Waioeka / Otara Catchment

• Geology and geography of the catchment
• Land use in the catchment
• Flood risk management in the catchment
• Significant floods in the catchment

Geology and geography of the catchment

The Waioeka-Otara catchment covers an area of approximately 1,130 km² the catchment is relatively steep with extensive bush cover.  As the catchment has such a large percentage of forest cover, it has a high capacity to absorb rainfall and delay runoff, reducing flood peaks downstream. However, being so rugged, the upper catchment is also vulnerable to erosion. Not only does erosion wash away vegetation (reducing the capacity to absorb rainfall), it also increases the amount of sediment and gravel travelling down the catchment. Once the sediment reaches the lower catchment, the amount of flood water the channel can carry is reduced by the sediment building-up in the river bed. As well as this, landslips in the steep upper catchment have the potential to temporarily dam the river, which can then burst and send a floodwave downstream. Anecdotal reports have been made of such phenomena in the Waioeka Otara catchment.
The lower reaches of both rivers meander across flood plains before joining at Opotiki and flowing out to sea. The Geology is characterised by alluvium on the floodplain, Kaharoa ash on the lower foothills and greywacke in the upper catchment.
The distinct feature which sets the upper Waioeka Catchment apart from other areas is the high rainfall intensities which occur in the south western portion of this catchment. Oponae recorded 326mm over a three day period in August 1967 which is one of the highest falls ever recorded in the Bay of Plenty.

Land use in the catchment

Roughly 80% of the catchment is covered by conservation estate in the form of the Urewera National Park and Waioeka Gorge Scenic Reserve. The main urban area in this catchmentis the coastal township of Opotiki. Surrounding the town on the floodplain the land is used for dairy farming with some horticulture on the surrounding foot hills.

Flood risk management in the catchment

The Waioeka-Otara Rivers Scheme is a comprehensive river control scheme, construction of the original scheme was completed in 1960, since then it has seen many changes and upgrades. It now includes substantial stopbanking of both the urban and rural sections of both rivers, one pump station (Duke Street) and considerable bank edge protection works including rock riprap, rubble and plantings.
The scheme also includes flood protection to the land adjoining Mill Stream and minor drainage works. These provide the benefits of flood protection to the township of Opotiki and surrounding land on the flood plain.

View map of Waioeka Otara stopbanks (pdf 18.6MB)
Link to more info on the Waioeka Otara river scheme and scheme liason group notes 

Significant floods in the catchment

The history of the catchment contains many flooding episodes. Over the last 100 years, major floods occurred in 1904, 1918, 1925 and 1964. The 1964 flood covered most of the floodplain and all but 24 of the 650 houses in the township were flooded. In the town, flood depths of over 1.5m were recorded.
Since that time a flood protection scheme has been completed, and has provided a degree of security to the floodplain. However that protection is not absolute, and in July 1998 floodwaters spilled over rural stopbanks in several locations and threatened the urban area. The peak flow in the Waioeka River was similar to that of the 1964, but the fact that the Otara was subject to a much lesser flow than in 1964 saved the floodplain from more extensive flooding. A further flood event in the Otara River of approximately 20% AEP (5yr) in November 1999 again overtopped some sections of the Otara rural stopbanks.