2016 QUDM Detention & Retention Basins PDF

  5.   Detention/retention  systems   5.1   General   In  the  absence  of  adequate  controls,  urban  development  can  modify  the  natural  drainage  regime,   potentially  altering  the  volume,  rate,  frequency,  duration  and  velocity  of  stormwater  runoff,  as  well   a...

  5.   Detention/retention  systems   5.1   General   In  the  absence  of  adequate  controls,  urban  development  can  modify  the  natural  drainage  regime,   potentially  altering  the  volume,  rate,  frequency,  duration  and  velocity  of  stormwater  runoff,  as  well   as  the  water  quality.  Stormwater  detention  and  retention  systems  can  be  used  to  help  minimise   any  such  changes  in  the  runoff  characteristics  of  a  drainage  catchment.     In  the  context  of  this  chapter,  detention/retention  systems  include  traditional  detention  basins,  on-­ site  detention  (OSD),  extended  detention  and  stormwater  retention  systems,  all  of  which  have  the   effect  of  reducing  and  delaying  peak  flow  rates.  A  definition  of  each  of  these  systems  is  contained   within  the  Glossary  (Chapter  13).   5.2   Master  drainage  planning   While  helping  to  reduce  many  of  the  adverse  impacts  of  urbanisation,  detention  and  retention   systems  can  also  introduce  various  problems  that  need  to  be  avoided  during  the  planning  phase.     Discussion  on  the  potential  problems  caused  by  detention/retention  systems  is  presented  within   the  Background  Notes.     If  catchment  modelling  is  being  used  to  investigate  a  single  development,  then  the  following  issues   need  to  be  given  appropriate  consideration:     a  single  development  cannot  be  considered  in  isolation  from  the  cumulative  effects  of  a  fully   developed  catchment     the  cumulative  effects  of  stormwater  detention/retention  should  be  determined  by  modelling  the   hydraulic  conditions  that  would  exist  if  all  future  land  developments  were  conducted  in   accordance  with  the  current  Planning  Scheme     appropriate  consideration  also  needs  to  be  given  to  the  likely  impacts  of  the  development   under  existing  catchment  conditions     the  potential  adverse  impacts  on  waterway  flooding  must  be  considered  over  all  reaches  of  a   waterway  where  flood  waters  are  likely  to  adversely  affect  either  the  value  or  potential  use  of   the  land.   5.3   The  uses  of  stormwater  detention  and  retention   Stormwater  ‘detention’  systems  are  typically  utilised  for  the  following  purposes:     controlling  or  attenuating  peak  discharges  to  minimise  the  potential  for  a  stormwater  nuisance   to  occur  within  down-­slope  properties     controlling  or  attenuating  peak  discharges  to  ensure  new  developments  do  not  cause  existing   drainage  systems  to  exceed  their  desired  operational  capacity  (this  issue  may  relate  to  the   hydraulic  capacity  of  a  piped  drainage  system,  or  the  depth-­velocity  safety  limits  of  an   established  overland  flow  path)     controlling  or  attenuating  peak  discharges  for  the  purpose  of  reducing  or  preventing  increases   in  downstream  creek  flooding     controlling  or  attenuating  peak  discharges  for  the  purpose  of  minimising  accelerated  channel   erosion  within  downstream  waterways.     Queensland  Urban  Drainage  Manual   2016  Edition   Detention/Retention  Systems    5-­1        Stormwater  ‘retention’  systems  include  a  broad  range  of  urban  water  features,  including  many   constructed  lakes,  wetland  and  water  quality  treatment  systems.  