Narellan Creek Overland Flood Study (Vol 1) - Flipbook - Page 58
Narellan Creek Catchment
Flood Study
It is unlikely that floods of equivalent frequency will occur simultaneously in the Nepean
River and Narellan Creek catchments due to the different characteristics of each catchment.
In recognition of this, the 8Narellan Creek Flood Study9 (Worley Parsons, 2015) adopted a
20% AEP Nepean River flood as part of each Narellan Creek design simulation (except for the
50% AEP catchment flood, which adopted a 50% AEP Nepean River flood).
For more recent overland flood studies that have been completed elsewhere along the
Nepean River (e.g., Peach Tree & Lower Surveyors Creeks Catchment Flood Study (CSS, 2018),
Emu Plains Overland Flow Flood Study (BMT, 2020) and Liverpool Overland Flow path Study
for Rural Catchments (Stage 2) (CSS, 2021) it was assumed that floods of equivalent severity
were occurring in the local catchment and the Nepean River during all events up to and
including the 5% AEP event. For local catchment events rarer than the 5% AEP, the 5% AEP
Nepean River level was retained.
It was considered desirable to maintain consistency with the other studies along the Nepean
River. Therefore, the combination of local catchment and Nepean River floods that
summarised in Table 14 were applied. It was noted that a 10% AEP Nepean River flood level
was not available. Therefore, the water level was interpolated between the 20% AEP and 5%
AEP flood levels.
Table 14
Adopted Nepean River Boundary Conditions
Nepean River
Design Flood Event
Narellan Creek
Flood Event
Nepean River Flood
Event
Water Level (mAHD)
50% AEP
50% AEP
50% AEP
63.40
20% AEP
20% AEP
20% AEP
66.23
10% AEP
10% AEP
Interpolation
between 20% AEP
and 5% AEP
67.89
5% AEP
5% AEP
5% AEP
69.55
2% AEP
2% AEP
5% AEP
69.55
1% AEP
1% AEP
5% AEP
69.55
0.5% AEP
0.5% AEP
5% AEP
69.55
0.2% AEP
0.2% AEP
5% AEP
69.55
PMF
PMF
5% AEP
69.55
The impact of alternate tailwater levels on design flood behaviour was subsequently
assessed as part of the sensitivity analysis (refer Section 6.3.4).
5.3.2
Blockage
Culvert and Bridge Blockage
8Base9 blockage factors for each bridge and culvert in the study were estimated based upon
recommendations in Chapter 6 of Book 6 of ARR2019. This document also recommends
adjusting the 8base9 blockage factors up or down depending on the severity of the event (i.e.,
higher blockage factors during larger floods and lower blockage factors during smaller
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