Appendix P Drainage, Flooding, Stormwater Management Part 5

Flooding, Drainage and Stormwater Management
Sanctuary Villages
A comparison of PRM and XP-RAFTS flow rate estimates at Millfield Bridge for the 1, 5,
10, 20, 50 and 100 year ARI storm events is provided in Table 3-1. The critical storm
duration that produced the peak discharge for the 1 year ARI event was the 720 minute
storm. For all other storm events, the 2160 minute storm duration produced the largest
peak discharge.
Note that these flow estimates do not account for temporary storage provided by Ellalong
Lagoon.
Table 3-1:
Comparison of PRM and XP-RAFTS Peak Discharges
ARI PRM Estimate (m 3 /s) XP-RAFTS Estimate
(m 3 /s)
Percentage Difference
1 Year 114 81 -29%
5 Year 267 289 +8%
10 Year 344 388 +13%
20 Year 444 525 +18%
50 Year 589 663 +13%
100 Year 729 809 +11%
The following features of the PRM could explain the minor discrepancies with modelpredicted
results:
Only a peak flow estimate is provided and the hydrograph shape is not considered.
A constant rainfall intensity is adopted with the temporal variability of rainfall not
accounted for.
The estimation of time of concentration and runoff coefficient is subjective.
The PRM does not explicitly allow for catchment surface storage and routing, which
may tend to produce high flow estimates in steep catchments.
From Table 3-1, XP-RAFTS model predicts higher discharges than the PRM for all
ARI events except the 1 year. The magnitudes of the differences, however, are
considered within limits of engineering best practice for flow estimation.
3.1.5 Existing peak discharges
Existing peak discharges allow for storage provided by Ellalong Lagoon. The heightstorage
and height-discharge relationship for Ellalong Lagoon is shown in Figure 4. The
height-storage relationship was interpolated from the Quorrobolong 1:25,000 topographic
map, which has a 10m contour interval. The height-discharge relationship is based on
culvert hydraulics for inlet control and allows for Congewai Road to function as a weir
during high flows. Also provided on Figure 4, for comparative purposes, is the
documented PBP height-storage-discharge relationship for Ellalong Lagoon.
The existing culverts have been estimated as having an invert level at 110.5m AHD with
the minimum elevation along Congewai Road being 113.2m AHD. It is estimated that
Congewai Road is depressed for a length of approximately 500m.
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A summary of existing peak discharges at Millfield Bridge (node CON.15) for the 1, 5, 10,
20, 50 and 100 year ARI storm events is provided in Table 3-2. Also provided are peak
discharge estimates documented in the Patterson Britton and Partners (PBP) flood study
(April 2004) at the same location. The critical storm duration that produced the peak
discharge for all events except the 1 year ARI event was the 2160 minute storm. The
peak discharge for the 1 year ARI event occurred for the 720 minute storm.
Estimated hydrographs at Millfield Bridge (node CON.15) for the 1, 5, 10, 20, 50 and 100
year ARI critical duration storm event are provided in Appendix D.
800
4,000
700
3,500
600
3,000
Discharge (m 3 /s)
500
400
300
2,500
2,000
1,500
Storage (ML)
200
1,000
100
500
0
0
110.5 111 111.5 112 112.5 113 113.5 114
Height (m AHD)
PB H-Q Relationship PBP H-Q Relationship PB H-S Relationship PBP H-S Relationship
Figure 4:
Height-Storage-Discharge Relationship for Ellalong Lagoon
Table 3-2:
Existing peak discharges at Millfield Bridge (Node CON.15)
ARI
Existing Peak
Discharge (m 3 /s)
Critical Storm
Duration (min)
PBP Peak Discharge
Estimate (m 3 /s)^
1 Year 62 720 -
5 Year 221 2160 255
10 Year 296 2160 -
20 Year 392 2160 434
50 Year 571 2160 -
100 Year 762 2160 654
^ PBP peak discharge estimate has been provided for comparative purposes only.
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Flooding, Drainage and Stormwater Management
Sanctuary Villages
Existing peak discharges at Millfield Bridge for the 5 and 20 year ARI storm events are
generally in close agreement with estimates documented in the Wollombi Valley Flood
Study prepared by Patterson Britton and Partners (April 2004). The predicted 100 year
discharge is 17% larger than that documented by PBP. This may be due to variations in
the height-storage-discharge relationship adopted by both Consultants.
