8

Negative  Surges  in  the  Southern  Baltic  Sea

the  above  period,  with  a  lower  frequency  in  the
last  two  decades  than  in  the  first  decades.  Under
extreme  conditions,  water  levels  may  drop  more
than  1.5  m  in  12  hours.  Water  levels  are  correlated ­
  with  local  wind  patterns,  and  the  best  wind
correlation  for  the  GDR  coast  was  found  at
Arkona.  Based  on  such  correlations,  it  has  been
possible  to  develop  different  statistical  prediction
models.  A  very  simple  model  is  wind  surge
curves,  with  water  levels  shown  as  a  function  of
wind  speeds  and  directions.  For  example,  with
southwesterly  winds  of  20  m/s  at  Arkona,  a  water
level  1  m  below  normal  has  to  be  expected  at
Warnemünde.  However,  such  simple  point  correlation ­
  inadequately  reflects  the  complicated  natural ­
  processes  taking  place.  Mewes  (1987)
distinguished  among  three  different  cylone  paths
causing  negative  surges  on  the  coast  of  the
GDR:  firstly,  cyclones  which  form  in  the  western
North  Atlantic  Ocean  and  track  across  the  North
Sea,  secondly,  cyclones  forming  roughly  south  of
Iceland  and  tracking  northeast  and,  thirdly,
cyclones  forming  off  Iceland  which  track  along
Scandinavia  toward  the  Nordic  Seas  but  whose
centre  does  not  cross  the  Baltic  Sea.  After  the
reunification  of  Germany,  Baerens  et  al.  (1995)
studied  the  frequency  of  negative  surges  on  the
German  Baltic  Sea  coast  between  Flensburg  and
Warnemünde  without  including  the  eastern  part
of  the  Baltic  between  Warnemünde  and  the
national  border  of  Poland.  In  contrast  to  the  declining ­
  number  of  negative  surges  on  the  coast  of
Mecklenburg-Vorpommern,  their  frequency  in  the
period  from  1900-1990  has  actually  increased  on
the  coast  of  Schleswig-Holstein.  This  contrasting
development  appears  to  be  linked  to  the  direction ­
  of  the  coast,  but  its  exact  cause  is  still
unknown.  In  both  areas,  negative  storm  surges
are  most  frequent  in  the  time  from  November
through  January.
Water  level  fluctuations  on  the  coast  of  Poland
and  in  the  eastern  part  of  the  German  coast  are
thought  to  be  closely  linked,  but  a  cross-border
analysis  of  negative  surges  has  not  been  carried
out  so  far.  Therefore,  the  development  of  negative ­
  surges  in  the  different  coastal  sections  and
the  exact  causes  of  differences  in  the  occurrence
of  negative  surges  in  different  parts  of  the  coast
still  are  not  fully  understood.

1.3.  Geographic  and  hydrodynamic
background
The  region  dealt  with  in  this  monograph  is  the
south  coast  of  the  western  and  central  parts  of
the  southern  Baltic  Sea.  The  western  part  of  the
Baltic  is  shaped  like  a  bay  opening  to  the  east,
whose  width  increases  from  25  nm  at  Wismar  to
120  nm  at  Kotobrzeg  (Fig.  1.3.  a).  The  westernmost ­
  part  of  the  southern  Baltic  coast  between
Wismar  on  the  Mecklenburg  Bay  and  Cape
Arkona  on  the  island  of  Rügen  extends  roughly  in
a  southwest  to  northeastward  direction.  This  part
of  the  coast,  covered  by  the  gauges  at  Wismar
and  Warnemünde,  has  a  highly  variable  topography ­
  with  shallow  water  and  a  multitude  of  creeks,
shoals,  and  sandbanks.
The  central  section  of  the  coast  extending  from
the  high  chalk  cliffs  of  Cape  Arkona  to
Swinoujscie  and  the  Odra  estuary  (gauges  at
Sassnitz  and  Swinoujscie)  is  oriented  from  northwest ­
  to  southeast  and  also  has  intricate  topographic ­
  features  comprising  small  sandy  coastal
islets,  narrows,  and  sandbanks.  In  the  adjacent
Pomeranian  Bay,  also  the  seabed  is  highly  variable, ­
  with  shallow  depths  below  10  m  prevailing.
A  particularly  wide  belt  of  shoals  exists  off  the
island  of  Uznam  (Usedom),  in  the  waters  close  to
the  Swinoujscie  gauge,  and  around  the  island  of
Wolin.
The  rather  straight  eastern  part  of  the  coast
between  Wolin  and  Kotobrzeg  runs  in  a  westsouthwest
  to  east-northeast  direction.  Also  the
10-m-isobath,  which  is  only  one  nautical  mile  off
the  coast,  runs  in  a  rather  straight  line.
Considering  the  subdivision  of  the  coast  into
three  parts  differing  in  their  topography,  the  probability ­
  of  extreme  water  levels  occurring  at  Sassnitz ­
  would  be  expected  to  be  on  the  same  order
as  at  Swinoujscie.  The  same  holds  for  the
gauges  at  Wismar  and  Warnemünde.  However,
because  of  the  bay  effect,  the  probability  of
occurrence  of  extreme  sea  level  events  decreases ­
  from  west  to  east,  as  is  clearly  shown  in
chapter  3.3.  One  of  the  main  causes  of  this  phenomenon ­
  is  the  size  of  the  area  of  open  water
relative  to  the  coast  length  and  the  widening  of
the  bay.
The  main  factor  influencing  coastal  sea  levels  in
the  Baltic  Sea  region  is  wind,  which  either
pushes  the  water  away  from  the  coast  or  towards
it,  whereas  tides  are  of  minor  importance.
Another  factor  of  lesser  importance  is  the  chang-