Even  though  flow  attenuation  may   not  be  a  key  feature  of  a  particular  lake,  pond  or  wetland,  each  of  these  features  are  likely  to   provide  one  or  more  of  the  following  benefits:     capturing  a  proportion  of  stormwater  runoff  from  minor  rain  events  for  the  purpose  of   minimising  potential  nuisance  within  down-­slope  properties  as  a  result  of  the  increased   frequency  and/or  duration  of  minor  surface  flows     treating  stormwater  runoff  to  improve  its  quality  and  thus  reduce  potential  adverse  effects  on   receiving  waterways     reducing  the  volume  of  stormwater  runoff  for  the  purpose  of  reducing  stresses  on  aquatic  life   within  receiving  waters  (e.g.  minimising  increases  in  ‘frequent  flows’)     reducing  the  volume  of  stormwater  runoff  for  the  purpose  of  reducing  the  risk  of  accelerated   channel  erosion  within  receiving  waters  (e.g.  minimising  increases  in  the  frequency  and   duration  of  near-­bankfull  flows)     reducing  the  volume  of  stormwater  runoff  for  the  purposes  of  reducing  the  annual  runoff  of   certain  pollutants     reducing  the  volume  of  stormwater  runoff  for  the  purposes  of  reducing  the  potential  for  the   overlapping  of  flood  hydrographs  discharged  from  basins,  thus  reducing  the  risk  of  increased   flooding  downstream  of  multiple  basins     collecting  rainwater  or  stormwater  runoff  for  the  purpose  of  reducing  a  region’s  dependence  on   town  water.     5.4   Limitations  on  the  use  of  stormwater  detention  and  retention   It  should  not  be  assumed  that  stormwater  detention  and  retention  systems  can  fully  mitigate  all  the   flooding  and  drainage  problems  associated  with  urban  development.  Some  problems  will  continue   to  exist  simply  because  there  is  no  perfect  solution,  while  other  problems  will  continue  because  the   potential  solutions  could  introduce  or  further  exacerbate  problems.     An  extensive  discussion  on  the  limitation  of  stormwater  detention  and  retention  systems  to  control   key  outcomes  is  provided  within  the  Background  Notes.     5.5   Design  objectives  and  standards   5.5.1   General   Design  standards  depend  on  the  required  functions  of  the  detention/retention  system.  If  the   detention/retention  system  is  required  to  satisfy  more  than  one  function,  e.g.  flood  control   and  the  control  of  creek  erosion,  then  appropriate  consideration  must  be  given  to  achieving   the  specific  flood-­control  and  erosion-­control  requirements  concurrently.     In  all  cases,  detention/retention  systems  must  not  cause  unacceptable  increases  in  flood  levels   upstream  or  downstream  of  the  system.  An  unacceptable  increase  in  flooding  would  include  any   change  in  flood  characteristics  on  surrounding  properties  that  could  present  a  significant  safety   risk,  damage  to,  or  adversely  affect  either  the  value  or  potential  use  of  the  land,  or  cause  problems   as  a  result  of  changes  in  flow  velocity  or  the  distribution  of  flow  velocity  across  that  land.   Queensland  Urban  Drainage  Manual   2016  Edition   Detention/Retention  Systems    5-­2        5.5.2   On-­site  detention  systems   There  are  generally  three  design  standards  set  by  regulating  authorities,  they  are:   (i)   A  specified  minimum  site  storage  requirement  (SSR)  and  permissible  site  discharge  (PSD)   relative  to  either  the  site  area,  land  use,  or  the  change  in  impervious  area.   (ii)   A  permissible  site  discharge  for  the  specified  design  storm  frequency  with  no  minimum   storage  volume  specified.   (iii)   A  requirement  not  to  exceed  pre-­development  peak  discharge  rates  for  a  range  of  design   storm  frequencies.     The  first  two  design  criteria  are  often  adopted  by  local  governments  following  the  development  of  a   regional  flood  control  strategy,  Master  Drainage  Plan,  or  Stormwater  Management  Plan.     Most  small  on-­site  detention  systems  incorporate  in-­ground  tanks.  When  appropriate  soil  and   groundwater  conditions  exist,  some  of  these  systems  can  be  converted  into  infiltration  systems.   Above-­ground  stormwater  detention  tanks  are  not  normally  recommended  for  single  residential   properties  because  of  the  risk  of  the  systems  being  decommissioned  or  being  converted  into   stormwater  harvesting  systems  that  no  longer  provide  the  required  discharge  control.   5.5.3   Discharge  restrictions  due  to  limited  down-­slope  drainage  capacity   The  objective  here  is  to  control  the  maximum  site  discharge  to  avoid  over  taxing  an  existing  down-­ slope  drainage  system.  