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Table 11: Adopted Manning’s Values
Table 11: Adopted Manning’s Values
Surface Type Manning’s Value (n)
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Roads 0.02
Residential 0.05 Parks 0.06
Commercial 0.02
7.5 Hydraulic Features
Hydraulic features that impacts on flood behaviour have been included in the TUFLOW hydraulic model. These features include:
• Road profiles; • Buildings; • Saltwater Creek bathymetry; and • Drainage network.
The road profile was included in the hydraulic model using breaklines along the road crests and gutter lines. This ensures that the representation of gutter flow, a critical component of urban drainage, best matches reality.
Buildings can block flood waters and therefore significantly impact flood behaviour. Accordingly, aerial imagery was used to outline building footprints in the Exile Bay catchment and these were blocked out of the TUFLOW model (shown in Figure 14).
Survey data of the Saltwater Creek bathymetry (see Section 3.3.2) has been included in the TUFLOW model using breaklines to represent the channel shape. This ensures that the capacity of the channel is accurately represented.
The pit and pipe drainage network was included in the TUFLOW model. This network and the relevant conduit sizes are shown in Figure 16. This network is described in more detail in Section 3.3.3.
7.6 Hydraulic Structure Blockage
The capacity of drainage systems and hydraulic structures can be significantly impacted by blockage. Blockage will tend to occur in the presence of suitably sized debris. In urban areas drainage systems can be subject to blockage from a variety of materials including garbage, vegetation, shopping trolleys, garbage bins, cars and sediments. Blockage of hydraulic structures to some degree is almost inevitable in the event of large and rare flood events.
ARR2016 provides guidance on the degree to which structures are blocked with a distinction between cross drainage structures over waterways (such as Saltwater Creek) and drainage system inlets and pipes. The design blockage is the blockage condition that is most likely to occur during a given design storm and needs to be an “average” of all potential blockage conditions to ensure that the calculated design flood levels reflect the defined probability.
