Wallis and Swamp Fishery Creeks Flood Study - main report

STUDY FINDINGS
- Swamp/Fishery Creeks System
The most significant flooding of the Swamp/Fishery Creeks system over the last 50 years
occurred in February 1990 and June 2007. Detailed investigations and modelling
undertaken for this study has shown that the June 2007 flood was larger or rarer than the
design 100 year ARI event, and that it was substantially rarer in the vicinity of Abermain.
The analysis shows that the June 2007 event was in the order of a 200 year ARI flood at
Abermain and slightly rarer (i.e., or larger) than a 100 year ARI event at Weston. This
event led to inundation of large areas of the adjoining floodplain of Swamp Creek and
significant property damage at Abermain and Weston, and in between.
The modelling of design events up to the 100 year ARI flood shows that flooding of
Swamp Creek is most severe between the Kline Street Bridge and the Government Road
Bridge crossings, with significant overbank inundation occurring during floods of the
magnitude of the 20 year ARI event. A number of properties are flooded in this area in
events of this severity, particularly in the vicinity of the Fourth Street Bridge crossing
(refer to Figure 18). Water depths in the vicinity of the houses along the right bank of the
Fourth Street Bridge crossing are estimated to be up to 0.8 metres in the 100 year ARI
flood and mean velocities are in the order of 0.5 m/s.
Flooding at Abermain can also be severe, as evidenced by the June 2007 event.
Floodwaters from the upper catchment are concentrated in the main channel of Swamp
Creek which is flanked by residential development that extends along both banks the
William Street and the Cessnock Road bridges. The flood modelling results show that this
reach of Swamp Creek experiences steeper flood surface gradients than further
downstream. As a result, there is greater potential for flood damage and risk to life due to
the high energy flow regime that can occur during major flooding.
Downstream of Loxford, Wentworth Swamps provide a massive flood storage area which
has the capacity to hold a volume of nearly 11 million cubic metres in a design 100 year
ARI event. This storage serves to provide significant attenuation of flows carried by
Swamp Creek and reduces the impact of these flows before they discharge into Wallis
Creek. Therefore, it is important that the Wentworth Swamps storage be retained so that
flooding downstream in the Louth Park and South Maitland area is not exacerbated.
- Wallis Creek
Flooding due to catchment runoff (with no fresh in the Hunter) is generally retained within
the defined floodplain downstream to Buttai Lagoon. Further downstream, in the Louth
Park area, floodwaters are controlled by local levees and spillways. These spillways are
overtopped in the 1% AEP local catchment runoff event.
However, flooding of the lower Wallis Creek Valley can be influenced by backwater
flooding from the Hunter River. That is, the worst flooding conditions downstream from
John Renshaw Drive typically occur when flooding of Wallis Creek occurs concurrently
with flooding of the Hunter River. Modelling has shown that when a flood of 20 year
recurrence or rarer occurs in the Hunter River, levees around Maitland within the Lower
Hunter Flood Mitigation System, will overtop and begin to fill backwater storages. As
these backwater storages fill, floodwaters back-up in a southerly direction along Wallis
Creek, generating higher flood levels than would occur in local catchment events.
Historical flood records indicate that a peak flood level of 10.3 mAHD was recorded
during the 1955 flood at Victoria Bridge, which is near the current site of the Wallis Creek
flood gates. A peak level of 6.2 mAHD was recorded at the Trappaud Road Bridge
crossing of Wallis Creek in 1977.
As a result, peak 100 year recurrence flood levels along Wallis Creek are generated by the
occurrence of coincident Hunter River flooding. As shown in Figures 22 to 30, these
backwater effects extend almost upstream to John Renshaw Drive.
- RECOMMENDATIONS
- Swamp/Fishery Creeks System
 Undertake a floodplain management study for Abermain and Weston aimed at
investigating planning and structural options that could be implemented to reduce flood
damages incurred by property owners adjacent to the creek.
Potential structural flood damage reduction options (in order of provisional priority)
include:
(i) creek improvements such as vegetation clearing, channel de-silting and channel
realignment;
(ii) construction of on-line flood control dams or retarding basins in the upper catchment
of Swamp Creek;
(iii) modification to existing off-line storages such as Hebburn Reservoir so as to increase
their flood storage capacity;
(iv) upgrading of bridge waterway areas for bridge crossings; and,
(v) installation of levees along the creek banks.
It should be noted that a preliminary assessment of potential structural flood
management options is being undertaken concurrently with the finalisation of this
report.
This assessment includes the consideration of the flood mitigation benefits that would
be afforded by channel vegetation clearing, channel lowering (dredging) and increased
off-line flood storage (Hebburn Reservoir). The findings from this preliminary option
assessment are to be documented in a separate report which will inform any future
Floodplain Risk Management Plan that is developed for the villages of Abermain and
Weston.
Potential non-structural measures that could be implemented to minimise flood damage (in
order of provisional priority) include:
(i) House raising.
(ii) Property buy back scheme.
(iii) Flood proofing of flood affected buildings
(iv) Planning controls that:
 Set minimum habitable floor levels based on the flood hazard
 Modify land-use zonings upstream of the perceived flood problem to ensure that
future development is not incompatible with flooding and that future development
does not worsen flooding in downstream areas
(v) Provide flood evacuation routes.
(vi) Undertake flood awareness campaigns and develop a suitable flood warning system.
 Re-commission the water level gauge recorder (Station No 210053) located at Horton
Road, Kurri Kurri, so that a continuous record of the water levels of future floods can be
recorded and used to generate a discharge hydrograph for Swamp Creek, which in turn
can be used to calibrate further hydrodynamic modelling, as required.
- Wallis Creek System
 Undertake a floodplain management study for the Buchanan Area aimed at determining
planning controls that could be implemented to manage potential future development
that may be promoted on or adjacent to the floodplain as a consequence of the F3
Extension to Branxton.
 Installation of a pluviograph within the Wallis Creek catchment to provide a continuous
record of rainfall for use in assessing the severity of future floods and the significance
of rainfall distributions to flood behaviour. It is suggested that the pluviograph be
located around Mulbring to provide a realistic measure of average rainfall patterns.
 Re-commission the water level gauge recorder located along Wallis Creek at Richmond
Vale, so that a continuous record of the water levels of future floods can be recorded
and used to generate a discharge hydrograph for Wallis Creek, which in turn can be
used to calibrate further hydrodynamic modelling, as required.

Tags: Fishery Creek, Kurri Kurri, Swamp Creek, Wallis Creek, Wentworth Swamp, hydraulic model, hydrologic model
Organization: Cessnock City Council