Wairau/ Awatere Resource Management Plan 17.3 Flooding 17.3.1 Issue Effects from flooding and erosion are major hazards, especially on the main Wairau floodplain where extensive river channel modification and other river control works have been carried out. Over the 100 years or so since river works were commenced on the Wairau floodplain, vast amounts of money have been spent to create the present river system for the prime purpose of flood control. Since the 1950’s alone, over $80 million in present terms has been spent - mostly with a high degree if success and benefit in the prevention of catastrophic flooding. The Wairau is the most powerful river in New Zealand for which a comprehensive river control scheme has been carried out. It is not possible to reverse these works and the Council and its ratepayers have a responsibility to at least maintain the current ‘artificial’ river pattern; or improve it to an appropriate standard. In particular the location of rivers has been changed by construction of diversions, or by blocking off alternative outlet channels. These diversions and blockages have meant that every river channel or floodway on the Wairau floodplain below the Waihopai confluence (Rural 3 Zone) is carrying a different flow regime or in a different position (or both), and usually substantially different from what it did prior to European settlement. The major rivers of this Wairau floodplain have an integrated contiguous stopbank system. The most significant of these works is the blockage of the Opawa distributary channel from the Wairau. This has increased flood flows down the mainstem Wairau by typically 50%. This has subsequently required substantial river control works on the mainstem Wairau - including the Wairau Diversion - to cope with these increased flood flows. It has also allowed development of Blenheim to go ahead without the regular flooding from the Wairau. More recently it has also allowed much of the land of the Upper Opawa channel to be developed for viticulture within a narrowed floodway. By 1991 the flood control standard achieved was capable of withstanding floods to a 1 in 30 year return period frequency over almost all of the floodplain. For other Wairau tributaries outside the floodplain the river control works that have been carried out are to much lesser standards. The majority of the work has been channel clearing and bank edge protection to prevent erosion and generally do not include contiguous stopbank works. Few river control works have been carried out for rivers outside the Wairau catchment. 17 - 4