slumping and erosion, and it is important to keep a good sole of grass cover on the drain banks to minimise this problem. Wherever access is available, drain banks are mown 3 times each year, using special long-reach mulching mowers. This controls weed growth and encourages the grass cover on the drain banks to develop into a dense mat that stabilizes the bank, keeps the waterways efficient, and provides a pleasing appearance. | 
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The use of herbicides is kept to a minimum, and drain banks are never sprayed.
Spraying is limited to weed growth in the drain invert, to control weeds such as Water Cress and Plantain, Sweetgrass etc. The spray is applied using a specially designed hydraulic arm that reaches right into the drain and applies only a narrow band of spray to the plant growth in the bottom of the drain. Spraying is usually done during the flush of growth in the Spring and Autumn.
Many of the drains within the Heretaunga Plains Scheme have flat grades, and repeated excavation with a large digger can be very destructive in the long term. It is also very expensive. Because the regime of mowing and spraying has been very successful in minimising bank slumping and siltation, the Council has been able to reduce the need for mechanical cleaning to about once in every 5-8 years for most drains in the network. Because much of the land is used for horticulture, the excavated material usually has to be carted by truck to dump sites some distance away, which is often the most expensive part of the operation. | 
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In a number of larger waterways where there is a significant depth of water, and a high permanent flow, the growth of submerged weeds is a serious problem. If left to grow unchecked, these weeds will gradually occupy all of the space in the channel thus raising the water to a level that disrupts drainage and reduces the flood capacity. The Council controls this type of weed growth using 2 weedboats that were designed and built locally. These are capable of working in shallow water, and will cut to a depth of about 1.5 metres. Cut weed is usually gathered at suitable locations for removal from the channel by an excavator. | 
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| Throughout the drainage network there are many hydraulic structures, including floodgates, control gates, weirs, pipelines, and detention dams or ponds. These are all inspected regularly and any necessary repairs are carried out to ensure that they continue to function properly. | 
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These range in size from just 160 litres/second capacity up to 18,000 litres /second. They are located in areas where it is not possible for drainage and floodwaters to escape naturally, and operate automatically according to the water level in the inlet channel.
Each pumping station is checked weekly, and the screens are cleared of weed. Electrical checks are done 3 times each year, and pumps are removed from the station for routine inspection and servicing according to a set programme based on their usage. Nine of the larger installations are remotely monitored using a radio telemetry system. The system keeps a constant check on water levels, electrical functions, pump status, and building security. It will warn of weed blockages on the screens, extreme changes in water level, and any electrical faults, so that action can be taken quickly to ensure that the pumps operate efficiently during times of heavy rain. | 
Computer controlling the Pumping Stations |
The Council provides a drainage advisory service to assist landowners in solving drainage and drain maintenance problems. In most case a site inspection will be necessary and the first visit is free of charge.
If the problem requires some survey, investigation or design work, Council staff will provide an estimate of these costs, carry out the necessary investigation and provide a report on options for solving the problem. Advice can also be given about any consent that may be necessary for the work. If the landowner wishes, the Council will then arrange and complete the project. If there is more than one landowner involved the Council can assist in developing a cost sharing agreement and a future maintenance programme.