Attachment 4a - Engineering Report Part 1 · MCU18/0015

7.2.3      Downstream Drainage Report


Refer to the attached Flood and Tide Maps, sourced from Council’s Online Mapping Portal
and attached as Appendix D.


The existing Weyba Creek Tributary discharges to Council stormwater infrastructure in
Halse Lane. A concrete lined channel conveys stormwater runoff 200m west towards the
tidal waterway Weyba Creek. Under existing conditions, this drainage network has
adequate capacity to convey minor 39% & 10% AEP) storm events. During major events
(1% AEP) this stormwater infrastructure is influenced by tailwater conditions within
Weyba Creek. Tidal Surge Levels generally exceed 1% AEP Flood Levels within the lower
reaches of the focus catchment.



7.2.4      Peak Flow Management


As indicated above, no detention measures are proposed for the site on the following
grounds;


  -        The increase to stormwater discharge (0.3%) is minor when considered as part
           of the overall contributing catchment.
  -        The subject site is located at the lower end of a larger catchment. Providing
           detention measures onsite would increase the likelihood of peak discharge from
           the subject site coinciding with peak event runoff from upstream external
           catchments.
  -        Stormwater runoff is directed to an existing flow path. Increases to flow depth
           (1mm) are minor and will not cause an actionable nuisance to downstream
           property owners.
  -        The eventual discharge point in Halse Lane is affected by tailwater levels within
           the tidal waterway, Weyba Creek. Onsite detention measures are considered
           redundant when discharging to stormwater networks affected by storm surge or
           tidal prisms.

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