This  may  occur,  for  example,  when  a  down-­slope  drainage  system  is   known  to  be  undersized,  or  was  designed  for  a  lesser  degree  of  catchment  development.     If  the  development  is  a  one-­off  modification  to  the  catchment,  then  the  design  standard  can  be   directly  linked  to  the  design  standard  for  the  downstream  drainage  system.     If  the  development  is  not  a  one-­off  modification  to  the  catchment,  then  either:     the  local  authority  would  provide  an  acceptable  drainage  standard  for  the  development  (e.g.   permissible  site  discharge,  PSD)—this  usually  requires  modelling  of  the  catchment  based  on   ultimate  development  conditions;;  or     the  ‘time  of  concentration’  (Rational  Method)  or  ‘critical  storm  duration’  (runoff-­routing  methods)   is  determined  for  the  undersized  drainage  system,  and  the  site’s  drainage  is  then  designed  to   ensure  that  the  peak  discharge  from  the  site  during  a  storm  of  such  duration  is  not  increased   above  pre-­development  conditions;;  or     the  ‘time  of  concentration’  (Rational  Method)  or  ‘critical  storm  duration’  (runoff-­routing  methods)   is  determined  for  the  undersized  drainage  system,  and  the  site’s  drainage  is  then  designed  to   ensure  that  the  peak  discharge  from  the  site  during  a  storm  of  such  duration  does  not  exceed  a   fraction  of  the  drainage  system’s  allowable  discharge  equal  to  the  ratio  of  development’s   effective  C*A  to  the  drainage  system’s  C*A  for  ultimate  development  conditions,  i.e.       Qsite  =  Qpipe  *  (C*A)site/(C*A)pipe   (5.1)   where:   Qsite   =   allowable  site  discharge  during  a  storm  of  duration  equal  to  the  critical  storm   duration  of  the  downstream  drainage  system  of  concern   Qpipe   =   allowable  discharge  of  the  downstream  drainage  system  during  a  storm  of  duration   equal  to  the  critical  storm  duration  of  that  system   Queensland  Urban  Drainage  Manual   2016  Edition   Detention/Retention  Systems    5-­3        (C*A)site   =   the  effective  product  of  the  coefficient  of  discharge  (C)  and  catchment  area  (A)  for   the  development   (C*A)pipe  =   the  effective  product  of  the  coefficient  of  discharge  (C)  and  catchment  area  (A)  for   the  ultimate  development  of  the  drainage  system’s  full  catchment   5.5.4   Controlling  local  stormwater  flooding   The  objective  here  is  to  minimise  the  risk  of  causing,  or  aggravating,  damage  caused  by   stormwater  runoff  entering  nearby  or  down-­slope  buildings  or  causing  soil  erosion.     Such  problems  commonly  exist  when  a  down-­slope  building  is  inappropriately  designed,  or  the   property’s  landscaping  collects  and  redirects  stormwater  runoff  in  a  manner  that  directs  it  towards   a  building,  specifically  the  doorway  of  a  building.  Problems  can  also  exist  if  a  down-­slope  building   or  fencing  is  constructed  across  a  natural  overland  flow  path.     The  design  standard  is  commonly  the  1  in  50  year  ARI  (2%  AEP)  event.   5.5.5   Controlling  downstream  creek  flooding   The  objective  here  is  to  minimise  potential  increases  in  creek  flooding  within  downstream   properties.  The  term  ‘creek  flooding’  is  used  solely  to  represent  watercourse  flooding  where  the   watercourse  is  of  sufficiently  small  size  that  flood  levels  could  measurably  be  impacted  upon  by   residential  development.  This  is  unlikely  to  be  the  case  for  most  rivers,  except  for  the  very  upper   reaches  of  these  waterways  where  the  river  looks  and  behaves  more  like  a  ‘creek’.     Of  course,  this  design  objective  only  needs  to  be  achieved  in  circumstances  when  downstream   creek  flooding  is  known  to  be,  or  expected  to  be,  a  problem  that  required  drainage  management.     The  design  standard  depends  on  whether  or  not  the  creek  catchment  has  been  modelled.  If  full   catchment  modelling  exists,  then  the  local  authority  may  be  in  a  position  to  specify  a  permissible   site  discharge  (PSD)  for  new  developments.  