Wairau/ Awatere Resource Management Plan 20 January 2011 9.16 Methods of Implementation Rules The Plan will allow the erection of structures as a Permitted Activity where they have little environmental effect (eg mai mais). Information The Council will provide information on the difficulties of building structures in this area. Structures and activities within, and occupation of the foreshore may also be subject to a Department of Conservation Management Plan where such a plan exists. 9.17 Issue Not opening the Wairau and Diversion River Mouth Bars has adverse effects. The Wairau River mouth bar is a natural feature that has a dominating effect on water levels in the Wairau estuary, the Wairau Lagoons, the lower Wairau to upstream of Ferry bridge, and the lower Opawa. This is particularly at normal and low flows, but flood flows are also affected. The bar is formed by a combination of marine forces, tidal flows into the Wairau Lagoons and river flows from the lower Wairau and to a lesser extent the lower Opawa. The marine storm wave forces are very important. In times past they formed a bar typically extending a kilometre to the north. When such a bar formed there would be significant water friction loss down this extra distance of coarse gravel bed channel. In these situations the water level in the whole lower Wairau to upstream of Ferry Bridge is kept at virtual high tide levels with little or no tidal variation. This has a significant effect on the lower Wairau, lower Opawa and Wairau Lagoons. With this partially closed bar the water there may stay almost completely devoid of saline water, or conversely stay with an extensive saline wedge. This deleteriously affects the ecological environment of the area. Gravity drainage of the extensive areas of flat lower plains into the lower Wairau is also prevented and expensive drainage pumping is then required. During flood flows, flood levels are higher upstream and can lead to stopbank overtopping until the 2-3 m high bar is overtopped and scoured out by the flood flow. On the other hand a directly open bar mouth is of obvious benefit. With a direct open mouth there is twice daily flushing of saline water, consistent tidal water level variation in the lower Opawa and lower Wairau and good gravity drainage. Boat access across the bar is also much better with a direct mouth outlet and was a concern of Harbour authorities when the Wairau and lower Opawa were important for shipping. The Wairau diversion bar has on a minor scale the problems that the lower Wairau mouth has. 9 - 14