2014022_71_SeaviewTce_SWMP-revC_full · OPW16/0021

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3 Stormwater!Quantity!
The!objective!of!this!component!of!the!study!is!to!investigate!the!potential!impact!
that! the! proposed! development! will! have! on! peak! flows! discharging! from! the! site.!!
Where! any! increases! of! peak! flows! are! found,! potential! mitigation! options! will! be!
investigated!in!the!form!of!a!detention!system.!!!
Hydrologic!modelling!of!the!catchment!has!been!carried!out!using!WBNM!software!
developed! jointly! by! the! University! of! Wollongong,! Rienco! Consulting! and! Balance!
R&D.! ! The! 2012! version! has! been! used! for! this! investigation.! ! A! peak! flow! rational!
method!calibration!has!also!been!carried!out.!
Modelling! has! been! carried! out! for! the! 2,! 5,! 10,! 20,! 50! and! 100! year! ARI! storm!
events.!!The!following!scenarios!have!been!investigated:!
        1.      Existing!Scenario!
        2.      Developed!Scenario!–!conventional!roof!(100!%!impervious)!
        3.      Mitigated!Scenario!
A! conventional! impervious! roof! type! was! modelled! in! order! to! conservatively!
quantify! the! likely! impact! the! ‘green’! roof! may! have! on! peak! flow! generated! from!
the!site.!!It!is!recognized!that!while!the!roof!will!be!covered!with!up!to!200!mm!of!
planted!media!it!will!not!act!entirely!as!a!catchment!with!0%!impervious!fraction.!!In!
reality,! the! peak! flows! generated! from! the! green! roof! will! likely! fall! between! a!
partially!impervious!catchment!and!a!fully!impervious!catchment.!!!

3.1     Site!Drainage!and!Catchment!Definition!
The! site! is! relatively! steep! with! runoff! flowing! in! a! north! easterly! direction!
predominately!as!sheet!flow!with!limited!channel!delineation.!As!noted!above!there!
is! an! external! catchment! to! the! west,! consisting! of! two! existing! residential!
allotments!on!McAnally!Drive,!which!currently!drains!through!the!site.!!!
There!is!an!existing!gully!and!drainage!line!located!to!the!north!of!the!site!boundary!
which!conveys!stormwater!from!the!outlet!at!the!turnaround!on!McAnally!Drive,!the!
national! park! and! likely! receives! flow! from! the! subject! site.! ! The! drainage! line!
appears!to!cross!the!site!at!the!northeastern!corner.!!
For!the!purposes!of!this!investigation!it!is!assumed!that!all!site!runoff!flows!across!
the! northern! boundary! and! for! calculation! of! nonTworsening! peak! flows! will! be!
measured!at!the!outlet!of!the!site!subTcatchment!at!the!boundary.!
SubTcatchments! delineated! for! the! WBNM! model! have! been! based! on! detailed!
survey!and!existing!contour!details!provided!for!the!site.!!The!WBNM!subTcatchment!
layout!is!shown!in!Figure!6.!!The!WBNM!model!has!been!split!into!4!subTcatchments!
in! order! to! represent! the! site.! The! external! catchment! to! the! west! of! the! site! has!
been!included!in!the!model!as!runoff!from!the!area!would!be!directed!through!the!
site!but!not!flow!into!the!proposed!developed!area.!A!small!portion!of!the!site!at!the!
north!western!corner,!which!is!proposed!not!to!be!developed,!flows!off!site!to!the!
north!and!has!not!been!included!in!modeling.!




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