Fig.  7.4.1:  Water  level  distribution  (input  signal:  3  positive  single  signals,  wave  height  5  m,
from  the  north,  North  Sea  2  km)
Left:  after  1.5  hours  with  realistic  depth  distribution  (T  1800  s)
Flight:  after  0.5  hour  with  constant  water  depth  (h  500  m,  T  600  s)

54

7.4  North  Sea  signal  from  the  north
Figs.  7.4.1-6  show,  in  their  left  part,  the  propagation  of  the  boundary  signal  described  in
section  7.1  (three  positive  signals,  71800  s,  H  5  m).  For  better  physical  understanding,
computations  were  additionally  carried  out  with  a  constant  water  depth  of  500  m  in  the  entire
North  Sea.  In  these  simulations,  the  same  kind  of  boundary  signals  were  used,  but  a  period
of  600  s.
7.4.1  Propagation
The  influence  of  realistic  bottom  topography  is  obvious  from  the  faster  propagation  in  deep
water,  in  this  case  the  Norwegian  Trench.  In  the  north,  the  simulation  shows  major  local
water  level  maxima  (Figs.  7.4.1  and  7.4.2,  left  figures).  They  are  caused  by  directional
changes  and  subsequent  superposition  among  the  single  signals  due  to  an  inhomogeneous
depth  gradient  (refraction).  These  maxima  do  not  occur  in  the  comparative  computations  with
a  flat  bottom  (right  figures).
Diffraction  and  reflection  produce  locally  very  high  water  levels  in  bays  and  estuaries,  and
near  islands  (e.g.  Lerwick  station,  Fig.  7.1.3).  Such  processes  are  governed  by  the  shape  of
the  coastline  and  the  location  of  islands.  They  may,  however,  be  subject  to  modification  by
variable  bottom  topography,  which  influences  the  direction  and  velocity  of  propagation,  cf.
Figs.  7.4.2  and  7.4.3,  left  figures.
The  German  coast  is  located  in  the  wave  shadow  of  Norway.  It  is  reached  first  by  extensions
of  the  input  signal  that  has  weakened  on  the  shelf  (Fig.  7.4.4).  Later,  a  secondary  signal
generated  by  superposition  of  a  diffraction  pattern  also  arrives  at  the  coast  (Fig.  7.4.6).  It  is
generated  mainly  by  diffraction  and  reflection  on  the  British  coast.  It  is  better  visible  in  the
flat-bottom  simulations  but  also  constitutes  a  significant  signal  in  model  time  series  using  a
realistic  topography.
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