This project assessed of the proposed wetland / retarding basin at Hull Road, Lilydale, and in particular, break risks associated with the proposed dam wall. It covers the hydrology, and hydraulic analysis of the system, and includes a consequence assessment as per the ANCOLD guidelines.
The raingarden / retarding basin site is located west of Hull Road, approximately 3 km south of the Lilydale township. The asset is to provide treatment and retardation for the stormwater runoff generated from the urbanisation from the upstream catchment.
Several important points emerged from the range of modelling scenarios run. In terms of dam hydraulics, the modelling found that:
- The site and surrounding areas are exposed to existing flood risk.
- The proposed retarding basin was able to alleviate overall flood risk, and particularly to lots along the main road.
- Pipe blockages did not worsen the flood risk in major events because the basin is able to hold the 1% AEP volume.
- Assuming that the dam break trigger occurs when water levels reach the spillway weir, the modelling found that no break is likely in the dam until rarer storm events (< 0.1% AEP).
- Fall-back Flood Capacity for the spillway was a means of providing additional risk protection against an overtopping failure. A Fall-back spillway design of between 0.01% and 0.001% should be considered for the Basin.
Key Features and Innovations:
- Understanding the dam’s behaviour in various storm events.
- Impact to existing downstream properties and how to manage this safely.