Chapter 9 Rivers and Flooding

Environmental  Geology   Chapter  9    Rivers  and  Flooding     • Flooding  in  Pakistan  2010  -­‐1600  killed/20000  affected   • The  hydrologic  cycle  is  powered  by  the  Sun  –  The  cycle  includes   evaporation,  precipitation,  infiltration,  runoff  &  transpiration  …..  This   chapter  is  about  the  “runoff”  part  of  the  cycle:  the  process  that  “sculpts”   Earth’s  surface   • Streams  =  Small  rivers.   • River  components:   – Network  of  streams   – Watershed  or  drainage  basin   – Base  level  and  slope/gradient   – Latitudinal  profile   – Longitudinal  profile   – Grading  processes   • Drainage  Basin  -­‐  The  land  area  that  contributes  to  the  flow  of  a  stream     • Streams  flow  down  to  their  base  level  =  the  low  point  at  which  the  water   stops  moving  and  collects.      Sea  level  =  the  ultimate  base  level.     • Lakes  and  reservoirs  =  temporary  or  local  base  levels.       • Lowering  the  base  level  steepens  the  overall  slope,  causing  streams  to   increase  in  velocity  and  to  erode.       • Raising  base  level  will  lessen  the  overall  slope,  causing  streams  to  slow   down  and  deposit  sediment.   • River  Sediment  Transport      Load  =  transported  material   – Bed  load  -­‐  Coarse  particles  moving  along  the  bottom  of  river  channel   (less  than  10  percent  of  the  total  load)   – Suspended  load:  Accounts  for  about  90  percent  of  its  total  load  and   makes  river  look  muddy   – Dissolved  load:  Carried  in  chemical  solutions,  such  as  HCO3-­‐,  SO42-­‐,  Ca   +,  Na  +,  Mg+   • Stream  Competence  &  Capacity   – Competence  =  the  maximum  size  transported  by  a  river  (largest   particle  it  can  move)   – Capacity  =  the  total  amount  a  river  capable  of  transporting  (the   maximum  load  of  solid  particles  it  can  carry)   – Note:    If  stream  velocity  doubles,  competence  increases  by  four   times.   • River  Dynamic  Characteristics     • Continuity  equation:     – Discharge:  The  volume  of  water  passing  through  a  given  location  of  a   river  per  unit  of  time   Q  =  W  *  D  *  V   – W  =  Width  of  flow   – D  =  Depth  of  flow   V  =  velocity   Q  =  Discharge  in  CUBIC  Meters  per  second   – – • • •   • • •     A  stream’s  ability  to  erode  and  carry  away  sediment  is  controlled  by  its   velocity,  which  is  a    function  of  gradient  (steepness  of  slope),  cross-­‐ sectional  shape,  channel  size  and  roughness     PROFILES  -­‐  Low  gradient  versus  High  gradient  streams     • V  shape  =  High  gradient  (downcutting  –  erosional)    =  high  velocity  &   competence   • Low  gradient  –  low  velocity  &  competence  (near  base  level)     • Low-­‐gradient  streams  (low  slopes)  typically  erode  only  side-­‐to-­‐side   in  their  floodplains,  producing  wide  valleys  with  flat  floors,  most   of  which  is  the  floodplain.       • Low-­‐gradient  streams  commonly  meander  back  and  forth  across   the  floodplain.         Floods  are  common  geological  events  and  involve  the  stream  leaving  its   channel  and  filling  part  or  all  of  the  floodplain,  blanketing  it  with  fresh   sediment  after  the  flood  subsides.  The  new  sediment  is  a  beneficial  result  of   flooding  –  related  to  farming.     When  meanders  get  “cut  off”,  an  oxbow  lake  results.     River  Erosion  Effectiveness  is  related  to:   • Stream  velocity   • Stream  discharge   • Stream  load   • Nature  of  the  rocks     • Regional  topographic  relief   • Base  level   • Climatic  conditions   River  Erosion   • Erosion  types   – Abrasion  by  sediments  transported  by  river   – Hydraulic  action  of  moving  water   – Chemical  corrosion   • Erosion  location  -­‐  types   – Downcutting   – Lateral:  Concentrating  on  the  outer  bends  (cutbanks)   – Headward  =  erosion  in  the  upstream  direction   • Streams  erode-­‐downcut  under  certain  conditions  such  as  when:   • sea  level  drop  during  an  ice  age       • a  stream  is  captured  (via  stream  piracy  –  arrrrgh!)  