30

BOKIv  FORMATION

BOKIv  FORMATION

Fig.  5.5.1:  Change  in  signal  shape  near  the  coast  (linear  Boussinesq  equations,  ij  surface
elevation,  h  undisturbed  water  depth,  10.5:  PEDpT).
Non-linear  effects  would  finally  cause  the  wave  to  break  and  form  a  bore.  Since  the  breaking
of  a  tsunami  involves  considerable  dissipation  of  energy,  shoaling  does  not  lead  to  a  further
increase  in  the  wave  height  of  the  bore  despite  decreasing  water  depth,  but  wave  height
even  decreases  again  (Fig.  5.5.2,  right).  Bore  formation  has  been  parameterised  in  the
course  of  the  exemplary  simulation  of  (10.5:PEDpT).

BORE  FORMATION  BORE  FORMATION

Fig.  5.5.2:  Bore  formation  (linear  Boussinesq  equations,  ij  surface  elevation,  h  undisturbed
water  depth,  10.5:  PEDpT).
A  tsunami  turning  into  a  bore  does  so  along  the  entire  shore.  Tidal  waves  in  estuaries  form
bores  mainly  under  the  influence  of  a  change  in  cross  section.  In  some  funnel-shaped  river
estuaries  narrowing  toward  upstream,  the  height  increase  of  the  tidal  wave  is  so  sudden  that
it  takes  the  shape  of  a  surf  wave  (Dietrich  et  al.  1975,  Figs.  8.45  and  10.6:  CAR04).  Also
tsunami  travelling  into  estuaries  may  develop  into  this  type  of  bore.
The  numerical  simulation  of  processes  close  to  the  coastline  and  the  propagation  of  tsunami
on  land  are  still  subject  to  intensive  research  (Geist  et  al.  2006).