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Page Title: Types of Tides and Reference Planes
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Types of Tides and Reference Planes Semidiurnal, Diurnal, and Mixed  Tides According  to  the  characteristics  of  the  tidal  pattern  occurring  at  a particular  place,  tides  are  classified  as  semidiurnal,  diurnal,  or  mixed. In  semidiurnal  tides,  there  are  two  high  and  two  low  tides  each  tidal day,  and  they  occur  at  fairly  regular  intervals.  Usually,  there  are  only relatively  small  variations  in  the  height  of  any  two  successive  high  or low  waters.  Tides  on  the  Atlantic  coast  of  the  United  States  are representative  of  this  pattern. In  diurnal  tides,  there  is  only  one  high  and  one  low  tide  each  tidal  day. The  water  levels  on  succeeding  days  usually  do  not  vary  a  great  deal. In  the  United  States,  diurnal  tides  occur  along  the  northern  shore  of  the Gulf  of  Mexico. In  mixed  tides,  the  tidal  pattern  is  characterized  by  wide  variations  in heights  of  successive  high  and  low  waters.  There  are  usually  two  high and  two  low  waters  each  day,  but  occasionally  the  tide  may  become diurnal.  In  the  United  States,  mixed  tides  occur  along  the  Pacific  Coast, Alaska,  and  Hawaii.  If  information  for  water  depths,  heights,  elevations of  topographical  features,  aids  to  navigation,  bridge  clearances,  and  so forth  are  to  be  meaningful  when  printed  on  nautical  charts,  standard reference  planes  for  their  measurements  must  be  used.  For  this  reason, standard  reference  planes  for  these  measurements  have  been  established. Generally  speaking,  heights  and  elevations  are  given  on  a  chart  in reference  to  a  standard  high-water  plane,  while  heights  of  tide  and charted  depths  of  water  are  given  with  respect  to  a  standard  low-water plane  (see  fig.  7-2).  The  charted  depth  is  simply  the  vertical  distance from  the  low-water  reference  plane  to  the  ocean  bottom;  it’s  the  depth figure  you  see  printed  on  nautical  charts.  The  charted  height  is  the vertical  distance  above  the  water  measured  from  the  high-water reference  plane. The  mean  range  of  tide  is  the  vertical  distance  between  the  high  water and  low-water  reference  planes  used,  and  represents  the  average  range of  tide  at  a  given  location. You  should  remember  that  the  water  level will  sometimes  be  below  the  reference  plane.  Put  another  way, sometimes  the  actual  depth  of  water  can  be  less  than  the  charted depth.  You  will  recognize  this  situation  because  there  will  be  a  minus sign  (-)  placed  before  the  height  of  tide  shown  in  the  Tide  Tables.  In this  case,  you  subtract  the  value  of  the  height  of  tide  from  the  charted depth  to  find  the  actual  depth  of  water. 7-5

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