Powells Creek Flood Study
A limited number of gaugings (height v velocity measurements) have been undertaken enabling the construction of a rating curve (height versus flow). Whilst in theory this approach appears very simple it becomes complex for several reasons, including: • the events occur within a few hours and thus it was very hard for the UNSW staff to get to the gauge whilst a flood was in progress. •
•
the above means that there are several low flow gaugings but very few high flow gaugings which are more relevant for use in a flood study. a gauging was taken by the UNSW at high flows which produced velocities above the rating of the instrument (say above 5 m/s). Thus, even this gauging could not confidently determine the peak flow.
Rating curves from various sources are provided on Figure 7.
2.9.2.
Sydney Water Gauge
This gauge, which is located on Powells Creek under the M4, has only recorded one significant flood (January 1996) since it was installed in 1995. The gauge zero is RL 2.15 m AHD and the January 1996 flood peaked at 2.04 m (4.19 m AHD) at 14:05 hours. Three streamflow gaugings have been undertaken. All gaugings are below 0.1 m gauge height (flow <2 m3/s). Extrapolation of the rating curve based on these data is not appropriate and as a result flow data from this gauge have not been used for calibration of the hydrologic model.
2.10.
Flood Levels from Debris or Other Marks
2.10.1. Resident Interviews As part of the 1998 Powells Creek Flood Study (Reference 1) and earlier studies (refer Table 1) questionnaires were distributed to residents to collect information about past flood events. Prior to the 1998 Powells Creek Flood Study the responses were generally concerned with drainage issues (blocked pits, minor overland flow) and not with identifying historical flood levels. The only exception to this was at Airey Park (Saleyards Creek) for the January 1996 event. Data obtained from residents should be used with caution for several reasons, including: • residents may have only been in the study area for a short period. • • • •
residents may have “missed” a flood whilst they were away. the more recent events are remembered more clearly than (say) a larger event several years ago. some events noted by residents may be because of a blocked drain or other local factors and are more typically referred to as local drainage problems rather than flood related. residents can easily forget the date of a flood or become confused about the extent and nature of the problem. Experience has shown that water entering a house may have resulted from a leak in the gutter or a local drainage problem in the yard rather than overbank flow from the main creek.
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