Large scale sewer obstruction - Real story and implications explored

Behaviour of the Riccarton Interceptor trunk sewer in Christchurch has confounded modelling and led to uncertainty in regard to future upgrade planning.

A Parshall Flume was recently re-discovered at the downstream end of this trunk sewer with a substantial obstruction blocking its throat. This obstruction and the collected debris, was removed resulting in a significant improvement in the hydraulic performance of the trunk sewer. A permanent flow monitor installed a short distance upstream provides continuous flow and level data before and after removal of the obstruction.

This paper explores and compares observed, measured and model predicted behaviour of the trunk sewer with the blocked flume and under normal clean conditions. This modelling is used to illustrate how far upstream the obstruction may have affected sewer performance. It also explores whether there could be significant benefit from demolishing the flume.

Implications for future growth planning around the Riccarton Trunk sewer will be discussed and recommendation made for a more robust future assessment of future upgrade options.