If  such  modelling  does  not  exist,  then  the  following   design  procedure  is  recommended.     Recommended  design  procedure  in  the  absence  of  a  specified  Permissible  Site  Discharge:     The  development  should  be  designed  such  that  the  peak  site  discharge  from  the  site  does  not   increase  above  that  listed  below  for  any  of  the  standard  storm  durations  that  are  considered  to  fall   within  the  range  considered  ‘critical’  for  those  downstream  locations  where  flooding  is  judged  to  be   a  concern.  Unless  otherwise  directed  by  the  local  authority,  the  maximum  recommended  storm   duration  is  3-­hours.  The  allowable  site  discharge  being  either:     pre-­development  peak  discharge,  or     that  which  would  be  expected  from  the  development  based  on  an  urban  density  or  percentage   impervious  surface  area  specified  by  the  local  authority.     The  standard  storm  durations  are:  10,  15,  30  &  45  minutes;;  1,  1.5,  2  &  3  hours.     For  example,  if  the  downstream  flood-­affected  reach  of  the  creek  covers  a  range  of  critical  storm   durations  from  30  minutes  to  2  hours,  then:     If  the  pre-­development  discharge  for  a  30  minute  storm  is  X  L/s,  then  post-­development  Q  120   B   13   200   10–120   C   6   125   1–10   D   3   75    1000     Extreme     Notes:     Sourced  from  DEWS  (2013)  (modified  from  ANCOLD,  2000b.)     It  is  unlikely  that  the  severity  of  damage  and  loss  will  be  negligible  where  one  or  more  houses  are   damaged.     Minor  damage  and  loss  would  be  unlikely  when  PAR  exceeds  10.     Medium  damage  and  loss  would  be  unlikely  when  the  PAR  exceeds  1000.     Not  used.     Change  to  High  C  where  there  is  the  potential  for  one  or  more  lives  being  lost.     Refer  to  ANCOLD  (2000b)  –  sections  2.7  and  1.6  for  explanation  of  the  range  of  High  Hazard   Categories.   5.10.2   Spillway  design   The  high-­level  outlet,  usually  formed  by  a  spillway,  must  be  designed  to  safely  convey  extreme   outflows  from  the  basin.  The  design  flow  should  consider  the  potential  for  full  or  partial  blockage  of   any  outlet  structures.  Wherever  practical,  design  of  the  spillway  should  assume  full  blockage  of  the   low-­flow  outlet.     Where  possible,  the  spillway  should  be  cut  into  virgin  ground  at  the  side  of  the  embankment,  or   otherwise  located  to  minimise  the  possibility  of  embankment  failure.     In  some  circumstances  the  high-­level  outlet  may  be  constructed  as  a  glory-­hole  inlet  (with  bar   screen  and  anti-­vortex  device  as  required)  leading  to  a  pipe  or  a  culvert  through  the  embankment.     The  spillway  chute  may  be  protected  by  riprap,  concrete,  paving,  or  other  suitable  coverings.  A   grass  or  reinforced  grass  cover  may  be  adequate  where  spillway  slopes  are  flatter  than  1  on  6   (1V:6H).  Care  should  be  taken  to  maintain  a  healthy,  continuous  grass  cover  on  grass  spillways.   Trees,  shrubs,  watering  tap  outlets,  or  any  other  fixed  structure  that  may  cause  turbulence  or   eddy-­induced  erosion  must  not  be  located  within  a  grassed  spillway  chute.  Design  information  for   grassed  spillways  is  described  by  the  U.S.  Soil  Conservation  Service  (1979).     5.11   Embankments   Detention  basins  are  intermittent  water-­retaining  storages  for  which  the  embankments  do  not  need   to  be  as  rigorously  designed  as  dams  unless  they  are  particularly  high  or  have  special  soil   problems.  Retention  basins  are  designed  to  have  a  permanent  or  semi-­permanent  water  storage   component—these  structures  need  particular  design  measures  if  the  retention  depth  is  significant.       The  embankment  design  of  all  detention  and  retention  basins  should  be  undertaken,  or  at  least   reviewed  by  a  suitably  experienced  Geotechnical  specialist.     