Author(s): Terry Van Kalken; Soren Tjerry; Colin Roberts
Linked Author(s):
Keywords: Confluence; Bend cut; Morphological modeling
Abstract: The township of Opotiki is located at the confluence of the Waioeka and Otara Rivers in New Zealand’s Bay of Plenty, about 1km from the coast. The township has been subjected to flooding in the past and is currently protected against a 100 year flood by high embankments on both rivers. Water levels near the confluence are elevated due to the alignment of the two rivers which approach each other head on, causing significant turbulence and head losses at the junction. Increasing the level of flood protection through raising of the existing embankments is not desirable. Instead additional flood protection could be realised through the construction of a bend cut channel, that will streamline the flow, reduce existing the flood levels, and essentially compensate for the water level increase due to the breakwater construction. The construction of a bend cut channel will however lead to morphological changes. Most importantly there is a risk that the construction of the bend cut channel will lead to a permanent capture of the Waioeka flow, leading to the abandonment of the existing channel. In addition other morphological effects associated with the bend cut channel needed to be identified and quantified, which called for the application of a morphological model. A curvilinear hydrodynamic model with morphological bed updating was applied with a graded sediment transport description in investigate the confluence. A curvilinear grid was developed to cover the rivers as well as the conveying portions of the floodplain located between embankments. . Several bend cut channel layouts were tested with different widths and invert levels, and the design flood level reduction was identified for each layout along with the morphological impact. Several morphological impacts were identified, including long-term lowering of the bed levels in Waioeka due to the lowered water levels and sedimentation in the existing confluence channel due to the reduced flows.
Year: 2010