RFI - Attachment B - Stormwater management Report · MCU20/0026

Initially, the ensemble of storms for each duration was simulated for the pre-developed model for the
entire site, and the median storm for each duration & AEP was determined utilising the ensemble
statistics utility. The resultant storms were utilised as the design storms for the 10 minute, 25 minute,
and 1 hour duration events for each AEP.

Once the design storms were determined, the pre-developed roof catchment was simulated for the 10%
AEP and 1% AEP events. There was found to be an approximately 30% increase in runoff in the 1%
AEP simulation. Based on the results, the catchment area conveyed to the rear of the site for both the
pre-developed and post-developed model for the 1% AEP event was artificially increased by 30%
(based on modelled roof area) to account for major flows bypassing the underground stormwater
network and discharging to the rear. Further detail on catchment setup is provided below.
Hydraulic Modelling

An integrated 1D/2D model was developed to simulate the hydraulics of the stormwater drainage
network. The 1D network was limited to modelling the infiltration/detention tank in the post developed
model. Physical parameters for the storage node were calculated based on data obtained from the
manufacturer. The following parameters were utilised in developing the 2D network:
       A 1m grid cell size was utilised.

       The DTM was derived from LiDAR data

       Manning’s roughness for of 0.06 was considered appropriate due to the land use of the site.

       A 2D head boundary line with a free outfall was used to define the 2D model extent.
Model Setup

Two models were developed for the site. These are summarised as follows:

Pre-developed model

       The existing site excluding the roof area was modelled as a single catchment linked to Node A.
        For the 1% AEP event, the impervious catchment area was artificially increased by 30% of the
        existing roof area to account for major flows bypassing the underground stormwater network.
        The hydrograph of the catchment was linked to and evenly distributed across a rainfall/ flow
        area grid at the rear of the site. The peak flow rate discharging across the property boundary
        was measured at a flow line at the rear of the site and the total runoff volume discharging across
        the property boundary was measured as the total runoff inflow at Node A.

Post-Developed Model
        The developed site excluding the roof area was modelled as a single catchment linked to Node
        A. Similar to the pre-developed model, the impervious catchment area was artificially increased
        by 30% of the existing roof area to account for major flows bypassing the underground
        stormwater network. Node A was set as a storage node to model the detention/infiltration tank.
        Node A was linked to the 2D network to allow surcharging to occur. The peak flow rate
        discharging across the property boundary was measured at a flow line at the rear of the site
        and the total runoff volume discharging across the property boundary was measured as the
        total outflow volume from Node A

The catchment data for each model is summarised in Table 2 below:




11 June 2020                           Stormwater Management Report                                     4
S20-023                              40 Park Crescent, Sunrise Beach