Queensland  Urban  Drainage  Manual   2016  Edition   Detention/Retention  Systems    5-­15        The  sides  of  grassed  embankments,  including  any  inner  basin  grassed  slopes,  should  generally  be   flatter  than  1  on  6,  and  never  steeper  than  1  on  4  for  reasons  of  mower  access.  The  top-­width   should  be  at  least  three  (3)  metres.  Steeper  slopes  may  be  used  on  embankments  or  basins  lined   with  structural  facings  or  low-­maintenance  ground  covers,  but  steps  must  be  provided  at   appropriate  intervals  if  the  steepness  of  the  slope  could  impede  the  egress  of  a  person  from  the   basin  during  a  flood.     5.12   Public  safety  issues   While  detention  and  retention  basins  are  generally  less  hazardous  than  drainage  channels  with   respect  to  water  velocity,  they  are  typically  much  deeper.  The  safety  hazards  associated  with  these   basins  are,  however,  likely  to  be  less  obvious  to  the  public.  Safety  hazards  associated  with   submerged  outlet  structures  can  be  significant—consequently,  measures  usually  need  to  be  made   to  prevent  the  public  approaching  these  structures  while  basins  are  in  operation.     The  hazards  associated  with  off-­stream  basins  (i.e.  basins  through  which  a  watercourse  does  not   flow)  are  likely  to  be  less  obvious  than  those  associated  with  on-­stream  basins,  thus  greater   consideration  may  need  to  be  given  to  safe  egress  from  off-­stream  basins.     The  most  important  safety  features  of  detention  and  retentions  basins,  which  includes  ponds,  lakes   and  wetlands,  are  likely  to  be:     the  avoidance  of  unprotected  steep  slopes  around  the  water’s  edge  that  increase  the  risk  of  a   person  falling  into  the  water,  especially  during  a  storm  event  when  water  levels  are  likely  to   have  increased  above  normal  water  level     the  provision  of  safe  egress  at  regular  intervals  around  the  circumference  of  the  water  body,   which  is  usually  a  function  of  bank  slope  and  vegetation  type     means  of  preventing  a  person  approaching  the  high  velocity  and  high  suction  pressure  region   immediately  in  front  of  outlet  structures.     The  side  slopes  of  ‘grassed’  basins  should  preferably  be  1  on  6  or  flatter  to  allow  easy  egress  up   the  likely  wet  surface.  Grassed  areas  with  slopes  steeper  than  1  on  4  will  require  steps  and  a   handrail  to  assist  egress.  Vegetated  slopes  that  allow  a  person  to  gain  assistance  or  safe  footing   as  a  result  of  the  shrubby  vegetation,  may  be  steeper.  These  recommendations  especially  apply  to   basins  that  incorporate  dual  use  activities  such  as  passive  or  active  recreation.     The  provision  of  exclusion  fencing  around  open  water  stormwater  detention/retention  systems   should  be  considered  a  last  resort.  Wherever  practical,  the  first  preference  should  be  to  minimise   the  safety  risk  through  appropriate  basin  design.     Special  attention  should  be  paid  to  basin  outlets  to  ensure  that  persons  trapped  in  the  basin’s   water  are  not  drawn  into  the  basin’s  outlet  system.  Rails,  fences,  anti-­vortex  devices,  trash  racks  or   grates  should  be  provided  where  necessary.  Outlet  systems  should  be  located  well  away  from  the   water’s  edge  of  the  flooded  basin  such  that  a  person  wading  along  the  edge  of  the  basin  cannot  be   drawn  into  the  basin’s  outlet.  This  usually  requires  the  outlet  system  to  either  be  located  well  away   from  the  embankments,  or  surrounded  by  fencing/bollards.     Where  suitable  areas  of  land  are  available,  it  is  recommended  that  designers  limit  basin  depths  to   1.2  m  at  the  1  in  20  year  AEP  level  and,  if  possible,  for  a  greater  recurrence  interval.  This   requirement  relates  to  the  safety  risks  associated  with  adults  wading  through  the  water  to  look  for,   and  rescue,  a  child  swept  into  the  water.  However,  it  is  acknowledged  that  this  depth  limit  is  rarely   practical.     