2014-10-14 Planning & Organisation Committee Agenda - Pages 1 - 50

PLANNING & ORGANISATION COMMITTEE MEETING AGENDA                              14 OCTOBER 2014


After detailed modelling and consideration of road geometry and drainage elements, the following
constraints limit the ability to achieve compliance with the condition:
1.   Garnet Street in its existing formation provides flood mitigation. Alteration of road levels
     and pipe culvert configuration must ensure non-worsening both upstream and downstream
     of Garnet Street.
2.   The ability to raise Garnet Street in combination with increased culverts to achieve the
     desired immunity has limitations in the form of:-
     a.    Maintaining the sag point at Cooroy Creek
     b.    Earthworks / embankment extents to be maintained within the road reserve
     c.    Close proximity of Cooroy Creek immediately adjacent to the road reserve (bend
           upstream)
     d.    Changing existing overland flow paths and
     e.    Impact on adjoining private properties.
The most recent design (Refer updated Civil Drawings) proposes to raise Garnet Street to what
is considered to be the maximum levels without changing or exceeding the above mentioned
limitations.
Given these constraints and limitations, configuration and sizing of culverts has been determined
to limit non-worsening of upstream and downstream conditions.
Current Inundation Times and Levels
The Hydrology and Water report (2014) titled "Garnet Street Culvert Upgrade Hydraulic
Assessment" indicates that currently Garnet Street experiences the following time of inundation
and flow depths:-
                     Table 1- Current Inundation Times & Peak Flow Depths
 Event                           Inundation time                     Peak Flow Depth (over Road)
 2-Year ARI event (39% AEP)      6 hours                             0.5m
 10-Year ARI (10% AEP)           12 hours, 20 minutes                0.8m
 100 Year ARI (1%AEP)            21 hours                            1.0m

Currently Garnet Street is not trafficable during any of these rainfall events and is inundated for
extended periods of time.
The most recent design being proposed and referenced in the Hydrology and Water report
(2014) titled "Garnet Street Culvert Upgrade Hydraulic Assessment" indicates significant
improvement of times of inundation and peak flood depths. Refer Table 2 below.
                   Table 2 - Proposed Inundation Times & Peak Flow Depths
 Event                           Inundation time                     Peak Flow Depth (over Road)
 2-Year ARI event (39% AEP)      Nil                                 Nil
                                 (Approximate 6 hour reduction)
 10-Year ARI (10% AEP)           2 hours, 40 minutes                 0.18m - Safely trafficable
                                 (Approximate 10 hour reduction)     (Peak Velocity-Depth Product =
                                                                     0.12)
 100 Year ARI (1%AEP)            5 hours, 20 minutes                 0.4m
                                 (Approximate 16 hour reduction)     (Peak Velocity-Depth Product =
                                                                     0.6)
                                                                     (Refer Emergency Vehicle Access
                                                                     references below)

Note that the above-mentioned improvements/reductions have been achieved with a blockage
factor of 20% applied to the existing and proposed culverts.


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