OPW16/0021- Operational Works Application 2 of 4

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3.3       Rational!Method!Calibration!
The! existing! scenario! WBNM! model! has! been! calibrated! to! the! Rational! Method!
derived!peak!flows!at!the!outlet!of!the!catchment!draining!to!northern!boundary!of!
the!site!(WBNM!catchment!node!‘Outlet’).!!The!adopted!calibration!parameters!are!
detailed!in!the!Section!3.1.!
A!summary!of!the!Rational!Method!calculation!input!parameters!is!provided!below!
in!Table!9!and!additional!details!of!the!Rational!Method!calculation!are!provided!in!
Appendix!2!of!this!report.!
Table!9:!Rational!Method!Calibration!Summary!(Total!Catchment)!

              Parameter!                                               Value!
              Time!of!Concentration!(tc)!                              6!min!
              Catchment!Area!                                        1.1784!ha!
              C10!Value!                                               0.48!
              100!year!Rainfall!Intensity!                          302.7!mm/hr!
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A! comparison! of! peak! Rational! Method! flow! rates! compared! with! peak! WBNM!
model!flow!rates!are!provided!in!Table!10!below.!
Table!10:!Rational!Method!Peak!Flows!Versus!WBNM!Peak!Flows!!

                      2yr!ARI!      5yr!ARI!   10yr!ARI!   20yr!ARI!     50yr!ARI!   100yr!ARI!
    Catchment!                      (m3/s)!
                      (m3/s)!                   (m3/s)!     (m3/s)!       (m3/s)!     (m3/s)!
      Rational!         0.24!         0.3!       0.33!       0.42!         0.54!       0.62!
      WBNM!            0.298!       0.387!      0.437!      0.506!        0.536!       0.598!
    %!Difference!       19%!         22%!        24%!        17%!          Z1%!        Z4%!
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The!above!results!show!that!a!reasonable!peak!flow!calibration!has!been!achieved,!
particularly!for!the!larger!events!of!the!WBNM!model!when!compared!to!a!Rational!
Method! calculation.! ! The! WBNM! model! estimates! slightly! lower! peak! flows! in! the!
smaller! events,! however,! this! would! be! considered! acceptable! in! assessing! nonZ
worsening!of!flows.!

3.4       Developed!Scenario!
The! proposed! development! has! been! incorporated! into! the! WBNM! model! by!
adjusting! the! site! subZcatchments! and! the! fraction! impervious! values.! ! Fraction!
impervious! values! of! the! existing! and! proposed! scenarios! are! provided! in! Table! 11!
below.!!The!modelled!development!case!adopts!conventional!impervious!fractions!as!
a! conservative! estimate.! The! developed! scenario! WBNM! model! layout! is! shown! in!
Figure!6!of!this!report.!!!It!is!noted!that!the!catchment!area!reporting!to!C1!outlet!is!
significantly!reduced!by!the!proposed!development!(Existing!=!2265!m2;!Developed!=!
1550!m2).!!The!supplied!hydraulic!design!from!FLOTH!(Appendix!1)!is!based!on!the!
capture! of! all! runoff! up! to! and! including! a! 100yr! ARI! event! within! the! internal!
stormwater!collection!network.!!This!network!is!outlet!to!the!south!eastern!corner!of!
the!site!and!flows!to!Outlet!C2.!!Therefore,!it!is!expected!that!the!peak!flows!from!


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