As described in the Floodplain Development Manual (Reference 1), the Floodplain Risk
Management Process entails four sequential stages:
- Stage 1: Flood Study
- Stage 2: Floodplain Risk Management Study
- Stage 3: Floodplain Risk Management Plan
- Stage 4: Implementation of the Plan
The Rose Bay Catchment Floodplain Risk Management Study constitutes the second stage in
the process. The Flood Study stage was completed in September 2010 with publication of the
Rose Bay Catchment Flood Study (Reference 2). A combination of hydrologic and hydraulic
models was used in that study to determine design flood levels for the Rose Bay catchment.
This study superseded a previous Catchment Management Study (Reference 3) completed in
1991.
A rainfall-runoff approach was adopted due to the absence of long term
historical flood data. This approach involved setting up a DRAINS hydrologic and hydraulic
computer model that simulated flow both in the pipe system and as overland through private
property and along roads. The DRAINS model covered the entire catchment. A two-dimensional
(2D) SOBEK computer model was established in the lower reaches to convert the upstream
flows obtained from DRAINS into flood levels and velocities.
Design rainfall data obtained from
Woollahra Council and design temporal patterns from Australian Rainfall and Runoff (Reference
4) were obtained and used in a DRAINS model to determine design event flows. The SOBEK
model was then used to determine design flood levels in the lower catchment. The downstream
regions of the Rose Bay catchment were predominately influenced by catchment flows and only
slightly by elevated water levels in Rose Bay. Sensitivity analyses were undertaken on the
DRAINS and SOBEK model results.