A summary of existing peak discharges, peak storage and peak water level at Ellalong
Lagoon (node QUO.7) for the 1, 5, 10, 20, 50 and 100 year ARI storm event is provided
in Table 3-3, and will be used as a baseline comparison for evaluating the impacts of the
development on existing flow rates, as well as for estimating the 100 year ARI flood levels
within the study area.
Table 3-3:
ARI
Existing peak discharge, peak storage and peak water level in Ellalong Lagoon
Existing Peak
Discharge (m 3 /s)
Critical Storm
Duration (min)
Peak Storage
(ML)
Peak Water Level
(m AHD)
1 Year 16 720 473 111.23
5 Year 64 2160 1,560 112.35
10 Year 80 2160 1,560 112.84
20 Year 141 2160 2,820 113.34
50 Year 231 2160 3,030 113.51
100 Year 305 2160 3,160 113.62
3.2 Developed hydrology
3.2.1 Potential hydrological impacts of development
Urban development typically results in an increase in the impervious surfaces within a
catchment and engineering works to collect and convey runoff. Without appropriate
management, several potential impacts result from development. These could include:
increased frequency of runoff events;
increased runoff volume, peak flow and velocity; and
decreased groundwater base flows and lowering of the watertable.
These impacts may in turn lead to:
inundation of downstream property and infrastructure;
change in existing channel form and sediment transport;
damage to riparian and aquatic vegetation; and
damage to existing aquatic ecosystems and natural wetlands.
3.2.2 Model parameters
The effect of urbanisation, of the proposed development site, was modelled using XP-
RAFTS and involved the inclusion of impervious areas within the existing model. The
model was then used to estimate developed peak flow rates at Millfield Bridge (node
CON.15).
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Flooding, Drainage and Stormwater Management
Sanctuary Villages
Details of XP-RAFTS developed model catchment data are summarised in Appendix B.
Node locations used in the XP-RAFTS developed model are identical to those for the
existing model and are shown in Figure 3.
The percentage of impervious area for the development site has been adopted as 60%.
This value has been adopted to represent the relative contribution of various land types
including; road reserves, commercial, residential and parkland. This assumption is valid
for a regional level study but a more detailed breakdown of land use will be considered at
the local strategy level.
3.2.3 Developed peak discharges
Predicted developed flow rates within Congewai Creek at Millfield Bridge for the 1, 5, 10,
20, 50 and 100 year ARI storm events are summarised in Table 3-4. Also shown are
existing peak flow rates at this location and the percentage change in flow resulting from
development. Runoff hydrographs for the critical duration storm that produced the largest
peak flow estimate are provided in Appendix E for developed conditions. The critical
storm duration that produced the peak flow rate at node CON.15 was the 720 minute
storm for the 1 year ARI event and 2160 minute storm for all other events.
Table 3-4: Developed peak flow estimates within Congewai Creek at Millfield Bridge (Node
CON.15)
ARI
Existing Peak Flow
Rate (m 3 /s)
Developed Peak
Flow Rate (m 3 /s)
Percentage Change
1 Year 62 62 0%
5 Year 221 221 0%
10 Year 296 295 -0.3%
20 Year 392 391 -0.2%
50 Year 571 571 0%
100 Year 762 761 -0.1%
Table 3-4 illustrates that the proposed development results in a negligible change in the
critical duration peak flow rate at Millfield Bridge for the 1, 5, 10, 20, 50 and 100 year ARI
storm events.
Examination of Figure 3 shows the proposed development area to be located near the
outlet of the analysed catchment, which extends as far as Millfield Bridge. The
introduction of impervious surfaces at this location increases the rate of runoff from the
lower portions of the catchment allowing runoff to enter Congewai Creek before the
overall catchment flood peak. This is further supported by examination of the developed
runoff hydrograph for the 1 year critical duration storm event contained within Appendix
E, which shows a small spike on the rising limb of the hydrograph when compared to the
existing runoff hydrograph at this location.
A comparison of existing and developed peak flow rates for the 1, 10 and 100 ARI storm
events run for a range of storm durations is shown in Figure 5. Examination of Figure 5
reveals negligible change in peak flow rate for the 10 and 100 year ARI storm events at
Millfield Bridge for all storm durations. However, there is a minor increase in peak flow
rate for the shorter duration 1 year ARI storm events. This increase in flow rate will
require detention storage to reduce the potential for scour of existing drainage lines.
