Report: TR05/33

Author: José Beyá

Abstract

The Rangiriri spillway and Lake Waikare are part of the Lower Waikato Waipa Control Scheme (LWWCS). The Rangiriri spillway is located on the right side of the Waikato River and it allows spillage from the river into the Lake Waikare during large flood events.

Lake Waikare provides flood storage when the Waikato River experiences floods of a magnitude exceeding the 50-year event. The lake is controlled at its northern foreshores by a stopbank and a control gate that will retain water within the lake during the event and divert the stored water into the Whangamarino wetland and back into the Waikato River after the water levels recede in the river. The lake is designed to accommodate flows of up to a 100-year event in the Waikato River, above which a spillway along the lake northern foreshore is designed to divert excess flows into the wetland. The design of this spillway in conjunction with a flood management regime was not confirmed following the completion of the construction works of the Lower Waikato Scheme.

The aim of this report is to identify the lake levels under extreme events and investigate different options for managing overflows from the lake into the Whangamarino wetland.

A 1D hydraulic model of the Rangiriri Spillway and Lake Waikare was developed, calibrated and validated. The model showed that the performance of the Rangiriri spillway is adequate compared to its design and that the Lake Waikare provides sufficient flood storage for events larger than design. A number of options were proposed for the structures located at the Lake Waikare northern foreshore. The options involve different configurations for the northern foreshore spillway and stopbanks as well as different ways for operating the Lake Waikare gate.

The findings of the report include the following:

• The Rangiriri Spillway will perform as designed and change is not required.
• The 100-year design flood level of Lake Waikare is RL 7.36 m as originally designed.
• The preferred solution is to manage increased lake levels in events exceeding the design event by constructing a 70m spillway along the northern foreshore of the lake, that will operate in conjunction with the control gate to reduce lake levels and ensure that the existing northern foreshore stopbanks will not be overtopped.

The Lake Waikare spillway is currently under construction and is due for completion by May 2006.

Note: This technical report is unavailable electronically on our website due to file size restrictions.
Contact us if you would like us to email you the report.