Overland Flow Assessment — Single Dwelling, Mosman
Council — Mosman Council
Development — Residential — Alterations and Additions
Services — Flood Risk Assessment & Reporting · Hydrology Assessment · Flood Modelling
Year — 2026
Outcome — Approved
Background
The development proposed alterations and additions, with extensions at street level.
The problem
The site sits low in its catchment and is inundated by overland flow during large storm events. Amuna was engaged to assess whether extensions at street level could safely accommodate the existing overland flow conditions — and to give the client confidence before committing to the design.
Our solution
We built a detailed DRAINS model of the catchment contributing flow toward the site, incorporating a custom road cross-section so the existing road profile and overland flow path were represented accurately rather than approximated.
The model routed upstream catchment flows toward the site and assessed potential inundation. It confirmed that even with the full upstream contribution directed at the site, water levels remained below the proposed extension floor levels — demonstrating the extensions could be built without risk of inundation under the assessed conditions.
Was it approved? Yes.
Custom DRAINS modelling section and estimated water level