draft revised kaihu river management plan - Northland Regional ...

Northland Regional Councilbottleneck where the riverbed flattens is still evident, serving as aponding area and sediment trap.2.4. Poplar Trees - Section 5Section 5 which is the channel between Waihue Road and Ahikiwi sitson a much wider floodplain which manifests a number of older riverchannels. In the early 1900s, Poplar trees were planted in this sectionfor the purpose of controlling stream bank erosion. At present, theover mature trees risk falling into the river, contributing to debris damformation and accelerating stream bank erosion. .2.5. Trapped Gravels - Section 6As Waima river approaches a narrow gorge, it joins Mangatu Streamto form Kaihu River. In Section 6, which is the channel betweenAhikiwi to the convergence with Waima River, coarse river gravel hasbuilt up in the bed causing a reduction of the capacity of the channeland the risk of an overflow in the settlement of Kaihu. The gravel loadin the river needs to be managed for about one kilometre upstream ofKaihu and three kilometres downstream so as to maintain sufficientcapacity in the channel to reduce the risk of an overflow. In thissection the management of willow trees (Salis fragilis) is required tominimise the formation of gravel islands which can exacerbate streambank erosion,3. River Maintenance PlanRegular maintenance work that has been done since 2006 comprising ofricegrass and willow spraying, machine cleaning of banks, blockageremoval and gravel management is proposed over the next six years,along with works to improve flow efficiency of the Kaihu River where thisshallows near the confluence with Okahu Creek.3.1. Spray RicegrassSpraying Manchurian ricegrass on both banks in the lower sectionfrom the confluence with the Northern Wairoa River to Duck Creek,where slips and the Dargaville floodwall obstruct machine cleaning, isproposed. This option is more efficient than cleaning with a hydraulicexcavator, given issues with access and bank instability associatedwith this section of the river. The objective is to eradicate the grasscompletely, which will reduce accumulation of sediments on thebanks. This option will require a long term commitment to ensureadequate control of the ricegrass, because of the tenacity of the plantto rapidly grow and recolonise areas.3.2. Machine CleanAnnual machine cleaning of the banks in the lower sections of the riveris proposed in order to maintain river cross section capacity, which isKaihu River Maintenance Plan 2010-2016 - 4 -