by  another   stream  with  a  lower  base  level   • there  is  gradual  uplift  and  steepening  of  mountain  slopes  through   plate  tectonics       • • • • • Deposition  Condition:  The  flow  of  a  river  slows  down  as  a  result  of:   – Decrease  of  stream  gradient   – Decrease  of  velocity   – Decrease  of  discharge   – Change  of  channel  shape   – Change  in  the  amount  of  stream  load  (e.g.,  land-­‐use  change,  vegetation   cover)   – Change  of  geologic  setting  (e.g.,  rock  types  along  the  river  banks)     • Construction  of  a  dam  raises  the  base  level  (creates  a  temporary   base  level)  resulting  in  a  depositional  condition.    A  wedge  of   sediment  accumulates  behind  the  dam  that  decreases  the  gradient   of  the  stream.     River  Dynamic  Characteristics  -­‐  Rock  types  along  river  channels     – Harder  rocks  will  produce  boulders   – Softer  rocks  will  produce  mud/sand   – Boulders  in  channel  slow  down  flow  the  most   – Mud  slows  down  flow  the  least   • Smaller  clasts  produce  less  friction  along  streambed  and  river   banks   River  Sediment  Deposition  -­‐  Deposition  features   – Floodplain   – Natural  levee   – Point  bar   – Island  bar     – Alluvial  fan   – Delta   – Note:    Stream  sediment  deposits  in  general  are  known  as   alluvium.       Flooding  in  Thailand  2010/2011  –  One  of  the  worst  floods  in  memory   – Heavy  rains   – Bangkok  below    sea  level   – Up  to  3m  water   – Devastation  to  homes  and  industry   – Deadly   – Expensive   – Expensive     Point  Bar  Deposition   • • • • • • • • • Stream  Velocities  and  Channels  in  Meander  –  the  highest  velocity  is  on   the  cutbank  side  of  the  river  in  a  meander.    Deposition  occurs  on  the   point  bar,  on  the  bank  opposite  the  cutbank.    Meanders  migrate  laterally   by  erosion  on  the  cut  banks  and  by  deposition  on  point  bars.    Channels   often  contain  a  series  of  regularly  spaced  pools  and  riffles     Braided  vs.  Meandering  Channels  –  Low  gradient  streams  -­‐  Braided   streams  result  from  an  abundance  of  coarse  bed  load  sediment.    The   channels  tend  to  be  wide  and  shallow  compared  with  meandering  rivers     Alluvial  fan  deposits  are  produced  by  velocity  decrease  at  the  edge  of  the   mountains  or  when  the  stream  enters  a  lake  or  sea       Drainage  patterns  (as  seen  from  above)  include  dendritic  -­‐  most  common   (uniformly  eroded  rock  types)  &  radial,  rectangular,  and  trellis.      produced   by  specific  geologic/landform  conditions.     Dynamics  of  River  Erosion  and  Deposition   • Ever-­‐changing  processes:  Time  and  rate  of  erosion  and  deposition     • Reasons  for  the  changes—Complex,  but  related  to     Ø Changes  in  river  channel  (width,  depth    and  slope)   Ø Composition  of  channel  bed  and  banks   Ø Vegetation  cover   Ø Variations  of  weather  and  climate  pattern   Ø Human  activities,  particularly  land-­‐use  changes   Effects  of  Land-­‐Use  Changes     • Changes  in  infiltration  rate:  Change  of  the  amount   of  water  flowing  into  a  river   • Soil  erosion:  Change  in  the  amount  of  sediments  in   a  river   • Amount  of  water  and  sediments  in  river:  Changes  in  the  velocity  of  water   flow     Changes  in  river’s  velocity:  Leading  the  change  in  river  dynamics   Effects  of  Land-­‐Use  Changes   • Forest  to  farmland   Ø Increases  soil  erosion,  stream  deposition   Ø Increases  gradient  and  velocity   Ø Increases  river-­‐channel  erosion   • Urban  build-­‐up   Ø Increases  impervious  cover     Ø Increases  certain  flood  frequency   Ø Reduces  the  lag  time  of  flood   Flooding   •Flooding:  Overbank  flow  condition,  discharge  greater  than  channel’s  holding   capacity   • • • • • • • • • •Stage:  The  height  of  the  water  level  in  a  river  at  a  given  location  at  a  given   time     •Hydrograph:  Graphic  representation  of  a  river’s  discharge  over  time   •Lag  time:  The  amount  of  time  between  the   occurrence  of  peak  rainfall  and  the  onset  of  flooding   Flooding  Data  Measurement   Frequency  and  Magnitude  of  Flood   Recurrence  interval       R  =  (N  +  1)/M       N  as  the  number  of  years  of  record,  M  is  the  rank    of  individual   flow  within  the  recorded  years   The  probability  of  a  given  magnitude  flood       P  =  1/R   Statistical  probability  versus  reality       Probability:  One  25-­‐year  flood,  on  average,  once     every  25   years       Reality:  Two  25-­‐year  floods  within  the  same  year   Types  of  Flooding   • By  stream  location   Ø Upstream  flood:  Shorter  duration,  smaller  area   Ø Downstream  flood:  Longer  duration,  greater  magnitude,  larger  area   • By  duration   Ø Flash  flood:  High  volume  of  flooding  water  in  very  short  duration,   characteristic  short  lag  time   • By  time/magnitude   Ø 100-­‐year,  50-­‐year,  25-­‐year,  10-­‐year  floods     Factors  Affecting  Flood  Damage   • Regional  land-­‐use  changes,  such  as  urban  development,  deforestation,   soil  erosion,  etc.   • Land  use  on  the  floodplain     • Frequency  and  magnitude  of  flooding   • Lag  time  and  duration  of  flooding   • Sediment  load   • Effectiveness  of  forecasting,  warning,  and   emergency  management   Urbanization  and  Flooding   • Impact  on  frequency  and  magnitude   Ø Increase  in  both  frequency  and  magnitude,  especially  in  small   drainage  basins   • Impact  on  a  river’s  discharge   Ø Increase  in  runoff,  without  an  increase  in  precipitation   • Significant  reduction  in  lag  time  or  flashy  discharge     • Flood  hazard  increases  as  percentage  of  impervious  area  increases  in  an   urban  setting   Impact  Effects  of  Flooding   Primary  effects   Ø Injury  and  loss  of  life,  damage  and  destruction   of  property,  erosion  and  deposition  of  sediments   • Secondary  effects   Ø Water  pollution   Ø Fire   Ø Diseases   Ø Displacement  of    people   Ø Interruption  of  social  and  economic  activities   Adjustments  to  Flood  Hazards   • The  structural  approach   Ø Physical/Engineering  barriers:  Levee  augmentation  (see  next  slide)     Ø Channelization   Ø River-­‐channel  restoration   Ø Reservoir  construction  –  to  store  excess  water  for  later  release   Ø Storm  sewers   • Flood  insurance:  Shared  responsibility  and  accountability   • Flood-­‐proofing:  Raised  foundation,  floodwalls,  waterproof  doors  and   windows,  pumps  -­‐  can  create  false  sense  of  security  of  living  in  floodplain   • Levee  Augmentation  -­‐  Levees  are  NOT  unbreakable  &  people  should   not  think  they  are.   After  levee  construction  –  land  behind  levees  farmed  &  wetlands  confined   between  levees   Channel  Restoration   Ø Viable  alternative  to  deepening  and  straightening  of  channels   Ø Less  aesthetically  and  ecologically  damaging  to  river  system   Ø Allows  for  variety  of  environments  (pools  vs.  riffles,  shaded  vs.  non-­‐,   high  vs.  low  velocity  flow,  gravelly  vs.  silty/sandy)  and  diverse  biology   Ø With  minor  engineering  (e.g.,  riprap  on  outside  of  stream  bends)  it   can  be  effective  at  protecting  adjacent  areas  from  some  flooding   Flood  insurance:  Shared  responsibility  and  accountability   • National  flood  insurance  available   Ø Properties  within  100-­‐year  floodplain  are  eligible  (100-­‐year  flood  =   largest  flood  to  occur  in  given  century)   Ø Properties  within  20-­‐year  floodplain  prohibited/not  insurable  as  new   sales   Ø Pre-­‐existing  properties  (before  floodplain  determination)  may  be   grandfathered  in,  but  would  be  uninsurable  for  any  rebuilding   Ø 30-­‐day  waiting  period  for  new  policies     Ø Floodproofing,  relocations,  and  buyouts/demolitions  are  now   promoted   Adjustments  to  Flood  Hazards   • Floodplain  regulation:  Obtaining  the  most  beneficial  use  of  floodplains   Ø Flood-­‐hazard  mapping   Ø Floodplain  zoning   • Government  buyout  and  relocating  people  from  floodplain   • • • • • • • •     Ø Allows  for  some  use  of  floodplains  that  provides  high  benefit  with   lower  risks   Ø Encroachment  on  floodplains  can  increase  heights  of  subsequent   floods!   • Personal  adjustments   Storm  Hazard  Mapping   Ø 100-­‐year  floodplain  evaluation/mapping   Ø Images  taken  during  flooding  are  key   Ø Historical  records  of  flooding  very  important   Ø Should  lead  to  improved  zoning  regulations   Individual  citizens  typically  have  a  harder  time  recognizing  (or   acknowledging)  flood  hazards  than  public  institutions/agencies   Perception  of  Flooding   • Individual  level:  Variable  in  their  knowledge  of  flooding,  anticipation  of   future  flooding,  and  willingness  to  accept  adjustments     • Local  and  state  level:  Mitigation  plans   • Federal  government  level   Ø Mapping  of  flood-­‐prone  areas   Ø Floodplain  management  plans   Ø Public  outreach