15

3.3  Duration  of  storm  surges  at  Swinoujscie

In  order  to  estimate  the  duration  of  a  storm  surge,  two  points  in  time  have  to  be  determined:
its  beginning  and  end.  For  example,  the  inflexion  point  of  the  tide  curve  may  be  used  as  the
beginning  of  a  storm  surge.  In  the  forecasting  routine  in  the  Baltic  Sea,  however,  this  point
cannot  be  predicted  in  advance  (in  real  time).  Therefore,  for  practical  purposes,  a  daily
reference  sea  level  indicator  parameter  is  applied  to  calculate  the  duration  of  a  storm  surge.
The  hour  at  which  the  sea  level  rises  above  the  daily  reference  sea  level  (before  reaching  the
maximum  value)  has  been  defined  as  the  beginning  of  a  storm  surge,  and  the  hour  at  which
it  drops  below  the  daily  reference  sea  level  (after  having  reached  the  maximum  value)  as  its
end.
At  Swinoujscie,  surges  lasting  21  -  30  hours  and  31  -  40  hours  had  the  highest  frequency
(26%  and  25%,  respectively),  which  amounted  to  about  half  of  all  surges.  19.2%  of  all  surges
lasted  11-20  hours,  and  about  12%  of  surges  were  in  the  41  -  50  hour  range.  Only  10.9%
of  the  surges  lasted  from  51  to  70  hours,  and  6.9%  more  than  70  hours.  The  surges  with  the
longest  duration  on  record  at  Swinoujscie  in  the  second  half  of  the  20 th  century  exceeded  80
hours  and  occurred  in  April  1997  (117  hours),  February  1979  (85  hours),  November  1981  (93
hours),  and  January  1987  (82  hours).

Fig.  3.5.  Storm  surge  duration  at  Swinoujscie  -  frequency  distribution
3.4  Warning  and  alarm  levels
The  warning  and  alarm  levels  for  a  particular  section  of  the  coast  are  determined  taking  into
account  the  height  and  frequency  of  occurrence  of  dangerous  surge  levels,  the  configuration
of  the  coastline,  and  existing  coastal  defence  structures  such  as  dikes  etc.  In  case  of  an
expected  storm  surge,  the  hydrological  forecasting  services  are  required  to  inform  the
responsible  authorities  of  the  expected  duration  of  the  dangerous  surge  level  above  the
warning  levels,  and  particularly  above  the  alarm  levels.  This  information  is  essential  for  the
organisation  of  protection  measures.
At  the  coasts  around  Swinoujscie,  the  warning  and  alarm  levels  have  been  set  at  560  cm
and  580  cm,  respectively.  Higher  levels  have  been  set  at  the  coasts  between  Sassnitz  and
Wismar,  at  600  cm.
The  period  during  which  emergency  levels  were  exceeded  is  indicated  in  hours  as  an  annual
total  of  each  stormy  season  (Table  3.4.).