For the residents and ratepayers of the Kawakawa - Taumarere areas:- Here is an up-date for you from the Regional Councils Environmental Management Committee agenda for Monday. Item 4 refers: Investigation and design works are progressing for flood mitigation options in the Kawakawa catchment. Results have been received for a flood model simulation of a spillway option (V1) at Otiria. The spillway option is located on the east side of Pokapu Road and extends from the South bank of the Otiria stream discharging to the Waiharakeke stream. Box culverts under Otiria road have been incorporated into the design. The spillway option is intended to divert overland flow that affects Otiria Road Properties and reduce overland flow towards Moerewa. This overland flow originates from both the Otiria and Waiharakeke streams. Model results show that the spillway diverts approximately 120 m3/s of peak flow into the Waiharakeke Stream downstream of the Pokapu Road Bridge and this reduces flood plain water level to the East of the spillway. The benefit is 0.4 reduction at 305m (50% of flood depth), 0.2m at 800m (25%of flood depth) and 0.1m at 1km (20% of flood Depth). There are no dwellings which benefit from flood level reduction exceeding 0.4m but 12 dwellings have flood level reduction between 0.1m and 0.3m. The Otiria Marae on Kingi Road has a 0.05m benefit and the cemetery at the end of Wahamiti Lane (South of the railway track) has a 0.35m benefit. Additional benefit includes substantial reduction in overtopping of the Otiria road between Moerewa and the Pokapu Road Junction. The spillway also reduces downstream flow in the Otiria Stream by around 10m3/s but this does not appear to significantly reduce flood level at the Turntable Hill SHW1 Bridge. The Kawakawa flood model is likely to under represent overland flow into Moerewa along the Otiria Road as model surface does not incorporate the detail of roadside drainage. The model results do however show a significant flow across Lucas Road towards Pembroke Street in Moerewa and the spillway simulation shows this flow would be reduced as a result of the spillway. Model results show that the spillway effectively protects Pembroke Street properties from this Otiria overland flow. it is likely that a refined spillway design would deliver more significant benefit to Moerewa. The cost of spillway construction is likely to be around $500,000 and it is considered that the design does not deliver sufficient benefit to justify the cost. The reasons for continued flooding to the east of the spillway are: diversion around the spillway due to insufficient capacity and overflow from the Otiria Stream east of the spillway. The spillway concept is still considered to be the most viable scheme option for flood management along the Otiria Road and staff are consider9ng design adjustments to maximise benefit. These include spillway enlargement, increasing capacity under the Otiria Road and reducing Otiria Stream berm crest levl at the overflow. Increasing spillway capacity under the Otiria Road is likely to require a bridge crossing. This will also mitigate predicted increase in flood level west of the Oritira Scheme stopbank. Geotechnical site investigations have not yet been undertaken to determine sub surface constraints to excavation. Attachments - to view please visit nrc.govt.nz
Posted on: Thu, 09 Oct 2014 23:30:02 +0000
Trending Topics
Recently Viewed Topics
© 2015