28

'7.5

Fig.  5.4.2:  Amplitude  development  on  a  1:2,500  shelf  slope  at  a  shelf  width  of  100  km  (left
scale:  depth,  “German  Bight”)

Fig.  5.4.3:  Amplitude  development  on  a  1:4,000  shelf  slope  at  a  shelf  width  of  800  km  (left
scale:  depth,  “North  Sea”)Topographic  effects
5.4.3  Topographic  effects
In  the  barotropic  two-dimensional  North  Sea  models  referred  to  in  sections  7.3  to  7.5,  the
dissipation  of  energy  through  internal  friction  and  bottom  friction  is  taken  into  account  in
complex  form  (Dick  et  al.  2001).  Fig.  7.4.9  shows  no  asymptotic  behaviour  of  wave  height,
but  a  limitation.  A  successive  increase  in  initial  wave  heights  by  1  m  leads  to  an  increase  of
just  0.05  m  in  wave  height  for  waves  arriving  near  the  coast  (at  Cuxhaven,  first  signal  in  Fig.
7.4.9).  Flowever,  Cuxhaven  is  not  a  well-chosen  example  because  at  this  location  even  the
primary  signal  is  caused  exclusively  by  diffraction  (compare  section  7.4.2).  Fig.  5.4.4  shows
peak  water  levels  at  selected  stations  along  the  North  Sea  coast.  The  lines  between  Wick
and  Borkum  are  comparable  to  those  in  Fig.  5.4.3.  The  high  values  at  North  Shields  and  the
additional  values  from  Ijmuiden  til  Hirtshals  indicate  an  influence  other  than  dissipation.
Bottom  topography  and  the  shape  of  the  coastline  influence  the  propagation  and  modification
of  tsunami-size  waves  in  various  ways.  Fig.  7.3.1  shows  a  circular  wave  in  the  northwest  that
has  been  caused  by  diffraction.  Figs.  7.4.1  to  7.4.6  are  examples  of  the  superposition  of  such
secondary  waves  (interference),  which  may  lead  to  very  high  water  levels,  especially  in  bays
where  wave  reflection  is  a  contributing  factor.
Waves  on  the  shelf  are  reflected  and  scattered  by  topographic  features,  just  like  they  are  in
the  deep  ocean  and  on  the  continental  slope.  Especially  pronounced  in  the  North  Sea  is  the
influence  of  topography  on  the  propagation  velocity  <Jgh  and  on  changes  in  the  direction  of
propagation  due  to  refraction  (cf.  figures  in  sections  7.3  -  7.5).
Such  wave  phenomena  are  understood  (e.g.  Masselink  2005)  and  have  been  discussed  with
respect  to  tsunami  (Camfield  1990,  Mofjeld  2000).  To  simulate  these  processes  in  detail,  a
good  representation  of  the  bottom  topography  is  indispensable.