15.3.2 Flood Protection and River Management
The objective of the Lower Waikato Waipa Control Scheme is to provide flood protection to rural agricultural land in the Waipa and Lower Waikato valleys and to provide protection for the towns of Te Kuiti, Otorohanga and Huntly. A large component of the existing scheme is in the Lower Waikato Management Zone.
The limiting design flood is the one percent Annual Exceedance Probability (AEP) event (100 year recurrence interval), except for specific areas where a lower standard of protection was adopted for economic or other reasons.
An additional objective was incorporated into the scheme works subsequent to the Tongariro Power Development. This objective aimed to offset the effects of increased river levels due to the additional water diverted into the Waikato River as a result of this scheme. These works are known as the Tongariro Offset Works.
Protection is achieved by a combination of the following:
- Stopbanks providing direct protection to specific areas, including floodgates and pump stations for evacuating water from behind stopbanked areas
- Willow clearing along river and stream channels where infestation severely restricted flood capacity
- Improvements to the channel of the Waikato River, including river training and dredging
- Control of the natural ponding and storage within Lake Waikare and the Whangamarino Swamp.
Since it was constructed, the Scheme has been successful in achieving its objectives and continuation of the service is justified because:
- The scheme provides a significant benefit to the economy and community of the greater Waikato Region.
- The scheme’s economic benefits outweigh the ongoing costs of providing the service by nearly 10 to one.
- The economic livelihoods of the communities in the protected areas are dependent on continuation of the scheme benefits.
- In addition to economic benefits, the scheme provides benefits in terms of reduced societal risk, security of access for national, regional and local transport and communication networks, enhanced resource management and recreational benefits.
The Lower Waikato Waipa Control Scheme was originally funded on the basis of 75 percent from Government subsidy, 12.5 percent from sand royalties and 12.5 percent from landowner contributions that were advanced in the form of loans which will be repaid in 2003.
Project Watershed maintenance costs for the scheme include depreciation, which will be used to fund annual refurbishment works
The Asset Management Plan for the scheme notes that the overall benefit cost ratio for ongoing management and maintenance of the scheme is 8. All sections of the scheme apart from the Mangawara section have a benefit cost ratio well in excess of one. For these sections the high benefit cost ratio shows that continued maintenance of the scheme works is sustainable. The Mangawara section works have a benefit cost ratio of 1.2 which indicates that this part of the scheme is economically viable. However, the relative benefits are not as great as for other parts of the scheme.
For Deroles, studies demonstrated sufficient economic benefit that the landowner agreed to the Council’s construction and operating funding proposals.