Flood Assessment - Turramurra Development
Our clients required an overland flow assessment prepared by a suitably qualified hydraulic engineer for the proposed development located at Turramurra. This overland flow assessment was to be submitted to Ku-ring-gai Council for the 100-year ARI design storm to determine the extent of the flow path and freeboard requirements for the development. In response to the request, a hydraulic analysis was undertaken to determine the 1% AEP (100-year ARI) flood level and flood afflux for the site, and an assessment of the development implications on the proposed development and surroundings was conducted. The analysis and study were conducted as per the requirements set by Ku-ring-gai Council.
The proposed development involved rebuilding the house and increasing the footprint, building a pool and a spa, and building a Gazebo at the south of the site.
Hydraulic analysis was carried out based on 2D modelling using the most up to date version of HEC-RAS (6 Beta). The hydraulic model was built with a 1m grid using Lidar data, and used data obtained from the Site Survey.
The developed hydraulic model for the proposed development was based on the site plan. Following council review, the driveway at the garage for the developed case was modelled in the Civil 3D package. This modelling showed that because of filling in front of the garage for the driveway, the runoff does not reach the garage. In addition, the installation of a grated drain alongside the Western side of the property and alongside the Gazebo to improve afflux was proposed.
Modelling was done for both the existing structure and for the developed structure. The modelling results found that the proposed development had minimal effect on flooding levels, with flood levels relatively unchanged. Furthermore, the afflux map shows that the proposed development would not increase the flood depth in the surrounding lots.
Communication between Neilly Davies, the client, Ku-ring-gai Council and other relevant parties ensured that the project was completed to a high standard and before the due date. The project required several changes after Council feedback, and Neilly Davies was able to update the relevant models and reports to satisfy the appropriate Council requirements and Australian Standards.