MCU14-0039 Flood report and flood man Plan rev August 2015

Como Glasshouse P/L C/‐ Martoo Consulting P/L
Flood Impact Assessment – 305 Lake McDonald Drive Cooroy


2.4               Hydraulic Results
Hydraulic results for the existing and developed cases are attached in Appendix b and include flood
depth and velocity maps for the 10% AEP and 1% AEP design flood events. Additionally, flood impact
maps are also included in Appendix b for the roughness condition of n=0.07 as well as the sensitivity
analysis using n=0.1 as the difference between existing versus developed case flood levels at the site.
The following summary is provided on the flood impact results: ‐
         The site is completely inundated during the 10% AEP and 1% AEP design flood events;
         There is no fundamental change in hydraulic characteristics at the site including flood
          magnitude, flood flow distribution, flood velocities or flood depths. This is primarily a result
          of water being allowed to flow through the greenhouse structure during times of flooding
          which fundamentally maintains existing flow characteristics and does not result in any re‐
          distribution of flows;
         Following initial model simulations and advice from Council, Bund 3 was moved to a location
          between 5 m and 10 m to the north of the vegetation lining the creek immediately south of
          the greenhouse. This was undertaken to ensure the maintenance of a riparian buffer in this
          location.
         The largest increases in flood levels were found to be limited to the site itself. The largest
          increase in flood level was estimated to be up to 200mm immediately upstream from Bund 3.
          This dissipated further upstream from the bund;
         Flood impacts external to the site were found to be unsatisfactory with Bunds 1 and 2 in‐place
          at the site. Accordingly, Bunds 1 and 2 should be removed from the site to otherwise minimise
          flood impacts;
         No flood impacts were found to occur on the western boundary of the property;
         Minor flood impacts were found to occur to the adjacent property located to the south west
          of the site (i.e. Lot 1 RP173884). The maximum increase in flood level was found to be up to
          100mm in the 10% AEP event and 50mm in the 1% AEP event and does not impact on any
          existing infrastructure or will compromise existing access provisions to and from the property.
          As the topography rises steeply from Six Mike Creek in this area, the increases will have a
          negligible impact in terms of the flood extent which fundamentally will not change. As such,
          it is contended that these impacts are non‐worsening and will not result in actionable
          nuisance;
         Minor flood impacts are also found to occur to the southern lot on Lot 2 SP108094. This site
          is used for industrial brick and paver manufacture by PGH Bricks and Pavers. The impacts to
          this site are very marginal and are limited to a very small area adjacent to the site boundary.
          Impacts were estimated to be up to 25mm for both the 10% AEP and 1% AEP flood events and
          again will have a negligible impact in terms of the flood extent which fundamentally will not
          change. It is also contended that these impacts are non‐worsening and will not result in
          actionable nuisance;
         Minor impacts were also estimated to occur to Lot 8 RP901387. These impacts were very
          isolated and were less than 25mm in the 10% AEP event and below 10mm in the 1% AEP event
          and are considered inconsequential, with no impact to any existing infrastructure or access
          provisions;
         The increased roughness sensitivity scenario using n=0.1 showed a very similar result and was
          not significantly different to the n=0.07 case. The extent over which flood impacts occurred
          were greater but were limited in magnitude to be very similar to the n=0.07 case. Flood
          impacts external to the site were therefore found to not be adversely affected by an increase
          in floodplain roughness; and




3917‐01 / R01 v02 ‐ 31/08/2015                                                                          7