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Sanctuary Villages
This study has considered the impact of the development on the major (regional) flood
events. Future stormwater strategies should consider the impact of the development on
local stormwater runoff flow rates. The results of the future stormwater management
strategy are likely to indicate the need for stormwater detention at local outfalls, which
should also resolve the impact of the development on shorter duration 1 year ARI
regional storm events.
Details of stormwater management measures that may be considered in future detailed
stormwater management strategies are provided in Section 5.
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900
800
700
600
500
400
300
200
100
0
Flooding, Drainage and Stormwater Management
Sanctuary Villages
30 45 60 90 120 180 270 360 540 720 1080 1440 1800 2160 2880
Storm Duration (min)
1 year Existing Flow 1 year Developed Flow 10 year Existing Flow 10 year Developed Flow
100 year Existing Flow 100 year Developed Flow
Figure 5: Comparison of 1, 10 and 100 year ARI storm events for a range of storm durations
PARSONS BRINCKERHOFF 2122870A-PR0116 Page 15
Peak Flow Rate (m 3 /s)

Flooding, Drainage and Stormwater Management
Sanctuary Villages
4. Hydraulic modelling
The HEC-RAS (Version 3.1.2, April 2004) computer model, developed by the Hydrologic
Engineering Centre, U.S. Army Corps of Engineers was used to determine the 100 year
water profile within Congewai and Quorrobolong Creeks south of the development precincts.
The programme uses the standard step method to determine water surface profiles for
gradually varied flow.
4.1 Modelling parameters
The hydraulic analysis has been undertaken as a steady state simulation. A brief discussion
on model parameters is provided below. Storage within Ellalong Lagoon has been
accounted for in the hydrologic model described in Section 3.
4.1.1 Flow regime
A mixed flow regime analysis was undertaken for the analysed portion of Congewai and
Quorrobolong Creeks situated within the study area.
4.1.2 Boundary conditions
For mixed flow, both upstream and downstream boundary conditions are required for model
simulation. The upstream boundary conditions assumed normal flow with a creek slope of
0.003. The sensitivity of the model to this slope has not been examined but is likely to be
negligible given that flow is subcritical.
The downstream boundary condition comprised a fixed water level that was obtained from
the Wollombi Valley Flood Study prepared by Patterson Britton and Partners. A level of
110.1m AHD, which is situated approximately 150m upstream of Millfield Bridge, was
adopted. The sensitivity of the model to this starting level has not been examined.
4.1.3 Peak flows
A summary of 100 year ARI peak flow rates used within the hydraulic model are provided in
Table 4-1. These flows were taken from the XP-RAFTS model for the 2160 minute critical
storm duration. Note that the existing and developed peak flow rate estimates for the critical
duration storm event are identical.
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Sanctuary Villages
Table 4-1:
XP-RAFTS
Node
100 year flows adopted in hydraulic model
Creek
HEC-RAS
Crosssection
100 Year Flow Rate
(m 3 /s)
QUO.5 Quorrobolong 2695.4 286.3
QUO.6 Quorrobolong 1905.17 299.9
CON.10 Congewai 6231.75 370.0
CON.11 Congewai 4363.81 700.0
CON.14A Congewai 1643.19 746.8
CON.15 Congewai 304.12 761
UNK.4 Unnamed 582.97 14.0
UNK.7 Unnamed 1864.4 60.1
UNK.9 Unnamed 1864.32 3.1
4.1.4 Energy loss coefficients
Several types of loss coefficients are utilised by the program to evaluate head losses. These
are:
Manning’s ‘n’ values for friction losses in the defined creek line and overbank regions.
The Manning’s ‘n’ for the defined creek line and overbank regions varied depending on
vegetation type (U.S. Army Corps of Engineers, 1998); and
Contraction and expansion coefficients to evaluate creek transition losses. For gradual
transitions, the contraction coefficient was assumed to be 0.1 with the expansion
coefficient being 0.3 (U.S. Army Corps of Engineers, 1998).
4.1.5 Cross section geometry
Boundary geometry for the analysis of flow in natural watercourses is specified in terms of
ground surface profiles (cross sections) and the measured distance between these profiles
(reach lengths).
Creek cross sections were determined from aerial survey of the site. The locations of all
cross sections utilised in this analysis are shown in Figure 6.
4.1.6 Reach length
The measured distances between cross sections are referred to as reach lengths and are
typically measured along the centreline of the defined creek line. Distances between
overbank regions are measured between upstream and downstream cross section overbank
regions.
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