WD.1368 RevC WCD dam break assessment report with appendices

Waimea Community Dam: Dam Break Analysis
Waimea Water Ltd
EXECUTIVE SUMMARY
In 2009 Tonkin and Taylor (T&T) completed an initial dam break analysis in the project feasibility
stage of the Waimea Community Dam project. The primary purpose of the 2009 study was to aid in
classifying the dam for preliminary design. In 2012 T&T published further breach mapping for
resource consent reporting.
This report describes an updated assessment conducted by Waimea Water Ltd (WWL) in 2020. This
updated analysis is a consent requirement to support development of the Emergency Action Plan
(EAP) and a condition of financing with Crown Irrigation Investments Ltd (CIIL). The primary purposes
of this 2020 study were to:
• review previous results
• update the hydraulic modelling to meet new (2015) guidelines
• incorporate the detailed design
• include the latest ground terrain and coverage data
• provide both rainy and sunny day scenarios for emergency management planning
Downstream flooding in a breach closely follows the Lee, lower Wairoa and Waimea River paths. This
includes the north and eastern parts of Brightwater township. The flood reaches the sea in an area
between Redwood and Swamp Road. These extents are very similar to the reports produced by T&T
in 2009 (Stage 2 for feasibility), then also in 2012 (Stage 3 for resource consent).
The existing maps prepared by T&T were, therefore, sufficient for use in the temporary Construction
Emergency Action Plan (CEAP), but could now be replaced by the more detailed mapping in this
report. The upcoming operational EAP should use these updated inundation maps and arrival times
for a current assessment.
The information presented in this assessment does not in any way reflect upon the structural
integrity or likelihood of failure of the Waimea Dam, but rather the consequences of the failure
should an unexpected event occur.