OPW16/0021- Operational Works Application 2 of 4

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3.5       Mitigated!Scenario!
When!comparing!the!peak!flows!in!the!existing!and!developed!scenarios,!increases!
in!the!peak!discharge!occur!at!the!catchment!outlets.!!As!a!result!a!detention!system!
will! be! required! in! order! to! prevent! worsening! of! peak! flows! from! the! site.!!
Detention!tank!locations!and!associated!catchments!are!detailed!in!Figure!6.!!
The! mitigated! case! model! includes! the! proposed! detention! volume! within! the!
Stormwater!Tank!specified!by!FLOTH!(Appendix!1).!!FLOTH!propose!that!a!90!KL!tank!
be!installed!of!which!the!effective!volume!to!be!used!for!site!reuse!is!to!be!only!60!
kL.!!Therefore,!the!additional!30!kL!has!been!modelled!as!available!detention!volume!
with! the! outlets! as! specified! by! FLOTH! (Appendix! 1).! ! The! Rainwater! Tank! will! not!
serve!as!a!detention!storage!and!thus!has!not!been!included!in!hydrologic!modelling.!!
Both!tanks/catchments!are!directed!to!the!proposed!detention!storage!basin.!
All! modelling! parameters! remain! consistent! between! the! developed! and! mitigated!
scenarios,! with! the! addition! of! a! detention! system! elements! proposed.! ! The!
detention!storage!details!included!within!the!model!are!provided!in!Table!13.!
Table!13:!!Storage!and!Outlet!Details!of!Detention!System!
                                                                                  Peak!100!yr!
                   Storage!          Storage!
    Catchment!                                            Outlet!Dimensions!      Water!Depth!
                 Volume!(m3)!       Height!(m)!
                                                                                     (m)!
     SW!Tank!
                                                  2!x!225mm!at!the!base!of!the!
      (FLOTH!         30!              0.8!                                           0.233!
                                                             tank!
      Design)!
                                                  1x100mm!at!the!base!of!the!
    Detention!                                     tank!(incorporated!as!the!
                      52!              2.6!                                            2.62!
      Basin!                                          overflow!from!the!
                                                      bioretention!basin)!
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A! comparison! of! the! peak! flows! between! the! existing,! developed! and! mitigated!
scenarios! are! presented! below! in! Table! 14.! It! can! be! seen! that! nonZworsening! of!
peak!flows!are!achieved!at!each!outlet!to!the!site.!!
Table! 14:! ! Comparison! of! Existing,! Developed! and! Mitigated! WBNM! Outlet! Peak! Flows! –!
Outlet!2!
                   Existing!Peak!      Developed!Peak!        Mitigated!Peak!   Change!in!Peak!
          !         Flow!(m3/s)!         Flow!(m3/s)!          Flow!(m3/s)!      Flow!(m3/s)!
       2!year!          0.11!               0.15!                  0.11!            0.00!
       5!year!          0.15!                 0.19!                0.14!            Z0.01!
      10!year!          0.17!                 0.22!                0.15!            Z0.02!
      20!year!          0.20!                 0.25!                0.18!            Z0.02!
      50!year!          0.21!                 0.26!                0.19!            Z0.02!
     100!year!          0.23!                 0.27!                0.22!            Z0.01!
This!tables!shows!that!the!MCU!condition!28a!is!met!with!no!increase!in!volumes!of!
stormwater!discharging!to!the!existing!outlet.!!!
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