Appendix P Drainage, Flooding, Stormwater Management Part 5

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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