Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 40
Narellan Creek Catchment
Flood Study
irregular culverts (e.g., arch culverts) were defined using a flow height versus flow width
relationship.
The bridge deck and road surfaces above each hydraulic structure were represented as part
of the 2D domain. Therefore, flow through each of the structures below the road level was
represented in 1D and flow across the road surface (i.e., once the capacity of the structure is
exceeded) was represented in 2D.
Blockage
During a typical flood, sediment, vegetation, and urban debris (e.g., litter, fence palings,
bins) from the catchment can become mobilised leading to blockage of downstream
culverts and bridges. Consequently, bridges and culverts will typically not operate at full
efficiency during most floods. This can increase the severity of flooding across areas located
adjacent to these structures.
In recognition of this, blockage factors were calculated for all bridges and culverts. The
blockage factors were calculated based on guidelines contained in 8Australian Rainfall and
Runoff: A Guide to Flood Estimation9 (Ball et al, 2019). The blockage calculations are
summarised in Appendix D for each culvert and bridge located within the TUFLOW model
area.
3.3.7
Stormwater System
The stormwater system has the potential to convey a significant proportion of runoff across
the urbanised sections of the catchment during relatively frequent rainfall events.
Therefore, it was important to incorporate the stormwater system in the TUFLOW model to
ensure the interaction between piped stormwater and overland flows was reliably
represented.
The full stormwater system contained within the catchment was included within the
TUFLOW model as a dynamically linked 1D network. This allowed representation of the
conveyance of flows by the stormwater system below ground as well as simulation of
overland flows in two dimensions once the capacity of the stormwater system is exceeded.
The inlet capacity of each stormwater pit was assigned using a water depth versus inflow
relationship. The 8Drains Generic Pit Spreadsheet9 (Watercom Pty Ltd, July 2005) was used
to develop the inlet capacity curves. The inlet capacity curves were developed to take
account of:
The different pit inlet types (e.g., grated, side entry, combination)
The different topographic locations (e.g., sag or on-grade), and
The different grate dimensions and lintel sizes.
Hydraulic
