2015-03-31 I&S Committee - Item 4 - Attachment 1 - Flood Mitigation Investigation Ashgrove Dr Cooroy

Flood Mitigation Investigation: Ashgrove Drive, Cooroy


6 KEY HYDRAULIC REQUIREMENTS & DESIGN INPUTS FOR THE
    PREFFERED OPTION
HWMC have developed a preferred flood mitigation concept for Ashgrove drive. Detail design of
this mitigation concept has not been included in the current scope of works because it is expected
that DTMR’s civil engineers will be best placed to finalise the design to ensure it ties in appropriately
with their design of the Highway Upgrade.

To assist DTMR’s civil engineers, HWMC provide the following information which may be used to
guide detail design of the mitigation works.


Channel Lining
Hydraulic modelling of the concept design has allowed for a manning’s ‘n’ hydraulic roughness
value of 0.1 which assumes the channel will become re-vegetated. This has been chosen on the
basis that it is conservative in relation to overall channel capacity and peak flood level predictions.

It is noted that the choice of channel lining can be selected during detail design of the diversion
channel with consideration given to cost and on-going maintenance requirements.

Due to the channel’s flat grade (approximately 0.5%) and the relatively shallow flow depths
(approximately 1m for 1% AEP event), peak flow velocities are predicted to be less than 1 m/s and
should therefore not require any scour protection. Peak flow velocities should be checked for the
final channel configuration developed during detail design and it is also noted that some localised
scour protection may be required at transitions into and out of the channel.


Peak Flow Rates:
Peak flow rates through the diversion channel for the 10%, 1% and 0.05% design events
respectively are; 0.9m3/s, 6.8m3/s, 10.4m3/s.


Peak Flood Levels:
Peak design event flood levels at key locations for the mitigated case are tabulated below and are
also plotted on the longitudinal channel profile on Image 6-1.

Table 6-1 Peak Flood Levels
  Location                                               10% AEP       1% AEP          0.05% AEP

  Downstream extent of diversion channel
                                                         111.2 m AHD   111.5 m AHD     111.7 m AHD
  (immediately adjacent to Cooroy Creek)


  Upstream extent of diversion channel                   111.7 m AHD   112.3 m AHD     112.5 m AHD



Peak Flow Velocities:
Peak flow velocities through the diversion channel for the 10%, 1% and 0.05% design events
respectively are; 0.5m/s, 0.9m/s, 1.0m/s. Peak flow velocities should be checked for the final
channel configuration developed during detail design and it is noted that some localised scour
protection may be required at transitions into and out of the channel.
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