Explanatory Notes about Flood Level Information
Wairau (including Waihopai and Branch)
For the Wairau River upstream of the Waihopai River, the Branch River recorder is a good indicator of flood conditions. A Branch flood level greater than 1 m above normal threatens Wairau River flats. Farmers living in the Wairau Valley area will get a Wairau flood about six hours later than for the Branch River. Rainfall at Top Valley rain-gauge is also a useful indicator of a likely flood. Rainfall at Top Valley exceeding 50 mm in the previous nine hours is a likely indicator of a flood in the Wairau or its tributaries above Waihopai. River flood level information is also available for the Waihopai River.
The Wairau at Tuamarina recorder is indicative of flood conditions for the main Wairau floodplain. The council can predict flood levels here up to nine hours in advance from rainfall. A flood level greater than 2 m above normal threatens Wairau floodway land. A flood rise greater than 3 m floods the Wairau Diversion and Lower Wairau floodway land. Rainfall at Onamalutu rain-gauge is also a useful indicator of a likely flood. Rainfall at Onamalutu exceeding 50 mm in the previous nine hours is a likely indicator of a flood in the Wairau on the floodplain. Rainfall exceeding 75 mm on the floodplain measured at Wairau rain-gauge is also an indicator of local flooding or drainage impairment.
Omaka (Ōpaoa , Taylor, Doctors, Fairhall, Ruakanakana Creek, etc)
The Omaka at Tyntesfield Gorge is a good indicator of expected floods in the Upper Ōpaoa some six hours or so later and Roses Overflow later still. A flood level of 0.8 m above normal will close the Old Renwick Road ford. A flood level greater than 1.8 m above normal will result in flooding of floodway berm land and closure of other road fords. An Omaka, flood level exceeding 1.8 m above normal is also indicative of flooding in other southern valley rivers such as Doctors Creek, Fairhall, Ruakanakana Creek etc. Floodway berm land in the Ōpaoa/Taylor becomes flooded as the Taylor Dam outflow exceeds 50 m/sec. Rainfall exceeding 50 mm in the previous nine hours at Tinpot rain-gauge in the Taylor catchment is also an indicator of likely flooding in the Southern Valleys and the Awatere area.
Sounds (Rai, Pelorus/Te Hoiere, etc)
The council has less information for the variable Marlborough Sounds rivers. A flood level rise of greater than 2.5 m in the Rai River at Rai Falls is indicative of a flood in the downstream Rai and Te Hoiere/Pelorus Rivers. Rainfall exceeding 75 mm in the previous nine hours at Tunakino rain-gauge is indicative of likely potential flood problems in the Marlborough Sounds.
It is best to view the flow graphs on Floodwatch where the flooding thresholds are shown in relation to current predicted flood flows.