Queensland  Urban  Drainage  Manual   2016  Edition   Detention/Retention  Systems    5-­16          Depth  indicators  should  be  considered  within  the  basin  and  in  the  channel  downstream  of  the   embankment  for  basins  with  a  storage  depth  of  greater  than  one  (1)  metre.  The  indicator  within  the   basin  should  have  its  zero  level  relative  to  the  lowest  point  in  the  basin  floor.     Suitable  safety  provisions,  such  as  fences  and  warning  signs,  should  be  provided  for  deeper   basins.  Ultimately,  the  owner  of  the  detention/retention  basin  is  required  to  accept  the  ongoing   responsibility  for  maintaining  the  above  safety  standards.     5.13   Statutory  requirements   Works  constructed  within  a  watercourse  generally  require  approval  under  the  Water  Act  2000  and   need  to  satisfy  all  legal  requirements  of  this  Act.  When  applying  this  Act,  the  definition  of  a   ‘watercourse’  as  presented  within  the  Act  must  be  applied,  not  the  definition  provided  within  this   Manual.     Under  the  Water  Supply  (Safety  and  Reliability)  Act  2008  (Water  Supply  Act)  and  under  common   law,  responsibility  for  the  safety  of  a  dam  rests  with  the  dam  owner.  Dam  owners  may  be  liable  for   harm,  loss  and  damage  caused  by  the  failure  of  a  dam  or  the  escape  of  water  from  a  dam.   Consequently,  dam  owners  need  to  be  committed  to  dam  safety  and  have  an  effective  dam  safety   management  program.  A  dam  safety  management  program  is  intended  to  minimise  the  risk  of  a   dam  failing  and  to  protect  life  and  property  from  the  effects  of  such  a  failure,  should  one  occur.     In  addition  the  embankment  for  a  detention  basin  may  require  a  failure  impact  assessment  under   the  Water  Supply  Act  and  development  approval  from  the  State  Assessment  and  Referral  Agency   under  the  Planning  Act  if:     the  dam  will  be  more  than  10  m  in  height  and  have  a  storage  capacity  of  more  than  1500  ML;;   or     the  dam  will  be  more  than  10  m  in  height  and  have  a  storage  capacity  of  more  than  750  ML   and  a  catchment  area  that  is  more  than  3  times  its  maximum  surface  area  at  full  supply  level;;   or     the  Chief  Executive  gives  the  dam  owner  a  notice  to  have  a  dam  failure  impact  assessed   (regardless  of  its  size)  because  the  Chief  Executive  reasonably  believes  the  dam  will  have  a   Category  1  or  Category  2  failure  impact  rating;;  or     if  works  proposed  to  an  existing  dam  will  result  in  the  dam  exceeding  certain  thresholds  (see   Section  343  of  the  Water  Supply  Act  for  details).     Referable  dams  are  classified  according  to  categories  which  are  based  on  the  population  at  risk  if   the  dam  fails,  that  being:     dams  with  a  Category  1  failure  impact  rating  have  2  or  more  persons  and  not  more  than  100   people  at  risk     dams  with  a  Category  2  failure  impact  rating  have  more  than  100  persons  at  risk.     If  less  than  2  people  are  at  risk  by  the  dam  failing  then  the  dam  is  not  referable  under  the  Water   Supply  Act.     People  are  considered  to  be  part  of  the  population  at  risk  if  there  is  a  300  mm  incremental  rise  in   water  level  at  a  place  of  occupation  in  the  event  of  dam  failure.     Queensland  Urban  Drainage  Manual   2016  Edition   Detention/Retention  Systems    5-­17        A  dam  is  referable  if:     a  failure  impact  assessment  is  required  to  be  carried  out  under  the  Water  Supply  Act     that  assessment  states  that  the  dam  has  or  will  have  a  Category  1  or  Category  2  failure  impact   rating     the  Chief  Executive  has,  under  the  Water  Supply  Act,  accepted  the  assessment.     In  addition,  some  dams  were  made  referable  by  the  transitional  provisions  in  the  Water  Supply  Act.     Once  a  dam  is  accepted  as  being  referable,  the  Chief  Executive  may  impose  safety  conditions  on   the  dam.  These  safety  conditions  effectively  require  a  dam  safety  management  program  to  be   developed  and  implemented  for  the  dam.       Queensland  Urban  Drainage  Manual   2016  Edition   Detention/Retention  Systems    5-­18